Post on 21-Apr-2015
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DESCRIPTIONIt shows basic controls of boilers and how the same is achieved.
Please visit http://powerplant-instrumentation.webs.com for further information about Control Systems
The task of planning and regulating a process, with the objective of performing it in an efficient, effective and consistent manner.
General feedback control loopED SP Controller FCE Process Controlled Output
The following control loops are used in Boiler#9
Drum level control Furnace draft control Steam temperature control- attemperator-1 Steam temperature control- attemperator-2 Combustion air pressure control BFG header pressure control Corex header pressure control Combustion control
Drum level control
Drum level is measured by 3 DP transmitters. Output of each transmitter is given to compensation block for calculating compensated drum level. Out of 3 compensated level signals 2 signals are taken for level control.
Drum level compensationP+H(Da-Ds)
The compensated drum level is calculated by the following formula Compensated drum level (h) = (Dw-Ds) Where, P = Differential pressure measured by transmitter. h = Compensated drum level signal. Dw = Density of water. Ds = Density of steam. H = Water head on LP side wet head leg, which is to be feed as constant=800mmWC. Da = Wet leg density; water density at 30 C (Constant=0.996 g/cm^3)
The density of water and steam depends on the pressure. Water and steam densities corresponding to pressure are given below. Pressure (Kg/cm^2) 40 68 90 100 Water density (g/cm^3) 0.798 0.7436 0.7051 0.6884 Steam density (g/cm^3) 0.02010 0.03537 0.04880 0.05540
Drum level is controlled by 2 modes:
Single element control mode (Drum level) Three element control mode (Drum level, Steam flow and Feed water flow)
Single Element Control ModeLT-1 LT-2 LCOM PV LSP LCOM 2oo3 LC
LCOM 30% CONTROL VALVE
PT-1 1oo2 PT-2
LT PT LCOM LC
= Level Transmitter = Pressure Transmitter = Level compensation = Level Controller
Three element control modeCompensated Drum level PV Compensated Feed water flow PV Reverse Feed forward summation block
30% CONTROL VALVE
Compensated Main Steam Flow
100% control valve
Compensated Main Steam flowPT-1 FT-1 1oo2 PT-2
Compensated main steam flow
FT-3 TT-1 FT PT TT = Flow Transmitter = Pressure Transmitter = Temperature Transmitter
Compensated Feed water flowFT-1
Compensated feed water flow
FT PT TT
= Flow Transmitter = Pressure Transmitter = Temperature Transmitter
Compensated steam Flow
Compensated Steam Flow = P1+1.029 P2+1.029 T2+273 T1+273
Actual steam flow x
Where, P1 P2 T1 T2
= = = =
Measured pressure signal. Design Pressure. ( P2=95 Kg/cm^2) Measured temperature signal. Design Temperature. (T2=540 C)
Auto Change over It is done by soft switch in DCS. The switch has two modes 1) 1E to 3E 2) 3E to 1E
Before choosing mode-1 Ensure the following: (30% A/M) in auto mode. 1E controller o/p should go to (30% A/M). 3E controller o/p should track 1E controller o/p. 3E controller o/p should not reach (100% A/M) and input to (100% A/M) should be zero. (100% A/M) should be in auto.
After choosing mode-1 following actions occur automatically: (30% A/M) o/p should ramp down to 0% in 15 Secs. 1E controller o/p should go to 3E Controller through feed forward summation block. (100% A/M) o/p should ramp up to 3E Controller o/p in 15 secs. 1E controller o/p should not reach (30% A/M). Once 15 secs ramp up time finished 100% control valve will be in action and 30% control valve will be a stand by.
Choosing the Mode-2 will be totally an inverse action of choosing Mode-1
Action Type LSP RSP LAL HAL
: : : : : :
Reverse PID From manual (50%) 0 mmWC From Feed forward summation block -150 mmWC +150 mmWC
Action Fail Action
: Air to close : Air fails to open.
Furnace draft control
Furnace draft control is performed by SPLIT RANGE CONTROL. The Hydro coupling and the damper actuator are in split range operation to cater the min and max ID fan air flow requirement.
ControllerAction Type Set point Low Alarm High Alarm : : : : : Direct PID -5mmWC -100mmWC +100mmWC
Damper ActuatorAction Fail Action : Double acting : Air fails to lock and tend to stay at last position
Furnace draft controlPT-1 PT-2 PT-3
2oo3 LSP -5mmWC 0% to 50% PV PC PT= Pressure Transmitter PC= Pressure Controller 50% to 100% Function block 0% to 100% ID fan A Fluid oil Coupling system 0% to 100% ID fan B Fluid oil Coupling system
Function block 0% to 100% ID fan A damper Actuator 0% to 100% ID fan B damper actuator
Steam Temperature control Attemperator-1
The temperature between PSH-1 and PSH-2 is controlled by spraying feed water into the steam after the PSH-1. The set point to this temperature controller is a RSP. The RSP is derived from the functional block, where it is calculated based on the load(%), steam flow and fuel.
Load(%) 20 50
Steam Flow (TPH) 40 100
BFG Alone ( Deg C ) 395 389
Corex Alone ( Deg C ) 375 371
BFG + Corex Alone ( Deg C ) 385 375
WATER SPRAY PSH-1 T/C-1 mV TT-1 4-20mA 1oo2 T/C-2 mV TT-2 4-20mA PSH-2
Compensated Steam Flow
T/C TT TC PSH
= Thermocouple = Temperature Transmitter = Temperature Controller = Primary Super Heater
ControllerAction Type Set point Low Alarm High Alarm : : : : : Direct PID From Function block 380 Deg C 400 Deg C
Control valveAction Fail Action : Air to close : Air fails to lock and then tend to open
Steam Temperature control Attemperator-2
Steam temperature after PSH-2 is controlled by spraying water into the steam after the PSH-2. The desired main steam temperature (i.e.) SSH outlet temperature is achieved by controlling the temperature of PSH-2 outlet. The set point is local set point. Controller Action Type Set point Low Alarm High Alarm Action Fail Action : : : : : Direct PID 540 Deg C 530 Deg C 550 Deg C Control valve
: Air to close : Air fails to lock and then tend to open
WATER SPRAY PSH-2 SSH T/C-1 mV TT-1 4-20mA 1oo2 LSP 540 Deg C T/C-2 mV TT-2 4-20mA
T/C TT TC
= Thermocouple = Temperature Transmitter = Temperature Controller
PSH = Primary Super Heater
Combustion air pressure controlCombustion air pressure control is performed by two modes 1) VFD mode 2) Damper mode VFD mode: Damper actuator A/M station is in manual mode and the damper is in 100% open condition. The air pressure is controlled by VFD.
Damper mode: The fan is started by bypass starter and it is to be run at full speed. The damper actuator is in auto mode and it controls the air pressure. Change over: The change over from VFD to Damper mode and Damper mode to VFD mode is to be carried out manually by the operator.
FD FAN DISCHARGE
LSP FD FAN A
PC FD FAN B
VFD A/M DAMPER A/M VFD DAMPER ACTUATOR FD FAN MOTOR DAMPER ACTUATOR DAMPER A/M
FD FAN MOTOR
ControllerAction Type Set point Low Alarm High Alarm : : : : : Reverse PI 335 mmWC 315 mmWC 355 mmWC
Control valveAction Fail Action : Double acting : Air fails to lock and tend to stay at last position.
BFG Header pressure controlBFG LINE TC-1 mV TT-1 4-20mA TC-2 mV TT-2 4-20mA 1oo2 1oo2 PT-1 PT-2 PT-1 PT-2
BFG Flow Compensation
ControllerAction Type Set point Low Alarm High Alarm : : : : : Reverse PI 250 mmWC 230 mmWC 270 mmWC
Control valveAction Fail Action : Air to open : Air fail to close
Corex header pressure controlCOREX LINE TC-1 mV TT-1 4-20mA 1oo2 TC-2 mV TT-2 4-20mA PI PC LSP 1oo2 1oo2 PT-1 PT-2 PT-1 PT-2
Corex Flow Compensation
ControllerAction Type Set point Low Alarm High Alarm : : : : : Reverse PI 3000 mmWC 2800 mmWC 3200 mmWC
Control valveAction Fail Action : Air to open : Air fail to close
It is a lead lag combustion control This control always maintain the air flow more than the fuel flow for proper combustion of fuel. The combination firing can be done strictly adhering to following operational procedures: All burners are loaded equally in normal running condition Same fuel is fired in all the running burners. The block diagram for the combustion control is given below.
Stoich Air fuel ratio computation
Corex main FlowCorex support Flow
Total heat value computation
a Air Fuel Ratio low Alarm Block From b Curve
O2 Analyzer a Excess Air Ratio O2 Controller From Curve
Main steam Pressure LSP Pressure controller Fuel firing limit block Flow controller
Total Air Demand
50% for furnace purge signal to BMS logic. Air flow >25%