Steam Traps Manual 1

STEAM AND STEAM TRAPS

PURGADORES DE CONDENSADO, S.L.

PURGADORES DE CONDENSADO, S.L. STEAM & STEAM TRAPS

INDEX 1. STEAM: BASIC CONCEPTS Page

1.1 Definitions 1 1.2 Flash steam 3 1.3 Differences between live steam and flash steam 6 1.4 Back pressure problems 7

2. INTELLIGENT STEAM TRAPS AND VALVES

2.1 The steam trap’s mission 11 2.2 The ideal steam trap 12 2.3 Intelligent steam traps: BiTherm SmartWatch™ 12 2.4 Intelligent valves: SmartWatch™ 16

3. MECHANICAL STEAM TRAPS

3.1 Introduction 19 3.2 Classification of steam traps 19 3.3 Cyclic and continuous trap systems 20 3.4 Orifice plate steam trap 21 3.5 Float steam trap 22 3.6 Inverted bucket steam trap 24 3.7 Thermodynamic disc steam trap 25 3.8 Impulse steam trap 27 3.9 Thermostatic steam trap 28


CHAPTER 1

STEAM: BASIC CONCEPTS 1.1. DEFINITIONS Water steam is a thermal fluid widely used in industry due to two main characteristics:

- High energetic content - Easy to transport

The union of this two properties provides with a very simple method for the supply of great amounts of energy to points located very far from the installation, taking advantage of the steam’s own internal pressure to pump the fluid. The water can be in three phases: solid, liquid and gas or steam. The transition from one state to the other is known as change of state and implies an energetic interchange in the shape of heat. When the transition is from solid to steam the process uses energy and when the transition is in the other way the process gives energy. The process schema is in Fig. 1.1. Fig. 1.2 represents the process of water evaporation. In the graphic, three zones are clearly differentiated:

- Zone 1: Water in liquid phase - Zone 2: Coexistence of water and steam - Zone 3: Superheated steam

Figure 1.1

water + Sensible heat = Boiling water

Boiling water + Latent heat = Saturatred Steam

Evaporation

100 Kca/Kg

640 Kca/Kg

PHASE CHANGES IN WATER The supplied energy in zone 1 is accumulated in the water in liquid phase, increasing its temperature till reaching the evaporation point. The amount of heat needed to elevate its temperature from 0ºC until boiling point is called sensible heat. Once reached the boiling point, the evaporati


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on begins, zone 2, which continues until the total evaporation of the water. During this process, the temperature remains constant while there is water and vapour in coexistence. The produced energy is used for the vapour’s formation. The amount of heat needed to evaporate totally the liquid water at the boiling temperature or saturation temperature receives the name of latent heat. The generated steam in this process is called saturated steam. Once the evaporation process is over we entry in zone 3, where any energy contribution produces a new increase in the steam’s temperature, thus obtaining reheated or superheated steam. The energetic content of steam is: Total Heat = Sensible Heat + Latent Heat + Overheating The graphic in Fig. 1.2 is different for each vapour pressure or operation pressure; representing this graphic for different pressure values in a tridimensional coordinate system, we would obtain a surface that would relate the three magnitudes. Thus, the sensible heat, the latent heat and the saturation temperature depend on the vapour’s pressure. Fig. 1.2

VARIATION OF TEMPERATURE AND ENERGY IN THE WATER’S EVAPORATION

Fig. 1.3 shows the variation of the vapour saturation temperature with the pressure. This variation can be also consulted in the table of vapour saturation at the end of this chapter. During the condensation, evaporation inverted process, the water vapour passes to liquid state, condensate, giving its latent heat, energy which is used in heat exchange processes. In certain applications even part of the condensate sensible heat is used, the condensate reaches a certain subcooling according the saturation temperature.


3

The heat unit is the calorie (Cal), it is the heat amount that should be supplied to a gram of water to elevate its temperature from 14.5ºC to 15.5ºC. The kilocalorie (Kcal) equals to 1000 Cal. The specific heat of a body is the quantity of heat needed to elevate the temperature of one mass unit of the body one degree Celsius. It is expressed in Kcal/KgºC. For water the specific heat is 1 Kcal/KgºC. The specific volume of a body is the volume occupied by a mass unit of the body. It is expressed in m3/Kg. The specific volume of steam is very big compared to the one of the water, for this reason a huge cloud of flash steam is observed in the steam trap discharge, even in correct functioning. This physical process of revaporation of the condensate is produced always as a consequence of the expansion or decrease of the condensate pressure (see 1.2) and should be differentiated from live steam or steam generated by heat supply.

Fig. 1.3

Pressure (bar)

Temperature (ºC)

2 6 14104 128

50

2018

150

100120

16

170

250

200

0

Water

Saturated steam

WATER STEAM SATURATION CURVE 1.2 FLASH STEAM The expansion process of the condensate is easily analysed in Fig. 1.4. The graphic shows the total energetic content of the flash steam (enthalpy) depending on the steam pressure (the table of saturated steam gives the real values in this graphic). As it is observed, the sensible heat increases when the pressure grows and, on the contrary, the latent heat decreases when the pressure grows. Therefore, point 1 represents the energetic content of the saturated steam at the entrance of a heat exchanger. When all the latent heat is transferred we arrive to point 2, where all steam has condensated; note that during state change (line 1-2) the steam’s temperature has remained constant; this is the theoretical state in which the condensate arrives to the steam trap to be


4

eliminated (in practice the condensate is cooled between 10ºC and 15ºC before reaching the steam trap). Fig. 1.4

ENTHALPY PRESSURE DIAGRAMM OF STEAM At the exit of the steam trap the pressure decreases suddenly until Pc, line 2-3, remaining the condensate energetic content. The decrease of pressure from Pv to Pc is known as differential pressure (Fug. 1.5). However, in point 3, the condensate energetic state is superior than at pressure Pc, point 4. The line 3-4 represents the excess of energy of the discharged condensate by the steam trap, due to the expansion of the condensate. This excess of energy is absorved as latent heat by the condensate, which suffers a partial revaporation, so that the energetic equilibrium between the steam trap entrance and exit remains. Summarising, in the condensate discharge some quantity of flash steam necessarily appears, which reestablishes the energetic balance of the fluid before and after the steam trap. The quantity of flash steam formed per mass unit of evacuated condensate is precisely the quotient between the enthalpy correspondent to line 3-4 and the enthalpy of line 5-4, that is, the quotient between the difference of condensate enthalpies before and after the steam trap (h3-h4) and the latent heat of evaporation at the pressure of the steam trap’s exit (h5-h4): Flash steam per mass unit = (h3-h4) / (h5-h4)

LATENT HEAT

SENSIBLE HEAT

STEAM TRAP

PRESSURE

ENTHALPY


Fig. 1.5

DIFFEREN For a quick calculation of the flash steam amocondensadte the graphic of Fig. 1.6 can be us Introduce the pressure value of the condensapressure) in the horizontal axis; from that poincondensate pressure after the expansion (prehorizontal line till it cuts the vertical axis, whersteam produced. The example of Fig. 1.7 shows the process of Special attention should be made to the incrsteam trap, which can induct to error in the di Note that though the amount of condensate inflash steam, when comparing the corresponflash steam compared to 0.840 m3/h of condsteam compared to the one of the condensate 1.3 DIFFERENCE BETWEEN LIVE STEAM A There is no difference between live saturatedin there generation process but once generatproperties. This makes the visual detection ofdifficult. With some experience is possible to differentisteam in the steam trap discharge, but a reliaadequate measuring equipment or leak detec

DIFF. P

S

BACKPRESSURE OPERATION PRESSURE

5

TIAL PRESSURE

unt per mass unit produced in the expansion of the ed in the following way:

te before the expansion (steam trap operation t draw a vertical line up until it cuts the curve of the ssure at the steam trap’s exit); from that point draw a e you can read the percentage in weight of flash

flash steam formation.

ease of volume of the revaporated at the exit of the agnostic of its operation.

weight is much bigger than the amount of produced dent volumes is the other way around (276 m3/h of ensate), this is due to the high specific volume of the .

ND FLASH STEAM

steam and flash steam. The only difference lies only ed both have the same physical and chemical live steam leaks in the steam trap discharge more

ate only by view the presence of live steam and flash ble result can only be obtained with the help of tors by ultrasound.

RESS. = P1 – P2

TEAM TRAP


6

Fig. 1.6

P2=

1bar

1.5

bar

2 ba

r

2.5

bar

3 ba

r

3.5

bar

4 ba

r

5 ba

r 6 ba

r

8 ba

r 15 b

ar

10 b

ar

20 b

ar30

bar

40 b

ar

50 b

ar65

bar

1 1.5 2 3 4 5 10 15 20 30 40 60 100

Condensate pressure at saturation temperature P1 (bar)

0

2.5

5

7.5

10

12.5

15

17.5

20

22.5

25

27.5

30

32.5

35

Atmospheric discharge

Example: Discharging 1 Kg of condensateat saturation temperature from P1=15 barto p2=5 bar a flowrate of10 % will be generated (0.1 Kg of flash steam)

Fla

sh s

tea

m (%

we

ight

)

FLASH STEAM

The flash stem goes always with condensate, presenting a more humid aspect than the live steam. Flash steam is lightly opaque and tends to float in the ambient, while saturated live steam is colourless and comes out with high speed and noise right at the exit of the steam trap. Fig. 1.8 can help to recognise the type of discharge in the steam traps. The ambient temperature and the relative humidity of the air affect very much the aspect of the steam trap discharge. In cold and humid days the flash steam is much more visible than in hot and shiny days. Tracing is an outstanding exception in the purge of condensates, because the discharge temperature oscilates between 80ºC and 100ºC. This case will be treated in more detail later due to its economical importance in the chemical industry. 1.4 BACKPRESURE PROBLEMS Backpressure is one of the most important problems in the condensate return collectors in all big steam installations. This problem is always present in oil refineries and big petrochemical industries and is an important source of economical losses because it affects not only the installation’s operation but also its energetic efficiency, maintenance, etc.


Fig. 1.7

FLASH STEAM IN Its origin is very diverse:

- Live steam leakage through ste- Flash steam in steam trap disc- Inadequate selection of steam - Incorrect dimensioning of steam- Incorrect temperature of discha- Steam leakage in steam trap b- Inefficient steam trap control - Scarce steam trap maintenanc- Incorrect dimensioning of cond- Successive amplifications of th

Its effects are very noxious:

- Decrease of the installation’s th- They contribute to the appeara- Decrease of evacuation capaci- Increase of steam losses in the- Difficulty in recuperating the res

INLET PRESSURE 10 BAR

STEAM FLOW 1000 KG/H

CONDENSA

FLAS

16

CONDENSAT

STEAM T

HEAT EXCHANGER

7

THE STEAM TRAP DISCHARGE

am traps harge traps traps

rge in steam traps y-pass valves

e ensate return collectors e installation without modifying collectors

ermal efficiency nce of waterhammering ty of steam traps rmodynamic steam traps idual energy of condensates

TE FLOW 0.84 M3/H

H STEAM 276 M3/H

ATMOSPH. PRESS.

% FLASH STEAM

E FLOW 1000 KG/H

RAP


8

- Increase of the installation maintenance costs Fig. 1.8

STEAM TRAP DISCHARGE

The solutions are various depending on the origin of the problem. Nevertheless, one of the most efficient measures that can be adopted to resolve the problem is to control the revaporation in the steam trap discharge. Thus, the flash steam produced in the steam trap discharge originates a strong local backpressure due to its huge specific volume, which affects nearby steam traps and the rest of the installation in different ways:

- Generally the evacuation capacity of the seam trap is reduced - In thermodynamic steam traps the discharges are prolonged and the cycle cadence

increases, therefore, live steam losses in the steam trap increase. - The recuperation of the condensate residual energy is more difficult

- The problem expands throughout the installation fast and progressively.

Therefore, the steam trap is a lot of times cause of the problem and the problem itself affects the steam trap very unfavourably, creating a circle of very difficult exit. The energy saving techniques used actually and the use of intelligent steam traps (see chapter 2) capable of selfdetecting internal leaks of live steam or any variation of the operation conditions of the steam trap, resolve this problem and increases the thermal efficiency of the installation at the same time.

LOW CAPACITY

SMALL LIVE STEAM LEAK DISCHARGE WITH STEAM LEAK

NORMAL OPERATION WITH FLASHING

NORMAL OPERATION HIGH FLOW

LIVE STEAM LEAK


9

Note that the absence of live steam leaks in steam traps doesn’t assure in any way the reduction of backpressure in big installation, due to that generally it is associated with the presence of high amounts of flash steam in the condensate return collectors. Remember that to reduce the formation of flash steam it is necessary to decrease line 3-4 in Fig. 1.4, which is the same as descending position of point 2. The situation of point 2 depends on the operation pressure, which cannot be changed easily because its imposed by the process itself, but also depends on the condensate discharge temperature, which can only be modified when using thermostatic steam traps with external adjustment of the operation temperature of the steam trap. This function is also monitored in intelligent steam traps. In summary, the solution to the problem requires various simultaneous actions:

- Elimination of live steam leaks - Efficient control of the formation of flash steam - Review of the steam trap design - Review of the dimensioning of condensate return collectors


10

TABLE OF SATURATED STEAM


11

CHAPTER 2

INTELLIGENT STEAM TRAPS AND VALVES 2.1 THE STEAM TRAP’S MISSION In all steam installation four functionally different parts can be considered (Fig. 2.1):

- Steam generation unit (boiler) - Steam distribution lines - Process equipment (steam consumers) - Condensate return collectors

There are two differentiated zones in the installation:

- Steam zone, of high energetic level - Condensate zone, of low energetic level

The steam trap’s mission is to establish a physical barrier of separation between both zones, avoiding the pass of energy from the high level zone to the low energetic level zone. Therefore, it is evident that the steam trap’s function is essential to reach a high efficiency in the installation.

Fig. 2.1

Boiler

Heat Exchanger

Steam traps

Condensate return collector

Steam net distribution

FeedTank

Pump

SIMPLIFIED SCHEME OF AN INSTALLATION

The steam trap, in its most basic concept, must evacuate condensate that arrives without letting any live steam escape. To achieve this function adequately the points of the installation where steam traps must be installed have to be correctly chosen. In its most general concept, the steam trap is an automatic regulation valve that must control the evacuation of the condensate to reach the maximum energetic efficiency and the optimum operation of all the equipment that is part of the installation. The amplitude of this concept joined to the diversity of applications of steam can be translated into very flexible requirements and, sometimes contradictory, which require different types of steam


12

traps to achieve the service specifications. For example, a dryer cylinder in the paper industry can require the steam trap to work with a small continuous steam leak while the tracing steam trap in the fuel line to the boiler generally will discharge condensate at 40ºC less than the steam saturation temperature. 2.2 THE IDEAL STEAM TRAP From a functional point of view, the steam trap must be capable of doing the following functions:

- Evacuate condensate without losing steam - Evacuate air or other incompressible gases - Adjust itself automatically to operation condition changes

Nevertheless, from an operative point of view the steam trap must incorporate additional performances, such as:

- High energetic efficiency - Low maintenance - Reliability, robustness and versatility - Self-detection of its own failures - Simple maintenance in line and if possible without interrupting its service - High quality and low price

Obviously, it is impossible in practice to fulfil all these specifications. The traditional steam traps are valves of purely mechanical type designed to fulfil the most necessary aspects for the applications to which they are assigned, sacrificing other secondary applications. Thus, the specification are more and more exigent and include actualised criteria of energy saving, environmental protection, safety, maintenance, inspection, etc., which causes the evolution of the classic mechanical steam trap to the modern intelligent BiTherm SmartWatch™ steam trap. 2.3 INTELLIGENT STEAM TRAPS: BiTherm SmartWatch™ They are automatic steam traps which incorporate externally the electronic continuous monitoring system SmartWatch™. Though the SmartWatch™ system can be applied to any type of steam trap its maximal potential is obtained when applied to bi-thermostatic steam traps with external adjustment in operation, described in chapter 3 (Fig. 2.2). The system has two operation modes:

- Autonomous mode, powered by solar or conventional energy - Network mode, powered by conventional energy

The intelligent steam trap is a device of high technology, unquestionnable technologic leader, which takes advantage of the synergy of the union of the BiTherm bi-thermostatic steam trap with the electronic continuous monitoring device SmartWatch™, controlled by microprocessor.


Fig. 2.2

INTELL

The result produces the high

- System’s self-dia- Continuous detec- Continuous detec- Continuous detec- Continuous detec- Correction possib- Reduction of flas- Discharge tempe- Reduction of insp- Minimum duratio

The SmatWatch™ device inon top of the steam trap’s ccoupled, the system can init The external mechanism oresolve any detected problethe operation. One of the most interesting of the condensate return co

SOLAR PANELS ALARM SIGNAL

PR

INDEPE

EXTERNAL ADJSTMENT DEVICE WHILE IN OPERATION

13

IGENT BITHERM-SMARTWATCH STEAM TRAP

er performances at the moment:

gnostic tion of live steam leaks tion of flash steam formation tion of the efficiency decrease tion of excessive condensate accumulation ility of live steam leaks while in operation

h steam formation while in operation rature adjustment while in operation ection and maintenance costs

n three times higher than the mechanical steam trap

corporates a screwed element, which is connected to the screwed part over. It can be also connected during the steam trap’s operation. Once iate its operation alerting about any incidence in its operation.

f adjustment of the steam trap’s temperature makes it possible to m while in operation, with no need to isolate the steam trap nor stop

functions of the intelligent steam trap is the control of the pressurisation llectors.

“Y” STRAINER

STRAINER VALVE

EXTERNAL CONNECTION

SMARTWATCH

UPPER BIMETALLIC THERMOSTATS

LOWER BIMETALLIC THERMOSTAT

BALANCED ESSURE VALVE

NDENT SEAT

BITHERM BI-THERMOSTATIC

STEAM TRAP


14

Thus, the intelligent steam trap incorporates a system of continuous ultrasound detection that normally is activated when a small live steam leak appears. The system also detects increasing revaporation levels even before reaching the live steam leak. On the other side, the external mechanism of the steam trap allows to adjust the temperature of evacuation of the condensate, that is, to fix the position of point 2 (Fig. 2.3). From this not only the flash steam formation in the steam trap’s discharge can be controlled very easily, but also its evolution in time. Suppose an application that allows to eliminate completely the flash steam in the steam trap’s discharge. In this case point 2 must be situated in point 2ª, in this way point 3 coincides with point 4 and there would be no residual energy to revaporate the condensate discharged by the steam trap. This would additionally mean to use the energy H2-H3 (condensate partial sensible heat) as useful energy, reducing in the same proportion the residual energy in the evacuated condensate. From this significant advantages can be deduced:

- Total decrease of revaporated in the steam trap’s exit - Backpressure reduction due to flash steam - Live steam saving (partial use of sensible heat) - Absence of thermal waterhammering caused by flash steam

Suppose now that with the pass of time the steam trap suffers internal damage due to erosion; then the evacuating temperature would increase, that is, point 2 would increase lightly and therefore point 3 would be in the flash steam area. This fact would immediately detected by the intelligent steam trap alerting the user, who in a couple of minutes could correct the situation using the external adjustment of the steam trap while it is in operation, with no need of substituting the steam trap or its spare parts. To avoid the risk that point 2 is situated under the desired point the intelligent steam trap monitors continuously this point generating the correspondent alarm when it decreases more than it should. Again new significant advantages are evident:

- Absence of inspection costs - Reduction of material costs - Reduction of maintenance costs - High energetic efficiency maintained in time - Detection of failures by excessive condensate accumulation

The exposed example is not a theoretical case nor even less infrequent, but the contrary. A high percentage of steam traps are used in the chemistry industry in steam tracing, where condensate is discharged at between 80ºC and 100ºC. The high number of these steam traps generate high backpressures in the condensate return collectors of very hard solution. The intelligent steam trap is the ideal solution to resolve all these problems in a profitable, safe and lasting way (Fig. 2.4). Other advantages of the intelligent steam trap are its great reliability, robustness and versatility. Reliability is achieved through the continuous self-inspection system, alerting of possible failures. The robustness is guaranteed because it is a bimetallic steam trap resistant to corrosive condensates, waterhammers, high pressures and high operation temperatures. It incorporates high technology materials such as Titanium Nitride. Additionally, its design as balanced pressure valve, independent from the differential pressure, guarantees its operation even in installation with great backpressures.


Fig. 2.3

The versatility isdischarge at theeach specific apossible that thlines or even tuelements and re 2.4 INTELLIGEN The concept of safety valves an The SmartWatcsupport for the f
ENTHALPY
15

ENTHALPY-PRESSURE DIAGRAMM OF WATER STEAM

assured by the external adjustment mechanism, which makes the condensate

ideal wished temperature possible. It is like having a steam trap manufactured for pplication, but conserving the advantages of a product in series. This makes e same device can be used for so different applications like tracing, distribution rbine protection, only by modifying its discharge temperature. This standardises duces costs of immobilised in spare part stocks.

T VALVES: SmartWatch™

intelligent steam trap can be applied for the monitoring of steam and gas leaks in d automatic on-off valves, etc.

h™ system is applied externally to these valves through a collar, which serves as ixation of the electronic element (Fig. 2.5).

LATENT HEAT

SENSIBLE HEAT

STEAM TRAP

PRESSURE


16

Fig. 2.4

Smart Watch Steam trapSteam leak detection

Zone of local pressurisationand waterhammering

Steam

CONTINUOUS STEAM LEAK DETECTION

SmartWatch™ can be integrated forming a powerful network of valve leakage surveillance, integral safety concept, improving safety significantly in industrial installations, in LPG spheres, etc. The system stays in continuous surveillance of the valve detecting any gas or steam leak as soon as it appears. Once detected the leak transmits the information to the central control unit, from where all programmed actions for each case are automatically generated (remote alarms, telephone calls to maintenance centres and safety departments if necessary, etc.). Bidirectional data transmission is used, which allows the remote configuration of the sensors as well as its activation or disactivation. Additionally, self-calibration and failure self-detection functions are available, generating the correspondent alarms to facilitate its identification.

Fig. 2.5

INTELLIGENT SAFETY VALVE


All the information is collected in databases for its analysis and posterior treatment according the user’s necessities. SmartWatch™ can be applied to on-off automatic valves to identify instantly process gas leaks, which would be burnt in the flare without being detected (Fig. 2.7). This allows to reduce process losses and, therefore, production costs as well as inspection and maintenance costs. Fig. 2.6

SMARTWATCH INTEGRAL SAFETY SYS

A notable characteristic of the SmartWatch™ system is the facilityuniversal alarm network with a practically unlimited growing capactraps, valves of different types and other elements share a unistructure. This standardisation means an important operative sinstallation and conservation costs.

Surveillance 24 h/day, 365 days/year Dangerous gas leak

SmartWatch

Safety valve

t Remote alarm

Simultaneous advise

to the Safety

TE

ofity quimp

Instant alarm to the Maintenance Dpt.

Central control uni

17

M

application integration in a (Fig. 2.8). This way, steam

e and powerful information lification and reduction of


Fig. 2.7

REDUCTION OF PRODUCTION COSTS

SMARTWATCH NETWORK STRUCTURE

FRefining tower

Process units Process units

Up to 256elements

Up to 256elements

Up to 256elements

Control

Unit

Fig. 2.8

lare

Up to 16.7 million elements and up to 4 parameters per element

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CHAPTER 3

MECHANICAL STEAM TRAPS 3.1 INTRODUCTION The intelligent steam trap has been excluded from this chapter due to the fact that, though a mechanical steam trap, it incorporates electronic technology. Nevertheless, the bi-thermostatic steam trap will be studied here in detail as a mechanical steam trap, basis of the intelligent steam trap. Since the first steam trap of history, from the orifice plaque to the intelligent steam trap, there has been a constant evolution of steam traps trying to satisfy all demands and improve there characteristics. The evolution of energy costs caused the new mechanical steam traps’ appearance, like the bi-thermostatic, to improve their energetic efficiency. 3.2 CLASSIFICATION OF STEAM TRAPS There are different criteria of steam trap classification, according the concept used to classify them. According to their operation principle, they can be classified in the following way: Tos Tdoto Acth

TYPE OF STEAM TRAP

Sensible to status changes

Sensible to velocity changes Orifice Plate

Disc Thermodynamic Impulse

Sensible to temperature changes

Thermostatic Bimetallic

Bi-Thermostatic Capsule

Sensible to density changes Float

Inverted Bucket Open Bucket

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he steam traps sensible to density changes are based upon the buoyancy of a float, closed or pen, that activates a valve in dependence with the level reached by the condensate inside the team trap.

he group of steam traps that are sensible to the fluid’s pass velocity are based on the great ifference between the specific volume of the steam and the condensate. This makes the velocity f the steam pass through an orifice much higher than the one of the condensate; this is translated pressure differences that are used to control the steam trap.

t last, the steam traps sensible to temperature changes take advantage of the condensate ooling in relation to the saturation temperature of the steam to activate a thermostat that controls e steam trap operation. They behave like automatic thermostatic valves.


20

According to the type of valve, steam traps can be classified in three big groups:

- Differential pressure valve - Balanced pressure valve - Piloted valve

The balanced pressure valve’s plug is submitted to a uniform pressure field that involves it completely (Fig. 3.1), so that the plug is floating and isn’t submitted to any force, while over the differential pressure valve’s plug always acts the pressure P = P1-P2. For this reason the balanced pressure valve can act independently from the backpressure in the steam traps discharge. In the piloted valve, a small valve commands the action of the principal valve. The steam traps with piloted valve are practically not used. Fig. 3.1

Differential pressure valve Balanced pressure valve

TYPES OF VALVES 3.3 CYCLIC AND CONTINUOUS TRAP SYSTEMS According to its operation mode the steam traps can be classified in two big groups:

- Steam traps of cyclic discharge - Steam traps of continuous discharge

In the group of steam traps of cyclic discharge are the following types:

- Inverted bucket - Impulse - Disc - Bellow - Capsule

In the group of continuous discharge are:

- Closed float steam trap - Bimetallic

The production of condensate in an industrial process occurs continuously, with no strong fluctuations, which makes think that the systems of continuous discharge offer advantages in relation to the cyclic discharge systems.

P1 P1


21

In the continuous discharge system, the discharge of the steam trap is adjusted always to the condensate production, allowing a constant equilibrium between steam trap and process, avoiding strong pressure oscillations in the condensate return system. On the other hand, in case of cyclic discharge the steam trap must be overdimensioned to compensate in the active part the capacity loss of the dead part of the cycle. The intermittent discharge provokes pressure oscillations and backpressure that can affect other steam traps and produce strong waterhammering. In a cyclic system it is normal to find live steam leaks before the steam trap closes. When it opens a cyclic system must rapidly eliminate the accumulated condensate; this causes a decrease of pressure before the steam trap and, with it, a small decrease of temperature. At the same time, in the discharge an increase of backpressure is produced, therefore the final differential pressure that acts upon the steam trap results decreased. In summary, the evident advantages of a continuous discharge system in comparison to other cyclic are:

- Equilibrium between the condensate charge of the process and the steam trap - Softer functioning of the installation - Higher energetic efficiency - Better control of the steam trap’s operation - Better control of steam leakage - Maintenance of a higher differential pressure in steam traps

3.4 ORIFICE PLAQUE STEAM TRAP It can be considered as the CAR steam trap in history. It is the simplest trap device. It is an orifice made on a metallic plaque, calibrated according the condensate flow that it is capable of evacuating (Fig. 3.2). The orifice plaque cannot be considered really as an automatic steam trap because it does not incorporate any pressure, temperature or flow regulation element. It is only capable of creating a charge loss that increases with the flow. The live steam pass at great speed through the orifice produces a charge loss that stops partially the current decreasing at certain level the great steam losses that this steam trap can present if it is not well dimensioned. The advantages of this steam trap are:

- Maximum simplicity - Wide pressure range - Low maintenance

Fig. 3.2

ORIFICE PLAQUE STEAM TRAP


Its disadvantages are evident:

- Very critic dimensioning - Low flexibility - Great live steam losses - Appearance of great backpressures in the condensate collectors.

3.5 FLOAT STEAM TRAP It was the first mechanical automatic steam trap used in industry. Basically it is a liquid level regulation valve (Fig. 3.3). Its mechanism is constituted by an articulated bar in one of its extremes of length (L) and a float on the other end that gives an impulsion (E). In an intermediate point the plug of a valve of area (S) is situated. The level of condensate in the steam trap activates the opening and closing of the valve (V). The articulated bar constitutes an arm bar (L). From the equilibrium of the applied forces to it, it is deduced that there is a limited differential pressure that acts on the valve and from which the floating pressure that acts on the float cannot open the valve. The opening force (Fa = E x L) must be always bigger than the closing force (Fc = P x S). Therefore: E x L > P x S For this reason in float steam traps it is always necessary to take into account the section of the discharge orifice and the maximal differential pressure of operation.

Fig. 3.3

To evacuate big flows a valve of grfloat’s diameter or the arm bar’s len To evacuate the retained air insidecapsule, bellow or bimetallic, or a bimetallic type if it works with overhorifice, which, like a by-pass, substhave a constant live steam leakage.Consequently, to dimension a float s

Shearing (E)

Pressure (P)

Valve

Thermostat

Float Float

22

FLOAT STEAM TRAP

eat pass section will be needed, which obliges to increase the gth and therefore the size of the steam trap.

, the steam trap can incorporate a thermostatic air vent (T) of small valve of manual deareation. The thermostat must be of eated steam. Sometimes the air vent is simply a small internal itutes the thermostatic air vent, in this case the steam trap will team trap, the following aspects must be considered:


23

- Maximal differential pressure. Must not be higher than the indicated by the

manufacturer - Minimum differential pressure. Must allow to evacuate the maximum operation flow in

start-up and continuous operation. - Maximum operation pressure. Maximal value indicated by manufacturer - Deareation type required

This type of steam traps normally incorporate an external bar that allows to raise the float opening the steam trap in case is needed, but it must be advised that this bar isn’t any mechanism of external flow adjustment, but an element that blocks the steam trap cancelling its regulation capacity. The characteristic advantages of these steam traps are:

- They support strong flow and differential pressure variations - Continuous condensate evacuation at saturation temperature - They discharge dirty and oily condensates easily - They are hermetic to steam

Their basic disadvantages are:

- They are very voluminous, heavy and expensive - Fixed mounting position - They normally lack of filter and check valve - They are sensible to freezing and to waterhammering - Their discharge temperature cannot be changed

3.6 INVERTED BUCKET STEAM TRAP The theoretical principle of operation of this type of steam trap is identical to the one of the float steam trap, nevertheless there are substantial differences between them. The float of this steam trap is a cylindrical bucket open in its bottom part (inverted bucket), which has a small orifice (C) in its top part to let pass of non-condensables or steam during the operation (Fig. 3.4). When it is installed the valve (V) is open and the bucket stays at the bottom of the steam trap. During start-up the air circulates free around the bucket and leaves through the valve on top (V). Afterwards comes cold condensate that rises through the exterior and interior of the bucket until it fills up the steam trap and is discharged through the top valve (V). When steam appears it is retained in the interior of the bucket due to its lower density, discharging in the bottom part partially the water it contained; a pressure on the bucket is then produced and the bucket floats closing the top valve (V). While steam is passing through the deareation orifice (C) out of the bucket, it is condensating allowing therefore the flow of new condensate inside the steam trap; the liquid level rises inside the bucket and the volume of the steam area decreases until it reaches a level where the weight of the bucket is higher than the pressure the steam produces inside it, therefore the bucket falls and the top valve (V) opens.


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It is observed then that the discharge in this steam trap is intermittent and its need of an hydraulic seal in its bottom part to establish the steam pressure inside the bucket, necessary to achieve its flotation.

Fig. 3.4

INVERTED BUCKET STEAM TRAP

Like in the float steam traps, the size of the valve’s orifice (V) is critical and determines the maximum differential pressure, at higher pressures the steam traps cannot work because the weight of the bucket that moves the bar isn’t capable of opening the valve (V). Therefore when dimensioning these steam traps the same mentioned indications for the float steam traps must be followed. The advantages of this steam trap are:

- Simplicity, with low possibilities of mechanical failures - Waterhammer resistant - It discharges dirty condensates without difficulty - It requires low maintenance

Its disadvantages are:

- Slow deareation, small orifice to avoid great energy losses in operation - It can lose its water seal and produce great steam losses - Fixed mounting position - Normally it doesn’t incorporate filter nor check valve - It works with steam losses by condensation in each cycle - Expensive maintenance and generally it is not repairable in line - Sensible to freezing due to the presence of water inside it - It admits only a small steam overheat if a check valve is installed in its entrance - Its discharge temperature cannot be varied


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3.7 THERMODYNAMIC DISC STEAM TRAP The disc thermodynamic steam trap deserves special attention because it has been the mostly used in the past. Its utilisation has descended considerably due to its low energetic efficiency. The operation of the disc steam trap is very negative because it produces steam losses that are origin of strong local backpressures in the return collectors that affect very negatively the good operation of the installation. The disc steam trap’s design is very simple. Its operation is based on the Bernouilli principle. It consist of a body, a cover and a disc. The cover has a protuberance to facilitate the formation of a control chamber between the disc and the cover when the disc is in the highest position, open. When the disc is in the lower position, closed, the control chamber remains also closed. Initially, in the installation start-up, the steam trap discharges with no difficulty until live steam reaches the steam trap. In its pass through the steam trap the steam runs under the disc at great speed to the exit of the steam trap generating an increase of the dynamic pressure and therefore a decrease of the static pressure in the bottom side of the disc, because according Bernouilli’s principle the total pressure in the fluid remains constant. At the same time a small amount of steam reaches the small control chamber between the disc and the cover. The steam flow speed in this chamber decreases and for the same reason before mentioned, but inversely, an elevation of the static pressure in the chamber is produced. As a consequence of these facts the disc falls violently against the seat of the steam trap, pressed from above and absorbed from below, producing the steam trap’s closure. In this position the control chamber, the entrance and exit orifice remain isolated one from each other. On all the disc’s top surface acts the pressure of the control chamber in the closing direction while in only a small part of the inferior side of the disc acts the steam pressure and the backpressure, both in direction of opening. The result is a net closing force. This unbalance of forces remains until the pressure in the control chamber decreases sufficiently, due to the condensation of the retained steam, to open the disc. This cycle is repeated successively. It is important to mention that the steam trap’s discharges are repeated in cycles independently from whether it flows condensate to the steam traps or not.

Fig. 3.5

THERMODYNAMIC DISC STEAM TRAP


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This fact is easily confirmed when some water drops fall on the steam trap; it immediately condenses the steam retained in the control chamber and the steam trap opens open over and over again even if there is no condensate in front of it. For this reason protection caps are installed on the steam traps when it is installed in cold or humid areas, to reduce it opening rhythm which would produce great energy losses. Advise that sometimes this steam trap’s operation is described wrongly when affirming that the its closing is produced by the condensate´s revaporation effect inside the steam trap. This is incorrect because the flash steam is produced after the steam trap and cannot progress counter-current where the pressure is higher. Therefore, it is the live steam escape, not flash steam, the one that produces the steam trap closure, like demonstrated experimentally (see work published in Petrogas, September 1979, p. 43). The advantages of a disc steam trap are:

- Wide pressure range - Robust construction and low price - Insensible to waterhammering - It works with overheated steam - Insensible to freezing - Very resistant to corrosion

Disadvantages of the disc steam trap:

- Low deareation capacity - It doesn’t admit a backpressure higher than 80% (50% maximum for low service

pressures) - Sensible to failures due dirtying - Cyclic operation. Fast deterioration of the disc and/or seat due to the violence of the

closing with important and increasing steam losses. - Important energy losses specially in low flow services like line and tracing with steam. - Very sensible to climate adverse conditions, rain and wind - Creation of high backpressures in return collectors

As observed, the disadvantages of this type of steam trap are based repeatedly on the excessive energy consume, being nowadays considered as one of the steam traps with the lowest energetic efficiency. 3.8 IMPULSE STEAM TRAP The mechanism of the impulse steam trap (Fig. 3.5) is constituted by a cylindrical piston or plug (P), that has a central orifice (O) along its symmetry axis, communicating the entrance with the exit of the steam trap. This plug (P) incorporates a circular horizontal wing in its top part. The plug (P) can move upwards and downwards inside a cylinder of conical internal surface. In the top part the plug closes the valve’s orifice (V). The condensate that reaches the steam trap flows through the central orifice of the plug. At the same time the condensate flows around the cylindrical wing of the piston onto the top control chamber of the steam trap, producing steam loss in the straight part between the horizontal plug and the conical cylinder.


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As a consequence the pressure in the top control chamber, over the plug, is lower than the pressure in the lower part of the plug wing and the plug rises opening the valve (V). When steam reaches the steam trap, the central orifice (O) of the plug produces a higher resistance to the steam pass due to its high flow speed compared to the condensate; this makes the pressure in the top control chamber increase and the plug (P) descends strangling the flow section of the valve (V). The free section between the plug and the guide cylinder varies with the movement of the first caused by the conicity of the second, acting as a tube of variable section. This allows certain flexibility to flow variations, resulting therefore to be an organ of regulation because the valve (V) depends on the vertical position that the plug (P) adopts each time, this position will depend on the condensate flow.

Fig. 3.6

IMPULSE STEAM TRAP

Note that the position of the steam trap must be always vertical not to interfere in the upward and downward movement of the plug. For more operative flexibility the steam trap normally incorporates in its top part a adjustment screw that fixes the position of the guide conical cylinder able to vary the top control chamber’s volume and, in consequence, the pressure in the chamber. It is evident that a watertight plug closure is never achieved, because the control orifice (O) maintains a live steam leak permanently (control steam), which makes the steam trap operation possible. For this reason, the leak detection through ultrasound in impulse steam traps is always positive. The advantages of this type of steam trap are:

- Small and robust - Air and incondensable discharge - Wide operation range - It can be used with overheated steam

Its disadvantages are:

- Live steam losses and low efficiency - Increase of backpressure in return collectors - Fast waste of internals by erosion - Doesn’t support well backpressures higher than 40% - Great sensibility to staining


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3.9 THERMOSTATIC STEAM TRAP It includes a wide group of steam traps that use a thermostat of solid expansion, liquid expansion (bellow or capsule) or bimetallic to control its operation. The thermostatic solid expansion steam traps use a resin of high dilatation coefficient that in its expansion activates the valve of the steam trap. Its use is very limited. The liquid expansion steam traps (Fig. 3.7) are formed by a capsule (C) or a corrosion resistant bellow of stainless steel or other materials, in which there is a mixture of water and alcohol or other liquid with a boiling point that is a few degrees under the one of the water. The operation is very simple; when condensate reaches the steam trap at a temperature close to the saturation point, the internal liquid of the capsule evaporates and the capsule dilates closing the steam trap’s valve. When the condensate cools, the internal fluid of the capsule condenses and the capsule contracts opening the valve. Advantages of the bellow or capsule steam traps:

- Great evacuation capacity - Great precision and fast response - Operation in any position - Automatic deareation - Insensible to staining - Insensible to freezing - They follow the steam saturation curve without readjustments - They admit great backpressures

Disadvantages:

- Fragility of the thermostatic element - They do not support well waterhammering and overheated steam - High maintenance cost; expensive spare parts and of short duration

Fig. 3.7

CAPSULE THERMOSTATIC STEAM TRAP


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The third group of thermostatic steam traps, the bimetallic, deserves a special mention due to its wide utilisation and the evolution grade they have suffered. They are constituted by a thermostat of bimetallic plates that react with the different condensate temperatures transmitting its movement to the internal valve plug of the steam trap. They are very versatile steam traps due to the possibility of adjusting their condensate evacuation temperature to the optimum value for the service they must fulfil and their use has allowed the achievement of great energetic efficiency of the installations. The bimetallic thermostat can present diverse configurations. Generally the internal regulating organ of the steam trap or bimetallic regulator is formed by a packet of bimetallic plates more or less robust, according the pressure they must support, a valve with its plug and a temperature adjustment device that is used also to join all the other elements. Fig. 3.8 shows a classic bimetallic steam trap with differential pressure valve. Its operation is as follows: When cold condensate reaches the steam trap the bimetallic plates are relaxed, plain, allowing the movement of the plug that is pushed in the direction of opening by the fluid own pressure. When the condensate’s temperature starts to rise the bimetallic sheets curve, they act in pairs one against the other, pulling up the plug that is pressed in opposite direction. The plug’s position, and therefore the opening of the valve, depends continuously on the equilibrium of the closing forces (thermal) and the opening forces (differential pressure over the plug). When the condensate temperature is close to the steam’s saturation point the bimetallic regulator closes the valve hermetically; this process is continuous and the closing point depends on the adjustment given to the thermostat. Fig. 3.8

CLASSIC THERMOSTATIC BIMETALLIC STEAM TRAP

The valve of this steam tarp is of differential pressure type; the plug in on the exit side of the valve, where the condensate expansion is produced and the fluid flow speed is very high and consequently the erosive action over the plug is very aggressive. The advantages of the bimetallic steam trap are:

- Optimum energy efficiency - Continuous discharge and wide pressure range - Great robustness, resistance to waste and to waterhammering - Insensible to corrosive condensates and freezing - Automatic deareation and great capacity of start-up in cold - Mounting in any position

Bimetallic package


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- It supports overheated steam - High reliability and versatility

The disadvantages of these steam traps are:

- Sensibility to staining - Slow response to strong pressure or operation condition changes

The most innovative of the bimetallic steam traps is the bi-thermostatic steam trap of balanced pressure (Fig. 3.9). One of the differences with respect to the classic bimetallic steam trap is that the bi-thermostatic steam trap incorporates two antagonistic thermostats. The thermostat on top consists of various bimetallic plate pairs guided by the plug’s stem, and the inferior thermostat is formed by one bimetallic sheet that leans and is guided in its edges by the body of the steam trap. The bimetallic plate pairs of the superior thermostat act one against the other; each pair is separated from the next by a ring that distances them 1mm and that protects the regulator from failures caused by staining. The fittings of the valve are covered with Titanium Nitride in steam traps of high pressure, increasing its surface hardness until 84 HRC to enlarge the steam trap’s life. The valve is guided in its flow direction, therefore the steam trap can work in any position (the steam trap works even if the entrance and exit connections are exchanged, in this case the steam trap would operate like a differential pressure valve with the thermostat in the exit side). The valve is of balanced pressure type, the plug is situated before the valve, in an area where there is only condensate circulating at low speed and, therefore, does not suffer the erosive effects of the flash steam, formed at the exit of the valve. As a consequence the duration of the bi-thermostatic steam trap is three times longer than the one of the classic bimetallic steam trap. The superior thermostat expands with the increase of temperature pressing the plug in the closing direction; at the same time, the inferior thermostat arches itself allowing the movement of the valve in the same direction. The combined result of both actions is the progressive strangulation of the condensate flow as the condensate temperature rises till it reaches its complete closure when it arrives to the previously adjusted temperature. When the condensate temperature decreases the thermostats act inversely; The superior thermostat contracts decreasing its pressure over the plug while the inferior thermostat decreases its curvature pushing the plug in the direction of valve opening. As a consequence the flow section of the valve increases. Both thermostats always fulfil an antagonistic equilibrium function without intervening in any way the differential pressure. The condensate discharge is modulated according the amount of condensate produced in the installation. The balance point can be fixed with the steam trap in operation through the external temperature adjustment mechanism, which allows to convert a series steam trap into a specific steam trap for each application. While in a classic bimetallic steam trap, the bimetal is constantly submitted to the opening force, fact that requires a more robust design, with higher thermal inertia and that limits the pressure range of the steam trap. In bi-thermostatic steam traps both thermostats are never submitted to pressure forces, but exclusively to thermal effects, which only affect the intrinsic property of the bimetal, thus its operation pressure range is much higher and its duration much longer.


Fig. 3.9

BI-THERMOSTATIC BAL

Also as a difference with the classic bimetalregulator are independent and can be subrepairing cost. Additionally, the external possible, with no need of substituting any incost. The failure of a classic bimetallic steam trapthermostat has been eliminated in the bi-the1mm between bimetallic plate pairs. Finally, the steam trap incorporates an extersystem, described in chapter 1, that converthigher performances than any other known As the principal advantages we can mention

- Optimum energetic efficiency - External adjustment mechanism - Possibility of repair while in opera- Very long duration - High reliability and versatility - Great robustness, resistance to w- Insensible to staining and to corr- Insensible to freezing - Automatic deareation - Great security coefficient in cold - They admit great backpressures - They support overheated steam - Reduced maintenance and spare

The disadvantages of these steam traps are

- They follow with small retard to s The bi-thermostatic steam trap adds therefoeliminating also its defects.

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Steam Traps Manual 1

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Transcript of Steam Traps Manual 1