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

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 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 M4-3

4. INSTRUMENTATION • PRESSURE TRANSMITTER • Where a remote pressure indicator required, an electrical signal used • Separate transducer • Mounted on same branch as pressure indicator M4-4

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 M4-5

6. Typical Water Level Gauge for Low Pressure Boiler M4-6

7. Electronic and digital water level indicators M4-7

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 M4-8

9. INSTRUMENTATION • TEMPERATURE MEASUREMENT • Flue gas exit temperature • An increase indicates fouling problems • Thermocouples, insulated from gas sampling tube M4-9

10. TYPICAL PROCESS AND INSTRUMENTATION DIAGRAM (P&ID) • COAL FIRED LP BOILER • Instrumentation • Control loops • OIL OR GAS FIRED LP BOILER • Instrumentation • Control loops M4-10

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) M4-11

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 M4-12

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 M4-13

14. COMBUSTION CONTROL • Two basic types of combustion control systems: • Positioning • Metering • Pneumatic or Electrical control drives 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 M4-15

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 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 27 500 Full metering (Steam /air flow) 18 000 – 36 000 Full metering with Oxygen compensation above 36 000 Fuel Oil & Gas Modulating positioning to 11 500 Full metering (Steam / air flow) 11 500 – 45 000 Full metering with Oxygen compensation above 45 000 M4-17

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 M4-18

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 M4-19

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 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 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 M4-22

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 M4-23

24. Schematic of Three Element Level Controller M4-24

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 M4-25

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 M4-26

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 M4-27

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” M4-28