Condensate and Flash Steam Recovery

85
TECHNICAL REFERENCE GUIDE Condensate and flash steam recovery

description

Condensate and Flash Steam Recovery

Transcript of Condensate and Flash Steam Recovery

Page 1: Condensate and Flash Steam Recovery

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ECondensate and

flash steam recovery

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Contents

Introduction 3

Condensate return 4

Why return condensate and reuse it? 4Condensate recovery cost saving example 6

Condensate return lines 9Drain lines to traps 9Discharge lines from traps 10• Discharging into flooded return lines 10Common return lines 11• Temperature controlled plant with steam traps draining into flooded lines 13Discharge lines at different pressures 13Discharge lines from vented pumps 14Sizing condensate lines 14Sizing drain lines to traps 15• From steam mains 15• From process applications 16Sizing discharge lines from traps 18• Recommendations on trap discharge lines 20• The condensate pipe sizing chart 21Sizing common return lines 29Sizing pumped return lines 31• Pumping traps and pump-trap installations 33

Condensate pumping from vented receivers 34Pumping terminology 34Electrical centrifugal condensate pumps 37Sizing an electrical condensate recovery unit 38Sizing the discharge pipework for an electrical condensate recovery unit 39Mechanical condensate pumps 41Sizing a mechanical condensate pump 43Sizing the discharge pipework for a mechanical condensate pump 45Longer delivery lines 45Fully loaded pumps and longer lines 46Consideration of a larger pump and smaller pipeline 47

Lifting condensate from steam mains drain traps 50Contaminated condensate 51Stall and the stall point 52

The stall cycle 52Temperature controlled plant 54• Condensate drainage to atmosphere 54• Closed loop condensate drainage 55Determining the stall point on controlled plant 57Using the stall chart 57A typical stall chart 60Constant pressure plant 61

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Contents

Flash steam 62What is flash steam and why should it be used ? 63How much flash steam ? 64Sub cooled condensate 64Pressurised recovery 65The flash vessel 66Sizing flash steam recovery vessels 66Requirements for successful flash steam applications 68Control of flash steam pressure 68

Typical applications for flash steam 70Flash steam supply and demand in-step 70Flash steam supply and demand not in-step 73Boiler blowdown heat recovery applications 74Spray condensing 76

Steam tables 78Further information 80Appendix 1 - Condensate line sizing chart 81

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Introduction

Steam is usually generated for one of two reasons :

to produce power, as in power stations and co-generation plants.

to carry energy for heating and process systems.

When a kg of steam condenses, a kg of condensate at the samepressure and temperature is formed. An efficient steam distributionsystem will make good use of this condensate. Failure to do somakes no financial, technical or environmental sense.

Steam, used for heating, gives up its latent heat, which is a largeproportion of its total heat. The remainder is held by the condensedwater. As well as having heat content, the condensate is also adistilled form of water, which is ideal for use as boiler feedwater. Anefficient installation will collect condensate and either return it to thedeaerator, boiler feedtank, or use it in another process. Only whenthere is a real risk of contamination should condensate not be returnedto the boiler. But then it may be possible to collect the condensate anduse it as hot process water or pass it through a heat exchanger whereits heat content can be recovered before discharging to drain.

Condensate is discharged through traps from a higher to a lowerpressure. As a result of this drop in pressure, some of thecondensate will then re-evaporate into 'flash steam'. The proportionthat will 'flash off' is determined by the pressure difference betweenthe steam and condensate sides of the system, and a figure of 10 %to 15 % by mass is typical. However, the percentage volumetricchange can be considerably more. Condensate at 7 bar g willlose about 13 % of its mass when flashing to atmospheric pressure,but the steam produced will require a space some 200 timeslarger than the condensate from which it was formed. This canhave the effect of choking undersized trap discharge lines, andshould be taken into account when sizing these lines.

The flash steam generated can contain up to half of the totalenergy of the condensate, hence flash steam recovery is anessential part of an energy efficient system. Condensate andflash steam discharged to waste means replacement feedwater,more fuel, and increased running costs.

This technical reference guide will look at two essential areas -condensate management and flash steam recovery. Some of theapparent problem areas will be outlined and solutions offered.Illustrations, together with tables and charts to which reference ismade, are included in the text. Basic steam tables can be found atthe end of this guide.

Note: the term 'trap' is used to denote a steam trapping devicewhich could be a steam trap, a pumping trap, or a pump-trapcombination. The ability of any steam trap to pass condensaterelies upon the pressure difference across it, whereas apumping trap or a pump-trap combination is able to removecondensate irrespective of pressure differences across it.

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Condensate return

Monetary value. Condensate is a valuable resource and eventhe recovery of small quantities is often economically justifiable.The discharge from a single steam trap is often worth recovering.

Unrecovered condensate is replaced by cold make-up water withadditional costs of water treatment and fuel to heat the water froma lower temperature.

Water charges. Any condensate which is not returned needs tobe replaced by make-up water, incurring further water chargesfrom the local water supplier.

Effluent restrictions. In the UK for example, water above 43°Ccannot be returned to the public sewer because it is detrimental tothe environment and may damage earthenware pipes. Condensateabove this temperature must be cooled if discharged, which couldincur extra energy costs. Similar restrictions apply in most countriesand effluent charges and fines may be imposed by water suppliersfor non-compliance.

An effective condensate recovery system, collecting the hotcondensate from the steam using equipment and returning it tothe boiler feed system, can pay for itself in a remarkably shortperiod of time. Fig. 1 shows a typical steam and condensatecircuit, where condensate is returned to the boiler feedtank.

Why returncondensate and

reuse it?

Fig. 1 A typical steam and condensate circuit

Feedtank

Make-up water

Feedpump

Steam

Boiler

Spaceheatingsystem

Process vessel

Condensate

Vat Vat

Pan Pan

Condensate

Steam

Steam

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Figure 2 shows the relative amounts of energy in steam andcondensate at various pressures.

Maximising boiler output. Colder boiler feedwater will reducethe steaming rate of the boiler. The lower the feedwatertemperature, the more heat,and thus fuel needed to raise steam.

Boiler feedwater quality. Condensate is a distilled water whichcontains almost no dissolved solids (TDS). Blowdown is used toreduce the concentration of dissolved solids in the boiler. Morecondensate returned to the feedtank reduces the need forblowdown and thus reduces the energy lost from the boiler.

Summary of reasons for condensate recovery.Water charges are reduced.

Effluent charges and possible cooling costs are reduced.

Fuel costs are reduced.

Boiler blowdown is reduced - less energy lost from boiler.

Chemical treatment is reduced.

kJ/k

g

Fig. 2 Heat content of steam and condensate

Pressure bar g

Saturated steam temperature °C

Heat available for flash steamrelease to atmospheric pressure

Latent heat (enthalpy of evaporation)

Total heat of steam

Heat in condensate at steam temperature

Heat in condensate at atmospheric pressure

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The following example demonstrates how savings are possibleby returning condensate to the boiler feedtank. Savings willobviously depend on the cost of fuel and water, and this examplegives typical costs in the UK at the time of writing.

The fuel used in this example is a heavy fuel oil with a grosscalorific value of 42 MJ/litre.

Fuel savings based on the following average temperaturesCondensate return temperature = 90°C

Make-up water temperature = 10°CTemperature difference = 80°C

Each kg of condensate not returned must be replaced by 1 kg ofcold make-up water that will need heating to the same temperature.

Heat required to raise 1 kg of cold make-up water by 80°C:

1 kg x 80°C x 4.19 kJ/kg °C = 335 kJ/kg

Basing the calculations on an average of 10 000 kg/h evaporationrate, and where none of the condensate is presently returned, 24hours a day, 7 days a week, 50 weeks of the year (8 400 h/year),the nett energy required to replace the heat in the make-up wateris:

10 000 kg/h x 335 kJ/kg x 8 400 h/year = 28 140 GJ / per year

If the average boiler efficiency is 85 %, gross energy needed toheat the make-up water

2 8140 GJ / year

= 33106 GJ/year0.85

With a calorific value of 42 MJ / litre, potential savings on fuel 33106 GJ / year

= 788 000 litres / year42 MJ / litre

With fuel at £0.15 / litre, cost savings = £ 788 000 x 0.15

Therefore, potential annual fuel savings = £ 118 200

Water savings. Total amount of water required in one year toreplace condensate which is not returned:

8 400 h x 10 000 kg/h = 84 000 m³1 000 kg/m³

Costed at £0.61 per m³: = £51 240

Therefore potential annual water savings = £51 240

Condensaterecovery

cost saving example

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Effluent savings. The condensate that was not recovered wouldhave to be discharged to waste which may also be charged by thewater authority.

Total amount of water to waste in one year also equals 84 000 m³

If effluent costs £0.45 per m³ =£37 800

Therefore, potential annual effluent savings =£37 800

Total potential savings. The total annual potential savings for10 000 kg/h evaporated based on none of the condensatepresently being returned are :

fuel savings = £ 118 200water savings = £ 51 240

effluent savings = £ 37 800

total savings = £ 207 240

It follows that for each 1% of condensate returned per 10 000 kg/hevaporated in the above example, a saving of 1% of each of theabove values would be possible.

To calculate relative savings based on the same reasoning, usethe formulae on the next page by putting figures in the blankboxes.

Fuel savings (on 80°C increase = £in feedwater)

Water savings = £

Effluent savings = £

Total = £

This sample calculation does not include a value for savings dueto correct TDS control and reduced blowdown which will furtherreduce water loss and boiler chemical costs. These can varysubstantially from location to location, but should always beconsidered in the final analysis. Consult Spirax Sarco for adviceregarding any specific installation.

Further information on how to calculate savings by automaticTDS control is available in the Spirax Sarco Technical ReferenceGuide TR-GCM-01, 'Water treatment, storage and blowdown forsteam boilers'.

Clearly, when assessing condensate management for a specificproject, such savings should be determined and included.

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Savings in currency used in 'D' =

£ 335 x A x B x C x DE x F

where:A = average evaporation rate in tonnes/hB = hours per yearC = percentage increase in condensate returnD = cost per unit of fuel ( £ / litre; £ / therm; £ / kg)E = calorific value of fuel per same unit ( MJ / litre; MJ / therm; MJ / kg)F = boiler efficiencyeg, consider the previous example, if a 30 % increase in condensatereturn is to be made, annual cost savings on fuel:

£ 335 x 10 x 8 400 x 30 x 0.1542 x 85

fuel savings = £ 35 470

Savings in currency used in 'C' =

A x B x C x D100

where:A = average evaporation rate in tonnes/hB = hours per yearC = cost per m³ of waterD = percentage increase in condensate return

eg, consider the previous example, if a 30 % increase incondensate return is to be made, annual cost savings on water:

£ 10 x 8 400 x 0.61 x 30100

water savings = £ 15 372

Savings in currency used in 'C' =

A x B x C x D100

where:A = average evaporation rate in tonnes / hB = hours per yearC = cost per m³ of effluentD = percentage increase in condensate return

eg, consider the previous example, if a 30 % increase incondensate return is to be made, annual cost savings on effluent :

£ 10 x 8 400 x 0.45 x 30100

effluent savings = £ 11 340

Fuel savings

Water savings

Effluent savings

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Condensate return lines

The subject of condensate piping will divide naturally into fourbasic sections where the requirements and considerations ofeach will differ. They are: Type of line Pipe sized to carry

Drain lines to traps condensate

Discharge lines from traps flash steam

Common return lines flash steam

Pumped return lines pumped condensate

The condensate must flow from the steam space outlet to the trap.The steam space and the body of the trap upstream of its orificewill usually be at the same pressure, and flow usually occurs dueto the force of gravity. As there is no significant pressure dropbetween the process and the trap, no flash steam is present in thepipe, and it can be sized to carry condensate only.

It should never be assumed that the plant outlet connectionindicates the correct size for the trap or condensate pipe, especiallyin the case of temperature controlled processes where lowdifferentials in pressure can occur across the trap under part-loadconditions. Each process will have its own system conditions,and should be treated with these in mind. Refer to the latersection 'Stall and the stall point' for further details. Stall is alsodiscussed in Reference Guides:Steam trapping and air venting - TR-GCM-11Condensate removal from heat exchangers - TR-GCM-23

Long drain lines from plant can fill with steam and preventcondensate getting to the trap. The effect is generally termed'steam locking'. To minimise this risk, drain lines should be keptshort (Fig. 3), first falling vertically wherever possible before anyhorizontal run, to ensure the trap is below the plant outlet. Thisalso encourages gravitational flow between the outlet and thetrap. Float traps are also available with steam lock release devicesto alleviate the problem.

Drain lines to traps

Fig. 3 Keep drain lines short

✗✔

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These carry condensate, incondensable gases, and flash steamfrom the trap to the condensate return system (Fig. 4). Flashsteam is formed due to the pressure drop across the trap orifice,caused by the difference in pressure between the steam andcondensate systems.

During start-up of a steam system, condensate will be cool withlittle or no flash steam, but the condensing rate will be maximum,and air will have to pass with the condensate. Soon, as thesystem heats up, full steam load may occur, the pressure in thesteam space will be at its highest, and the amounts of flashsteam released in the discharge line immediately after the trapwill be at their greatest. Trap discharge lines are sized on full loadconditions because of this. In so doing, the pipe will be adequatelysized for start-up loads, including the efficient purging of non-condensable gases.

Discharging traps into flooded return mains is best avoided,especially from blast action traps draining steam pipelines atsaturation temperature. Pumped and rising condensate linesoften follow the same route as steam lines, and it is tempting tosimply connect drain trap discharge lines into them. The highvolume of flash steam released into long flooded lines will violentlypush the water along the pipe, causing waterhammer, noise, andin the extreme, mechanical failure of the pipe. The solution is toavoid discharging into flooded lines by returning condensate andflash steam in lines that slope at least 1 in 70 down to a ventedcollecting receiver, from which it can be pumped.

Fig. 4 Trap disharge lines pass condensate, flash, andincondensables

Discharge lines fromtraps

Discharging intoflooded return lines

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Common return lines

Fig. 5 A swept tee connection

Steam main

Condensate main

Where condensate from more than one trap flows to the samecollecting point such as a vented receiver, it is feasible to run acommon line into which the individual lines can discharge, aslong as certain conditions are met, and the pipework is adequatelysized. When connecting to the common line, swept tees will helpto reduce mechanical stress and erosion at the joint (Fig. 5).

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If this is not possible, use a float trap to discharge into theflooded line (Fig. 6). The energy dissipated from the relativelysmall continuous flow from the float trap can usually be absorbedby the flooded line, especially when fitted with a diffuser suchas the DF2.

Fig. 6 Float trap with diffuser into a flooded line

Steam

Condensate

Diffuser

Another alternative is to use a thermostatic trap which holds backcondensate until it cools below the steam saturation temperaturethus reducing the amount of flash steam formed (Fig. 7). To avoidwaterlogging the steam main, the use of a generous collectingpocket on the main, plus a cooling leg of 2 to 3 m of unlagged pipeto the trap is essential. The cooling leg gives storage for condensatewhile it is cooling to the discharge temperature. If there is anydanger of waterlogging the steam main, do not use this method.Always consult expert advice from Spirax Sarco if in any doubt.

Fig. 7 Thermostatic trap with cooling leg into a flooded line

Diffuser

Condensate

Thermostatic trap setwith cooling leg

Steam

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Temperature controlled plant with steam trapsdraining into flooded lines

Take care if condensate from steam traps on temperature controlledplant is discharged into flooded lines. The back pressure couldhave a derogatory effect on the performance of the trap and theefficiency of the process (Fig. 8).

Heat exchanger

Steam trap Pumping trap

Heat exchanger

Floodedcommon line

Fig. 8 Discharge from steam traps into non-flooded lines if possible.

Non - floodedcommon line

Discharge lines atdifferent pressures

However, condensate from more than one temperature controlledprocess may join a common line as long as this line is:

a) designed to slope in the direction of flow to a collection point

b) sized to cater for the cumulative effects of any flash steam fromeach of the branch lines at full load.

The concept of connecting the discharges from traps at differentpressures is sometimes misunderstood.

If the branch lines and the common line are correctly sized, thepressures downstream of each trap should be virtually the same.However, if these lines are undersized, the flow of condensateand flash steam will be restricted due to a build up of backpressure caused by the increased friction along the pipe.Condensate flow from traps operating at lower pressures willtend to be restricted first.

Each part of the discharge piping system should be sized to carryany flash steam present at acceptable velocities. The dischargefrom a high pressure trap will not interfere with that from a lowpressure trap if the discharge lines and common line are properlysized and sloped in the direction of flow. A later section "Sizing ofcondensate lines" gives further details.

✗ ✔

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Discharge lines fromvented pumps

Sizing condensatelines

Fig. 9 Condensate recovery from a vented receiver

Plant PlantPlant

Receiver

Pump

Vent

pumpedcondensate

As mentioned previously, the four main situations for sizingcondensate lines are: Type of line Pipe sized to carry

Drain lines to traps condensate

Discharge lines from traps flash steam

Common return lines flash steam

Pumped return lines pumped condensate

Flash steam may ultimately be separated from the condensateand used in a recovery system, or vented to atmosphere from asuitable receiver (Fig. 9). The residual hot condensate from thelatter can be pumped on to a suitable collecting tank such as aboiler feedtank. When the pump is served from a vented receiver,the return line will be fully flooded with condensate having little orno tendency to create flash steam.

Flow in a pumped return line is intermittent as the pump startsand stops according to needs. The pump discharge rate will behigher than the rate at which condensate enters the pump. It isthe pump discharge rate that determines the size of the dischargeline.

Pumping will be further covered in a later section.

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Sizing drain lines totraps

From steam mains

Mains diameter - D Pocket diameter - d1 Pocket depth - d2

Up to 100 mm nb d1 = D Minimum d2 = 100 mm

125 - 200 mm nb d1 = 100 mm Minimum d2 = 150 mm

250 mm and above d1 = D / 2 Minimum d2 = D

d2d1

Steam mainD

Condensate return

A simple rule is to make the line to the trap the same size asthe trap connections. This presupposes, however, that the trapitself has been sized on sound technical reasoning. A briefsynopsis follows:

Steam traps basically fall into two distinct areas of application,steam mains or process applications.

The condensate load per trap is affected by various factors suchas the size of the pipe, pressure, degree of insulation, ambienttemperature, number of traps used along a defined length, positionand situation of the pipe. The Technical Reference Guide 'SteamDistribution' (TR-GCM-03), gives information for condensate loadswith different sized pipes at various pressures.

It is sufficient to consider a condensate load for each drain trapbased on 1% of the steam capacity of the main and traps placedevery 50 m if insulated, and 5% and 25 m if not. Whatever the sizeof the main and traps, it is important they are served by an adequatelysized drain pocket. As a guide, see below (Fig. 10):

Fig. 10

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From process applications

On most constant pressure applications, sizing the trap to passapprox twice the rated design load at the working pressure (minusany back pressure) will allow it to cope with both start-up andrunning loads.

Fig. 11 Typical constant pressure application

Air vent

Jacketed pan

Reducing valve

Trap set

The drain line off-take should be at least 25 to 30 mm from thebottom of the pocket for mains up to 100 mm, and roughly a thirdto centre of the pocket for larger mains. This allows a space belowthe outlet for dirt and scale to collect, and the bottom may be fittedwith a blowdown valve for cleaning purposes.

On most drain points, by sizing the trap to pass approx twice therated design load at the working pressure (minus any back pressure)will allow it to cope with both start-up and running loads.

The method of selecting and sizing the trap depends on whetherthe process is temperature controlled or not, but in either case thepipe should be sized as below on the worst condition.

i) Applications on constant steam pressureSome applications work on a constant pressure supply, such aspresses, ironers, ovens, unit heaters, radiant panels, boiling pansetc. When an adequate steam supply is provided, the workingpressure tends to remain fairly constant even under varying loadconditions. The worst condition will apply at start-up when thesteam pressure will tend to drop and the condensation rate is atits highest due to the large difference in temperature between thesteam and cold metal.

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ii) Applications with temperature controlIf the process is temperature controlled, the system operatingparameters and layout need to be considered in greater detail asthe heat load may change during normal operation. The steampressure and condensate load in the heat exchanger will alter asthe steam control valve modulates to meet this change, and asthe steam pressure reduces, so does the trap's capacity.

Take the case of an air heater battery which is designed to heat airfrom -5oC to 25oC using steam at 3.2 bar (145oC). If the incoming airtemperature rises to 5oC, the �T and heat load will be reduced by30%. The steam temperature will reduce by ratio, and onceestablished, its pressure can be established from steam tables.

Steam temp. at full load = 145 oC (a)Steam temp. at no load = 25 oC (b)ie, steam temperature range = 120 oC (a - b)30% of range = 40 oC (c) = ( [a - b] >< 0.3)steam temp. at 30 % reduction = 105 oC (a - c)steam pressure at 105 oC = 0.2 bar g (from steam tables)

The pressure in the heat exchanger has reduced from 3.2 bar g to0.2 bar g, and will reduce the trap's capacity. If the trappingdevice were a float trap and sized on the full load at 3.2 bar g,then it is possible that its capacity may be below that needed atthe lower pressure. It is for this reason that it is important to sizethe float trap on the minimum heat load rather than the full load.Should the steam space pressure reduce enough to approachthe condensate pressure, stall will occur and the trapping deviceis selected and sized on the load at stall point.

Not all temperature controlled applications will stall. Stall will notoccur if the steam space pressure at the minimum heat load ishigher than the condensate back pressure.

Whether the trapping device is a float trap, a pumping trap, or amechanical pump and float trap in combination, will depend onthe system operating requirements and the piping infrastructure.

The drain line can usually be the same size as the trap especiallyon shorter lines, but on lines over 5 m, should be checked onthe table in Fig. 37 on page 48, against a pressure drop of up to160 Pa / m.

The size of the trap discharge line needs to be determined by adifferent set of rules, and this is considered next.

Stall and its implications on trap sizing is discussed in furtherdetail in a later chapter, and in the Technical Reference Guide"Condensate Removal from Heat Exchangers" (TR-GCM-23)

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The section of pipeline downstream of the trap will carry bothcondensate and flash steam at the same pressure and temperature.This complex situation is called "two phase flow", where the mixtureof fluids will have the characteristics of both steam and water inproportion to how much of each component is present. Considerthis by example where 10% of condensate forms flash steam :

Sizing dischargelines from traps

As each kg of condensate at 4 bar g passes through the trap, 0.1kg will become steam at 100°C, and 0.9 kg will become water at100°C. However, the respective volumes will depend on thespecific volume of each at the pressure in the line (0 bar g).

0.9 kg of condensate will have a volume of = 0.0009 m³0.1 kg of flash steam will have a volume of0.1 kg >< 1.673 m³/kg (spec. vol.at 0 bar g) = 0.1673 m³Total volume of 1 kg of the mixture = 0.1682 m³

Therefore, 0.0009 >< 100 = 0.5% is volume of water in the line 0.1682

and, 0.1673 >< 100 = 99.5% is the volume of flash steam 0.1682

It follows that the flow of fluid through this line will have more incommon with steam than water, and it is sensible to size onreasonable steam velocities rather than the relatively small volumeof condensate. If lines are undersized, the flash steam velocityand back pressure will increase which can cause waterhammerand reduced trap capacity.

Fig. 12 Quantity of flashsteam from condensate

13

12

11

10

9

8

7

6

5

4

3

2

1

0

Pre

ssur

e on

trap

s ba

r g

0 0.02 0.06 0.10 0.14 0.18 0.22

Flash steam pressures

Example

kg Flash per kg condensate

2.5

bar g

2.0

bar g 0 ba

r g

0.5

bar g

1.0

bar g

1.5

bar g

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Steam lines are sized with attention to maximum velocities. Drysaturated steam can safely travel up to 40 m/s. Wet steam needsto travel somewhat slower (15 to 25 m/s) as it carries moisturewhich can have an erosive and damaging effect on fittings andvalves if travelling too fast. Similarly, trap discharge lines can beregarded as steam lines carrying very wet steam, and should besized on similar velocities.

Condensate discharge lines from traps are notoriously moredifficult to size than steam lines due to the two phase flowcharacteristic. In practice, it is impossible to determine what isgoing on inside the pipe with any certainty.

Although the amount of flash steam produced is related to thepressure difference across the trap, there are other factors thatwill have some bearing on what is happening inside the pipe. Forexample,

If, for some reason, the condensate on theupstream side of the trap is cooler than the saturation temperature,the amount of flash formed after the trap is reduced. This canreduce the size of the line needed.

If the line slopes down from the trap to its termination, thedegree of slope will have an effect on the flow of condensate, butto what magnitude, and how can this be quantified?

On longer lines, radiation losses from the line may condensesome of the flash, its volume will decrease along with its velocity,and there may be a case for reducing the line size. But at whatpoint should it be reduced and by how much?

If the discharge line lifts up to an overhead return line, therewill be times when the lifting line will be full of cool condensate,and times when flash steam from the trap may evaporate someor all of this condensate. Should the line be sized on flash steamvelocity or the quantity of condensate?

Most processes operate some way below their full loadcondition for most of their running cycle, which reduces theamount of flash produced for most of the time. Should thedesigner size on the full load condition when it may not bewarranted due to the frequency and small amount of time itoccurs?

On temperature controlled plant, the pressure differential acrossthe trap will itself change depending on the heat load. This willaffect the amount of flash steam produced in the line.

Due to the conflicting nature of all the above, an exact calculationof line size would be complex and probably inaccurate. In practice,experience has shown that if trap discharge lines are sized oncomfortable flash steam velocities and certain recommendationsare adhered to, few problems will arise.

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Correctly sized trap discharge lines that slope in the directionof flow and are open-ended are non-flooded and allow flashsteam to pass unhindered over the condensate, (Fig. 13). Aminimum slope of 1 in 70 (150 mm drop every 10 m) isrecommmended. A simple visual check will usually confirm if theline is sloping - if no slope is apparent it is not sloping enough!

Recommendationson trap discharge

lines

If unavoidable, non-pumped rising lines (Fig. 14) should bekept as short as possible and fitted with a non-return valve tostop condensate falling back down to the trap. They shoulddischarge into the top of overhead return lines to allow easypassage of flash steam into them. It is sensible to considerslightly larger pipes having lower flash steam velocities to reducethe risk of waterhammer and noise from the steam trying to findpassage through the liquid condensate in the rising line.

Important: A rising line should only be used where the loweststeam pressure in the process is guaranteed to be higher thanthe total condensate back pressure. If not, the process willwaterlog unless a pumping trap or pump/trap combination isused to provide proper drainage against the back pressure.

Fig. 13 Discharge line sloping 1 : 70 in the direction of flow

1 : 70 slope = 150 mm per 10 m run

Process

easy passage for condensate

easy passage for flash steam

vented receiver and pump

vent

pumpedcondensate

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Process

Fig. 14 Keep rising lines short and connect to the top of return lines

1 : 70 slope = 150 mm per 10 m run

ventedreceiver

and pump

vent

pumped condensate

Return lines themselves should also slope down and be non-flooded (Fig. 14). To avoid flash steam occurring in floodedreturn lines, hot condensate from trap discharge lines shoulddrain into vented receivers (or flash vessels where appropriate),from where it can be pumped on to its final destination via aflooded line at a lower temperature.

The Condensate pipe sizing chart (Fig. 15).The condensate pipe sizing chart can be used to size any type ofcondensate line.

Lines containing two-phase flow, such as trap discharge lines,are selected according to the pressures either side of the trap.The chart works around acceptable flash steam velocitiesaccording to the pipe size and percentage flash steam formed.

The chart can be entered on lower temperatures than thesteam saturation temperature, such as may be the case whenusing thermostatic steam traps for condensate discharge.

Flash steamhas to passthrough thecondensate

cpapa
Highlight
cpapa
Highlight
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Pipe sizes can be estimated for pumped lines containingcondensate below 100°C, as shown by example 5. Also, shortdrain lines to traps (less than 5 m) can be determined in a similarway. Note: in the case of pumped lines, the pressure drop andvelocity must always be checked by referring the condensateflowrate to the pipe size against the table provided in Fig.37(pages 48 and 49).

The chart is used to size trap discharge lines on full loadconditions. It is not necessary to consider any oversizing factorsfor start-up load or the removal of non condensable gases.

On the lower chart, establish the point where the steam andcondensate pressures meet. Move vertically up to the upperchart to choose the selected condensate rate. If the dischargeline is falling (non-flooded) and the selection is on or betweenlines, choose the lower line size. If the discharge line is rising(flooded), choose the upper line size, (Fig. 15).

Some examples for sizing trap discharge lines follow.

Note: The reasoning behind sizing a trap and a discharge line isdifferent, and it is perfectly normal for a trap discharge line to be adifferent size than the trap it is serving. However, the normalancillary equipment associated with the steam trap set, such asthe isolation valves, strainer, trap testing chamber, and checkvalve can be the same size as the trapping device whatever thedischarge line size.

A condensate line sizing chart is provided for photocopying inAppendix 1 (page 81).

Using the chart

cpapa
Highlight
cpapa
Highlight
cpapa
Highlight
Page 24: Condensate and Flash Steam Recovery

23

Fig. 15 Condensate line sizing chart

100,000

50,000

20,000

10,000

5,000

2,000

1,000

500

200

100

50

20

10

Con

dens

ate

rate

kg/

h

Condensate system

pressure bar gC

ondensate line size mm

150

100

80

65

40

32

25

20

15

10

6

30

10

0

0.51

2345

50

5

2

1

3

4

6

500 400 350 300 250 200

20

61

243

250

100

120

140

160

180

200

Ste

am te

mpe

ratu

re °

C

Ste

am s

yste

m p

ress

ure

bar

g

30

10

5

0

0.5

1

2

2015

50

Page 25: Condensate and Flash Steam Recovery

24

Example 1

Fig. 16 Example 1 - non-flooded pressurised trap discharge line

H.P. steam

6 bar

Float trap set

Shell and tube heat exchanger

L. P. steam

1.7 bar25 Ø

Flash vessel

A steam trap passing a full load of 1 000 kg/h at 6 bar g saturatedsteam pressure through a sloping discharge line down to a flashvessel at 1.7 bar g.

As the discharge line is non-flooded, the lower figure of 25 mm isselected from the chart.

Discharge line being sized

Page 26: Condensate and Flash Steam Recovery

25

Example 2

Fig. 17 Example 2 - flooded trap discharge line

Add the 0.5 bar static pressure (5 m head) to the 3.5 barcondensate pressure to give 4 bar g back pressure.

As the discharge line is rising and thus flooded, the upper figureof 32 mm is selected from the chart.

H.P. steam

18 bar3.5 bar

32 Ø

Air vent

Float trap

SA control valve actingas air vent and

condensate drainon start up

5 m

A steam trap passing a full load of 1000 kg/h at 18 bar g saturatedsteam pressure through a discharge line rising 5 m up to apressurised condensate return line at 3.5 bar g.

Discharge linebeing sized

Page 27: Condensate and Flash Steam Recovery

26

Example 3

Fig. 18 Example 3 - non-flooded vented trap discharge line

A steam trap passing a full load of 200 kg/h at 2 bar g saturatedsteam pressure through a sloping discharge line falling down to avented condensate receiver at atmospheric pressure.

As the line is non-flooded, the lower figure of 20 mm is selectedfrom the chart.

H.P. steam

Plate heat exchanger

20 Ø

Vent

25 Ø

To high levelcondensate return line

Discharge linebeing sized

2 bar

Page 28: Condensate and Flash Steam Recovery

27

Example 4 A pumping trap passing a full load of 200 kg/h at 4 bar g saturatedsteam space pressure through a discharge line rising 5 m up to anon-flooded condensate return line at atmospheric pressure.

Fig. 19 Example 4 - flooded trap discharge line

The 5 m static pressure contributes the total back pressure of0.5 bar g.

As the trap discharge line is rising, the upper figure of 25 mmis selected from the chart.

H.P. steam

4 bar

Air flow

5 m

Discharge linebeing sized

25 Ø

Page 29: Condensate and Flash Steam Recovery

28

The automatic condensate pump shown in example 3 can alsohave its discharge line sized by the chart. The pump dischargerate is sized on 6 times the maximum expected inlet rate, in thiscase 6 >< 200 kg /h = 1 200 kg /h.

Example 5

Example 6

Fig. 20 Example 5 - pumped discharge line

Because the condensate will have lost its flash steam content toatmosphere via the receiver vent, the pump will only be pumpingliquid condensate. In this instance, it is only necessary to use thetop graph as shown in the example. As the line from the pump isrising, the upper figure of 25 mm is chosen.

A useful tip for lines of 100 m or less is to choose the dischargepipe the same size as the pump. Also refer to the later section oncondensate pumping for further details.

A balanced pressure thermostatic steam trap draining a hot tableoperating on a constant steam pressure of 2.6 bar g dischargescondensate at 20°C below saturation temperature from a 2 metrecooling leg up to an overhead non-flooded condensate line 2metres above the trap. The full load is 100 kg/h.

The saturation temperature of steam at 2.6 bar g is 140°C, so thedischarge temperature from the trap will be around 120°C. Thechart is then entered on the temperature scale at 120°C ratherthan the pressure scale.

The 2 m back pressure contributes the total back pressure of 0.2 bar g.

As the trap discharge line is rising, the upper figure of 15 mm isselected.

Condensate in

Vent

25 Ø

Pumpedcondensateout

Sloping non-flooded return line

Discharge linebeing sized

Page 30: Condensate and Flash Steam Recovery

29

Example

Sizing commonreturn lines

It is sometimes required to connect several trap discharge linesfrom separate processes into a common return line. Problems willnot occur if the following considerations are met:

a) the common line is not flooded and slopes in the direction offlow to an open end or a vented receiver, or a flash vessel if theconditions allow.b) the diameter of the common line is sized on the cumulativesizes of the branch lines.

The common line size downstream of two connected trapdischarge lines is the root of the sum of the squares of theconnected lines.

The example shows three heat exchangers, each separatelycontrolled and each operated at the same time. Loads shown arefull condensate loads and occur at 3 bar g in the steam space.The common line slopes down to the flash vessel at 1.5 bar gsituated in the same plant room. Condensate in the flash vesselfalls via a float trap down to a vented receiver from where it ispumped direct to the boiler house.

The trap discharge lines are sized on full load with steam pressureat 3 bar g and condensate pressure of 1.5 bar g, and as each isnot flooded, the lower line sizes are picked from the graph.

Line 1 picked as 20 mm, 2 picked as 20 mm, 3 picked as 15 mm

Common line for 1+2 = � 20²+20² = 28 mm

Common line for (1+2)+3 = � 28²+15² = 32 mm

Fig. 21 Calculating the common line size from the discharge lines

3 bar g HE 1

Full load750 kg/h

1" FT14HC20 Ø

3 bar g HE 2

Full load750 kg/h

1" FT14HC

28 Ø

3 bar g HE 3

Full load375 kg/h

1" FT14

20 Ø 15 Ø

32 Ø

Flashsteam

1.5 bar g

To receiverand pump

Page 31: Condensate and Flash Steam Recovery

30

Line Size (mm) Commercial size (mm)A 40 40

B 15 15

C 402+152 = 42.7 40

D 15 15

E 152+42.72 = 45.2 40

F 15 15

G 152+45.22 = 47.6 50

H 15 15

J 152+47.62 = 49.9 50

K 32 32

L 322+49.92 = 59.3 65

The commercial pipeline size is taken as the nearest available tothe calculated size. This may mean downsizing in certaininstances, but this will not normally cause problems in practicedue to the diversity of loads in the other lines.

Fig. 22 Trap discharge lines connecting to a common line

Example

?

32 Ø 15 Ø 15 Ø 15 Ø40 Ø

?

A B

C

D

E

F

? ?

H

G

K

J L

?

15 Ø

The theoretical dimension of 28 mm for the common line 1+2 doesnot exist as a nominal bore in commercial pipe sizes. The internaldiameters of pipes can be larger or smaller than the nominal boredepending on the pipe schedule. Eg. for a DIN 2448 steel pipe, theinternal diameter for a 25 mm nb is about 28.5 mm, while that for a25 mm nb Schedule 40 pipe is about 26.6 mm.

For most practical purposes, a 25 mm nb pipe may be comfortablyselected. If in doubt, seek expert advice.

Further example of calculating the common line size (Fig. 22)

Page 32: Condensate and Flash Steam Recovery

31

Flash steam, separated from the condensate, will be used in aflash steam recovery system or simply vented to atmosphere. Theremaining hot condensate should be pumped to the boiler housewhere its energy content and purity can be used to good effect.

The pumped return line will only carry condensate but at lowervelocities (typically 1 - 2 m/s) than those experienced in the trapdischarge and common lines. As seen in example 5, the pumpdischarge line can be selected from the condensate line sizingchart, or often simply sized the same size as the pump outlet.Refer to the following section "Condensate Pumping" for moredetail.

It is important to remember that the flow in a pumped line isintermittent, as the pump usually cycles. The instantaneousflowrate while the pump discharges is higher than that whichenters the pump. It is the instantaneous discharge figure that hasto be considered on discharge lines.

Water in lines longer than 100 m will develop larger forces ofinertia due to the larger mass of water that is moved during thepumping stroke. It is advisable to add the effects of inertia topressure drop calculations on sizing these longer lines whenmechanical pumps are used. Refer to the section 'longer deliverylines' at a later stage in this document for further details. As ageneral rule, the pipe should be at least one size larger than thepump outlet check valve.

Sizing pumpedreturn lines

Pumped lineslonger than 100 m

Fig. 23 An additional check valve 1 pipe length from thepump body to reduce the effect of backflow

Mechanical pump

Line over 100 m

Additional check valve1 pipe lengthfrom pump

At the end of the pumping stroke, the condensate will tend tokeep moving and can often cause a vacuum to be createddownstream of the pump outlet check valve. As the momentumof the condensate falls, the vacuum creates a sudden backflowonto the check valve which can, in extreme cases, cause severewaterhammer and noise. An additional check valve fitted 1 pipelength after the pump outlet check valve tends to dampen theeffect and protect the pump check valve from damage (Fig.23).

Page 33: Condensate and Flash Steam Recovery

32

Fig. 25 alternative choice - lift after the pump to a break tank

Mechanical pump

fallVacuum breaker

Fig. 24 best choice - lift after the pump

Automatic air vent

fall due toobstruction

Mechanical pump

Tank

Should the falling line have to fall anywhere along its length toovercome an obstruction, then an automatic air vent fitted at thehighest point will assist flow around the obstruction (Fig. 24).

If there is any choice, it is always best to lift immediately after thepump to a height allowing a gravity fall to the end of the line (Fig.24). If the fall is enough to overcome the frictional resistance of thepipe (Fig. 26), then the only back pressure onto the pump is thatformed by the initial lift. A vacuum breaker can be installed at thetop of the lift not only to assist the flow along the falling line but alsoto prevent any tendency for backflow at the end of the stroke.

Page 34: Condensate and Flash Steam Recovery

33

Alternatively, any question of back pressure caused by thehorizontal run can be entirely eliminated by an arrangement as inFig. 25 in which the pump simply lifts into a breaktank. The pipefrom the tank should fall in accordance with the table in Fig. 26.

Fig. 26 Pipe fall to overcome frictional losses

Discharge lines from pumps vented to atmosphere are sized onthe discharge rate of the pump. Condensate passing throughpumping traps and pump/trap combinations in closed loopapplications will often be at higher pressures and temperaturesand flash steam will be formed in the discharge line.

Because of this, discharge lines from pumping traps (such as theAPT14), and pump/trap combinations (such as an MFP14 and FTfloat trap) are sized on the trapping condition at full load and notthe pumping condition, as the line has to be sized to cater for flashsteam. Sizing on flash steam will ensure the line is also able tocope with the pumping condition.

Vented pumps,pumping traps and

pump-trapinstallations

Pipefall need Pipe size (DN mm)

to overcome 15 20 25 32 40 50 65 80 100 125 150

pipe friction Litres of water per hour

25 mm in 15 m 48 140 303 580 907 1 950 3 538 5 806 12 610 22 906 37 284

25 mm in 10 m 59 177 381 694 1 134 2 449 4 445 7 257 15 680 28 576 46 492

25 mm in 8 m 69 204 442 800 1 310 2 834 5 148 8 391 18 159 33 089 53 862

25 mm in 6 m 79 231 503 907 1 487 3 220 5 851 9 525 20 638 37 602 61 223

25 mm in 5 m 86 256 553 1 007 1 642 3 551 6 441 10 568 22 770 41 821 67 538

25 mm in 4 m 93 279 598 1 093 1 778 3 878 7 030 11 521 24 811 45 994 73 571

25 mm in 3 m 113 338 730 1 329 2 168 4 672 8 527 13 925 30 073 54 073 89 356

25 mm in 2 m 140 419 907 1 655 2 694 5 851 10 614 17 327 37 421 68 039 111 128

25 mm in 1.75 m* 152 454 984 1 793 2 923 6 327 11 498 18 756 40 573 73 708 120 426

25 mm in 1.5 m 165 490 1 061 1 932 3 152 6 804 12 383 20 185 43 726 79 378 129 725

25 mm in 1 m 206 612 1 324 2 404 3 923 8 482 15 422 25 174 54 431 99 019 161 476

*(1:70)

Page 35: Condensate and Flash Steam Recovery

34

In nearly all steam using plants, as much condensate as possibleshould be returned to the boiler house to use again. Even ifgravity drainage can be used from the plant to the boiler house,often the condensate must be lifted into a boiler feedtank.

Before looking at the types of pump available for condensatepumping, it may be helpful to discuss some basic pumpingterminology.

Vapour pressure. This term is used to define the pressurecorresponding to the temperature at which conversion of a liquidinto vapour takes place. In other words, it is the pressure at whicha liquid will boil i.e.

At atmospheric pressure, water will boil at 100°C

At a pressure of 7 bar g, water will boil at 170.5°C

At a pressure of 0.75 bar abs, water will boil at 92°C

The vapour pressure is a very important consideration whenpumping condensate. Condensate is usually close to its boilingpoint, which may cause difficulties where a centrifugal pump isconcerned. This is because as the condensate is drawn into thepump impeller, it is accelerated and so experiences a drop inpressure. If this drop in pressure takes the condensate below thevapour pressure or saturation pressure for its temperature, thecondensate will boil and some of the condensate will be releasedas flash steam bubbles.

As the bubbles are carried along within the water, they reach aregion of increased pressure, as they leave the pump impeller.This increased pressure brings the steam bubbles back abovethe saturation pressure causing the bubble to implode rapidly. Ifthis occurs while next to a solid surface, the forces exerted by theliquid rushing in to fill the spaces creates very high localisedpressures. This is known as cavitation and is capable of doing agreat deal of damage to a pump impeller and housing within ashort period of time. It also creates noise, similar to that of gravelrotating in the pump.

It is often recommended that electrical pumps are not used topump condensate at temperatures above 100°C. Some will havelimits as low as 94°C or 96°C, depending on the design of thepump and the suction head provided by the receiver.

Head (h) Head is a term used to describe the potential energy ofa fluid at a given point. There are several ways that head can bemeasured.

Condensate pumping fromvented receivers

Pumping terminology

Page 36: Condensate and Flash Steam Recovery

35

Pressure head (hp). Pressure head is simply the fluid pressureat the point in question. e.g. A pump is required to dischargeagainst a pressure head of 3 bar g. The pump fills from a pressurehead of 0.1 bar g. Where water is the fluid, a 1 bar pressure headis equivalent to approximately 10 m of static head.

Static head (hs). Static head is the equivalent vertical height of fluidabove the point in question. The following example best explains themeasure of static head. The pump inlet in Fig. 28 is subjected to astatic head (known as the suction or filling head) of 1 m, and dischargesagainst a static head (known as the static delivery head) of 30 m.Note that in this case, the water in the bottom of the header tank isabove the pump inlet (this situation is called a flooded suction),With an electrical pump the suction head is subtracted from the staticdelivery head, to give the net static head against which the pumphas to work. With a mechanical displacement pump (Fig.29),the filling head simply provides the energy to fill the pump, andhas no effect on the head against which the pump has to operate.

Net static head29 m

0.1 bar g 3 bar g

Fig. 27 Pressure head

Fig. 28 Net static head for an electrical pump

Collecting tank

Pump inlet

Pumpinlet

Header tankStaticdeliveryhead30 m

Static suction head1m

Page 37: Condensate and Flash Steam Recovery

36

Fig. 29 Net static head for a mechanical pump

hs = netstatic head

Condensate flow

floodedsuction head

Collecting tank

Pump receiver

Friction head (hf). The friction head is more accurately definedas the pressure loss due to friction, and is the head required toactually move the liquid along the pipeline, and, in simple terms,increases proportionately to the square of the velocity.

Pressure loss can be found from tables showing the liquid flowrate,the pipe diameter and the pipe length. To be precise, the resistanceto flow encountered by the various pipe line fittings must also betaken into account. Tables are available to calculate the equivalentlength of straight pipe for various pipe fittings.

This extra 'equivalent length' for pipe fittings is then added to theactual pipe length to give a 'total equivalent length'. However, inpractice, if the pipe is correctly sized, it is unusual for the pipefittings to represent more than an additional 10 % of the actualpipe length.

A general rule which can be applied is:Total equivalent length ( le ) = Actual length + 10 %

Within this reference guide, a figure of 10 % will be used as theextra equivalent length considered for calculating pressure lossdue to friction.

Tables are available which give head loss per metre of pipe forvarious flowrates, pipe diameters, and velocities. The standardS.I. units are Pascals per metre (Pa/m) or millibars per metre (mbar/m). An example of such a table is given in Fig. 32 page 40.

Page 38: Condensate and Flash Steam Recovery

37

Total delivery head (hd). The total delivery head hd againstwhich the pump needs to operate is the sum of :

Pressure required to raise the water to the desired level hsPressure required to move the water through the pipes hfPressure in the condensate system hp

ie

Total delivery head, hd = hs + hf + hp

Pump operation. Centrifugal pumps utilise centrifugal force,which imparts a high velocity to the liquid (condensate) beingpumped. Pressure energy is obtained by the rotation of an impellerfitted within a casing.

Liquid enters the pump and is directed to the centre of therotating impeller vanes. As the impeller rotates, the liquid ispassed along the impeller vanes and increases in velocity.

Pump application. The electrical pump is well suited toapplications where large volumes of liquid need to be moved.

Electrical pumps are usually built into a unit, often referred to as acondensate recovery unit (CRU). A CRU will usually include:

A receiver.

A control system operated by probes or floats.

One or two pumps.

The instantaneous flow from the CRU can be up to 1.5 timesgreater than the rate at which condensate returns to the receiver.It is this pumping rate that must be considered when calculatingthe friction loss in the discharge line.

On twin pump units, a 'cascade' control system may also beemployed which allows either pump to be selected as the 'lead'pump and the other as a 'stand-by' pump to provide back up if thecondensate returning to the unit is greater than one pump canhandle. This control arrangement also provides back up in thecase of the one pump failing to operate; the condensate level inthe tank will increase and bring the second pump into operation.Cascade type units usually pump at a rate of 1.1 times the returnrate to the receiver, allowing a smaller discharge line to beconsidered.

It is very important that the manufacturer's literature is readregarding the discharge pumping rate. Failure to do so couldresult in undersizing the pump discharge pipe work.

Electrical centrifugalcondensate pumps

Page 39: Condensate and Flash Steam Recovery

38

Fig. 30 A typical electrical condensate recovery unit (CRU)

Condensate in

Condensate out

Condensate in

Receiver

Electric pump

Vent

Level sensor

Overflow with"U" seal

To size an electric condensate recovery unit, it is necessary toknow:

The amount of condensate reaching the receiver in kg/h atrunning load.

The temperature of the condensate. This must be below themanufacturer's specified ratings to avoid cavitation, however,manufacturers usually have different impellers to suit differenttemperature ranges, eg. 90°C, 94°C and 98°C.

The total discharge head required. (Will need to be calculatedfrom the site conditions)

The pump discharge rate in order to size the return pipework.(Be sure to read the manufacturer's data properly to determine this).

Sizing an electricalcondensate

recovery unit

Page 40: Condensate and Flash Steam Recovery

39

Sizing the dischargepipework for an

electric condensaterecovery unit

Temperature of condensate = 94°CCondensate to be handled = 1 800 kg/h

Static lift ( hs ) = 30 mLength of pipe work = 150 m

Condensate back pressure = friction losses only ( hf)

Using the data below, an initial selection of a condensate recoveryunit can be made from the manufacturer's sizing chart, such asthe one in Figure 31. From the chart, CRU 1 should be the initialchoice subject to frictional losses in the delivery pipework.

Example

Fig. 31 Typical CRU sizing chart

35

30

25

20

15

10

5

2 0001 000500300 400200100

Pumpdelivery head

in metres

Condensate to be handled at 94°C kg/h

CRU 3

CRU 2

CRU 1

1800

From the chart in Fig. 31, it can be seen that CRU 1 is actuallyrated to handle 2 000 kg/h of condensate.

Reading the manufacturer's data shows that the CRU will actuallypump 1.5 times the maximum return rate shown on the sizingchart. i.e.:

1.5 x 2 000 kg/h = 3 000 kg/h

This ensures start-up loads can be handled without overflowing,and this is what the discharge pipe work must be sized on.

As in the earlier example it is now possible to determine theoptimum size for the return line.

Page 41: Condensate and Flash Steam Recovery

40

Fig. 32 Section of typical friction loss table for fully flooded pipelines (flowrates in L/h)

Actual length of pipe work = 150 mEquivalent length of pipe work = 150 m + 10 % = 165 m

From the pressure drop table above, using a 40 mm nb. pipe will allowa flowrate of 3 000 kg/h (L/h) and incur a pressure drop of between120 and 140 Pa per metre. For this example 128 Pa/m is about right.

Therefore the head loss to friction can be calculated;

Headloss to friction = 128 Pa / m x 165 m= 21 kPa= approx 2.1 metres

The total delivery head required by the pump is:

30 m (hs) + 2.1 m (hf) = 32.1 metres

The figure of 32.1 metres needs to be checked against themanufacturer's sizing chart for the CRU to confirm that there issufficient head available - there is in this case, but had theallowable head been exceeded, then the options are to re-calculateusing a larger pipe or to select a CRU with a greater lift capacity.

Alternatively, it can be seen that the selected CRU1 can pumpagainst a total head (hd) of 35 m. With an actual static head (hs) of30 m, 5 m are "available" for pipe friction loss (hf). It may bepossible to install a smaller pipe and take up a larger friction loss.

Reference to the pipe sizing table on page 42 will show that, if thenext lower sized pipe is used (in this case 32 mm), the unit frictionloss (hf) to pass 3000 kg/h (or 3000 L/h) is 300 Pa/m, and thevelocity is just over 1 m/s which is suitable for this application.

hf is 300 Pa/m x 165 m = 49.5 kPa (or 4.95 m)Therefore, total delivery head = hs + hf

= 30 + 4.95 m = 34.95 m

The conclusion is that 32 mm pipe can be used, as the CRU 1pump can handle up to 35 m total delivery head.

Pressure drop Pipe size (mm)

Pa/m mbar/m 15 20 25 32 40 50 65 80 100

95 0.95 176 414 767 1 678 2 560 4 860 9 900 15 372 31 104

97.5 0.975 180 421 778 1 699 2 596 4 932 10 044 15 552 31 500

100 1.00 184 425 788 1 724 2 632 5 004 10 152 15 768 31 932

120 1.20 202 472 871 1 897 2 898 5 508 11 196 17 352 35 100

140 1.40 220 511 943 2 059 3 143 5 976 12 132 18 792 38 160

Page 42: Condensate and Flash Steam Recovery

41

Pump operation. Mechanical pumps consist of a body, intowhich condensate flows by gravity, containing a float and anautomatic mechanism, operating a set of changeover valves.

Condensate is allowed to build up inside the body, which raises afloat. When the float reaches a certain level, it triggers a ventvalve to close and an inlet valve to open to allow steam to enterand pressurise the body to push out the condensate. Thecondensate level and the float both fall. The steam inlet valvethen shuts and the vent valve opens allowing the pump body torefill. Check valves are fitted to the condensate ports to ensurecorrect directional flow.

It should be noted that a receiver is needed when using a pump(Fig. 33), due to its cyclical action. When the pump is dischargingit is not filling, so there is a need to store the condensate which isbeing produced between pumping cycles.

Mechanicalcondensate pumps

Fig. 33 A typical mechanical condensate pump

Pumpedcondensate

Steam supply to pumpCondensate in Vent

Receiver

Page 43: Condensate and Flash Steam Recovery

42

Pump application. Generally, mechanical pumps handle smalleramounts of condensate than electrical pumps. They are however,particularly valuable in situations where:

Condensate temperature causes cavitation.

Condensate is in vacuum.

Space is at a premium.

Low maintenance is required.

The environment is hazardous, humid or wet.

Electrical supplies are not at hand (operated by steam, air orany inert gas).

Condensate has to be removed from individual items oftemperature controlled equipment which may be subjected tostall conditions.

As with electrically driven pumps, they are sometimes, but notalways, specified as packaged condensate recovery units. Amechanical condensate recovery unit will comprise a condensatereceiver and the pump unit. No additional control system is requiredas the pump is fully automatic and only operates when needed.This means that the pump is self regulating.

Mechanical pumps are, however, a little more involved to sizebecause the flow in the return line is intermittent. The pumpcycles as the receiver fills and empties. The instantaneous flowratewhile the pump is discharging can often be up to six times thefilling rate and it is this instantaneous flowrate which must beused to calculate the size of the discharge pipe. Always refer tothe pump manufacturer for data on sizing the pump and dischargeline.

Page 44: Condensate and Flash Steam Recovery

43

To size a mechanical condensate pump, the following informationis required:

The maximum condensate flowrate reaching the receiver.

The motive pressure of steam or air available. The selection ofsteam or air depends on the application and site circumstances.

The filling head available.

The total back pressure of the condensate system.

The sizing of mechanical pumps varies from manufacturer tomanufacturer, and is usually based on empirical data, which aretranslated into factors and nomographs. The following is a typicalexample on how to size a mechanical pump. (The pipe length isless than 100 m and friction loss is taken as being negligible):

Condensate load = 2 200 kg/hSteam pressure available for operating pump = 5.2 bar g

Vertical lift from pump to return piping = 9.2 mPressure in the return piping (piping friction negligible) = 1.7 bar g

Available filling head on the pump = 0.3 m

Sizing a mechanicalcondensate pump

Example

Total plant condensate 2 200 kg/h

1.7 bar g return mainpressure

5.2 bar goperatingpressure

Fig. 34 Mechanical pump sizing example

Filling head0.3 m

Reservoir 9.2 m lift

Pump

Vent

Condensatemanifold

Page 45: Condensate and Flash Steam Recovery

44

Calculate the total back pressure (delivery head hd), against whichthe condensate must be pumped:

Total back pressure (hd) = lift (hs)+ condensate pressure (hp)(friction loss neglected as line is shorter than 100 m)

lift (hs) = 9.2 mcond. pressure (hp) = 1.7 bar g = 17 m headTotal = 9.2 + 17 m

= 26 m

Reference to Fig. 35 below shows that a DN50 pump at 5.2 bar gmotive pressure will pump 2600 kg/h against a 26 m head, andwill thus be the correct choice for this example.

Note: the pump is sized on the filling rate.

Fig. 35 - DN50 MFP 14 pump sizing chart Fig. 36 - DN80 MFP 14 pump sizing chart

14

13

12

11

10

9

8

7

6

5.25

4

3

2

1

0Mot

ive

pres

sure

bar

g

1000 2000 2200 2500 2600 3000 4000 5000Flowrate kg/h

DN50 size capacities

80 m

lift

50 m

lift

40 m

lift

30 m

lift

20 m

lift

10 m

lift

4 m

lift

32 m 27 m26 m14

13

12

11

10

9

8

7

6

5.25

4

3

2

1

0

Mot

ive

pres

sure

bar

g

1000 2000 2500 3000 4000 5000 6000Flowrate kg/hDN80 x DN50 size capacities

80 m

lift

50 m

lift

40 m

lift

30 m

lift

20 m

lift

10 m

lift

4 m

lift

32 m 26 m

Page 46: Condensate and Flash Steam Recovery

45

Longer delivery lines

Sizing the dischargepipework for a

mechanicalcondensate pump

Below 100 m long, the discharge pipe from a mechanical pumpcan usually be taken as the same size as the pump body. Thefrictional resistance of the pipe is relatively small compared to theback pressure caused by the lift and condensate return pressure,and can usually be disregarded. Above 100 m, a general rulewould be to select one pipe size larger than the pump outletcheck valve.

On delivery lines over 100 m, and/or where the condensate flowis near to the pump maximum, it is advisable to check the pipesize to ensure that the total friction loss (including inertia loss)does not increase above that which effects the pump's capability(or installation costs).

With 5.2 bar g motive steam and 26 m delivery head,from Fig. 35, for a DN50 pump,Maximum pump capacity = 2 600 kg/hActual condensate flowrate into pump = 2 200 kg/hagain, from Fig. 35, for a DN50 pump,Max. back pressure permissible at 2 200 kg/h = 32 mtherefore, max. frictional resistance allowable = 32 - 26 m

= 6m (60 kPa)Inertia lossOn lines over 100 m, a considerable volume of liquid will be heldwithin the pipe. The sudden acceleration of this mass of liquid at thestart of the pump discharge can absorb some part of the pumpenergy, and this needs to be considered within the friction losscalculation by reducing the allowable friction loss by 50%, thus,

Total allowable friction loss = 50 % × 60 kPa= 30 kPa

Consider delivery pipe length to be 250 m+ 10% for additional fittings = 275 m

then, max. frictional resistance allowable / m = 30 kPa 275 m

approx. = 109 Pa/m

Taking delivery flowrate as 6 times filling rate = 6 × 2 200= 13 200 kg/h

Referring to Fig. 37 (the table), a frictional resistance of 109 Pa/mreveals that an 80 mm pipe is required to give an acceptableflowrate of 13 200 kg/h. In fact, the table shows that this size pipewill pass about 16 500 kg/h with this frictional resistance.

By rising up the '80 mm column', it can be seen that, byinterpolation, the flowrate of 13 200 kg/h actually induces africtional loss of about 72 Pa/m in an 80 mm pipe.

Page 47: Condensate and Flash Steam Recovery

46

Fully loaded pumpsand longer lines

Should the condensate filling rate have been near the maximum 2600 kg/h for the above example, say 2 500 kg/h, then less head isavailable for friction loss, and progressively less so for longer lines.

Sizing on a filling rate of 2 500 kg/h, and a 250 m (+10%) line,referring to Fig. 35, for the DN50 pump, it can be seen that acondensate filling rate of 2 500 kg/h equates to a max. backpressure of about 27 m, hence in this instance,

available head left for friction losses = 27 - 26 m= 1 m (10 kPa)

frictional resistance allowable = 10 kPa 275 m= 36 Pa / m

minus allowance of 50% for inertia loss = 50 % × 36 Pa/m

therefore, max. frictional resistance allowable = 18 Pa / m

As before, the discharge pipework has to be sized on theinstantaneous flowrate from the pump outlet, which is taken as 6 ×the filling rate. In this instance, the pipe would have been sizedon 6 × 2 500 kg/h = 15 000 kg/h with a friction loss of 18 Pa/m.

Fig. 37 (the table) reveals that this would require a pipe largerthan 100 mm to allow the pump to operate within its capability.

Although the system would certainly work with thisarrangement, it may be more economical to consider a largerpump with smaller pipework.

Page 48: Condensate and Flash Steam Recovery

47

Consideration of alarger pump andsmaller pipeline

Fig. 36 reveals that a DN 80 pump under the same conditions of5.2 bar g motive steam and 26 m back pressure would allow thefollowing friction losses:

Back pressure = 26 m

At a filling rate of 2 500 kg/h, max. allowed = 35 mhead available for friction loss = 35 - 26 m

= 9 m (90 kPa)

90 kPa over 250 m and inc. inertia loss = 50 % × 90250

max. frictional resistance allowable = 180 Pa/m

Fig. 37 (the table) shows that an 80 mm pipe will accommodate21420 kg/h with a friction loss of 180 Pa/m. Hence, in thisinstance, the larger pump will comfortably allow a pipe two sizessmaller than that for the smaller pump. Always check that velocityis within recommendations. The 80 mm pipe will handle theabove condition at just under 1 m/s, and is therefore suitable.

The DN80 pump would cost about 10% more than the DN50pump, but these costs could well be recovered with thedifference in installation costs on longer delivery lines betweenan 80 mm and 100+ mm pipe plus fittings and insulation etc.

Page 49: Condensate and Flash Steam Recovery

48

kg/h15 mm 20 mm 25 mm 32 mm 40 mm 50 mm 65 mm 80 mm 100 mm

Pa/m mbar/m <0.15 m/s 0.15 m/s 0.3 m/s10 0.1 50 119 223 490 756 1447 2966 4644 943212.5 0.125 58 133 252 554 853 1634 3348 5220 1065615 0.15 65 151 277 616 943 1807 3708 5760 1173617.5 0.175 68 162 302 670 1026 1966 4032 6264 1274420 0.2 76 176 328 720 1105 2113 4320 6732 1368022.5 0.225 79 187 349 770 1177 2254 4608 7164 14580 0.525 0.25 83 198 371 814 1249 2387 4860 7596 15408 m/s27.5 0.275 90 209 389 857 1314 2513 5112 7992 1620030 0.3 94 220 410 900 1379 2632 5364 8352 1695632.5 0.325 97 230 428 940 1440 2747 5616 8712 1771235 0.35 101 241 446 979 1498 2858 5832 9072 1843237.5 0.375 104 248 464 1015 1555 2966 6048 9396 1911640 0.4 112 259 479 1051 1609 3071 6264 9720 1976442.5 0.425 115 266 497 1087 1663 3175 6480 10044 2041245 0.45 119 277 511 1123 1717 3272 6660 10368 2102447.5 0.475 122 284 526 1156 1768 3370 6876 10656 2163650 0.5 126 292 540 1188 1814 3463 7056 10944 2221252.5 0.525 130 299 558 1220 1865 3553 7236 11232 2278855 0.55 130 306 572 1249 1912 3636 7416 11520 2336457.5 0.575 133 317 583 1282 1958 3744 7596 11808 2390460 0.6 137 324 598 1310 2002 3816 7776 12060 2444462.5 0.625 140 331 612 1339 2048 3888 7920 12312 2498465 0.65 144 338 626 1368 2092 3996 8100 12600 2548867.5 0.675 148 346 637 1397 2131 4068 8280 12852 2599270 0.7 151 353 652 1422 2174 4140 8424 13068 2649672.5 0.725 151 356 662 1451 2218 4212 8568 13320 2700075 0.75 155 364 677 1476 2257 4284 8748 13572 2746877.5 0.775 158 371 688 1505 2297 4356 8892 13788 2797280 0.8 162 378 698 1530 2336 4464 9036 14040 28440 182.5 0.825 166 385 709 1555 2372 4536 9180 14256 28872 m/s85 0.85 166 389 724 1580 2412 4608 9324 14472 2934087.5 0.875 169 396 734 1606 2448 4680 9468 14724 2977290 0.9 173 403 745 1627 2488 4716 9612 14940 3024092.5 0.925 176 407 756 1652 2524 4788 9756 15156 3067295 0.95 176 414 767 1678 2560 4860 9900 15372 3110497.5 0.975 180 421 778 1699 2596 4932 10044 15552 31500100 1 184 425 788 1724 2632 5004 10152 15768 31932120 1.2 202 472 871 1897 2898 5508 11196 17352 35100140 1.4 220 511 943 2059 3143 5976 12132 18792 38160160 1.6 234 547 1015 2210 3373 6408 12996 20160 40680180 1.8 252 583 1080 2354 3589 6804 13824 21420 43200 1.5200 2 266 619 1141 2488 3780 7200 14580 22644 45720 m/s220 2.2 281 652 1202 2617 3996 7560 15336 23760 47880240 2.4 288 680 1256 2740 4176 7920 16056 24876 50400260 2.6 306 713 1310 2855 4356 8244 16740 25920 52200280 2.8 317 742 1364 2970 4536 8568 17388 26928 54360300 3 331 767 1415 3078 4680 8892 18000 27900 56160320 3.2 342 796 1465 3182 4860 9180 18612 28836 58320 2340 3.4 353 821 1512 3287 5004 9504 19224 29772 60120 m/s360 3.6 364 846 1559 3388 5148 9756 19800 30636 61920380 3.8 374 871 1602 3492 5292 10044 20340 31500 63720400 4 385 893 1645 3578 5436 10332 20880 32364 65160420 4.2 396 918 1688 3672 5580 10584 21420 33156 66960440 4.4 407 940 1732 3744 5724 10836 21924 33984 68400

Fig. 37 - Flow of water in heavy steel pipes (with velocities)

cpapa
Highlight
Page 50: Condensate and Flash Steam Recovery

49

kg/h15 mm 20 mm 25 mm 32 mm 40 mm 50 mm 65 mm 80 mm 100 mm

Pa/m mbar/m <1 m/s 1 m/s 2 m/s440 4.4 407 940 1732 3744 5724 10836 21924 33984 68400460 4.6 414 961 1771 3852 5868 11088 22464 34776 70200480 4.8 425 983 1811 3924 5976 11340 22932 35532 71640500 5 432 1004 1850 4032 6084 11592 23436 36360 73080520 5.2 443 1026 1886 4104 6228 11808 23904 37080 74520540 5.4 450 1048 1926 4176 6372 12060 24372 37800 75960560 5.6 461 1066 1962 4212 6480 12276 24840 38520 77400580 5.8 468 1087 1998 4356 6588 12492 25272 39240 78840600 6 479 1105 2034 4428 6732 12708 25740 39960 80280620 6.2 486 1123 2070 4500 6840 12924 26172 40680 81720640 6.4 493 1145 2102 4572 6948 13140 26604 41040 83160660 6.6 500 1163 2138 4644 7056 13356 27000 41760 84240680 6.8 511 1181 2171 4716 7164 13572 27432 42480 85680 3700 7 518 1199 2203 4788 7272 13788 27828 43200 86760 m/s720 7.2 526 1217 2236 4860 7380 13968 28260 43920 88200740 7.4 533 1235 2268 4932 7488 14184 28656 44280 89280760 7.6 540 1249 2300 5004 7560 14364 29052 44640 90360780 7.8 547 1267 2333 5076 7704 14544 29412 45360 91800800 8 554 1285 2362 5112 7812 14760 29808 46080 92880820 8.2 562 1303 2394 5184 7884 14940 30204 46440 94320840 8.4 569 1318 2423 5256 7992 15120 30564 47160 95400860 8.6 576 1336 2452 5328 8100 15300 30924 47880 96480880 8.8 583 1350 2480 5400 8172 15480 31284 48600 97560900 9 590 1364 2513 5436 8280 15660 31680 48960 98640920 9.2 598 1382 2542 5508 8388 15840 32004 49680 99720940 9.4 605 1397 2567 5580 8460 16020 32364 50040 100800960 9.6 612 1411 2596 5616 8568 16200 32724 50760 101880980 9.8 619 1429 2624 5688 8640 16380 33084 51120 1029601000 10 623 1444 2653 5760 8748 16524 33408 52560 1040401100 11 655 1516 2786 6048 9180 17352 35064 54360 1094401200 12 688 1588 2912 6300 9612 18144 36720 56880 114120 41300 13 716 1652 2894 6588 10008 18900 38160 59040 m/s1400 14 745 1717 3154 6840 10404 19656 39600 612001500 15 770 1782 3269 7128 10764 20340 41040 633601600 16 799 1840 3380 7308 11124 21024 42480 655201700 17 824 1901 3485 7560 11484 21672 43920 676801800 18 850 1955 3589 7776 11808 450001900 19 871 2012 3708 7992 12132 464402000 20 896 2066 3780 8208 12456 47520

Fig. 37 - Flow of water in heavy steel pipes (with velocities)

Page 51: Condensate and Flash Steam Recovery

50

Lifting condensate fromsteam mains drain traps

A frequent requirement is to lift condensate from a mains draintrap to a higher level return line (Fig. 38), using the steampressure within the trap.

Pressure can be related to the increase in lift by using the followingconversion;

1 m increase in pipework lift = 0.1 bar g back pressure

If a head of 5 m produces a back pressure of 0.5 bar, then this reducesthe differential pressure available to push water through the trap,although under running conditions the reduction in trap capacity is likelyto be significant only where low upstream pressures are being used.

It is recommended that a check valve be fitted after any steam trapwhere there is the case of condensate being lifted. This willprevent condensate from falling back into the trap.

At start-up, the steam pressures are likely to be very low for awhile, and it is common to find water backing up before the trap.This can lead to waterhammer in the space being drained, if ameans of removing the condensate is not provided until sufficientsteam pressure is present to overcome the back pressure. Theliquid expansion thermostatic trap can often be used, dischargingcold condensate to waste but closing to hot condensate, which isthen forced through the trap to the return line.

The discharge line from the trap to the overhead return line ispreferably turned over on to the top of the main as shown, ratherthan simply teed to the underside. This assists operation, becausealthough the riser is probably full of water at start-up, it sometimescontains little more than flash steam once hot condensate underpressure passes through it. If the discharge line were fitted to thebottom of the return line, it would fill with condensate after eachdischarge and increase the tendency for waterhammer and noise.

High level condensate return

Steam main

Trap

Fig. 38 Use of liquid expansion trap

Liquid expansion trap

Drain to waste

Page 52: Condensate and Flash Steam Recovery

51

Contaminated condensate

Occasionally, condensate is discharged from sources where itmight have become contaminated by corrosive process liquids,and it becomes unsuitable for use as boiler feed because of thedangers of foaming, scaling or corrosion which can occur in theboiler and in the steam pipes.

However, although contaminated, the condensate still carries thesame useful heat as clean condensate and could be recovered ifproper contamination detection equipment is employed

Such systems detect changes in condensate conductivity. When achange occurs then it may mean that the condensate is contaminated.When this happens a dump valve opens, allowing condensate to flowto drain.

In some countries, continuous monitoring of condensate is a legalrequirement.

Fig. 39 Condensate contamination detection equipment

Contaminated condensate

to waste

Controller

Dump valve

Sensor

Drain

Condensate in Condensate out

Page 53: Condensate and Flash Steam Recovery

52

Stall and the stall point

Steam has many benefits when used as a heat medium, some ofwhich are its high heat content, its versatility for use in many differentapplications, and its ease of control. In temperature controlled plant,a condition commonly called stall can result in poor systemperformance. With proper arrangements, problems do not occur, butif they do, it is quite a simple matter to put things right. A basicunderstanding of these problems may benefit the practitioner.

The stall condition occurs when the steam space pressureapproaches, equals, or is less than the condensate back pressure.This hinders the positive flow of condensate through a steam trapas its capacity reduces relative to the pressure drop across it. Itcan be identified on a graph known as a stall chart, where it isreferred to as the stall point. When stall is inevitable, an activemethod of condensate removal, such as a pumping trap, willensure proper drainage of condensate under all loadconditions.

It is often thought that the steam pressure in a steam / liquid heatexchanger is always enough to force the condensate out. However,the relationship between two process conditions can preventcondensate draining out of a heat exchanger, they are:

High condensate back pressure.

Low steam pressure in the heat exchanger steam space.

When either occurs relative to the other, there can be insufficientpressure difference to move the condensate from the heatexchanger through the trap and into the condensate return line.Condensate will not drain, and the exchanger will begin to fill withwater. Symptoms are poor temperature control, waterhammerand noise in the short term, and erosion and corrosion of the heatexchanger in the long term.

To maintain good process temperature control and preventpremature mechanical or corrosive failure of heat exchangers, itis essential that condensate is removed from the exchanger asquickly as it forms.

The following is an example which shows how the stall situationdevelops (refer to Fig. 40):

When heat exchange occurs, the secondary fluid temperaturewill rise. This increase in temperature is detected by the controlsystem and the steam valve closes down.

The steam pressure falls, and the heat exchanger begins toflood because the back pressure in the condensate return pipe(P2) is greater than the steam pressure after the control valve(P1).

The stall cycle

Page 54: Condensate and Flash Steam Recovery

53

Temperature sensor

To condensate main

P2

P1

Fig. 41 Continuation of the stall cycle

steam trap

sec.return

sec. flow

Heat exchanger

To condensate main

Temperature sensor

P2

P1

Fig. 40 The onset of stall in a heat exchanger with temperature control

Flow

Return

steam trap

When the heat exchanger floods, the secondary fluidtemperature falls and the flow of steam through the control valveincreases.

The steam pressure rises (P1 > P2) and discharges thecondensate, but leaves the heater filled with steam at a higherpressure than is needed to maintain a stable secondarytemperature, as depicted in Figure 41. The cycle then repeats.

Page 55: Condensate and Flash Steam Recovery

54

Condensate drainage to atmosphereStall can also occur in temperature controlled heaters even whenthe condensate falls down to a steam trap and down again to avented receiver or open ended pipe. The back pressure would beatmospheric, but it would be wrong to assume that there wouldalways be enough steam pressure to push the condensate throughthe steam trap.

In this instance, where the secondary control temperature is lessthan 100°C, the steam temperature for some part load conditionswill also be lower than 100°C (the saturation temperature of steamat atmospheric pressure). Here, the steam space pressure wouldbe in vacuum, making it difficult for condensate to drain from thesteam space and pass through the steam trap. A stall condition willexist and special arrangements are needed to drain the condensate.

On smaller heat exchangers draining to atmosphere, a simple remedyis to install a vacuum breaker on the steam inlet to the heat exchanger.When vacuum is reached in the steam space, the vacuum breakeropens to allow the condensate to drain down to the steam trap. Thetrap itself must be placed a discrete distance below the exchangeroutlet, and must be sized to pass the condensate stall load on thestatic head created by the height of the outlet above the trap inlet.The condensate pipe from the trap should slope down so that nofurther back pressure is exerted on the trap. (Fig. 42)

Often, especially on larger plant, it is usually preferred not tointroduce air into the steam space, and the use of a vacuumbreaker may not be tolerated. Also, if the condensate lifts after thesteam trap up to a higher level, a vacuum breaker cannot assistdrainage. In these situations, a pumping trap or pump/trapcombination should be used.

Fig. 42 Shell & tube heat exchanger draining to atmosphere

Steam in

Condensate out to atmosphere

Secondary flow

Secondary return

Shell & tubeHeat exchanger

Vacuumbreaker

Temperature control system

Static head

Temperaturecontrolled plant

Page 56: Condensate and Flash Steam Recovery

55

If stall is inevitable and a vacuum breaker cannot be used, an activemethod of condensate removal must be used to give good systemperformance, as shown in Fig. 43. A pumping trap performs as asteam trap if there is sufficient steam pressure in the steam space toovercome the back pressure. If there is not, it acts as a pump.The device is fully self contained and automatic in its operation.

The pumping trap is also extremely valuable where there isrestricted space below the heater, such as is often the case on airhandling units which are often positioned close to the plant roomfloor. Fig. 44 shows an example draining single and multi- heaterbatteries to avoid both freezing and corrosion of the coils.

Closed loopcondensate drainage

Fig. 43 Shell & tube heat exchangerwith pumping trap arrangement

Secondary flow

Control valve

Condensate from heater to APT

Steam in

Motivesteam line

to pump

Balance line

Autoair vent

Fig. 44 Pumping traps on heater batteries with low suction heads

Heater batteries

Motive line trap set

Air flow

APT14s

Steam in Steam in

When a pumping trap arrangement is used, condensate willalways be removed from the heater under all pressure conditions,ensuring maximum system efficiency at all times, with no escapeof flash steam in the plant room.

Page 57: Condensate and Flash Steam Recovery

56

Where plant capacity is too large for the pumping trap, it can bereplaced by a separate pump and steam trap in combination,such as that shown in Fig. 45. A mechanical fluid pump isdedicated to a single heater, connected so that the pump chamber,piping, and the steam side of the heater tubes form a commonsteam space. When the steam pressure is sufficiently high,condensate flows from the steam space and through the pumpbody and steam trap and away. When the pressure is lowered asthe control valve closes down, condensate fills the pump chamber.Admittance of motive steam at high pressure is triggered, pushingcondensate out of the chamber and away through the trap.

The pump exhaust line is connected to a reservoir and acts as abalance pipe when the pump is filling. The small amount ofexhaust steam is then contained within the system, and pumpingoccurs with no waste of steam to atmosphere, making the systemenergy efficient, and the plant room free from flash steam.

Fig. 45 Shell & tube heat exchanger with pump and trap arrangement

Steam in

Condensate againsta back pressure

Secondary return

Secondary flow

Pump

Reservoir

Shell & tubeHeat exchanger

Motive steamto pump

Air vent & check valve

Trap

Page 58: Condensate and Flash Steam Recovery

57

In any heat exchanger, the heat flow Q, at any time, may beexpressed by Q = U . A . �Tm.where,Q is heat transfer in watts

U is the overall heat transfer coefficient (W/m²°C)

A is the area of the heat transfer surfaces (m²)�Tm is the mean temperature difference between steam

and water (°C)

If the heat flow in a steam/water or steam /gas heat exchanger isto be varied, a thermostatic control valve changes the steamflow. This leads to a change of pressure in the steam space, andso too a change in temperature. This immediately changes thevalue of �Tm in the heat flow equation. Since A and U virtuallyremain constant, the heat flow Q varies directly with �Tm.

For maximum heat load, �Tm must be at its greatest value, iemaximum steam temperature (and pressure) and minimumsecondary temperature with secondary fluid flow at its greatest.

For no load, �Tm must be zero, ie the steam temperature mustbe the same as the fluid leaving the heat exchanger.

For 50% load, �Tm must be 50% of maximum �Tm and so on. Thisallows two straight lines to be drawn on a chart (see Fig. 46), A - Band C - B which are furthest apart at full load, and meet at no load.

The upper line A - B represents the changing steam temperature(and pressure), and the lower line C - B represents the secondaryinlet fluid temperature. The chart can relate any secondary inlettemperature and corresponding steam temperature to the heatload. If a horizontal line is drawn, D - E, representing the pressuredownstream of the trap, it is possible to see when the steampressure approaches the back pressure and the system starts tostall. Further reduction in load increases the waterlogging.

Q. Why use a stall chart?

A. It is a simple way of telling whether a steam trap or pumpingtrap is needed for the application!

In order to select and size the trapping device correctly, thefollowing information needs to be known:

Steam flowrate at full load.Steam pressure in the steam space at full load.System back pressure.Secondary inlet temperature at full load.Secondary outlet temperature at full load.Minimum load condition

Determiningthe stall point

on temperature

controlled plant

Using the stall chart

..

.

.

Page 59: Condensate and Flash Steam Recovery

58

Steam flowrate at full load = 600 kg/h

Steam pressure in heater at full load (A) = 7 bar g

Condensate system back pressure (E) = 2 bar g

Inlet secondary medium temperature (C) = 25°C

Outlet secondary medium temperature (B) = 80°C

Minimum load condition (% of full load) = 40 %= 240 kg/h

Plot the above information on the stall chart (Fig. 46) in thefollowing sequence.

1. Plot the incoming secondary inlet temperature (C) and outletsecondary temperature (B) to give the secondary temperatureline C - B.

2. Plot the full load steam pressure (A) on the left side of thechart, and connect the line A - B.

3. Plot the condensate back pressure (point E)

4. Plot the point where the line A - B intersects with the systemback pressure line (D - E), and drop straight down to the bottomof the chart to determine the percentage load (F) at which thestall condition will occur. Any loads lower than 60% will requirethe use of a pump or pumping trap.

From the chart, the stall condition is at 60 %. From the maximumsteam load of 600 kg/h, this will be approximately 360 kg/h. As theexpected minimum load condition of 40 % is lower than the stallload, a pumping trap will be required to provide proper andcomplete drainage.

NOTE: if the minimum load were higher than the stall load, (at say70 %, ie 30% reduction) then the system will never stall. Here, asteam trap only is needed, and is sized on the minimum loadcondition, ie 70 % of 600 kg/h = 420 kg/h, at the pressure differentialacross the trap at this point - see below to establish this:

Steam temp. at full load = 170°C (a)Steam temp. at no load = 80°C (b)ie, steam temperature range = 90°C (a - b)30% of range = 30°C (c) = ( [a - b] x 0.3)steam temp. at 30 % reduction = 140°C (a - c)steam pressure at 140 oC = 2.6 bar g (from steam tables)condensate back pressure = 2.0 bar gMinimum load diff. pressure = 0.6 bar

Float trap sized to pass 420 kg/h with 0.6 bar diff. pressure.

Example 1

Page 60: Condensate and Flash Steam Recovery

59

The stall load can also be calculated mathematically. This maybe best explained using the information from the previousexample:

(Equivalent back pressure temp) - (Outlet secondary temp) (Full load steam space temp) - (Outlet secondary temp)

= 134 - 80170 - 80

Percentage stall load= 60 %

Actual stall load = 60 % x 600 kg/h

= 360 kg//h

As the 40% minimum load (240 kg/h) is going to be lower thanthe 60 % stall load (360 kg/h), the system will require a pumpingtrap or pump for correct condensate drainage.

Correct selection and sizing procedure of condensate removaldevices is further explained in the Spirax Sarco TechnicalReference Guide TR-GCM-23 'Draining condensate from heatexchangers'.

An easier way of determining whether a system will stall andwhether a steam trap or pumping trap is needed is achieved bythe APT14 pumping trap computer sizing program, whichaccurately plots a bespoke stall chart for any installation.

For further details please contact any local Spirax Sarco office, orsales engineer.

Example 2

Example 3

Page 61: Condensate and Flash Steam Recovery

60

Fig. 46 A typical stall chart

100 90 80 70 60 50 40 30 20 10 0

Pre

ssur

e - b

ar a

bsol

ute

(vac

uum

)P

ress

ure

bar g

auge

T °C

250

240

230

220

210

200

190

180

170

160

150

140

130

120

110

100

90

80

70

60

50

40

30

20

10

0

-10

-20

-30

-40

-50

39.0

33.0

27.0

22.0

18.0

14.5

11.6

9.0

7.0

5.2

3.8

2.6

1.7

1.0

0.4

0

0.7

0.5

0.3

0.2

0.12

0.07

0.05

Back pressure (2 bar g)

Stall point

Secondary outlet temperature (80°C)

Steam supply pressure (7 bar g)

Percentage load

C

B

A

39.0

33.0

27.0

22.0

18.0

14.5

11.6

9.0

7.0

5.2

3.8

2.6

1.7

1.0

0.4

0

0.7

0.5

0.3

0.2

0.12

0.07

0.05

Pre

ssur

e ba

r gau

geP

ress

ure

- bar

abs

olut

e (v

acuu

m)

250

240

230

220

210

200

190

180

170

160

150

140

130

120

110

100

90

80

70

60

50

40

30

20

10

0

-10

-20

-30

-40

-50

T °C

D E

F

Page 62: Condensate and Flash Steam Recovery

61

Constant pressureplant

Although stall is unlikely to occur on items of plant supplied with aconstant steam pressure above the condensate back pressure,any pressure in the condensate line will reduce the steam trapcapacity. It is important that the steam trap is sized on the differencein the steam and condensate pressures, and not just the steampressure, as depicted in Figure 47.

With constant pressure plant, a general rule is to size the steamtrap on twice the full load rating of the plant to allow for thecombination of low steam pressures and high condensation ratesat start-up.

Steam

30 kWUnit heater

Fig. 47 Trap capacity reduced due to an elevated return line

Condensate rising toa non-pressurisedsloping return line

In this example, the differential pressure across the trap is 70 %of the steam pressure. The trap is sized to pass twice the full loadcondition with a differential pressure of 1.0 - 0.3 bar g = 0.7 bar.

Full load = 30 kW2 x 30 kW = 60 kW

Steam load = 60 kW2 201 kJ /kg

x 3 600 = 98 kg /h

Size trap to pass 98 kg/h on a �p of 0.7 bar.

1.0 bar

0.3 bar

3 m

Page 63: Condensate and Flash Steam Recovery

62

'Flash Steam' is released from hot condensate when its pressureis lowered. Even water at an ordinary room temperature of 20°Cwould boil if the pressure was lowered below 0.02 bar a, andwater at 170°C will boil at any pressure below 6.9 bar g.

The steam released by the flashing process is just the same asthe steam released when heat is added to saturated water undera constant pressure.

For example, if a load is applied to a boiler, and the boilerpressure drops a little, then some of the water content of theboiler will flash-off to supplement the 'live' steam which is beingproduced by the supply of heat from the boiler fuel. Because it isall being produced in the boiler, all this steam is regarded as livesteam.

Only when flashing takes place at relatively low pressure, as atthe discharge side of steam traps, is the name 'flash steam'widely used. Unfortunately, this usage has led to the erroneousconclusion that flash steam is in some way different from, andless valuable, than so called live steam .

In any steam system seeking to maximise efficiency, flash steamwill be separated from the condensate, where it can be utilised atlow pressure, to supplement any low pressure load. Every kilogramof flash steam used in this way is a kilogram of live steam whichdoes not need to be supplied by the boiler. It is also a kilogramconserved and not vented to atmosphere, where it would simplybe lost.

The reasons for the recovery of flash steam are just as compelling,both economically and environmentally as those for recoveringcondensate.

Flash steam

What is flash steamand why should

it be used?

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Page 64: Condensate and Flash Steam Recovery

63

If use is to be made of flash steam, we need to know how much ofit will be available. The quantity is readily determined bycalculation, or it can be read from simple tables or charts. As anexample, consider the jacketed vessel shown in Fig. 48.

The condensate enters the trap as saturated water, at a gaugepressure of 7 bar and a temperature of 170.5°C. The amount ofheat in the condensate at this pressure is 721.4 kJ/kg.

After passing through the steam trap, the pressure in thecondensate return line is 0 bar gauge. At this pressure, themaximum amount of heat the condensate can hold is 419.0 kJ/kgand the maximum temperature is 100°C. Where does the excess302.4 kJ of heat go? In fact, it evaporates some of the condensate,but by how much?

The heat needed to produce 1 kg of saturated steam from water atthe same temperature at 0 bar gauge is 2 257 kJ. An amount of302.4 kJ can therefore evaporate:

302.4 = 0.134 kg of steam 2257

from each kg of condensate, and the proportion of flash steamgenerated therefore equals 13.4 %.

How much flashsteam?

Steam at 7 bar g

Air vent

Condensate at 0 bar ghf = 419.04 kJ/kg

Ball valve

Constant pressuresteam at 7 bar g.

Fig. 48 Excess heat in condensate produces flash steam

Condensate at 7 bar g hf = 721.40 kJ/kg

Excess heat at 0 bar g= 721.40 - 419.04 kJ/kg= 302.4 kJ/kg

Page 65: Condensate and Flash Steam Recovery

64

Sub cooledcondensate

If the equipment using steam at 7 bar g were condensing 250 kg/h,then the amount of flash steam released by the condensateat 0 bar g would be:

0.134 x 250 kg/h = 33.5 kg/h

Alternatively, as a short cut, Fig. 50 can be read directly for manyof the moderate and low pressures which will be met in manyplants.

The previous example is shown on Fig. 49, indicating 0.134 kg offlash per kg of condensate.

Fig. 49 Quantity of flash steam

15

14

13

12

11

10

9

8

7

6

5

4

3

2

1

0

Pre

ssur

e on

trap

s ba

r g

0 0.02 0.06 0.10

0.134 kg

0.14 0.18 0.22

Flash steam pressures

Example

2.5

bar g

2.0

bar g

1.5

bar g

1.0

bar g

0.5

bar g

0 ba

rg

kg Flash per kg condensate

If the steam trap is of a thermostatic type which holds backcondensate until it has sub-cooled below saturation temperaturebefore discharging, then the heat in this cooler condensate willbe less, and the amount of flash steam produced would be less.

If the trap in the previous example discharged condensate at15°C below the steam saturation temperature, then the heat in thecondensate would be less.

Page 66: Condensate and Flash Steam Recovery

65

temp. of saturated condensate at 7 bar g = 170.5°Camount of sub cooling = 15.0°C

temp. of sub-cooled condensate at 7 bar g = 155.5°C

(from steam tables)amount of heat in condensate at 155.5°C = 656 kJ/kg

minus heat in condensate at 0 bar g = - 419 kJ/kgSurplus = 237 kJ/kg

heat in steam at 0 bar g = 2 257 kJ/kg

Proportion of flash steam = 2372 257

= 0.105 kg/kgof condensate

Therefore, in this example, a reduction of condensate temperatureon the upstream side of the trap by 15°C has reduced theproportion of flash steam produced on the downstream side from13.4 % to 10.5%.

If the return line were connected to a vessel at a pressure of 1 bar g,then the maximum heat in the condensate at the trap dischargewould be 505.6 kJ/kg and the enthalpy of evaporation at 1 bar gwould be 2 201.1 kJ/kg.

The proportion of the condensate which flashes off as steamat 1 bar g would then be calculated in the following way:

heat in condensate at 7 bar g = 721.4 kJ/kgminus heat in condensate at 1 bar g = - 505.6 kJ/kg

Surplus = 215.8 kJ/kg

heat in steam at 1 bar g = 2 201.1 kJ/kg

Proportion of flash steam = 215.82 201.1

= 0.098 kg/kgof condensate

If the equipment using steam at 7 bar g were condensing 250 kg/hof steam, then the amount of flash steam released by thecondensate at 1 bar g would be 0.098 x 250 = 24.5 kg/h.

Therefore, the amount of flash steam produced can depend onthe type of steam trap used, the steam pressure onto the trap, andthe condensate pressure after the trap.

Pressurisedcondensate

Page 67: Condensate and Flash Steam Recovery

66

Sizing flash steamrecovery vessels

The flash vessel The flash vessel separates the flash steam from the condensatein a condensate return line. Fig. 50 shows a typical flash vesselconstructed to BS 5500, category 3 standard.

When condensate and flash steam enter the vessel, condensatefalls by gravity, to the base of the vessel, where it is drained backto a convenient condensate recovery system, usually a ventedreceiver collecting condensate ready for pumping. The flash steamin the vessel can then be piped to low pressure steam usingequipment.

In order to size the flash vessel, the following information isrequired:

Pressure onto the steam traps supplying the vessel.

The condensate flowrate.

The flash steam pressure (desired or existing).

Using this information, and a flash vessel sizing chart, the size ofthe vessel required can be determined. (Fig. 51).

The following example best demonstrates flash vessel sizing,using a simple chart.

Fig. 50 A Typical flash vessel constructed to BS 5500, Category 3 standard

CondensateIN

CondensateOUT

FlashsteamOUT

Page 68: Condensate and Flash Steam Recovery

67

Fig. 51 Flash vessel sizing chart

The pressure on the steam traps is 12 bar g with a total condensateflow of 2 500 kg/h. The flash steam from the vessel is to besupplied to equipment using low pressure steam at 1 bar g.

1. From the 'Pressure on steam trap' axis at 12 bar g, movehorizontally to the flash steam pressure curve at point A.

2. Drop down vertically to the condensate flowrate level, point B,and follow curved line to point C.

3. Move right from point C to meet the flash line at point D.4. Move upwards to the flash vessel size and select the vessel.

In this case, an FV8 flash vessel is required.

Example

Method

Flash steam pressure bar g20

18

16

14

12

10

8

6

4

300400500

1000

2000

300040005000

10 000

15 00020 000

30 000

Pre

ssur

e on

ste

am tr

aps

bar

gC

onde

nsat

e flo

wra

te k

g/h

250

0 2 4 6 8 10 12 14 16 18 20 %

FV 18FV 15

FV 12

FV 8

FV 6

0

0.2

7 6 5 4 3 2 1

Flash vessel size

0.5

0.20

0.511.523457

Fla

sh s

team

pre

ssur

e ba

r g

A

B

CD

Page 69: Condensate and Flash Steam Recovery

68

Control of flashsteam pressure

If full use is to be made of flash steam, some basic requirementsmust be satisfied.

It is essential to have a sufficient supply of condensate, fromloads at sufficiently higher pressures, to ensure that enoughflash steam is released for economic recovery.

The steam traps and the equipment they are draining must beable to function satisfactorily against the back pressure appliedto them by the flash system.

In particular, care is needed when attempting flash steamrecovery with condensate from temperature controlledequipment. At less than full loads, the steam space pressurewill be lowered by the action of the control valve. If it approachesor even falls below the required flash steam pressure, recoveryfrom this condensate requires active condensate removal.

A major requirement is a suitable use for the low pressure flashsteam. Ideally, it should be a low pressure load(s) which requiresa supply of steam that either equals or exceeds the amount ofavailable flash steam. Any deficit can be made up through apressure reducing valve. If the supply of flash steam exceedsits demand, the surplus may then have to be vented to wastethrough a surplussing valve.

It is possible to utilise the flash steam from condensate on aspace heating installation - but savings will only be achievedduring the heating season. When heating is not required, therecovery system becomes ineffective.

Wherever possible, the best arrangement is to use flash steamfrom process condensate to supply process loads - and flashsteam from heating condensate to supply heating loads. Supplyand demand are therefore more likely to remain 'in-step'.

It is also preferable to select an application for the flash steamwhich is reasonably close to the high pressure condensatesource. Piping for low pressure steam is inevitably of a relativelylarge diameter. This can mean costly installation if longdistances are involved

The next consideration is a method of controlling the pressure ofthe flash steam.

In some cases this pressure will find its own level and nothingmore needs to be done. When supply and demand are alwaysin-step, and particularly if the low pressure steam is used onthe same equipment producing the high pressure condensate,the simplest solution is to pipe the flash steam to the lowpressure plant without any other control.

Requirements forsuccessful flash

steam applications

Page 70: Condensate and Flash Steam Recovery

69

Fig. 52 shows the application of flash steam recovery to a multi-bankair heater battery which is supplying high temperature air to a process.Condensate from the high pressure sections is flashed to low pressure,and the low pressure steam is used to preheat the cold air enteringthe battery. The surface area of the pre-heater section, and therelatively low temperature of the incoming air, will mean that the lowpressure steam is readily condensed. Depending on operatingtemperatures, the flash steam will settle at a low pressure. It can evenbe sub-atmospheric. If site conditions and layout permit, the flashvessel and the low pressure coil trap should be located far enoughbelow the condensate outlet to give a hydrostatic head which canpush the condensate through the trap. If not, pumping traps can beused to drain both the pre-heater coil and the flash vessel.

Steam condensation may mean that a vacuum breaker is required.This will prevent the pressure in the battery becoming sub-atmospheric,assisting condensate flow to the trap. Drainage from the traps isinduced by gravity flow.

Fig. 53 shows an application where the flash steam system iskept at a determined constant pressure by steam fed from areducing valve. This ensures a reliable source of steam to the lowpressure system if there is a lack of flash steam to meet the load.

It should be remembered that this will decrease the differentialpressure across the high pressure steam traps, which should besized with this in mind.

LPcondensate

HP traps

HP steam supply

Flash vessel

Temperature control valve

Air flow

Flash steam

Flash vessel bypass line

Fig. 52 Flash steam recovery on a multiple air heater battery

Page 71: Condensate and Flash Steam Recovery

70

Typical applications for flash steam

With 10 % of the units supplied with steam at a lower pressurethan formerly, the total heat output of the system is marginallyreduced. However, it is rare to find an installation which does nothave a sufficient margin of output above the normal load to acceptthis small reduction. Where the output of all the heaters isinadequate, it would be advantageous to install additional heatercapacity so as to gain the benefit of using flash steam whichotherwise would be lost.

Sometimes an apparent problem arises where the use of availableflash steam may require more than one heater but less than two. Itwould be better in this case to connect two heaters to the flashsteam supply, rather than vent the excess flash steam off towaste. Two heaters together will usually pull the flash pressuredown to a lower level, even to sub-atmospheric levels. To copewith this, the supply of flash steam can be supplemented througha pressure reducing valve.

Flash steam supplyand demand in-step

This gives maximum utilisation of the available flash steam. Theair heater battery discussed previously is one such system, butsimilar arrangements are practical with many other applicationssuch as space heating installations using either radiant panels,or unit heaters.

Figure 53 shows a system where a number of heaters aresupplied with high pressure steam. The condensate fromapproximately 90 % of the heaters is collected and taken to aflash recovery vessel. This supplies low pressure steam to theremaining 10 % of heaters.

����

Fig. 53 Flash steam supply and demand in step

HPsteamsupply

PRV set

H P heatersLP Heaters

Trap set

Flash vessel

LPcondensate

HP traps

LP traps

Flash vessel bypass line

Page 72: Condensate and Flash Steam Recovery

71

Another example where supply and demand are in-step is thesteam heated hot water storage calorifier. Some of theseincorporate a secondary coil, fitted close to the bottom where thecold feedwater enters. Condensate and flash steam from the trapon the primary coil, is passed directly to the secondary coil. Herethe flash steam is condensed, while giving up its heat to thefeedwater. The arrangement is shown in Fig. 54.

An extension of this idea is shown in Fig. 55. Here a 'packagedcalorifier unit' is used with a normal steam-to-water calorifierdraining through a float trap to a smaller shell-and-tube exchanger,in which the flash steam is condensed into sub-cooled condensate.The unit is fitted in series with the calorifier, to enable it to preheatthe return water from the system, reducing the demand for livesteam.

A mechanical pump is used to lift the condensate to the returnline, and the exhaust steam leaving the pump is itself condensedin the condenser. The pumping of the condensate is then achievedat virtually no energy cost. Consideration must be given to thepump filling head in that it needs to be greater than the pressuredrop across the condenser tubes under full load conditions. Aminimum head of 600 mm will usually achieve this.

Fig. 54 Secondary flash steam coil in a storage calorifier

Condensate

Secondary coilacting as a flashcooler

Primary coil

Steam

Flow

Return

Page 73: Condensate and Flash Steam Recovery

72

Fig. 55 Packaged calorifier and flash condenser unit

Flow to heating

Steam trap

Shell and tubeflash condenser

Heating calorifier

Motivesteam

Balance line

Return from heating

Temperature control

Condensate return

Steam

Page 74: Condensate and Flash Steam Recovery

73

The arrangement in Fig. 56 is an example of flash steam recoverywhere the supply and demand are not always in-step.

Condensate from process plant releases flash steam, but the onlyuse to be found for it is to augment the supply of steam to the spaceheating installation. This is quite satisfactory during the heatingseason, as long as the heating load exceeds the availability offlash steam.

During the summer season the heating equipment will not be inuse, and even during spring and autumn the heating load may notbe able to use all the available flash steam. The arrangement ismuch less than ideal, although it is quite possible for the steamsavings made during the winter to justify the cost of the flash steamrecovery equipment.

Flash steamsupply and demand

not in-step

Fig. 56 Flash steam supply and demand not in step

Flash vessel

Flash steam

Condensate

Surplussing valveReducing valve

Steam

Page 75: Condensate and Flash Steam Recovery

74

Sometimes, surplus flash must be vented to atmosphere, and, asindicated, a surplussing valve is more suitable for this purposethan a safety valve, which usually has a 'pop' or 'on/off' action and aseat designed for infrequent operation. The surplussing valve willbe set so that it begins to open slightly above the normal pressurein the system. When the heating load falls and the pressure in thesystem begins to increase, the pressure reducing valve supplyingthe make-up steam closes down. A further increase of pressure,perhaps 0.15 or 0.2 bar, is then allowed before the surplussingvalve begins to open.

A safety valve may still be required should the surplussing valvefail. It must be set between the surplussing valve set pressure andthe system design pressure.

Occasionally, during summer conditions it may be preferable tobypass the flash system with a manual valve. The condensate andits associated flash steam will then pass directly to a condensatereceiver, where the flash steam will be vented to atmosphere.

Continuous blowdown of boiler water to control the level of TDS(total dissolved solids) within the boiler, is becoming increasinglycommon. It lends itself to the recovery of the heat content of theblowdown water and enables considerable savings to be made,since they continue all the time the boiler is steaming.

Boiler blowdown contains massive quantities of heat which caneasily be recovered as flash steam. After it passes through theblowdown control valve, the lower pressure water flows to a flashrecovery vessel. There the contaminant free flash steam releasedis separated from the condensate, and becomes available forheating the boiler feedtank, (Fig. 57)

Boiler blowdownheat recovery

applications

Page 76: Condensate and Flash Steam Recovery

75

Steam

Hot welltank

Boiler

Blowdownvalve

Feed pump

Floattrap

Drain

Heat exchanger

Flash vessel

Cold water

Fig. 57 Typical heat recovery from continuous blowdown

Make-up tank

Steam supplyto injector

Level controller

Condensate

Page 77: Condensate and Flash Steam Recovery

76

Finally, consideration must be given to those cases where flashsteam is available at low pressure, but where no suitable load isavailable which can make use of it.

Rather than simply discharge the flash steam to waste, thearrangement in Fig. 58 can often be adopted.

A lightweight but corrosion resistant chamber is fitted to the receivertank vent. Cold water is sprayed into the chamber in sufficientquantities to just condense the flash steam. The flow of coolingwater is controlled by a simple self acting temperature control,responding to the air temperature at the outlet side of the spraynozzle. It will amount to roughly 6 kg of cooling water per kg offlash steam condensed

If the cooling water is of boiler feed quality, then the warmed wateris added to the condensate in the receiver and re-used. This willmake water savings throughout the year.

Condensing water which is not of boiler feed quality, must be keptseparate from the water in the receiver, as shown by the dottedlines.

Spray condensing

Overflowwith 'U' seal

Self actingtemperature control

Condensed waterto waste

Centrifugal pump

Condensate receiver

Fig. 58 Flash steam condensing and water saving by spray

Vented toatmosphere

Condensate

Pumpedcondensate

Water in

Page 78: Condensate and Flash Steam Recovery

77

Page 79: Condensate and Flash Steam Recovery

78

Steam tables

Specific enthalpy SpecificPressure Temperature volume

Water (hf) Evaporation (hfg) Steam (hg) steambar kPa °C kJ/kg kJ/kg kJ/kg m3/kg

absolute0.30 30.0 69.10 289.23 2 336.1 2 625.3 5.2290.50 50.0 81.33 340.49 2 305.4 2 645.9 3.2400.75 75.0 91.78 384.39 2 278.6 2 663.0 2.2170.95 95.0 98.20 411.43 2 261.8 2 673.2 1.777

gauge0 0 100.00 419.04 2 257.0 2 676.0 1.6730.10 10.0 102.66 430.2 2 250.2 2 680.2 1.5330.20 20.0 105.10 440.8 2 243.4 2 684.2 1.4140.30 30.0 107.39 450.4 2 237.2 2 687.6 1.3120.40 40.0 109.55 459.7 2 231.3 2 691.0 1.2250.50 50.0 111.61 468.3 2 225.6 2 693.9 1.1490.60 60.0 113.56 476.4 2 220.4 2 696.8 1.0880.70 70.0 115.40 484.1 2 215.4 2 699.5 1.0240.80 80.0 117.14 491.6 2 210.5 2 702.1 0.9710.90 90.0 118.80 498.9 2 205.6 2 704.5 0.9231.00 100.0 120.42 505.6 2 201.1 2 706.7 0.8811.10 110.0 121.96 512.2 2 197.0 2 709.2 0.8411.20 120.0 123.46 518.7 2 192.8 2 711.5 0.8061.30 130.0 124.90 524.6 2 188.7 2 713.3 0.7731.40 140.0 126.28 530.5 2 184.8 2 715.3 0.7431.50 150.0 127.62 536.1 2 181.0 2 717.1 0.7141.60 160.0 128.89 541.6 2 177.3 2 718.9 0.6891.70 170.0 130.13 547.1 2 173.7 2 720.8 0.6651.80 180.0 131.37 552.3 2 170.1 2 722.4 0.6431.90 190.0 132.54 557.3 2 166.7 2 724.0 0.6222.00 200.0 133.69 562.2 2 163.3 2 725.5 0.6032.20 220.0 135.88 571.7 2 156.9 2 728.6 0.5682.40 240.0 138.01 580.7 2 150.7 2 731.4 0.5362.60 260.0 140.00 589.2 2 144.7 2 733.9 0.5092.80 280.0 141.92 597.4 2 139.0 2 736.4 0.4833.00 300.0 143.75 605.3 2 133.4 2 738.7 0.4613.20 320.0 145.46 612.9 2 128.1 2 741.0 0.4403.40 340.0 147.20 620.0 2 122.9 2 742.9 0.4223.60 360.0 148.84 627.1 2 117.8 2 744.9 0.4053.80 380.0 150.44 634.0 2 112.9 2 746.9 0.3894.00 400.0 151.96 640.7 2 108.1 2 748.8 0.3744.50 450.0 155.55 656.3 2 096.7 2 753.0 0.3425.00 500.0 158.92 670.9 2 086.0 2 756.9 0.3155.50 550.0 162.08 684.6 2 075.7 2 760.3 0.2926.00 600.0 165.04 697.5 2 066.0 2 763.5 0.2726.50 650.0 167.83 709.7 2 056.8 2 766.5 0.2557.00 700.0 170.50 721.4 2 047.7 2 769.1 0.2407.50 750.0 173.02 732.5 2 039.2 2 771.7 0.2278.00 800.0 175.43 743.1 2 030.9 2 774.0 0.2158.50 850.0 177.75 753.3 2 022.9 2 776.2 0.2049.00 900.0 179.97 763.0 2 015.1 2 778.1 0.1949.50 950.0 182.10 772.5 2 007.5 2 780.0 0.185

10.00 1 000.0 184.13 781.6 2 000.1 2 781.7 0.17710.50 1 050.0 186.05 790.1 1 993.0 2 783.3 0.17111.00 1 100.0 188.02 798.8 1 986.0 2 784.8 0.16311.50 1 150.0 189.82 807.1 1 979.1 2 786.3 0.15712.00 1 200.0 191.68 815.1 1 972.5 2 787.6 0.15112.50 1 250.0 193.43 822.9 1 965.4 2 788.8 0.14813.00 1 300.0 195.10 830.4 1 959.6 2 790.0 0.14114.00 1 400.0 198.35 845.1 1 947.1 2 792.2 0.13215.00 1 500.0 201.45 859.0 1 935.0 2 794.0 0.12416.00 1 600.0 204.38 872.3 1 923.4 2 795.7 0.11717.00 1 700.0 207.17 885.0 1 912.1 2 797.1 0.11018.00 1 800.0 209.90 897.2 1 901.3 2 798.5 0.10519.00 1 900.0 212.47 909.0 1 890.5 2 799.5 0.10020.00 2 000.0 214.96 920.3 1 880.2 2 800.5 0.099 421.00 2 100.0 217.35 931.3 1 870.1 2 801.4 0.090 622.00 2 200.0 219.65 941.9 1 860.1 2 802.0 0.086 823.00 2 300.0 221.85 952.2 1 850.4 2 802.6 0.083 224.00 2 400.0 224.02 962.2 1 840.9 2 803.1 0.079 725.00 2 500.0 226.12 972.1 1 831.4 2 803.5 0.076 826.00 2 600.0 228.15 981.6 1 822.2 2 803.8 0.074 027.00 2 700.0 230.14 990.7 1 813.3 2 804.0 0.071 4

Page 80: Condensate and Flash Steam Recovery

79

Specific enthalpy SpecificPressure Temperature volume

Water (hf) Evaporation (hfg) Steam (hg) steambar kPa °C kJ/kg kJ/kg kJ/kg m3/kg

28.00 2 800.0 232.05 999.7 1 804.4 2 804.1 0.068 929.00 2 900.0 233.93 1 008.6 1 795.6 2 804.2 0.066 630.00 3 000.0 235.78 1 017.0 1 787.0 2 804.1 0.064 531.00 3 100.0 237.55 1 025.6 1 778.5 2 804.1 0.062 532.00 3 200.0 239.28 1 033.9 1 770.0 2 803.9 0.060 533.00 3 300.0 240.97 1 041.9 1 761.8 2 803.7 0.058 734.00 3 400.0 242.63 1 049.7 1 753.8 2 805.5 0.057 135.00 3 500.0 244.26 1 057.7 1 745.5 2 803.2 0.055 436.00 3 600.0 245.86 1 065.7 1 737.2 2 802.9 0.053 937.00 3 700.0 247.42 1 072.9 1 729.5 2 802.4 0.052 438.00 3 800.0 248.95 1 080.3 1 721.6 2 801.9 0.051 039.00 3 900.0 250.42 1 087.4 1 714.1 2 801.5 0.049 840.00 4 000.0 251.94 1 094.6 1 706.3 2 800.9 0.048 541.00 4 100.0 253.34 1 101.6 1 698.3 2 799.9 0.047 342.00 4 200.0 254.74 1 108.6 1 691.2 2 799.8 0.046 143.00 4 300.0 256.12 1 115.4 1 683.7 2 799.1 0.045 144.00 4 400.0 257.50 1 122.1 1 676.2 2 798.3 0.044 145.00 4 500.0 258.82 1 228.7 1 668.9 2 797.6 0.043 146.00 4 600.0 260.13 1 135.3 1 666.6 2 796.9 0.042 147.00 4 700.0 261.43 1 142.2 1 654.4 2 796.6 0.041 248.00 4 800.0 262.73 1 148.1 1 647.1 2 795.2 0.040 349.00 4 900.0 264.00 1 154.5 1 639.9 2 794.4 0.039 450.00 5 000.0 265.26 1 160.8 1 632.8 2 793.6 0.038 651.00 5 100.0 266.45 1 166.6 1 626.9 2 792.6 0.037 852.00 5 200.0 267.67 1 172.6 1 619.0 2 791.6 0.037 153.00 5 300.0 268.84 1 178.7 1 612.0 2 790.7 0.036 454.00 5 400.0 270.02 1 184.6 1 605.1 2 789.7 0.035 755.00 5 500.0 271.20 1 190.5 1 598.2 2 788.7 0.035 056.00 5 600.0 272.33 1 196.3 1 591.3 2 787.6 0.034 357.00 5 700.0 273.45 1 202.1 1 584.5 2 786.6 0.033 758.00 5 800.0 274.55 1 207.8 1 577.7 2 785.5 0.033 159.00 5 900.0 275.65 1 213.4 1 571.0 2 784.4 0.032 560.00 6 000.0 276.73 1 218.9 1 564.4 2 783.3 0.031 961.00 6 100.0 277.80 1 224.5 1 557.6 2 782.1 0.031 462.00 6 200.0 278.85 1 230.0 1 550.9 2 780.9 0.030 863.00 6 300.0 279.89 1 235.4 1 544.3 2 779.7 0.030 364.00 6 400.0 280.92 1 240.8 1 537.3 2 778.5 0.029 865.00 6 500.0 281.95 1 246.1 1 531.2 2 777.3 0.029 366.00 6 600.0 282.95 1 251.4 1 524.7 2 776.1 0.028 867.00 6 700.0 283.95 1 256.7 1 518.1 2 774.8 0.028 368.00 6 800.0 284.93 1 261.9 1 511.6 2 773.5 0.027 869.00 6 900.0 285.90 1 267.0 1 501.1 2 772.1 0.027 470.00 7 000.0 286.85 1 272.1 1 498.7 2 770.8 0.027 072.00 7 200.0 288.75 1 282.3 1 485.8 2 768.1 0.026 274.00 7 400.0 290.60 1 292.3 1 473.0 2 765.3 0.025 476.00 7 600.0 292.41 1 302.3 1 460.2 2 762.5 0.024 678.00 7 800.0 294.20 1 311.9 1 447.6 2 759.5 0.023 980.00 8 000.0 295.96 1 321.5 1 435.0 2 756.5 0.023 382.00 8 200.0 297.66 1 330.9 1 422.5 2 753.4 0.022 684.00 8 400.0 299.35 1 340.3 1 410.0 2 750.3 0.022 086.00 8 600.0 301.00 1 349.6 1 397.6 2 747.2 0.021 488.00 8 800.0 302.61 1 358.8 1 385.2 2 744.0 0.020 890.00 9 000.0 304.20 1 367.8 1 372.7 2 740.5 0.020 292.00 9 200.0 305.77 1 376.8 1 360.3 2 737.1 0.019 794.00 9 400.0 307.24 1 385.7 1 348.0 2 733.7 0.019 296.00 9 600.0 308.83 1 394.5 1 335.7 2 730.2 0.018 798.00 9 800.0 310.32 1 403.2 1 323.3 2 726.5 0.018 3

100.00 10 000.0 311.79 1 411.9 1 310.9 2 722.8 0.017 8102.00 10 200.0 313.24 1 420.5 1 298.7 2 719.2 0.017 4104.00 10 400.0 314.67 1 429.0 1 286.3 2 715.3 0.017 0106.00 10 600.0 316.08 1 437.5 1 274.0 2 711.5 0.016 6108.00 10 800.0 317.46 1 445.9 1 261.7 2 707.6 0.016 2110.00 11 000.0 318.83 1 454.3 1 249.3 2 703.6 0.015 8112.00 11 200.0 320.17 1 462.6 1 237.0 2 699.6 0.015 4114.00 11 400.0 321.50 1 470.8 1 224.6 2 695.4 0.015 0116.00 11 600.0 322.81 1 479.0 1 212.2 2 691.2 0.014 7118.00 11 800.0 324.10 1 487.2 1 199.8 2 687.0 0.014 4120.00 12 000.0 325.38 1 495.4 1 187.3 2 682.7 0.014 1

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Further information

This technical reference guide has been designed to give worksengineers or energy managers, an introduction into the subject ofcondensate and flash steam recovery. It is quite impossible tocover all aspects of this subject, as almost every installation isunique.

We have tried to cover most alternatives exist, but it may be thatwe have omitted some options. Advice is always freely availablefrom our team of regional engineers, or by telephone or letterfrom head office as required.

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81

100,000

50,000

20,000

10,000

5,000

2,000

1,000

500

200

100

50

20

10

250

100

120

140

160

180

200

Ste

am te

mpe

ratu

re °

C

Con

dens

ate

rate

kg/

h

Ste

am s

yste

m p

ress

ure

bar

g Condensate system

pressure bar gC

ondensate line size mm

150

100

80

65

40

32

25

20

15

10

6

30

10

0

0.51

2345

50

50

30

10

5

0

0.51

2

2015

500 400 350 300 250 200

20

Appendix 1Condensate line sizing chart

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CM Issue 1TR-GCM-05

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