(48)Recommended Schemes & Writeup of Auto Controls

7/31/2019 (48)Recommended Schemes & Writeup of Auto Controls

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HINDALCO MAHAN & ADITYA, 2X(6X150MW)WRITE-UP ON RECOMMENDED AUTO CONTROLS

Page 1 of 7

WRITE-UP ON BOILER AUTO CONTROL SYSTEMS

THE FOLLOWING CONTROL LOOP DESCRIPTIONS ARE COVERED IN THIS WRITE UP

Sl.

No.Description Drg. Ref.

(Sheet No.)

01 FEEDWATER CONTROL 2

02 FURNACE DRAFT CONTROL 3.1,3.2

03 PRIMARY AIR HEADER PRESSURE CONTROL 4

04 MILL OUTLET TEMPERATURE CONTROL 5.1,5.2

05 COMBUSTION CONTROL 6

06 AIR FLOW CONTROL 7.1,7.2

07 FUEL FLOW CONTROL 8.1,8.2

08SUPERHEATER AND REHEATER STEAM TEMPERATURE

CONTROL9.1,9.2,9.3

09 LIGHT OIL FLOW CONTROL 10

10 HEAVY OIL FLOW CONTROL 11


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Page 2 of 7

01.FEED WATER CONTROL (DRUM LEVEL CONTROL)

The objective of this control system is to maintain the drum level to the normal waterlevel of the drum at all loads. At lower loads (less than 30% MCR), the start up feedcontrol valve will be used as final control element and at higher loads, speed control of

Boiler Feed water Pumps (BFPs) will be used. Drum level is measured by threetransmitters through temperature compensated constant head unit. The pressurecompensated drum level signal may be selected by Mid Value Auto Selection (MVAS)circuit for control.

Low load: The drum level measured signal is compared with the drum level set point.The error signal will have a proportional, integral and differential action in the singleelement controller. This controller output will be the position demand signal for the startup feed control valve. Auto/manual station is provided for auto/manual selection andoperation. Position indicator is provided for the start up feed control valve.

High load: At higher loads the start up control valve shall be closed. The steam flowshall be measured. In order to prevent sudden response due to drum swell and shrink onload change, a time lag unit shall be included in the steam flow signal. The temperaturecompensated feed water flow signal is computed by adding feed water flow ateconomiser inlet and superheater spray water flow. The error signal produced betweendrum level measured signal and drum level set point shall have proportional, integral anddifferential action in the three element drum level controller. This will be added withsteam flow signal which is the feed water flow demand signal (set point for feed waterflow). This will be compared with the feed water flow in the feed water controller.Deviation if any will have a proportional and integral action in the feed water controller.This controller output will be the desired speed signal for the individual Boiler FeedwaterPump(BFP) speed control system. Auto/manual station is provided for auto/manual

selection and operation. Position indicator is provided for the motor driven BFP hydrauliccoupling scoop position indication and speed indicator is provided for turbine drive BFPspeed indication.

02. FURNACE DRAFT CONTROL

The main objective of the control is to maintain the furnace pressure constant at thedesired set value at all loads. This is achieved by changing the flow of flue gas bymodulating the inlet guide vane or inlet damper and varying the speed of the ID fan byvariable fequency drive system. Furnace pressure is measured by three transmitters.One signal is selected by mid value auto selection circuit for control. Excessive furnace

pressure is monitored for directional block on Induced Draft(ID) and Forced Draft(FD)fans. Furnace pressure is compared with set point and error, will have proportional andintegral action. Fuel demand signal is added as a feed forward feature. Master FuelTrip(MFT) feed forward feature is provided to minimise negative furnace pressure excur-sion. Separate auto/manual station, and the position indicator for each ID fan regulatingdevice is provided.

To have equal loading of the ID fans each ID fan motor current (sum of channel 1 andchannel 2 current) is measured averaged and compared. The difference is used for


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Page 3 of 7

taking corrective action. The corrected signal is used to position the ID fan inlet damper.The ID fan inlet damper positions between a maximum and a minimum position limit foroptimised control action. If ID fan position goes outside these limits, an error signal goesto a controller, whose output is used to vary fan speed to bring back the inlet damperwithin the set limits.

Separate auto/Manual station and position/speed indicator are provided for each ID fanregulating device (damper/VFD)

03. PRIMARY AIR HEADER PRESSURE CONTROL

The main objective of this control is to adjust the primary air header pressure accordingto the feeder speed. That is, out of all the feeders, the feeder speed which is higher thanthat of others is considered as set value for this control.

Primary air header pressure is measured with three transmitters. One signal is selectedby mid value auto selection circuit for control. The measured signal is compared with theselected feeder speed signal through a high signal selector to maintain the minimumheader pressure. Deviation if any will have proportional and integral action. Separateauto/manual station and position indicator are provided for each Primary Air(PA) fanregulating device.

To have equal loading of two running PA fans, the PA fans motor current is measured,averaged and compared. The difference is used for taking corrective action. Thecorrective signal is used to position the PA fan regulating unit.

Refer the 'notes' in the control scheme for the interlocks.

04. MILL OUTLET TEMPERATURE AND AIR FLOW CONTROL

The objective of this control system is to adjust the mill air flow according to the feederspeed and to maintain the mill outlet temperature at the constant set value.

Mill air flow is maintained by adjusting the hot air regulating damper while the mill outlettemperature is maintained constant by adjusting the cold air regulating damper. Thetemperature compensated mill air flow is linearised by the square root extractor. This airflow signal is compared with variable air flow set point as a function of feeder speed. Anyerror between these two signals will have proportional plus integral action. Rate ofchange of fuel demand signal is added to provide feed forward feature. An auto/manualstation with position indicator is provided.

Mill outlet temperature is measured using a thermocouple with tungsten carbidethermowell to avoid erosion. The mill outlet temperature is compared with constant setpoint and error will have proportional, integral and derivative action. An auto/manualstation with position indicator is provided.


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Page 4 of 7

05. COMBUSTION CONTROL

The objective of this control is to maintain the turbine throttle pressure constant at thedesired value by adjusting the firing rate(both fuel flow and air flow).

Turbine throttle pressure is measured with primary and redundant transmitters. Deviation

is alarmed and the controller is tripped to manual. The selected signal is compared withthe set point and any error will have proportional and integral action. A proportionalvalue of total steam flow and derivative of drum pressure signal are taken as feedforward feature for the control. An auto/manual station is provided. The output of A/Mstation is the air flow demand and fuel flow demand signal.

Refer the 'notes' in the control scheme for interlocks.

06. AIR FLOW CONTROL

The secondary air flow is measured at left and right side of the secondary air ducts towind box by means of aerofoils. Each flow will have temperature compensation. The flowis linearised by means of square root extractors. The total PA flow measured for eachmill in service, is added to obtain total air flow to the boiler. This signal is compared withthe developed set point. The air flow demand from coordinated control and actual fuelflow whichever is high (lead lag system) is selected to ensure enriched combustion air.The oxygen in the flue gas at the inlet of AH is measured as primary or redundant.Transfer switch can be selected for either average value or individual value. This signal iscompared with excess air set point and any error will have proportional and integralaction to have better combustion efficiency. High/low limiters are used to limit the value incase the oxygen analyser is out of service. Under any circumstance the air flow shouldnot be less than 30% MCR flow. This signal is the developed set point and the air flowsignal will have proportional and integral action in the air flow controller.

This position demand signal will be selected to the corresponding FD fans in servicethrough' auto/manual station. To have equal loading of FD fans the FD fan motorscurrent is measured. The difference is used for taking corrective action. The correctedsignal is used to position the FD fan regulating damper. Necessary interlock from FSSS,Boiler auxiliaries interlock system Maximum Deviation Limit (MDL) etc. are provided.Separate auto/manual station and position indicator for each FD fan regulating deviceare provided.

Refer notes in the drawing for interlocks.

07. FUEL FLOW CONTROL

Fuel flow demand from combustion control and air flow signal from air flow controlcorrected for fuel air ratio are compared and the lower is selected for the set point of thefuel flow controller. (lead-lag system). This is to ensure that under any circumstance thefuel flow should be lesser than the air flow.

Fuel flow is measured by adding the feed signal of the feeders (or mills) in service andthe Light oil flow corrected for calorific value. The feeder speed/rate measured signal is


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Page 5 of 7

hooked up to the control after a delay to suit the process lag. The actual fuel flow signalis compared with the developed set point signal above and any deviation will haveproportional and integral action. The controller's output signal is the position demandsignal for feeder speed regulating device. Bias unit is provided to modify the signalwhenever required. An auto/manual station is provided for each feeder.

To ensure air rich furnace at all times, a maximum deviation limit system (MDL) is used.i.e. whenever the fuel flow is more than the air flow this will automatically reduce the fuelflow and increase air flow to a safe value and both the air flow and fuel flow control istransferred to manual.

Refer the 'notes' in the control scheme for interlocks.

08. SUPERHEATER/REHEATER STEAM TEMPERATURE CONTROL

General

Steam temperature control is provided by a combination of burner nozzle tilt positioningand Superheater(SH), Reheater(RH) de-superheating spray. Steam temperature ismaintained by allowing nozzle tilt to respond to the lower of either SH or RH outlettemperature, with spray responding to the higher. Auto manual stations are to be provid-ed for modulating the following.

1 SH spray water valve

2 RH spray water valve

3 Burner tilt power cylinder.

SH, RH and DESH temperature are measured with three sensors. One signal is selectedby mid value auto selection circuit for control

SH spray water valve control

In the automatic mode, each SH spray water valve is controlled by a cascade control.Under normal conditions average SH outlet temperature (Tsho), the primary controlledvariable, is in the outer loop of the cascade control for each valve; SHDesuperheater(DESH) outlet temperature (Tshdso), an index of the immediate effect of

spray valve operation, is in the inner loop. The outer loop PID controller receives an errorsignal equal to the average deviation of Tsho set point and each measured final SHoutlet temperature (Tsho). The Tsho set point is programmed as a function of unit steamflow for constant pressure operation (or sliding pressure operation). The outer loopcontroller establishes a set point for the SH DESH outlet temp. This set point iscompared to measured Tshdso and the resulting error is used to position the SH sprayvalve.


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Page 6 of 7

If the deviation between Tsho set point and measured Tsho exceeds a preset high limit,control of the outer cascade loops is transferred from average Tsho to the final SH outlettemperature, Tsho. Strong feed forward to the SH temperature control which interactwith each other are provided. These feed forwards are needed to compensate for overfiring and under firing which may be required initially. When changing unit load, feedforward signals are utilised as follows (using unit load increase as an example):

1. The rate of change of fuel demand increase (an indication of over firing) is used toopen the SH spray valve to counter the initial temperature rise.

2. Drum pressure and steam flow (applied inversely) to spray valve demand and tiltposition (applied directly) are used as an index to recognize the tendency for temperatureto drop after item-1 above occurs, and is used to close the SH spray valves to counterthe subsequent temperature drop.

Interlocks are provided to close the SH spray valves at loads less than 20% MCR.

RH spray water valve control

In the automatic mode, each RH DESH spray water valve is controlled by a cascadecontrol. Under normal operating conditions RH outlet temperature (Trho), the primarycontrolled variable, is in the outer loop of each cascade control; RH DESH outlet temper-ature (Trhdso), an index of the immediate effect of spray valve operation, is in the innerloop.

The outer loop PID controller receives an error signal final RH outlet temperature (Trho).The Trho set point is programmed against the main steam flow curve, for constantpressure operation (or sliding pressure operation). The outer loop controller output is

modified with tilt position to establish a set point for the RH DESH outlet temperature.This set point is compared to measured Trhdso, and the resulting error is used toposition the RH DESH spray water valve.

If the deviation between the Trho set point and the measured final RH outlettemperature exceeds a preset high limit, control of the outer cascade loops is transferredfrom average Trho to the measured final RH outlet temperature (Trho) to allow sprayingof the hot lead.

Interlocks are provided to keep the spray valves closed at loads less than 20% MCR.

Nozzle tilt control

The average Tsho and Trho error signals are compared and the greater of the two errors(i.e., the lower of the two temperatures) is selected for nozzle tilt control. An interlock isprovided to keep the tilts at their base (horizontal) position at loads less than 25% MCR.If any spray water valve opens fully, the tilt control is interlocked to disallow furtherraising of the tilts, and thus prevent possible over heating of the SH and RH outlet lead.


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Refer notes in control scheme for interlocks.

09. LIGHT OIL FLOW CONTROL

Light oil flow is regulated by maintaining the pressure at the header. Two pressuretransmitters are used to measure the pressure and the healthy signal is applied to thecontroller where it is compared with the set point. The error signal will have proportional& integral action in the controller. The controller output is the position demand signal forlight oil pressure control valve.

10. HEAVY OIL FLOW CONTROL

Heavy oil flow is regulated by maintaining the pressure at the header. Two pressuretransmitters are used to measure the pressure and the healthy signal is applied to thecontroller where it is compared with the set point. The error signal will have proportional& integral action in the controller. The controller output is the position demand signal forheavy oil pressure control valve.

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(48)Recommended Schemes & Writeup of Auto Controls

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