1. BOILER PLANT OPERATION Presented By Mr. Sivanand Ray
Fleet Management Training Institute

2. INSTRUMENTATION USED ON A SHELL BOILER
Fire-tube type boiler reviewed for simplicity
Instrumentation:
Items that provide operator with indication of condition within the boiler
Ensure indications are within SAFETY limits and design OPERATIONAL PARAMETERS
Operating pressures, temperatures, flows, water level
WHAT IS THE PURPOSE OF INSTRUMENTATION?
To tell us what is happening INSIDE the boiler. Solid metal so how must we know what is happening!
M4-2

3. INSTRUMENTATION PRESSURE GAUGE Normally Bourdon type of gauge
Large with large numbers
Graduated approximately double the pressure of safety valve setting, not < than 1,5
Located on top of shell/drum on an independent nozzle
Supplied with a siphon (“pigtail”) to develop a water seal
Must be calibrated at least once a year
A value or test cock should be placed adjacent to the gauge. (1,5”) 40 NB dedicated convection for attaching a test gauge when the boiler is in service.
May be located on a gauge board at the remote location, but in this case two pressure gauges required.
Must be easily seen and read by operator.
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4. INSTRUMENTATION PRESSURE TRANSMITTER
Where a remote pressure indicator required, an electrical signal used
Separate transducer
Mounted on same branch as pressure indicator
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5. INSTRUMENTATION LEVEL COLUMNS Provides VISIBLE water level indication
Minimum 25NB pipe line connecting column to boiler
TWO water gauge glasses for boiler pressure >28 bar
Normally simple gauge glass type
Installed such that with lowest reading on gauge glass, 75 mm water over highest point of tubes
Visible range of water level approx. 125mm from NWL
Daily routine blow down to remove debris, scale, sludge
NWL - normal water level
Level column - uses electric sensors to give a LED type of display at a remote position. Used on HP boilers
Gauge glass - Glass or ‘bulls’ eye type of fixed water gauge glass used on LP and HP boilers.
At least 3 or more test cocks per boiler not < 15NB. To be located within visible length of gauge glass. Three general types:
Tubular direct reading : Pyrex glass
Flat glass transparent : clamped plates
Refractive type.
M4-5

6. Typical Water Level Gauge for Low Pressure Boiler
At least 3 or more TEST COCKS per boiler. Not < than 15 NB. Test cocks to be located WITHIN visible length of gauge glass.
THREE general types off gauge glass :
1. TUBULAR direct reading : Pyrex glass
2. Flat glass transparent : clamped plates
3. Refractive type
Typical Water Level Gauge for Low Pressure Boiler
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7. Electronic and digital water level indicators
Used for REMOTE level indicators I.e. where the water gauge glass is distant from the operating level.
Electronic and digital water level indicators
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8. INSTRUMENTATION LEVEL SWITCHES Float type (Mobrey)
Single-element control (on-off or modulating)
Normally located in an external chamber
TWO chambers mandatory
Low level alarm
Low-low-level shut down
Weekly routine test to check correct operation of switches
Float movement transmitted by means of a magnetic coupling.
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9. INSTRUMENTATION TEMPERATURE MEASUREMENT Flue gas exit temperature
An increase indicates fouling problems
Thermocouples, insulated from gas sampling tube
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10. TYPICAL PROCESS AND INSTRUMENTATION DIAGRAM (P&ID)
COAL FIRED LP BOILER
Instrumentation
Control loops
OIL OR GAS FIRED LP BOILER
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11. CONTROL AND SYSTEM OPERATING PHILOSOPHY
CONTROLS :
Regulate the various quantities indicated by the instruments
Shut down (with interlocks) the plant if parameters exceeded
Manual operation to sophisticated automatic computer operation (DCS)
Control relates to a specific system or CONTROL loop as pertaining to a certain system:
Controls are diverse and continually developing. This section covers the principles rather than specific details.
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12. CONTROL AND SYSTEM OPERATING PHILOSOPHY
To control a boiler, following quantities require to be regulated :
Fuel/air ratio to maintain optimum combustion conditions
Water flow to match steam flow from boiler
Steam flow and pressure to regulate heat input
Combustion chamber pressure (balanced draught boilers)
Combustion safety
To maintain fuel air ratio, implies the fuel supplied and the air supply should also be measured and controlled; larger HP boilers this is the case.
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13. COMBUSTION CONTROL
Manual control cannot continuously control combustion in a reliable and efficient manner
Automatic control systems used :
maintains fuel-air ratio to furnace at an optimum
maintains a relatively constant steam pressure
Combustion control mechanisms vary slightly depending on fuel fired
No matter what fuel, four aspects that are regulated:
Air supply; Fuel supply; Ratio of fuel to air; Flame safety
An operator,performing a MANUAL operation, is unreliable.
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14. COMBUSTION CONTROL Two basic types of combustion control systems:
Positioning
Metering
Pneumatic or Electrical control drives
Positioning : A CHANGE in the steam pressure reading RESULTS in a proportional changes in fuel and air feeds.
DETAILS FOLLOW IN NEXT SLIDE
M4-14

15. COMBUSTION CONTROL Positioning Type Control:
Designed to control based on a change in steam pressure
Components :
Master steam pressure controller
Fuel control valve with cam for oil / gas or feed control drive for coal
Combustion air fan inlet vanes / outlet damper operator
Simplistic
cannot maintain fuel – air ratio at optimum over full load range
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16. COMBUSTION CONTROL Metering Type controller :
Components as per Positioning type with :
Fuel flow measuring device
Air flow signal
Fuel-air ratio is controlled by a relay
Capable of adjusting fuel-air ratio and where fuel pressure may vary
REFER TO THE P&ID AS VISUAL.
M4-16

17. General Selection Chart for Combustion Control Systems
Type of Fuel Type of Control Steam Generation
System Capacity Ranges
(kg/hr)
Coal Modulating positioning to
Full metering (Steam
/air flow) –
Full metering with
Oxygen compensation above
Fuel Oil & Gas Modulating positioning to
/ air flow) –
Oxygen compensation above
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18. STEAM PRESSURE CONTROL
Main purpose of a steam boiler is to produce steam within a definite pressure range
Steam pressure dictated by :
process plant steam demand
heat input to the boiler furnace
Pressure fluctuates between minimum and maximum settings
Steam pressure primary or major control.
Firing rate varied to increase / decrease rate of steam production.
More complex systems : burner firing rate modulated based on steam flow and pressure plus combustion air proportioned to the gas flow.
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19. STEAM PRESSURE CONTROL
Modulating control linked to upper and lower pressure limits
On-off pressure switch – low or high fire burner
Definite limitation exists on burner minimum firing rate
turndown ration varies : 4:1 gas; 2:1 oil
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20. FEED WATER SUPPLY CONTROL
Purpose is to provide sufficient feed water flow to ensure all heat transfer surfaces are cooled with water
Visible water level provided by :
Two simple gauge glasses
More sophisticated devices (electrodes, capacitance or conductivity type)
Water sensor controls :
Normally two independent
Float operated
Thermostatic
Or boiler Water sensor controls :
Normally two independent
Float operated : simple
Thermostatic : automatic, simple, dependable, maintenance neglible.
water level control.
Float common on ALL small LP boilers and large drum types.
M4-20

21. FEED WATER SUPPLY CONTROL
Typical arrangements of feed water plant :
Two or more pumps (standby) from hot well tank (10-15 min. storage)
Either on-off operation or a feed valve station
Feed valve station :
Check valve
Stop valve
Feed control valve c/w isolation and bypass valves
Typical arrangements of plant depends on complexity of controller system.
With feed water pump and valve station, the feed pump must be supplied with this arrangement, pumps require leak-off devices or pressure relief bypass
M4-21

22. FEED WATER CONTROL SYSTEMS
ON AND OFF FEED WATER PUMP
Small boilers with relatively large water storage, slow load changes
Typical float operated water level controller, mounted at water line
Also serves either as low-water level alarm or low water cut off
Type of level controller depends on the complexity of the feed water control system.
Small volume – steam demand termed :high turnover rate”.
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23. FEED WATER CONTROL SYSTEMS
MODULATING LEVEL CONTROLLERS
Boilers with relatively small water volume relative to steam demand
Thermostatic type regulator feeds water continuously to the boiler, stabilising the water level
THREE types of modulating controllers :
Single Element Level Controller
Two Element Level Controller
Three Element Level Controller
SMALL VOLUME implies a steam demand termed “high turnover rate”
Single element controller :
No flow measurement, only level indication
Two Element Controller :
As per single but now measure water level and steam flow
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24. Schematic of Three Element Level Controller
Used where several boilers ( 3 or more) supplied by ONE suitably sized pumping system.
Feed water flow measured, compared with steam flow signal, compensates of changes in system characteristics
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25. SOME CONSIDERATIONS REGARDING WATER LEVEL CONTROL
Rapid changes in steam flow rate can cause SURGING of the water level and carry-over
VERY IMPORTANT that the :
steam qualities
characteristics of boiler (size, firing system)
MUST be properly matched to the OPERATING characteristics of water level controlling system
Highest water level, NWL, lowest level for safe operation should be MARKED on boiler outer shell
Surging:
Steam flow rapidly increases – valve opens suddenly – pressure drops – Sat. temp drops (10 – 15 degrees) – water produces excessive flash steam causing water in boiler to swell up – water level rises i.e. gives false indication to controller and reduces FW flow. Steam temperature drops to lower sat. temp – steam flashing drops – water level drops below NWL – too sensitive control start to open FW flow – surging and carryover.
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26. FURNACE PRESSURE CONTROL
Only really required on coal fired boilers with FD and ID fan
Required to :
prevent excessive negative pressure and uncontrolled air entering furnace
products of combustion to percolate back to coal supply
Controller transmitter range not too narrow
typically +250 to –1250 Pa
ideal control point –100 Pa
REFER TO P&ID for COAL FIRED boiler unit :
Furnace draft measurement is subject to considerable “noise”.
Error signal to either side of ‘set point’ regulates movement damper/vanes on ID fan.
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27. AUTOMATIC CONTROLS
Instrumentation employed same as that for manual operation
Rapid and continuous development of electronic control systems means systems for boilers are now DIVERSE
Selection shall depend on owner / purchaser present complexity of control systems
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28. AUTOMATIC CONTROLS ASPECTS that do not vary whether auto/manual :
Principles of operation
Areas of control (control loops)
Direct reading of instruments
Safety devices and interlocks
Selector for “manual” or “automatic”
Selector for “manual” or “automatic” required for system failure or problematic operation.
M4-28