Boiler Manual

Cochrane Engineering (Pvt) Ltd Stirling MK7 S4500-S20000 Range of Boilers Boiler Manual Issue 3

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Chapter 1

General Information

Contents Introduction Page 2 Conversion to Gas or Oil Firing Manufacturers Detail Plate Serial number plates Design and Performance Data Page 2 Design and construction Clearance certificate Turndown ratio Smoke and grit emission Dryness fraction Physical data Page 3 General data Services Required Page 3 Type of coal required Feedwater Electrical power requirements Associated Equipment Page 3 Definition Combustion equipment Steam and furnace pressure control Steam pressure gauge Page 4 Furnace pressure gauge Control panel Boiler feedwater pumps Water level monitoring instruments Water level controls Ash removal trolley Grit collector Page 4 Sootblowers Inspections Page 5 Maintenance Tools and Consumable Spares Page 5 Optional Extras Page 5 Labour Requirements Page 5 Annexure 1 Table 101 - General Data Page 1 Boiler dimensions drawing single flue Page 2 Boiler dimensions drawing twin flue Page 3


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Chapter 1

General Information Introduction 1. The Stirling MK7 boiler is a high efficiency, wetback and fully packaged coal fired unit. The

boiler has been specifically designed for use with a Chaingrate Stoker. A cut-away view of a single stoker Stirling MK7 boiler is given in Appendix A to this manual.

2. The boiler is a horizontal three pass unit comprising :

a. A flue housing the stoker. Twin stoker packages have two separate flues and two separate stokers.

b. A wetback return chamber.

c. Two convection tube passes.

Conversion to Gas or Oil Firing 3. The Stirling MK7 boiler can be converted to fire gas or oil. Details will be supplied on request. Manufacturers Name Plate 4. A manufacturers detail plate is fitted to each boiler. The details stamped on this plate must be

quoted when ordering spares or making general enquiries concerning the boiler. (Ref. Appendix A page 3.)

Serial Number Plates 5. A serial number plate is fitted to the stoker hopper. The details stamped on this plate must be

quoted when ordering spares or making general enquiries concerning the stoker. 6. A serial number plate is fitted to the forced draught (FD) fan casing. The details stamped on this

plate must be quoted when ordering spares or making general enquiries concerning the FD fan. 7. A serial number plate is fitted to the induced draught (ID) fan casing. The details stamped on this

plate must be quoted when ordering spares or making general enquiries concerning the ID fan. Design and Performance Data Design and Construction 8. The boiler has been designed and constructed in accordance with an approved code and

complies with the Factory and Works Regulations 1976, and the Mining (Management & Safety) Regulations 1990.

9. All pressure parts are manufactured under the supervision of a recognised Approved

Independent Inspection Authority (AIIA). Clearance Certificate 10. On completion of manufacture, the Inspection Authority (see. Para. 9) issues a Release Note and

signs the Pressure Vessel Certificate confirming that the design and the materials used in construction and manufacture comply with the relevant Code. The Quality Assurance file is submitted to the relevant Regulating Authority with a copy to the Client, by Cochrane Engineering (Pvt) Ltd.


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Turndown Ratio 11. Under normal operating conditions, the automatic combustion control has a turndown ratio of 3 :

1 without altering the fuel thickness. Under certain conditions a turndown ratio of 4 : 1 can be achieved. If the boiler is to be operated on lower loads, it should be panel controlled or manually controlled.

Smoke and Grit Emission 12. The design of the boiler is such that

smoke is eliminated provided that the boiler is operated correctly and that the correct fuel is used (Ref. Appendix B to this manual). Grit emission is minimal as the fuel bed is quiescent at all times.

Dryness Fraction 13. A large steam space ensures a dryness fraction of approximately 0.98 when the boiler is

operating at full load. Physical Data 14. Dimensions and masses (operating and dry) of the boilers are shown in the drawing (Ref.

Annexure 1 to this chapter) and table (Ref. Annexure 1 to this chapter) attached. General Data 15. Table 101 (Ref. Annexure 1 to this chapter) lists general data applicable to the Stirling MK7

boiler range. Services Required Type of Coal Required 16. The boiler is capable of burning a wide variety of coals. The main factors which affect the choice

of coals, are detailed in Appendix B to this manual. Feedwater 17. Provision must be made for the supply of feedwater. The chemical content of the boiler water

and the feedwater must be maintained within close limits. Guidance on recommended qualities is given in Appendix C to this manual.

18. Because of the importance attached to the correct analysis and treatment of the boiler feedwater,

it is recommended that specialists carry out these functions. Electrical Power Requirements 19. Standard electrical power connections to the control panel are as follows :

a. 380V, 3 phase, 4 wire line

b. Installed power (kW) required for the complete Stirling MK7 boiler range is given in Table 101 (Ref. Annexure 1 to this chapter).

Associated Equipment Definition 20. For the purpose of this publication, associated equipment is defined as being standard items of

equipment normally supplied with a fully packaged boiler. Optional extras, which may be obtained from Cochrane Engineering (Pvt) Ltd., are listed in para.42.

Combustion Equipment


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21. In its standard form as a coal fired boiler, the Stirling MK7 boiler is equipped with the following combustion equipment :

a. Chaingrate stoker. b. Forced draught (FD) fan and damper.

c. An FD damper control unit.

d. An induced draught (ID) fan and damper.

e. An ID damper control unit.

22. The combustion equipment is described in the Cochrane Chaingrate Stoker Manual. Steam and Furnace Pressure Control 23. Control of the steam pressure and furnace pressure can be effected in three different ways as

follows :

a. Manually at the mechanical control units mounted on the boiler (manual).

b. Manually at the control panel (panel)

c. Automatically (auto). 24. The automatic combustion controls used on the Stirling MK7 boiler ensure :

a. Maximum efficiency under fluctuating load conditions.

b. Correctly controlled air/fuel ratios to give smokeless combustion.

c. Boiler pressure is maintained within close limits.

d. Immediate response to changing load conditions.

e. A high turndown ratio to meet practically all conditions and demands.

f. Minimal maintenance. Steam Pressure Gauge 25. Steam pressure is measured by a standard Bourdon type pressure gauge fitted with an isolating

cock and an Inspectors test connection. The gauge is mounted on the boiler name plate. Furnace Pressure Gauge 26. Furnace pressure is monitored by a draught gauge which is calibrated in Pascals. The gauge is

mounted on the fascia of the control panel and coupled to the furnace by a steel tube and rubber pipe. The single stoker version employs one furnace pressure gauge, whereas the twin stoker version employs two gauges.

Control Panel 27. The control panel provides a central point from which the boiler can be manually controlled

through the control regulators on the fascia (panel) or automatically controlled (auto). It also serves as a connection point between the various circuits of the Stirling MK7 boiler. Housed inside the control panel are the electric motor and start circuits, thermal overloads, fuses etc.


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Boiler Feedwater Pumps 28. Two electrically driven feedwater pumps are supplied with the boiler. Each pump is complete

with motor and coupling and has a capacity in excess of the boiler output at maximum continuous rating (MCR). The pumps are designed to handle non-aggressive boiler feedwater. One pump is used for standby purposes while the other is operating.

29. On single stoker versions, the pumps are mounted on a bracket attached to the boiler. On

double stoker versions the pumps are supplied as separate units

which are mounted in the vicinity of the boiler.

30. A central pump station is sometimes used for multi-boiler installations. Water Level Monitoring Instruments 31. The boiler water level is monitored by two water gauges with top and bottom isolating cocks,

gauge glass protectors and tri-valves. Water Level Controls 32. The water level in single stoker boilers is controlled by a float operated switch which starts and

stops the selected feedwater pump. Audible and visual alarms are incorporated to warn the operator when the water level in the boiler is at an unsafe level.

33. The water level in double stoker boilers is controlled by a modulating controller which actuates a

valve in the feedwater line. The feedpump operates continuously. Audible and visual alarms are incorporated to warn the operator when the water level in the boiler is at an unsafe level.

34. A second float operated switch is fitted to switch off the fan and stoker motors when the extra

low water level is reached. This level is never lower than the Lowest Legal Water Level (LLWL) which is marked on the boiler. This is the same as the Lowest Working Water Level (LWWL) in the boiler regulations.

Ash Removal Trolley 35. Mechanical ash extractors in the boiler flue have been eliminated by the incorporation of a

central ash discharge port in the boiler shell. Ash from the stoker drops through this port into the ash sump. Emergency de-ashing can be effected by opening a door beneath the front of the stoker and manually raking the ash out.

Grit Collector 36. To reduce grit emission a multicyclone

grit collector is supplied as standard equipment. A wet sump is supplied at the bottom of the collector hopper. Grit collects in the sump and may be removed in a wheelbarrow.

Sootblowers 37. A set of sootblowers is supplied as standard equipment at the rear of the boiler to clean the

second pass tubes. A set of sootblowers can be supplied fitted in the front smokebox, to clean the third gas pass.

Inspections (See Appendix F to this manual) 38. To comply with relevant Regulating Authority regulations, the boiler must be inspected and/or

hydraulically tested whenever a Government Inspector decides. A boiler log book must also be kept in which boiler maintenance work, inspections and tests must be recorded.


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Note - Inspection and testing must be carried out by a Government Inspector. 39. It is general practice to have an internal inspection at regular yearly intervals and an external

inspection and hydraulic test at regular two yearly intervals. The boiler should also have an external inspection and a hydraulic test when the following occur :

a. After positioning and before initial commissioning.

b. After the boiler has been moved from its original position.

c. After pressure part repairs.

40. The hydraulic test pressure is given on the manufacturers name plate (Ref. Appendix A page 3). Maintenance Tools and Consumable Spares 41. A number of maintenance tools and consumable spares are provided with each installation.

(Ref. chapter 5 to this manual) Optional Extras 42. The following items are available as optional extras :

a. Swinging coal chute b. Coal handling plant c. Ash conveying equipment d. Water sample cooler e. Modulating water level controls on single stoker boilers. f. Deaerator g. Front sootblowers.

43. Details of these optional extras can be

obtained from Cochrane Engineering (Pvt) Ltd.

Labour Requirements 44. Because of the boiler mass an experienced team consisting of one artisan and four labourers

with rigging tackle is required to off-load and position the boiler. It is strongly recommended that only a company that has considerable experience in handling large boilers be engaged to off-load and position the boiler. Cochrane Engineering (Pvt) Ltd. offers this service.

45. When fully automatic facilities are provided, one man can operate the boiler. When coal and/or

ash has to be handled manually additional labour is required. One man can maintain the boiler provided that heavy assemblies or sub-assemblies do not need to be removed.


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Chapter 2

Operating Information Contents Introduction Page 3 CONTROLS Component Location Page 3 Water Level Controls and Monitoring Instruments Page 3 Water level gauge glasses Feedpump selection switches Water level controls (single stoker boilers) Page 4 Water level controls (double stoker boilers) Combustion Controls and Monitoring Instruments Page 4 Steam pressure controls Steam pressure gauge Page 5 Firing rate control Furnace pressure control Furnace pressure gauge/switch Page 6 Stoker Controls Page 6 Coal cut-off gate Guillotine door Undergrate dampers Page 7 Ash and Grit Removal Page 7 Primary de-ashing port Emergency de-ashing door Grit collector Sootblowers Rear sootblowers Front sootblowers Valves and Mountings Page 7 Boiler blowdown valve Safety valves Main steam valve Automatic sequencing blowdown valves Main feedwater check valve Air release valve Page 7 Inspectors test gauge connection

Operating Procedures Introduction Page 8


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Sootblower Operation Page 8 Rear sootblowers Front sootblowers Blowdown Procedures Page 9 Blowdown of gauge glasses Blowdown of float-operated switches Boiler blowdown Page 10 Ash and Grit Removal Page 11 Ash removal Grit removal Page 11 Normal Water Level Control Page 11 Filling the boiler (on/off feedwater controls) Lighting A Fire Page 12 Notes on Combustion Page 14 Low Load Firing Techniques Page 15 Shutting Down Page 16 Standby periods of up to 18 hours (banking) Page 16 Normal shutdown Page 19 Emergency shutdown (power failure or extra low water level, ID fan, FD fan and stoker motors stopped) Page 19 Emergency shutdown (extra low water level, ID fan, FD fan and stoker motors stopped) Page 20


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Chapter 2

Operating Information Introduction 1. For the convenience of the user, this chapter is divided into two parts : CONTROLS OPERATING PROCEDURES 2. The first part describes the controls, gauges and valves fitted to the Stirling MK7 boiler. The

second part describes the actual procedures to be followed when operating the boiler. 3. The heart of the Stirling MK7 boiler is the chaingrate stoker. Due to its wide appeal, to both

boiler and furnace users alike, the stoker is supplied with its own operating and maintenance manual. To avoid unnecessary duplication, full descriptions of the stoker are not given in this manual.

Meaning of Operating Terms 4. Throughout the text, reference is made to the following :

a. Manual Control - This means that control is effected manually at the mechanical control units.

b. Panel Control - This means that control is effected manually through the control regulators on the panel.

c. Auto Control - This means that control is effected automatically through the electro

mechanical control systems.

CONTROLS Component Location 5. The location of the various components fitted to a standard single stoker boiler is shown in

Appendix A to this manual

(Pages 4 and 5). Location of alternative water level controls sometimes fitted to single stoker boilers as an optional extra, is also shown. The location of components on a standard twin stoker boiler is as shown in Appendix A to this manual (Pages 6 and 7).

6. Although component locations are the same for both single and double stoker boilers, several

controls are duplicated on the double stoker version. The standard feedwater control system fitted to single stoker boilers employs a float operated switch, which switches the feedpump on and off as the water level in the boiler varies with steam demand. The standard feedwater control system fitted to double stoker boilers employs a modulating water level control to regulate the flow of feedwater to the boiler in proportion to steam demand. This system can also be fitted as an optional extra to single stoker boilers.

7. Operators are advised to familiarise themselves with the controls and monitoring instruments as

fitted to their particular boiler, before any attempt at operating the boiler is made. Water Level Controls and Monitoring Instruments


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Water Level Gauge Glasses 8. Two gauge glasses are fitted to the boiler to enable the operator to visually monitor the water

level. One gauge faces to the front of the boiler and the other faces to the side of the boiler. A common indicating plate indicates the water level in the boiler. It should be noted that extra low water level has been reached when water is just visible in the glass. Each gauge is fitted with test cocks and gauge glass protectors.

Feedpump Selection Switches 9. A selection switch for the feedpumps is mounted on the control panel fascia. This two position

switch enables the selection of either feedpump No. 1 or No. 2. The selected feedpump is controlled by a float-operated switch, which is described in para. 12. An indicating lamp illuminates when either pump is operating.

10. When modulating controls are fitted, the water level in the boiler is controlled by a modulating

valve which adjusts the flow of feedwater in proportion to steam demand. A two position switch enables either pump No. 1 or pump No. 2 to be selected. The modulating valve cannot be by-passed for manual control except as described in para. 15.

Water Level Controls (Single Stoker Boilers) 11. The water level control system fitted to a single stoker Stirling MK7 boiler is known as on/off

feedpump control (two switch control). In this system two separate float operated control switch units are used. One of these switch units contains two switch contacts actuated by a single float. The function of the two switch units is as follows :

a. Control the water level in the boiler by switching the selected feedpump on and off.

b. Cause audible and visual alarms to be operated if the water level in the boiler falls to the low

water level mark. 12. The second float operated switch has only one set of contacts, which open when the extra low

water level is reached. Since the combustion equipment control circuits are wired through this switch, complete shutdown is initiated if the water level in the boiler falls to the extra low water mark. It should be noted that this level corresponds to water just being visible in the gauge glasses mentioned in para.8. The combustion equipment cannot be restarted until a safe water level has been re-established.

Water Level Controls (Double Stoker Boilers) 13. In place of the two switch control a modulating controller is fitted which actuates a valve in the

feedwater line. This valve continuously controls the flow of water in proportion to steam demand. Should the water drop to the low water level mark, the same audible and visual alarms mentioned in para.11.b are operated.

14. A float operated control switch identical to that described in para.12, is fitted to all boilers. 15. Manual Control - Under certain circumstances, it will be necessary to over-ride the automatic

function of the modulating valve. To do this, it is necessary to open the by-pass valve and close the globe valve feeding the modulating valve. The modulating valve is then fully open and the water level in the boiler can be controlled by the main feedwater check valve. When the boiler is filled to the required level, the globe valve feeding the modulating valve should be opened and the by-pass valve closed.

16. Pressure Relief Valve - Under low load conditions, the modulating valve is sometimes closed

for long periods and this could cause the feedpump to overheat since it is not designed to operate against a fully closed discharge. To guard against this, when a modulating water level controller is fitted to the Stirling MK7 boiler, a pressure relief valve is connected in the feedpump


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discharge pipe. The pressure relief valve allows feedwater to bypass the boiler and return to the feedwater tank when the modulating valve is closed.

Combustion Controls and Monitoring Instruments Steam Pressure Controls 17. Steam pressure is controlled through the ID damper control unit (Ref. Appendix A page 11). The

damper control linkage is brought down to the control unit at the rear of the boiler.

18. A pulling motor is fitted to enable auto control to be effected and a clutch lever is incorporated to enable either auto and panel control (panel position) or manual control (manual position) to be selected.

19. Manual Control - To enable the ID damper control unit and hence the steam pressure, to be

manually controlled, the clutch lever is set to the manual position. The ID damper control lever can then be set in a selected position by turning the locking handle clockwise turning the handle counter-clockwise releases the locking action.

20. Panel and Auto Control - To enable the ID damper control unit and hence the steam pressure,

to be panel controlled or auto controlled, the clutch lever is set to the panel position. The locking handle is left loose with the control lever in the zero position.

21. When the ID damper manual/auto selection switch on the control panel is set to MANUAL the ID

damper control unit is controlled from the control panel through the panel mounted control regulator. When the selection switch is set to AUTO, the ID damper control unit, and hence the steam pressure, is automatically controlled. Automatic control is effected through a pressure detector which, via a control relay and a pulling motor, converts pressure changes into mechanical movement of the ID damper. The pressure detector, which is mounted on the side of the control panel, incorporates a setting screw and scale indicator. A transmission range slide screw and indicator is also incorporated and increases the sensitivity of the transducer if the band width is decreased and decreases the sensitivity of the transducer if the band width is increased.

Steam Pressure Gauge 22. Steam pressure is monitored on a standard Bourdon tube type pressure gauge, which is

calibrated in kilopascals (kPa) and mounted on the boiler nameplate. A red mark on the gauge indicates the authorised working gauge pressure. For purposes of repair, replacement, etc., an isolating valve is fitted to isolate the gauge from the steam pressure.

Firing Rate Control

23. Firing rate is controlled through the VSD (variable speed drive) and the Sauter motor..

24. The FD damper control unit relates the position of the FD damper to the grate speed. The Sauter motor adjusts the position of the FD damper through a direct coupling and the potentiometer in the Sauter motor provides the signal to the VSD to adjust the grate speed.

25. The maximum speed range is 0 to 15 metres per hour and the minimum speed range is 0 to 5.5 metres per hour.

26. Increasing the speed range of the grate decreases the air/fuel ratio (i.e. more fuel will be fed to

the boiler for a given amount of air) and the CO2 content of the exhaust gases will be increased. Conversely, decreasing the speed range will increase the air/fuel ratio and the CO2 content of the exhaust gases will be decreased.


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27. If the air/fuel ratio is too low (CO2 content greater than 14 percent) the coal will not be completely burnt off before it reaches the end of the grate and it is possible then that black smoke will be emitted. If the air/fuel ratio is too high (CO2 content less than 11,5 percent) the load carrying capacity of the boiler will be impaired. Optimum results are obtained at a CO2 content of approximately 12,5 percent.

28. A Sauter motor is fitted to enable auto control to be effected and a clutch lever is incorporated to

enable either auto and panel control or manual control to be selected. 29. Manual Control - To enable the FD damper control unit and hence the firing rate to be

manually controlled, the clutch lever is set to the manual position. The FD damper control lever can then be locked in a selected position by turning it clockwise : turning the lever counter-clockwise releases the locking action.

30. Panel and Auto Control - To enable the FD damper control unit, and hence the firing rate, to

be panel controlled or auto controlled, the clutch lever is set to the panel position. 31. When the FD damper manual/auto selection switch on the control panel is set to MANUAL, the

FD damper control unit is controlled from the control panel through the panel mounted control regulator. When the manual/auto switch is set to AUTO, the FD damper control unit, and hence the firing rate and furnace pressure, is automatically controlled. Automatic control is effected through a transducer incorporated in the draught gauge. Two set-point adjusters and an indicator are also incorporated on the draught gauge. The transducer through control relays and a pulling motor, converts changes in the furnace pressure into mechanical movement of the FD damper control unit and consequently varies the grate speed and FD damper opening.

Furnace Pressure Control 32. Adjusting the position of the FD damper through the FD damper control unit controls the air

supplied to the fuel bed. This will vary the firing rate and affect the furnace pressure. Opening the damper increases the furnace pressure while closing the damper decreases the furnace pressure.

33. Adjusting the position of the ID damper can also control furnace pressure. This is controlled

through the ID damper control unit. Normally, when increasing the firing rate, the ID damper should be opened first. When decreasing the firing rate, the FD damper should be closed first.

Furnace Pressure Gauge/Switch 34. Furnace pressure is monitored by a photohelic draught gauge which is calibrated in Pascals

(Pa). The gauge is mounted on the side of the control panel. The instrument is connected to the furnace by a copper or steel tube and a rubber pipe. Constricting the rubber pipe with a clamp can dampen the signal to the instrument. When the needle is positioned to the right of zero a negative pressure is indicated, whereas to the left of zero indicates a positive pressure. Automatic control is effected through a transducer incorporated in the draught gauge. Two set point adjustors are fitted. The right hand one should be set to minus 15Pa and the left hand one to minus 3Pa.

Stoker Controls

C Coal Cut-off Gate

35. A coal cut-off gate is incorporated in the stoker hopper. To isolate the coal supply to the gate, the gate is lifted. A retaining pin holds the gate in the lifted position. When the gate is lowered, it allows coal to flow onto the grate and simultaneously closes off the front of the stoker.

Guillotine Door


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36. The guillotine door enables the depth of the fuel bed to be adjusted. The door is raised and lowered by the handwheel. The fuel bed depth is shown on the handwheel indicator. Recommended fuel bed depths for various conditions are shown in Appendix B to this manual. If the depth of the fuel bed is varied the air/fuel ratio will change and the fuel trim knob will have to be adjusted accordingly. Increasing the fuel bed depth decreases the air/fuel ratio, decreasing the fuel bed depth increases the air/fuel ratio.

Undergrate Dampers 37. Two adjustable undergrate dampers are provided and can be adjusted while the boiler is

operating. They enable the air distribution to be controlled along the length of the stoker. The front undergrate damper is provided to control combustion under the ignition arch. The rear undergrate damper is provided to adjust the length of the active fuel bed. If the damper is opened, the active fuel bed will lengthen. If the damper is closed, the active fuel bed will shorten. Both dampers should be closed when lighting a fire or when operating on low loads. When operating at more than 30 percent load, the position of the dampers should be adjusted to give optimum combustion conditions.

Ash and Grit Removal

Primary De-ashing Port 38. Ash is discharged from the end of the stoker grate through the ashport located in the boiler shell.

The ash port should always be open and clear of ash.

Emergency De-ashing Door 39. An emergency de-ashing door is provided beneath the front of the stoker. This door should

always be kept closed and secured unless ash is actually being removed through it. A special shovel is provided for this purpose.

Grit Collector 40. The multicyclone is provided with a grit removal sump.

Sootblowers 41. Rear sootblowers are fitted as standard equipment. The layout of the front (if fitted) and rear

sootblowers is as illustrated in Appendix A to this manual (pages 13 and 14). Sootblowers must only be operated when the boiler is on low loads and only for short periods. It is recommended that the sootblowers are used once every 8 hours or once during a shift.

Rear Sootblowers 42. Steam is supplied to the sootblowers through a sootblower isolating valve. The left and right

hand blowers, when operated, allow steam to blow through the first pass fire tubes and remove deposits. The drain cock is used to drain water out of the system.

Front Sootblowers 43. Steam is supplied to the sootblowers through a sootblower isolating valve. This valve has an

associated isolating cock and a drain cock : the latter is used to drain water out of the sootblower pipe.

Valves and Mountings

Boiler Blowdown Valve 44. This valve serves two different purposes

a. It allows controlled blowdown of boiler water during operation. This procedure is necessary in

order to keep the chemical content of the water within the required limits.


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b. It allows the boiler contents to be discharged if the boiler is to come off range for service or layup.

Safety Valves 45. These valves relieve excess steam pressure. Their settings are locked and must not be adjusted

during operation. It is recommended that the valves are tested once a week as detailed in Chapter 4 paras.30 and 31 .

46. Steam relieved by the safety valves is blown through the take away pipes. These pipes must not be blocked and must vent to atmosphere.

Main Steam Valve 47. This valve is used to isolate the boiler from the main steam range. Automatic Sequencing Blowdown Valves 48. The float operated controls must be blown down regularly to ensure that they operate correctly.

These valves automatically provide sequenced blowdown of the controls. Main Feedwater Check Valve 49. This valve is used to isolate the boiler from the feedwater supply and incorporates an accessible

check valve. Air Release Valve 50. This valve is used to vent air during filling and steam raising and to prevent a vacuum forming

when taking the boiler off range or when emptying. Inspectors Test Gauge Connection 51. The inspectors test gauge connection is positioned adjacent to the boiler pressure gauge to

enable both the test gauge and the boiler gauge to be viewed from one position. When the gauge is not connected the gauge connection aperture is sealed off with a plug.

OPERATING PROCEDURES Introduction 52. In this second part of Chapter 2 the operating procedures are given in the order usually carried

out (i.e. filling the boiler, lighting a fire from cold, etc.,). However, since sootblowing, blowdown operations, ash and grit removal are carried out at various stages of operating the boiler, these procedures are described first. Periods at which these operations should be implemented are left to the discretion of the operator who must decide for himself when these are required.

53. The operating procedures also assume that the operator has read the first part of this chapter

and is therefore, familiar with the controls as fitted to his particular boiler. Sootblower Operation 54. Before carrying out sootblowing operations, the following conditions should be observed :

a. Sootblowers should only be used when the boiler is on low load and near its maximum working pressure.

b. Panel control should be selected.


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c. Induced draught control dampers should be opened sufficiently to maintain balanced draught conditions during operation.

d. If the steam pressure falls during sootblowing operations, allow sufficient time between

successive operations to enable the boiler to regain its normal working pressure. Rear Sootblowers 55. These are operated as follows:

a. Open the drain cock.

b. Crank open sootblower isolating valve.

c. When all residual water and condensate has drained off, close the drain cock.

d. Fully open main sootblower isolating valve. e. Over a period of approximately 15 seconds, turn the right hand blower handwheel to the fully

open position and then reverse slowly to the fully closed position.

f. Repeat step e. three times and, finally leave the handwheel in the fully closed position.

g. Repeat steps e. and f. with the left hand blower handwheel.

h. Close the main sootblower isolating valve and open the drain cock.

i. When all residual water has drained off, close the drain cock. Front Sootblowers 56. These are operated as follows :

a. Open left hand drain cock. b. Crank open left hand main sootblower isolating valve.

c. When all residual water and condensate has drained off, close drain cock.

d. Fully open left hand main sootblower isolating valve.

e. Over a period of 15 seconds, turn the left hand lever operated blower valve to the fully open

position and then reverse slowly to the fully closed position.

f. Repeat step e. three times and, finally leave the valve in the fully closed position.

g. Close the left hand main sootblower isolating valve and open drain cock.

h. When all residual water has drained off, close the drain cock.

i. Repeat steps a. to h. inclusive using the right hand front sootblower. Blowdown Procedures Blowdown of Gauge Glasses 57. To blow down the gauge glasses the following procedure should be adopted :

a. Close the water leg valve.


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b. Open the tailcock and allow steam to blow through the gauge.

c. Close the tailcock and open the water leg valve.

d. Close the steam leg valve. Open the tailcock and allow water to blow through.

e. Close the tailcock and open the steam leg valve.

Blowdown of Float-Operated Switches 58. Three different versions of float operated switches are used, depending on the boiler. When

these units are blown down, low water conditions are simulated.

59. Two Switch Control - The two switch control is blown down as follows :

Note - This control should be blown down only when the feedpump is in the normal cut-out condition. This enables the operation of the switch to be proven.

a. Turn the sequencing blowdown valve spindle clockwise until the valve is at the mid-travel position as marked. Keep the valve in this position for 5 to 10 seconds to ensure full blowdown of the water leg.

b. Continue turning clockwise to the full extent of travel. Keep the valve in this position for 5 to 10

seconds to ensure full blowdown of the float chamber and the steam leg.

c. Check that the low water level indicating lamp on the control panel is illuminated and that the alarm bell is sounding. Check also that the pump has started.

d. Turn the sequencing valve counter-clockwise. Pause at the mid-travel position for a few

seconds and then turn counter-clockwise to the full extent of travel.

e. If the control unit is operating correctly and the boiler water level is correct, the low water level indicating lamp should extinguish and the alarm bell cease sounding. The feedpump should switch off.

60. Single Switch Control - the single switch control is blown down as follows :

Caution - When this unit is being blown down, the fans and stoker(s) are automatically shutdown. Therefore, blowdown should only be carried out at low load to prevent the steam supply from being seriously disrupted. After blowdown, it will be necessary to restart the fans and stoker(s) from the control panel.

a. Turn the sequencing blowdown valve clockwise until the valve is in the mid-travel position as

marked. Keep the valve in this position for 5 to 10 seconds to ensure full blowdown of the water leg.



c. Check that the ID fan, FD fan and stoker motors have stopped operating and that the appropriate indicating lamps are extinguished.

d. Return the sequencing blowdown valve to its normal operating position by turning counter-

clockwise. Pause at the mid-travel position for a few seconds before continuing counter-clockwise to the full extent of travel.


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e. Restart the fans and stoker drive motors as follows :

Note - If automatic combustion controls are fitted, set the manual/auto switches on the control panel to MANUAL and the control regulators to the zero position.

1) Check that the ID damper is closed.

2) Check that the FD damper control unit is set at zero.

3) Start the ID fan and check that its indicating lamp illuminates.

4) Check that the FD fan has started and that its indicating lamp has illuminated.

5) Start the stoker and check that its indicating lamp illuminates.

6) Reset the ID damper to its normal operating position,

7) Reset the FD damper control unit lever to its normal operating position.

8) If the boiler pressure is now within 50kPa of normal operating pressure, all combustion

controls can be set to AUTO. If the boiler pressure is low, build up the pressure gradually until the normal operating pressure is reached before setting the combustion controls to AUTO.

61. Modulating Control - This unit is blown down as follows :

Note - This check should be carried out on low load and only when the water level is just above the average water level (i.e. when the control valve is approaching the closed position with the spindle moving down).

a. Turn the sequencing blowdown valve spindle clockwise until the valve is in the mid-travel position as marked. Keep the valve in this position for 5 to 10 seconds to ensure full blowdown of the water leg.



c. Check that the low water level indicating lamp is illuminated and that the alarm bell is sounding. Check that the control valve spindle has moved upwards.

d. Turn the sequencing blowdown valve counter-clockwise, pausing at the mid-travel position for a

few seconds, and then screw counter-clockwise to the full extent of travel.

e. If the control unit is operating correctly and the boiler water level is correct, the low water level indicating lamp should extinguish and the alarm bell cease sounding. The control valve spindle should move downward.

Boiler Blowdown 62. Under the following circumstances it will be necessary to blow down the boiler :

a. To control the quality of the boiler water.

b. To control the boiler water level under abnormal conditions.

c. To empty the boiler.


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d. To promote circulation during start up from cold.

63. Boiler Water Quality - Feedwater entering the boiler contains dissolved solids. Steam leaving

the boiler will be virtually free of solids, thus solids entering the boiler will build-up in the boiler water. To control the total solids concentration it is necessary to blowdown the boiler at regular intervals.

64. Abnormal Water Level - If the boiler water level rises well above the normal operating level it

may be necessary to drain off the surplus. Draining procedures are exactly the same as water quality control procedures except that the blowdown valve is kept open until the control water level is established.

65. Procedure - To control the quality or the level of the water in the boiler, the following procedure

should be adopted :

Note - During routine blowdown procedures, always check that the valve is operating freely and is not sticking.

a. Insert the blowdown valve key into the valve.

b. Check that the water level is at least above normal water level.

Caution - Under no circumstances should the boiler be blown down if the water level is below normal.

c. Open the blowdown valve and allow the water level in the gauge glass to fall by approximately 25mm.

d. Close the blowdown valve.

Caution - If the blowdown valve key cannot be removed, then the valve is not properly closed. Make sure that the valve is always fully closed.

e. If necessary, repeat the blowdown procedure. Before doing so ensure that the water level is at least above the normal water level.

66. Emptying the Boiler - To empty the contents of the boiler the following procedure should be

adopted :

Caution - Under no circumstances should the water be drained from the boiler unless the fire has been completely run off.

a. Allow the boiler pressure to fall to approximately 50kPa.

b. Open the air release valve (do not close this valve until the boiler is cold).

c. Insert the blowdown key into the blowdown valve.

d. Open the blowdown valve and empty the boiler.

e. When the boiler has been fully drained, leave the valve in the open position with the blowdown

key inserted in the valve. This procedure indicates that the boiler is empty.


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Ash and Grit Removal Ash Removal 67. The frequency of emptying the ash sump is dependent on the size of the boiler and

the boiler load. The sump should not be allowed to overfill otherwise the ash port may become blocked. It is therefore, important that the sump is cleared and the contents removed at regular intervals as determined by the Engineer in charge of the boiler.

Grit Removal 68. The frequency of emptying the grit sump is dependent on the size of the boiler, the boiler load

and the type of coal being burnt. The intervals at which the grit sump should be removed are determined by the Engineer in charge of the boiler.

Normal Water Level Control Filling the Boiler (on/off feedwater controls) 69. To fill the boiler when standard on/off feedwater level controls are fitted, proceed as follows :

a. Ensure that the feedwater tank is full and water has been treated (Ref. Appendix C to this manual). Check that a free running supply of make-up water is available.

b. Open all suction-line valves between the feedwater tank and the feedpump suction manifold.

c. Open the feedwater delivery valve on the discharge side of both feedpumps.

d. Crank open the main feed check valve.

Caution - If the valve is not opened sufficiently the pump will be damaged. Conversely, if it is

opened too much at this stage the motor will trip out on overload.

e. Open the air release valve.

f. At the control panel :

1) Set the main isolating switch to the ON position. Check that the mains on (red) indicator lamp illuminates.

2) Check that the low water alarm lamp illuminates and that the alarm bell is sounding.

3) Set the pump selection switch to pump No. 1. Check that the feedpump ON lamp

(green) illuminates and that the pump is operating.

g. Monitor the water level in the gauge glasses until the extra low water level is reached (water just appearing in the gauge glass). Set the pump selection switch to pump No. 2 check that the feedpump ON lamp is illuminated and that the boiler continues to fill.

h. Note the level at which the low water alarm lamp extinguishes and the alarm bell stops sounding, this level should be about 25mm above the low water level mark. Stop the pump.

Lighting a Fire 70. To light a fire, proceed as follows :

a. Ensure that the boiler is filled with water as described in Para. 69. Start one of the feedpumps.


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b. Ensure that all dampers and firing controls are free to move.

c. Set the FD damper control unit and the ID damper control unit for manual operation.

d. Close the coal cut off gate to isolate the coal hopper. Check that the coal hopper is full and that a plentiful supply of coal is available.

e. Ensure that the water feedline valves are set as described in Para. 69.

f. Set the other valves on the boiler as follows :

Main steam valve Closed Pressure gauge Isolating valve Open Air release valve Open Blowdown valve Closed

Caution - The blowdown key must be kept in its storage rack position to indicate that the blowdown valve is closed.

g. Check the draught gauge for correct operation as follows :

1) Remove the plastic pipe connection from the copper pipe at the rear of the control panel.

2) Blow the copper pipe through (see Caution below) preferably with compressed air.

Caution - Do not blow air into the plastic pipe connected to the draught gauge.

3) Check that the draught reads zero. Re-zero if necessary.

4) Re-connect the plastic pipe connection to the copper pipe.

h. Lower the guillotine door to grate level. Check that the fuel bed depth indicator reads zero.

Adjust the guillotine door until it is 80mm above the grate.

i. Close the undergrate dampers, both front and rear.

j. Set the FD damper to zero (closed).

k. At the control panel : 1) Switch on the main isolator switch.

2) Check that the mains ON indicator lamp illuminates.

3) Set the control regulators for panel operation.

4) Start the stoker. Check that it operates and that its indicating lamp illuminates.

5) Set the fuel feed timer knob to the 50% position.

l. Fully open the FD fan damper.


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m. Crank open the coal cut-off gate and allow coal to run in until the length of the fuel bed is about 750mm from the guillotine door. Close the coal cut-off gate. Close the FD fan damper.

n. At the control panel, stop the stoker.

o. Open the guillotine door and spread live coals on the fuel bed. Alternatively, light a wood fire on top of the coal. Lower the guillotine door to within 80mm of the grate and seal the gap with coal.

p. Close the ID damper.

q. At the control panel : 1) Start the ID fan. Check that its indicating lamp illuminates.

2) Check that the FD fan has started and check that its indicating lamp has illuminated.

r. Open the FD fan damper approximately 40%.

s. Open the ID damper until a slight draught is evident above the fuel bed (this can be observed through the spy hole in the ignition arch). The draught gauge should read approximately minus 3Pa.

t. When the coal has ignited and the fire has burned back to the guillotine door, start the stoker and run in 75mm of coal.

u. Stop the stoker.

v. Repeats steps t. and u. until there is a good fire over at least half the length of the grate, and ignition occurs continuously.

w. Re-adjust the guillotine door to about 120mm above the grate and open the coal cut-off gate fully.

x. At the control panel, start the stoker.

y. Check continuously that ignition is taking place not more than 50mm from the guillotine door. If the ignition line tends to move away from the door, the stoker must be stopped.

z. Re-start the stoker when ignition has been re-established. Gradually increase the rate of

burning.

Note - Do not provide too much draught at this stage as this will retard burning. Caution - It is normal for the grate to expand during operation and for a slight hump to form where it passes over the front sprockets. If however, the humping effect becomes pronounced, it will be necessary to re-tension the grate. Re-tensioning must only be carried out when the grate is cold.

aa. When satisfactory combustion is achieved, set the guillotine door to give the required fuel bed

thickness. (Ref. Appendix B to this manual). Set the ID damper control lever to approximately 1/3 open and adjust the FD Fan damper lever to maintain balanced draught conditions, i.e. the draught gauge should read approximately minus 3Pa.

bb. Allow the fire to stabilise for approximately twenty to thirty minutes.


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cc. As the boiler warms up, the water will expand, therefore, it will be necessary to keep a continuous watch on the water level as indicated by the gauge glasses. Note - If the water level is above the normal working level blowdown until the correct level is reach (Ref. para.65).

dd. Close the air release valve when the boiler pressure is approximately 150kPa. ee. Allow the pressure to build up to normal working pressure. If necessary, increase or decrease

the firing rate as required (Ref to para.71). Note - Steam raising from cold should take at least five hours and should not exceed a rate of 250kPa per hour.

ff. During steam raising, it will be necessary to blowdown the boiler regularly to assist circulation.

Blowdown the boiler as described in Para. 65.

gg. At this stage the float operated switches should be blown down as described in Paras. 58 to 61 and as appropriate.

hh. When the boiler is at operating pressure, crank open the main steam valve in order to warm up the steam range. Make sure that the steam range drains are functioning correctly.

ii. When the steam range is sufficiently warmed-up, fully open the main steam valve.

jj. Adjust firing conditions to meet load. Notes on Combustion 71. The following notes should assist operators to obtain optimum combustion efficiency.

a. Combustion in the furnace takes place in three stages :

1) Firstly, the volatiles or tarry fraction of the coal is distilled off and the coal ignited. Then

2) The coke formed during the first stage is burnt leaving a layer of ash on the grate and, finally

3) The ash is transported to the ash port through a high temperature oxydising atmosphere to

burn off the remaining carbon.

b. For efficient combustion to take place, the air provided by the FD fan must be evenly distributed across the fuel bed. There are two devices inside the stoker for controlling air distribution. These are :

1) Fixed guide vanes fitted to the stoker to distribute air across its width. Whenever the grate

is opened it is advisable to check that these vanes are in their correct positions.

2) An undergrate damper to regulate the amount of air which is allowed to flow to the tail end of the stoker. This damper should be closed during initial firing and when operating on low loads. As the load increases above 30 percent, the fire will lengthen and it will be necessary to open this damper in order to ensure complete burn-off of the coal.

c. Apart from the vanes mentioned above, air distribution is also affected by the depth and grading

of the fuel bed. Good distribution is obtained when the fuel bed provides a moderately high resistance to air flow. A thick bed has a higher resistance than a thin one. Fine coal produces a higher resistance than coarse coal. When burning pea coal, the fuel bed should, therefore, be thicker than when burning mixed smalls.


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d. If the ignition line at high loads tends to move away from the guillotine door, the fuel bed

thickness should be increased. Changes in thickness should be made in increments of 10mm until stable conditions are achieved. One hour should be allowed to elapse after a change has been made for conditions to stabilise before making any further change.

e. When burning coal is discharged off the end of the grate a long fire condition exists. This condition occurs either through over-loading or through operating with an air fuel ratio which is too low.

f. If a long fire is obtained with the stoker operating within its design capacity, too much fuel is being fed onto the grate for the amount of air being supplied (i.e. the stoker is operating with an air/fuel ratio which is too low). To correct this, the feed timer knob setting should be decreased. This adjustment should be done in stages. Allow a minimum period of one hour to elapse between adjustments.

g. A short fire occurs when the coal is burnt to ash well before the end of the grate. This happens either because the load is too low or because too much air is being provided. If the load is going to remain low for some considerable time, the undergrate damper must be closed. If short fires are experienced at loads above 50 percent full load, the feed timer knob must be turned clockwise to increase the amount of fuel being fed onto the grate for the amount of air being supplied. The adjustments should be done in stages.

h. If the load on the boiler increases, the steam pressure will be decreased. Conversely, if the load

decreases, the steam pressure will increase. As a boiler should be operated at a steady steam pressure, it is necessary to adjust the firing rate to follow changes in load. If the pressure falls, the firing rate can be increased as follows :

1) Open the ID damper control lever. This unbalances the furnace draught.

2) Re-set the FD fan damper to balance the furnace draught and increase the fuel feed

timer setting.

3) If the pressure continues to fall after allowing firing conditions to stabilise for approximately two minutes, steps 1) and 2) must be repeated until the pressure remains steady.

i. If the load on the boiler decreases and the pressure rises above the required limit, the firing rate

can be decreased as follows : 1) Re-set the FD fan damper to depress the furnace pressure and decrease the fuel feed timer

setting.

2) Close the ID damper control lever until the required balanced draught conditions are obtained in the furnace.

3) If the pressure continues to rise after allowing combustion conditions to stabilise for two

minutes, steps 1) and 2) must be repeated until the pressure remains steady.

Notes - With reference to 71.h. and 71.i., adjustments must be made in small increments. Never make a first adjustment of more than half a division on any of the indicator plates. The combustion controls should be set up to meet the average load. Such a setting will very frequently result in pressures fluctuating by plus/minus 50kPa without further adjustments being required. When steaming on manual, this pressure fluctuation is quite acceptable.


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j. Combustion efficiency is determined by measuring the carbon dioxide (CO2) content of the exhaust gases. This should normally be between 12 and 13,5 percent. Check the CO2 with a Fyrite or equivalent CO2 analyser.

Low Load Firing Techniques 72. Under normal operating conditions the stoker has a turndown ratio of 3 : 1 without altering the

fuel bed thickness. Under test conditions a ratio of 4 : 1 can be achieved. 73. If the boiler is to be operated at below 30 percent load for long periods, proceed as follows :

a. Lower the guillotine door to reduce the fuel bed thickness.

b. Adjust the ID damper control lever to maintain the required conditions.

c. Adjust the FD fan damper to maintain a balanced draught condition.

d. If necessary, adjust the air/fuel ratio using the fuel trim knob.

Note - If the load falls to zero, the stoker can be stopped for short periods, but care should be taken to ensure that the fire does not burn back into the coal hopper. The grate should not be allowed to overheat.

74. If the load is to be increased from below 30 percent load to full load, proceed as follows :

a. Raise the guillotine door to increase the fuel bed thickness to the previous full load setting.

b. If necessary, re-adjust the fuel trim knob to the previous full load setting.

c. At the control panel, gradually increase the firing rate by increasing the ID damper control and single knob combustion control unit control regulators. The over fire draught gauge reading should be maintained at approximately minus 3Pa.

Note - The speed at which the firing rate can be increased is largely dependent on the operators ability to hold the ignition line under the stoker arch.

d. Switch controls to auto when stable conditions have been achieved and when the steam

pressure is within 50kPa of the operating pressure. Shutting Down 75. For very short standby periods, when the load on the boiler has been reduced to zero, the

following procedure should be adopted :

a. Set the FD damper control unit and the ID damper control unit for panel operation.

b. At the control panel :

1) Stop the ID and FD fans.

2) Set the ID damper control regulator to zero. 3) Close the FD fan damper and reduce the fuel feed percentage timer (in this position the

stoker should just be moving).

4) Adjust the ID damper control regulator to maintain a furnace pressure of at least minus 3Pa.


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Caution - Check frequently that the fire does not burn back into the hopper. If furnace pressure is allowed to rise above minus 3Pa (i.e. more positive than minus 3Pa), the grate will overheat.

76. To re-start after a short standby period, proceed as follows :

a. At the control panel :

1) Set the ID damper control regulator to zero.

2) Start the ID and FD fans. Check that the fans operate and that their respective indicating lamps illuminate.

3) Adjust the control regulators to re-establish the desired conditions.

Standby Period of up to 18 Hours (banking) 77. The boiler can be shut down for periods of 18 hours by banking the fire. Immediately prior to

coming off load, carry out the following preliminary steps :

a. Close the guillotine door gradually until it is 50mm above the grate.

b. Reduce the firing rate and allow the fire to burn to ash over the last half of the grate, leaving a fire approximately 900mm to 1200mm long.

78. When the load on the boiler has been reduced to zero, proceed as follows :

a. Close the main steam valve. b. On/off feedpump control :

1) Check that the selected feedpump is operating.

2) Allow the water level in the boiler to rise until the gauge glass is 75 percent full, then stop

the feedpump at the MCB..

c. At the modulating control : 1) Open the by-pass valve and close the globe valve feeding the modulating valve.

2) Using the main feedwater check valve allow the water level in the boiler to rise until the

gauge glass is 75% full. 3) Open the globe valve feeding the modulating valve and close the by-pass valve.

At the control panel :

d. Stop the stoker, ID and FD fans

e. Open the FD damper fully.

f. Adjust the ID damper control regulator until balanced draught conditions are obtained over the

fire (draught reading about minus 3Pa).

g. Shut the coal cut-off gate and allow the fire to die down.

h. Set the guillotine door to 100m above the grate.


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i. At the control panel, start the stoker. Check that it is operating and that its lamp illuminates.

j. Increase the fuel feed setting and run coal onto the grate.

k. When the coal bed is approximately 750mm long, stop the stoker at the control panel.

l. Open the guillotine door fully (see Caution below) and rake the live coals back over the green coal to form a bank.

Caution - Never open the guillotine door too soon. The sudden inrush of cold air will adversely affect the grate, refractories, brickwork and tube sealing.

m. At the control panel start the stoker. Check that it is operating and that its indicating lamp illuminates.

n. Run the stoker until the bank is about 300mm from the guillotine door. Stop the stoker.

o. Lower the guillotine door to within 25mm of the grate and seal the gap with green coal.

p. Leave the undergrate dampers open.

q. Adjust the ID damper until balanced draught conditions are obtained in the furnace.

r. At the panel, turn the main isolating switch off. Check that its indicating lamp goes out.

s. Close the feed check valve on the boiler. 79. While banked, the boiler and bank should be examined at 3 hourly intervals when the following

should be checked :

a. Furnace pressure - this should be maintained at approximately minus 3Pa. If necessary, the position of the ID damper should be adjusted to balance the furnace pressure.

b. Water level - if the water level falls, it will be necessary to fill the boiler as described in

para.70.

c. Bank conditions - the bank should not be allowed to burn up. This condition will normally occur if the furnace pressure is lower than minus 3Pa.

d. Boiler pressure - an excessive drop in steam pressure must be avoided. Pressure can fall

due to one or more of the following reasons

1) Blowdown - the boiler should never be blown down during periods of banking.

2) Excessive stack losses - this is generally due to the ID damper being opened too far. Keep the damper opening as small as necessary to maintain balanced conditions.

3) Leakage at valves, drains etc., - any leakage must be repaired as soon as possible.

4) Cold air circulating around the boiler - doors and windows should be kept closed. Make

sure, however, that there is sufficient ventilation in the boiler room to provide an adequate supply of fresh air.

80. The following guidance notes will assist the operators in making a bank and will help them

prevent damaging conditions occurring.


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a. Avoid rapid cooling of the arch.

b. When making a bank do not allow too much hot ash to accumulate at the rear of the stoker.

This can damage the grate.

c. When the fire is banked, make sure that a small quantity of air passes through the grate to prevent it from overheating and being damaged.

81. To re-start from bank, proceed as follows:

a. If necessary, use a slicing bar to remove clinker fused to the refractory arch and side seals. Take care not to damage the refractories.

b. Open the feed check valve on the boiler.

c. At the control panel :

1) Turn the main isolating switch on. Check that its indicating lamp illuminates.

2) Switch on the selected feed pump. Set the feed pump manual/auto switch to AUTO.

d. Open the guillotine door fully. e. Open the coal cut-off gate to allow coal to fall onto the grate surface.

f. Close the coal cut-off gate and spread the green coal to a depth of about 150mm over the grate

surface just beyond the guillotine door.

g. Rake hot coals from the bank over the green coal.

h. Lower the guillotine door to about 120mm above the grate surface.

i. Open the coal cut-off gate.

j. Close the ID damper.

k. At the control panel, start the ID fan. Check that it is operating and that its lamp illuminates.

l. Open the ID damper 20%.

m. Close the undergrate damper.

n. Close the FD damper fully. At the control panel, start the FD fan. Check that it is operating and that its lamp illuminates.

o. Adjust the FD damper to balance the furnace pressure.

p. Turn the fuel feed timer clockwise to the end of its travel to give maximum stoker ON time.

q. When the fire has burned up and the arch is warm, start the stoker at the control panel. Check

that it is operating and that its lamp illuminates.

r. Check regularly that ignition is taking place at not more than 50mm from the guillotine door. If the ignition line tends to move away from the door, stop the stoker. Re-start the stoker when ignition has been established.


__________________________________________________________________________________________ Chapter 3 Page 28 of 28

Note - Do not provide too much draught at this stage as this will retard burning.

s. When satisfactory combustion is achieved, set the guillotine door to give the required fuel bed thickness.

t. Increase the rate of burning by opening the ID damper to 30% and adjusting the FD damper to maintain balanced draught conditions.

u. Open the undergrate dampers.

v. Re-set the fuel feed timer to its normal position.

w. Adjust the burning rate and furnace pressure as required.

Normal Shutdown 82. If the boiler is to be shutdown for a period of more than 18 hours, the fire should be run off.

When the load on the boiler has been reduced to zero, proceed as follows:

a. Close the main steam valve.

b. On/off feedpump control :

1) Ensure that the selected pump is operating.

2) Allow the water level in the boiler to rise until the gauge glass is 75 percent full, then stop the pump at the MCB.

c. At the modulating control :

1) Open the by-pass valve and close the globe valve feeding the modulating valve.

2) Using the main feedwater check valve allow the water level in the boiler to rise until

the gauge glass is 75% full. 3) Open the globe valve feeding the

modulating valve and close the by-pass valve.

d. Close the feed check valve and the pump delivery valves.

e. Close the coal shut off gate.

f. Lower the guillotine door gradually until it is 24mm above the grate.

g. Reduce the FD damper and fuel feed timer and set the ID damper to maintain balanced draught conditions.

h. When the fire has burned out, stop the stoker, FD and ID fans at the control panel.

i. At the control panel, turn the main isolating switch OFF . Check that the indicating lamp goes

out.

j. Open the guillotine door fully.

Caution - Do not open the guillotine door too soon. The sudden inrush of cold air will adversely affect the grate, refractories, brickwork and tube sealing.


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k. If necessary use a slicing bar to remove clinker from the refractory arch and side seals.

l. Remove all the ash and clean the boiler and its surroundings. Emergency Shutdown (Power failure or extra low water level ID fan, FD fan and stoker motor(s) stopped) 83. If either of these conditions occurs, shutdown the boiler as follows :

a. Close the main steam stop valve.

b. Set the ID damper 30% open.

c. Close the FD damper and set the fuel feed timer to 0.

d. Close the coal cut-off gate.

e. Turn the guillotine door operating handle until the indicator reads 25mm above the grate.

Note - The door must be lowered gradually while the grate is being rotated by hand. (Ref. g. 3 below).

f. Open the undergrate dampers.

g. Run the live coals off the grate as follows :

1) Remove the grate movement indicator disc and cover.

2) Remove the shear pin. 3) Connect the crank handle to the squared end of the worm shaft and rotate the grate by

hand until all the live coals have been run off. Remove the live coals from the ash removal system immediately.

h. Allow the stoker to cool for approximately 30 minutes, then open the guillotine door fully.


i. If necessary, use a slicing bar to remove clinker from the refractory arch and side seals.

j. Remove all the ash and clinker from the stoker and its surroundings.

k. Set the ID damper to the closed position.

l. Close the feedwater check valve on the boiler when the water level has been restored to its normal level.

m. At the control panel :

1) Turn the main isolator switch to the off position.

2) Set the FD damper control and ID damper control regulators to zero.

Emergency Shutdown (extra low water level, ID fan, FD fan(s) and stoker(s) operating) 84. This condition is extremely dangerous as it is a sign that the single switch float operated control

has not operated. To shut down the boiler, proceed as follows :


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a. Close the main steam top valve.

b. At the control panel, stop both the ID and FD fan(s).

c. Set the ID damper to 30% open.

d. Close the FD damper and set fuel feed timer to zero.

e. Close the coal cut-off gate.

f. Turn the guillotine door operating handle until the indicator reads 25mm above the grate.

Note - The door must be lowered gradually while the grate is moving.

g. Open the undergrate damper.

h. Set the fuel feed timer to 100% and open the FD damper (to run the fire off the grate).

i. Remove the ash and live coals from the de-ashing system at regular intervals.

j. Allow the stoker to cool approximately 30 minutes after grate has been cleared, and then open

the guillotine door fully.


k. If necessary use a slicing bar to remove clinker from the refractory arch and side seals.

l. Remove all the ash and clinker from the stoker and its surroundings.

m. Close the FD damper and set the fuel feed timer to 0%.

n. Close the ID damper. o. Close the feedwater check valve on the boiler when the water has been restored to its normal

level.

p. At the control panel : 1) Stop the stoker. 2) Turn the main isolating switch off. Check that the panel ON light extinguishes.

Daily Log Sheet 85. A typical daily operating log sheet is shown in Appendix A page 15, to this manual.


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Chapter 3

Technical Description Contents Introduction Page 2 Meaning of Operating Terms Page 2 Combustion Page 2 Combustion Controls Page 2 Automatic combustion control Page 3 Panel control Mechanical Description Page 3 Boiler ID Fan Page 4 ID Damper Control Unit Instruments and Controls Page 4 Feedwater controls Steam pressure Page 6 Furnace pressure Control panel Electrical Circuit Description Page 6 Interlocking Protection circuits Feedwater pump motors Modulating feedwater level controller Page 7 Stoker, FD and ID fan circuits Page 8 Extra low water alarm Page 9 Automatic controls Page 9


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Chapter 3

Technical Description

Introduction

1. The Stirling MK7 boiler is a high efficiency, wetback and fully packaged coal fired unit. The boiler has been designed for use with a chaingrate stoker. The size of the stoker varies with the size of the boiler. Two stokers are used in the larger twin flue boilers. The stoker and associated equipment are fully described in the Cochrane Chaingrate Stoker Manual.

2. The boiler is a horizontal three pass unit comprising the following :

a. A flue which houses the stoker. A double stoker boiler has two flues and two stokers.

b. A wetback return chamber.

c. Two convection tube passes.

3. A forced draught (FD) fan is fitted to the side of the stoker and an induced draught (ID) fan is

fitted to provide balanced draught conditions. Both the FD and ID fans rotate at a constant speed. Each fan is fitted with contra rotating dampers which control the air flow from the fan.

4. Two electrically driven feedwater pumps are supplied with the boiler.

5. The water level in the boiler is controlled by either a float operated two switch control (on/off

feedwater control) or, alternatively a modulating feedwater level controller. Both systems employ an audible alarm which warns the operator when the water in the boiler falls to a low level. In addition to either water level control, a float operated single switch is used to stop the fans and stoker if the water level in the boiler falls to a dangerously low level.

6. A control panel, housing the motor switch gear, control switches and lights, etc., is mounted

on one side of the boiler. Meaning of Operating Terms

7. Throughout the text, reference is made to the following :

a. Manual Control - This means that control is effected manually at the mechanical control units.

b. Panel Control - This means that control is effected manually through the control regulators on the panel.

c. Auto Control - This means that control is effected through the electro mechanical control

systems. Combustion

8. Combustion in the Stirling MK7 boiler takes place under balanced draught conditions in three stages. Firstly, the volatile or tarry fraction is distilled off and the coal ignited. Then the coke formed during the first stage is burnt, leaving a layer of ash on the grate. Finally, the ash is transported to the ash port in a high temperature oxidising atmosphere, which burns off most of the remaining carbon. As the volatiles released in the first stage pass through the high temperature second stage zone, they are consumed thus eliminating black smoke. Grit emission is minimal as the fuel bed is quiescent at all times.

9. The products of combustion are cooled to at least 300C below the ash deformation

temperature before they enter the first tube pass. This design feature ensures that tube


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plate slag formation is minimal. The temperature of the flue gas, when leaving the boiler, is only 75C above the boiling point of water and, therefore, as

much heat as is economically possible is transferred to generate steam. Combustion Controls

10. An FD fan supplies the air required for combustion, to the fuel bed. An ID fan extracts the combustion gases through the boiler. Since both fans run at a constant speed, air flow from the FD fan and gas flow from the ID fan can only be controlled by the respective contra rotating dampers. In principle, when the ID dampers are closed and the FD dampers are open, the combustion chamber is pressurised (positive pressure). When the FD dampers are closed and the ID dampers are open, the combustion chamber is under a vacuum (negative pressure). Automatic combustion controls maintain balanced draught conditions in the combustion chamber. Under ideal conditions the FD air flow and the ID gas flow are balanced to achieve a zero pressure. In practice, a negative pressure of approximately minus 3Pa is required to prevent intermittent pressurising of the combustion chamber.

11. For efficient combustion, the amount of fuel fed to the furnace must be strictly controlled and

be related to the FD air- flow. On the Stirling MK7 boilers, the air/fuel ratio is controlled by an FD damper control unit which automatically relates the amount of fuel fed to the furnace to the amount of air necessary to complete combustion. A trimming assembly is incorporated to vary this ratio by enabling the grate speed to be varied independently of the air supply. Turning the trim knob counter-clockwise decreases the speed range and turning the trim knob clockwise increases it. The maximum speed range is 0 to 9 metres per hour and the minimum speed range is 0 to 4,5 metres per hour.

12. Another important factor for efficient combustion is that the air provided by the FD fan must

be evenly distributed across the fuel bed. There are two devices inside the chaingrate stoker for controlling air distribution. These are fixed guide vanes fitted to the stoker to distribute the

air across its width, and two undergrate dampers to regulate the amount of air which is allowed to flow to the tail end of the stoker. Air distribution is also affected by the depth and grading of the fuel bed. Raising or lowering a guillotine door fitted between the coal hopper and the ignition arch can regulate the depth of the fuel bed. Further details of these combustion controls can be obtained from the Cochrane Chaingrate Stoker Manual.

Automatic Combustion Control

13. The automatic combustion control system is formed by a series of control loops, each comprising a transducer, a control relay and a pulling motor.

14. The primary control loop varies the position of the ID damper in relation to steam pressure.

As the steam pressure in the boiler falls, the ID damper is made to open. As the steam pressure rises, the damper is made to close. By opening and closing the ID damper, the ID fan draws more or less gas through the boiler. If more gas is drawn through the boiler, the furnace pressure will be decreased, while if less gas is drawn through the boiler the furnace pressure will increase.

15. The secondary control loop varies the position of the FD damper to maintain constant

furnace pressure conditions. This ensures that ignition is controlled and that the correct amount of air is supplied to the fuel bed to carry the load. As the secondary control loop pulling motor positions the FD damper, via the single knob combustion control unit, it simultaneously varies the grate speed and hence the amount of fuel entering the furnace.

Panel Control


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16. The primary control loop transducer and the secondary control loop transducer can be switched out of circuit and control regulators switched into the control loops.(Select MANUAL at the ID Damper and FD Damper selection switches on the control panel) This facility enables combustion to be controlled manually from the con

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