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

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

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. M4-3

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

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

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 M4-6

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 M4-7

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. M4-8

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

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

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. M4-11

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. M4-12

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. M4-13

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

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

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

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 / air flow) 11 500 – 45 000 Oxygen compensation above 45 000 M4-17

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. M4-18

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

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

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

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”. M4-22

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 M4-23

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 M4-24

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. M4-25

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. M4-26

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

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