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MANICA BOARDS AND DOORS (MBD) Pvt Ltd

To: Mr. G. T. Zindove

Cc: Mr. S Mukwamba, Mr. Saungweme, Mr. C Nyamugure

From: Mr. Tawanda Bushu (Chemical and Processing System Engineering Attaché).

Date: 7 February 2012

REF: REPORT ON THE BOILER

1.0 INTRODUCTION

A boiler is a heat exchanger that converts chemical energy in fuel to heat energy under

controlled conditions, for the purpose of generating steam. The boiler section is one of the

sections of critical importance in the fiber board production as its operations affect the

whole process of fibreboard manufacture. The main objective of the boiler section is to

provide a steady and constant supply of steam at all times. All this has to happen within the

statutes and parameters that govern the operation of boilers.

2.0 THE BOILER AND ITS SUPPORTIVE ANCILLARY SYSTEMS.

Fig1.1 boiler and its ancillary systems.

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Fig 1.1 shows what goes into the boiler and what comes out and it is imperative that each

aspect of that system is working in order.

2.1 BOILER.

The type of boiler used at MBD is a Composite Boiler. It consists of a fire-tube boiler and a

John Thompson Chaingrate Stoker (Stoker). A fire-tube boiler is a type of boiler in which

hot gases from a fire pass through one or more tubes running through a sealed container of

water. The heat of the gases is transferred through the walls of the tubes by thermal

conduction, heating the water and ultimately creating steam. The heat source is inside a

furnace or in this case it is the John Thompson Chaingrate Stoker that has to be kept

permanently surrounded by the water in order to maintain the temperature of the heating

surface just below boiling point. (Specifications of boiler found in appendix)

2.2 AIR SUPPLY

Air is drawn in by forced draft fans (FD) which blow onto the feed stocker which will be

carrying burning coal. The volume of feed air is controlled by (FD) dampers but the speed

of the inlet air is constant. Secondary (FD) fans also blow onto the stocker to assist in the

combustion of coal. The heated air is then drawn by a large Induced fan (ID) into fire tubes

on the second section of the boiler. The hot air which in this case is now called flue gas is

further drawn into a grit arrester which in practice is a centrifugal separator to separate grit

from the flue gases. The grit is then collected at the bottom section and the lighter gases

then goes into the chimney where they are discharged into the atmosphere.

2.3 BOILER WATER SUPPLY AND PURIFICATION SYSTEM.

2.3.1 INTRODUCTION

The water supply system must achieve the following.

� Constant supply of safe make up water for the boiler.

� Prevention of corrosion from taking place at all cost or at list keeping it at the lowest

possible level.

� Prevention of formation of scale and scum in the boiler.

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The system is represented by the flow chart shown below. The water used in the Boiler at

MBD is sourced from the Mutare City Council. The water is first stored in reservoir tanks

then pumped to storage tanks through pipes with strainers to remove suspended solids in

the water. This is the first operation of boiler water treatment. From the storage tanks water

is pumped to the water softeners. From the softeners, water is pumped to 2 treated water

storage tanks. Water from these treated water tanks is fed to the de aerator were it is

scrubbed off the dissolved oxygen in water. Water from the de-aerator is the again treated

with dosing chemicals from the dosing plant en-route to the boiler where steam is

generated in a composite boiler.

Key: A main reservoir

B storage tanks (2)

C, D softeners

E, F storage tanks

G dosing tank

H, I de-aerator water pumps (2)

J de-aerator

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K, L feed water pumps (2)

M boiler

Fig 1.2 flow chart showing feed water system.

2.3.2 WATER TREATMENT

At (MBD) the chemical laboratory department is the one responsible for caring out all the tests and

analysis of the boiler water. The water is supposed to be tested on two hour intervals .In the event

that there is a large deviation from the normal average then tests are carried out on hourly interval

and a corrective measure is taken in each case until the anomaly is rectified. Boiler water has

certain specifications that it should meet for it to be used in the boiler.

Table 1.1 boiler water specifications

BOILER WATER SPECIFICATIONS

TOTAL

HARDNESS(TH)

NIL

TOTAL DISOLVED

SUBSTANCES(TDS)

2000-3000ppm

Ph More than 10.5

PHOSPATE 30-90ppm

SULPHATE 30-90ppm

CHLORIDE 800ppm

OH 250-600ppm

IRON 0-5ppm

These measurements are critical for the operations of a boiler. Below the different aspects that are

found in water are defined and how they might affect our boiler.

2.3.2.1 DISSOLVED SOLIDS

These are substances that will dissolve in water. The principal ones are the carbonates and sulphates

of calcium and magnesium, Effects:

1. Scale-forming when heated which reducing heat transfer and the effectiveness of the boiler it can

also causing over heating of pipes and eventually the pipes end up bursting.

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2. Forming and scum formation which can lead to carry overs, damaging of the steam pipes by water

hammers and lowering the steam quality by producing wet steam.

2.3.2.2 SUSPENDED SOLIDS

These are substances that exist in water as suspended particles. They are usually mineral, or organic

in origin. These substances are not generally a problem as they can be filtered out. In the case of

(MDB) strainers are inserted along the pipe lines to trap all suspended solids before the feed water

goes into the boiler. Effects:

1. The effects of suspended solids are the same to those of dissolved solid with the addition that they

damage control valves and heat exchange systems

2.3.2.3 DISSOLVED GASES

Oxygen and carbon dioxide can be readily dissolved by water. Effects:

1. These gases are aggressive instigators of corrosion. According to the design at (MBD) the amount

of oxygen going into the boiler is supposed to be substantially reduced by the de-aerator.

2.3.2.4 SCUM FORMING SUBSTANCES

These are mineral impurities that foam or scum. One example is soda in the form of a carbonate,

chloride, or sulphate. Effects:

1. Scum can cause carry-over of suspended solids which has the same negative effects as those of

suspended solids.

2.3.2.5 HARDNESS

Water is referred to as being either 'hard' or 'soft'. Hard water contains scale-forming impurities

while soft water contains little or none. Hardness is caused by the presence of the mineral salts of

calcium and magnesium and it is these same minerals that encourage the formation of scale. There

are two common classifications of hardness:

1. Alkaline hardness (also known as temporary hardness) - Calcium and magnesium bicarbonates are

responsible for alkaline hardness. The salts dissolve in water to form an alkaline solution. When

heat is applied, they decompose to release carbon dioxide and soft scale or sludge. Alkaline or

temporary hardness.

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2. Non-alkaline hardness and carbonates (also known as permanent hardness) - This is also due to the

presence of the salts of calcium and magnesium but in the form of sulphates and chlorides. These

precipitate out of solution, due to their reduced solubility as the temperature rises, and form hard

scale, which is difficult to remove. The purpose of the softener is to remove this hardness. In

addition, the presence of silica in boiler water can also lead to hard scale, which can react with

calcium and magnesium salts to form silicates which can severely inhibit heat transfer across the

fire tubes and cause them to overheat.

2.3.2.6 TOTAL HARDNESS

Total hardness is not to be classified as a type of hardness, but as the sum of concentrations of

calcium and magnesium ions present when these are both expressed as CaCO3. If the water is

alkaline, a proportion of this hardness, equal in magnitude to the total alkalinity and also expressed

as CaCO3, is considered as alkaline hardness, and the remainder as non-alkaline hardness

\

2.3.2.7 NON-SCALE FORMING SALTS

Non-hardness salts, such as sodium salts are also present, and are far more soluble than the salts of

calcium or magnesium and will not generally form scale on the surfaces of a boiler

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2.3.2.8 pH VALUE

This is a numerical value representing the potential hydrogen content of water - which is a measure

of the acidic or alkaline nature of the water. Acids and alkalis have the effect of increasing the

conductivity of water above that of a neutral sample. For example, a sample of water with a pH

value of 12 will have a higher conductivity than a sample that has a pH value of 7.Effects:

1. If the pH value is low then the feed water will be highly corrosive and can not be tolerated.

2. If pH is high say its 13 then it cause corrosion again in the form of crack corrosion.

It is therefore very crucial to make sure that the water is treated so as to avoid the above mentioned

problems. To achieve the above specifications water from the city council goes through a number

of stages so as to meet the above stages. The water is suppose to firstly pass through softeners to

reduce its hardness but currently the water softeners are not working, this is then followed by de-

aeration by a de-aerator which is functioning and lastly trough chemical treatment.

2.4 WATER SOFTENING

At MBD water softening was achieved by using resin softeners to remove hardness of water.

Hard water has high concentrations of Ca2+ and Mg2+ ions in form of salts that encourage

the formation of scale (fouling). Technical specifications of softeners found in

appendix.Water flows into the softener from the top making a fine spray on top of the

resins. The magnesium and calcium ions in the water are adsorbed on the surface of the

resin and sodium ions are released into the water. In principal this is basically the softening

process. The softening action continues until the ion exchange material contains no more

sodium which can be exchanged for calcium and magnesium.

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After 24 hours the resins becomes saturated with impurities and needs to be regenerated.

The regeneration takes place in four stages that are given below. The whole regeneration

process takes an average of 1 hour 30 minutes.

� Backwash stage

This is the first stage in the regeneration process. Water enters the softener in a

reverse direction all the impurities that are trapped in the softener are washed

out buy this reverse flow into the drain

� Sodium injection

Brine which is sodium chloride solution is pumped into the softener. Sodium

ions are adsorbed on the surface of the resin at the same time magnesium and

calcium ions are discharged into solution the solution. This is preferential

adsorption. During this stage no water enters the softener.

� Fast rinse

Water enters the softener in the normal way and the calcium and magnesium

ions in the solution are then washed out into the drain.

� Slow rinse.

This time water enter into the softener with a very slow rate all the calcium and

magnesium ions are then washed away and the softener is now ready for use

.MBD had two softeners so that during the regeneration of one softener the

other softener will be working but at present none is functioning.

From the softeners the water goes to the water storage tanks. The storage tanks have a glass

water level indicator and must be kept full at all times. In the storage tank EDITA is added

as an ant-scelent. From the storage tanks water goes into the de- aerator.

2.5 THE DE-AERATOR

De-aeration is based on two scientific principles. The first principle can be described by

Henry's Law. Henry's Law asserts that gas solubility in a solution decreases as the gas

partial pressure above the solution decreases. The second scientific principle that governs

de-aeration is the relationship between gas solubility and temperature. Easily explained, gas

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solubility in a solution decreases as the temperature of the solution rises and approaches

saturation temperature. A de-aerator utilizes both of these natural processes to remove

dissolved oxygen, carbon dioxide, and other non-condensable gases from boiler feedwater.

This process can be classified as a unit operation called scrubbing. The feedwater is

sprayed in thin films into a steam atmosphere allowing it to become quickly heated to

saturation. Spraying feedwater in thin films increases the surface area of the liquid in

contact with the steam, which, in turn, provides more rapid oxygen removal and lower gas

concentrations. This process reduces the solubility of all dissolved gases and removes it

from the feedwater. The liberated gases are then vented from the de-aerator. The de-aerator

saves two purposes

1. To remove dissolved gases in the boiler feed water.

2. To raise the temperature of the feed water to a temperature of about 750 C

Reasons:

� Dissolved gases lead to corrosion of the boiler.

� Dissolve gasses can accumulate in the steam section of the boiler forming

bubbles.

� To avoid heat stress in the boiler so as to avoid thermal stress failure in the

boiler.

� To increases solubility of dosing chemicals.

� To save energy during steam production since the water will be at elevated

temperatures already.

The de-aerator that is being used uses scrubbing mechanism. The diagram below shows

how this process works

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Water from the storage tanks is pumped into the de-aerator. The de-aerator has a control

valve and a pressure gauge to regulate the rate at which the water goes into the de-aerator

so that flooding or wiping will not take place in the boiler. The feed water enters the de-

aerator in the form a fine spray on top of the tray and flows down onto the trays where it

comes in contact with the steam which will be rising through the perforated trays. The

steam will strip the water of dissolved gases as it rises. Heat exchange also takes place

between the steam and the water resulting in an increase in the temperature of the water as

a result the solubility of dissolved gases decrees and the water gives up the dissolved gases.

The water then accumulates in the water storage section where a further heating occurs to a

temperature of about 80oC. From the de-aerator the water is then pumped in the boiler by

boiler feed pumps. Before it is pumped dosing chemicals are introduced between the de-

aerator and the feed pumps.This conditioning the water is then feed into boiler

2.6 DOSING CHEMICALS

Three chemicals are dosed online into the feed water. One is for scale and deposit control,

the other one for oxygen corrosion control and the last one is a pH booster.

� scale and deposit control

To control this, a phosphate based treatment (chematron 155) is used. Chematron

155 is synergistic blend of organic and inorganic scale inhibitors.

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� Oxygen corrosion control

The oxygen scavenger used has sulphite as the active component.

� pH buster

Caustic Soda is used as the pH buster

These chemicals are feed into the feed water stream by an adjustable pump which regulates

the rate at which these chemicals are feed into the feed water.

2.7 CONTROL SYSTEM

2.6.1 BOILER CONTROL SYSTEM

The boiler has a control system that regulates it operations. The system has gauges that self

regulate according to set points. (Copy of boiler control system attached)

� Water level

The boiler has a self regulating system that controls the amount of water in the boiler as

per set range of set points set by the operator. It also has a gauge known as the mobri

switch that gives an alarm when the water level reaches a critical low level. It also

operates within a range of set points set by the designer. If it reaches the lower set point

it automatically shuts the boiler off. The main feed water modulating control valve is an

electro pneumatic control valve which actuates level into pneumatics. It opens when the

level is below the lower value set point and closes when the level goes above the upper

set parameter. The system has a bypass in case of failer of modulating valve which can

be used during repairs to the valve

� Pressure control

The boiler has a valve that regulates the pressure within design set points. This gauge

shuts off the ID fans when pressure reaches the upper set point. The pressure release

valves are design feature for failure of the pressure control valves. It also controls the

amount of heart that goes through to the shells reducing the amount of steam generated

and thus controlling pressure.

� Flow control

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The boiler has a valve that controls flow rate as per designed parameters. The feeder

pipe has a designed capacity of nominal flow 16.5t.h and a maximum flow of 20.0t.h.

� Combined control system

The above control systems are connected altogether in controlling the boiler. The main

objective of a combined control system is to for them to complement one another in

case one fails the other can regulate the system.

2.6.2 DE-AERATOR CONTROL SYSTEM

The de-aerator has a control system that regulates its operations. The system has gauges that

self regulate according to set points. (Copy of boiler control system attached)

� Water control.

It has an electro pneumatic control valve which controls the level of water in the de-

aerator within two set points. It opens when the level goes below the lower limit and

closes when it passes the upper limit. There is also a bypass system which is used in

case the valve malfunctions.

It also has an over flow valve that opens when the water level surpasses the upper set

value of level and closes when it reaches the lower limit. It compliments the level

control valve.

� Steam control

The de-aerator has a pressure electro pneumatic pressure control valve which maintains

the pressure in the de-aerator within given set. There is also a bypass system which is

used in case the valve malfunctions.

2.8 STEAM GENERATION

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Hot gases from a fire pass through one or more tubes first running through a sealed container

of water. The heat of the gases is transferred through the walls of the tubes by thermal

conduction, heating the water and ultimately creating steam. The heat source is inside a furnace

or in this case it is the John Thompson Triumph Chaingrate Stoker that has to be kept

permanently surrounded by the water in order to maintain the temperature of the heating

surface just below boiling point. With the aid of the ID fan the flue gas moves to the shell side

of the boiler where it travels through 2 tube passes transferring heat by convection to the

surrounding water creating steam

2.9 STEAM USES

1. Pre-heating the de-aerator

2. Source of energy during pressing

3. Pre-treatment of the chips before defibration process

4. Source of energy during defibration process

5. Heat water at the accumulator

.

2.10 HEAT GENERATION

Coal is used as source of fuel in the stoker and it is washed pear size currently sourced

from Hwange. Washed peas coal is used and an average consumption of

1200tonnes/month. From the stoker hopper coal is laid onto the Chaingrate which is

rotating at a preselected speed. The Chaingrate then transfers the cola into the boiler via the

guillotine door which regulates the fuel bed thickness. After passing under the guillotine

door the coal is heated by radiation from the hot refractory ignition arc. Release of

volatile’s constituents of coal commences at about 1500C. The volatiles mix with the air

supplied by the FD fans and burn above the fuel bed. The burning volatiles heat the ignition

arc and radiate directly onto the oncoming fuel and supplement the effect of the arc. A step

in the arc increases turbulence in the ignition zone. This promotes mixing of the volatiles

and the air, increasing flame temperature and so improving the overall effect of the system.

The coke formed in the ignition arc the move s into the furnace arc where burns through a

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high temperature oxidizing atmosphere leaving a layer of ash on the chain grate. The coal

burns according to the following equation

C + O2 → CO2 + heat

The heat produced is of importance and is quantified by equation

Q=UAdT

Where

Q is the heat transferred per unit time,

A the area available for the flow of heat,

dT the difference in temperature between the flame and the boiling water, and

U is known as the overall heat transfer coefficient (W/m2 K).

Heat is either transferred by conduction to the water tubes surrounding the stoker or by

convection and radiation to the fire tubes of the boiler

For conduction:

Q= kA(T1-T2)/x [3]

Where:

k is the thermal conductivity of the material.

x wall of thickness

T1 and T2 Temperatures of stoker inside and outside walls respectively

For convection (forced convection):

Q= hA(T1-T2) [4]

Where:

k is the thermal conductivity of the material.

x wall of thickness

T1 and T2 Temperatures of stoker inside and outside walls respectively

The heated water will then produce steam and the amount of heat can be quantified by:

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Q = mCpdT [5]

Where:

m mass flow rate of top up water (Kg/s)

Cp specific heat capacity of water

dT temperature rise of top up water

The following energy balance equation will then apply assuming that there are no further

energy losses.

Ms hfg = Q¯ = m Cp dT [6]

Where:

Ms = mean steam consumption rate (Kg/s)

hfg = specific enthalpy of evaporation of steam (kJ/Kg)

Q¯ = mean heat transfer rate (kW)

Heat is also lost in a number of ways and these have effects on thermal efficiency of the

boiler.Boiler efficiency simply relates energy output to energy input, usually in percentage

terms:

Boiler efficiency, in the simplest terms, represents the difference between the energy input

and energy output. The following design issues should be considered during a boiler

efficiency evaluation.

1. Number of boiler passes.

The number of boiler passes simply represents the number of times the hot

combustion gas travels across the boiler (heat exchanger). A boiler with two passes

provides two opportunities for the hot gasses to exchange heat to the water in the

boiler. A 4-pass unit provides four opportunities for heat transfer. The stack

temperature of a 4-pass boiler will be lower than the stack temperature of a similar

size 2- or 3-pass boiler operating under similar conditions.

2. Repeatable air/ fuel control

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The efficiency of the boiler depends on the ability of the burner to provide the

proper air to fuel mixture throughout the firing rate, day in and day out, without the

need for complex set-up or adjustments. Many burner designs can deliver the

required air-to-fuel mix with enough time provided to adjust the burner or for a

single test period.

3. Combustion Efficiency

Combustion efficiency is an indication of the burner’s ability to burn fuel. The

amount of unburned fuel and excess air in the exhaust are used to assess a burner’s

combustion efficiency. Burners resulting in low levels of unburned fuel while

operating at low excess air levels are considered efficient.

4. Thermal Efficiency

Thermal efficiency is a measure of the effectiveness of the heat exchanger of the

boiler. It measures the ability of the exchanger to transfer heat from the combustion

process to the water or steam in the boiler. Because thermal efficiency is solely a

measurement of the effectiveness of the heat exchanger of the boiler, it does not

account for radiation and convection losses due to the boiler’s shell, water column,

or other components. Since thermal efficiency does not account for radiation and

convection losses, it is not a true indication of the boilers fuel usage and should not

be used in economic evaluations.

5. Fuel-To-Steam Efficiency

Fuel-to-steam efficiency is a measure of the overall efficiency of the boiler. It

accounts for the effectiveness of the heat exchanger as well as the radiation and

convection losses. It is an indication of the true boiler efficiency and should be the

efficiency used in economic evaluations. when dealing with fuel to steam efficiency

2.11 HEAT LOSSES

� Heat losses due to unburned carbon

� Size of the coal, fired in boiler should be controlled. For example for

sticker fired coal, it should not be more than 20 mm.

� Proper amount of excess air should be supplied to the boiler. Boiler

should not be overloaded.

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� Homogenous mixing of air and fuel should be maintained.

� Heat losses due to formation of carbon monoxide

� Excess air should not be less than 60%.

� Air should be entered with higher turbulence so that each carbon atom

comes in contact with oxygen atoms.

� Ratio of Actual air to theoretical air should be more than 1.3.

� Heat losses due to moisture in the fired coal

� Proper storage of coal should be done. (Especially in rainy season). Coal

preheating

2.12 HEALTH AND SAFETY

The boiler is both the source of energy for most unit operations in fibreboard production as

well as a source of indirect and direct risk. The operations of the boiler are guided by the law

• The Factories and Works Act 14:08/1996

• Regulations that include

1. RGN 279 of 1976 (Boilers).

2. RGN 303/1976 (Pressure vessels).

It is the best interest of the company that these regulations are followed for the safety of the

operator and the boiler itself.

2.13 FINDINGS

� Water analysis

The primary objective of the boiler is to supply steam to the production line, this supply

if primarily affected by things such as boiler efficiency and the efficiency is affected a

number of things and boiler water is one of them. Boiler water samples were taken for a

number of days to analyze the effect of the boiler water to the boiler.

MONDAY TUESDAY

property FW SW ST BW DW FW SW ST BW DW

pH 6.4 6.2 6.6 9.2 7.0 6.8 6.6 6.7 9.9 8.1

conductivity 26 14 24 700 22 20 20 22 84 89

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TDS 18 22 16 540 14 14 14 18 580 62

TH 17 11 20 ND ND 20 9 23 ND ND

Ca H 6 NA 11 10 4 NA

CL 84 70

Sulphite 11 9

PH 30 20

WENESDAY THURDAY

FW SW ST BW DW FW SW ST BW DW

pH 7.73 7.90 6.69 10.65 9.15

conductivity 20 20 20 900 34 20 20 20 920 26

TDS 18 20 20 540 14 14 14 18 580 62

TH 9 9 9 6 7 7 10

Ca H 6 2 5 7 5 7 6 4

CL 84 67

Sulphite 35 40

OH 31 34

A close analysis of these results will help us appreciate the effects of the water top the

boiler. The main are of concern is hardness, as the softeners are not working. The

results of the city council water show that it is not hard water as total hardness is fairly

low, lucky for us. Analysis of the feed water and the softeners water produces relatively

the same results.

The pH is generally within the parameters playing around alkalinity which is good for

the boiler. The next other thing very important is the sulphite concentration in the boiler

water; a low concentration would mean that little oxygen was removed from the boiler

water resulting in the corrosion of the shells. The results show that this was happening

on monday and tuesday with a corrective measure being taken the day after. The major

issue that was highlighted was the boiler operators were conducting blow downs before

the chemicals added to counter this effect had worked.

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It can be said that the dosing chemicals are achieving their desired objective and it can

be highlighted by the results above

� Boiler operations

Boiler operations have deviated from their designed operations. Control mechanisms

that were put in pace to make boiler operations safe and within desirable parameters are

no longer functional. The majority of the valves on the boiler and the de-aerator are not

functioning properly. Shear pins that seem to be so little to the running of the boiler and

which by design must be made of brass were changed and steel ones were put in place.

They have caused a lot of damage in the running of the chain grate.

The boiler and steam flow should be almost closed a closed system with a little of make

up water but currently make up water is now the major source of water being used. This

shows us that the piping system is now in bad shape as it is loosing a lot of stem and

water which in turn increase the operational costs.

� Redundant equipment

It was observed that the redundant equipment is no longer in the designed specifications

resulting in unnecessary stoppages due to failer of some of the equipment. Below is a

list of the design specification of the equipment.

Boiler Design current

1 Storage tank pumps 1running, 1 standby 1running, no

standby

2 De-aerator feed water

pumps

1running, 1 standby I running, 1

standby

3 Feed water pumps 1running, 1 standby I running, no

standby

4 softeners 1running, 1 standby No running

5 Dosing pumps 1running, 1 standby I running, no

stand by

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� Analysis of coal used at MBD

Coal used at MBD comes from a different places and an analysis of the coal used

for the past two years was done. Analysis of the results was done (see copy of the

results attached). It can be said that the ideal coal for use in the burner should have

low sulpher contents which are good for the boiler and the atmosphere, high volatile

content which release sufficient heat energy to ignite the coal, and an average

moisture content as it helps to limit segregation. It can be seen that at times the

buying of the coal is done not paying much attention to the previous records of

which coal is best for boiler.

� Soot blowing.

Soot blowing was condemned, as the boiler is not producing dry steam which is a cause

of concern. Removing of soot is now done during plant shut downs.

2.14 RECOMMENDATION

� Control system repair needs to be restored. It is imperative that the boiler and de-

aerator control systems be functional. This not only reduces pressure on the

operator but also on the system as a whole. All valves on the boiler and de-aerator

need to be repaired.

� Redundant equipment need to maintain as per designed specifications so as to

reduce dead time.

� New softeners need to be bought and installed to further reduce the effects of

hardness on the boiler.

� Finding the perfect coal with the right constituents is a bit difficult but blending two

typical coals is recommended for example Hwange coal and sengwa coal. Hwange

coal is typical low moisture, high sulphur, high volatile content coal and low ash

fusion temperature, with sengwa being a low sulphur, moderately to high moisture

content, high volatile content but high ash fusion temperature. Blending the two

colas produces excellent results as the shortfalls of the other are complimented by

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the other. Blending enables Hwange coal to burn to ash reducing clinkage and

significantly reducing costs.

� Proper maintenance of the boiler needs to be done as this can reduce future

operational costs which at times can even result in the whole boiler being replaced.

� It needs to be investigated why the boiler is producing wet steam.

2.15 APPENDIX

� Boiler specifications

Type Water tube and Fire tube

Manufacturers Cochrane engineering

Working pressure 32 bars

Design efficiency 75%

Size Capacity output 20 tones per hour

Type of fuel Coal (washed peas)

Firing methods chain grate stroker

Draught plant ID and FD draughts

� De-aerator specifications

Model Chematron T.6

Storage volume 3700 kg normal operation

Outlet capacity 1 800-2 150 kg/hr

Operating pressure 410 kPa

Design pressure 600 kPa

Outlet temperature 152 0C

Heating steam volume 1 200 kg/hr

Heating steam pressure 2 900 kPa

Heating steam temperature 233 0C

� Softeners specifications

MODEL AC 300 Aquamatic

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Maximum working pressure 550 kPa

Minimum working pressure 250 kPa

Maximum softening flow 12 m3/hr

Minimum softening flow 1.5 m3/hr

Resins Ambelite IR12 sodium cations

softening resins

� Blow down Blow down is the removal of water in the boiler. Feed water impurities, oxygen scavengers,

caustic soda and other chemicals that are injected in the boiler form sludge in the boiler. If

uncontrolled the sludge will cause “foaming” and “carryover” which is a threat to the

piping system. Blow down is necessary to avoid total dissolved solids from exceeding 3500

mg/ l.