monnet - recommended scheme and write up of auto controls

7/23/2019 Monnet - Recommended Scheme and Write Up of Auto Controls

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MONNET POWER COMPANY LIMITED, 2X525 MW

RECOMMENDED AUTO CONTROL WRITE-UP

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 feed

control valve will be used as final control element and at higher loads, speed control ofBoiler Feedwater 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 and

operation. 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 shrinkon load change, a time lag unit shall be included in the steam flow signal. Thetemperature compensated feed water flow signal is computed by adding feedwater flowat economiser inlet and superheater spray water flow. The error signal producedbetween drum level measured signal and drum level set point shall have proportional,integral and differential action in the three element drum level controller. This will beadded with steam flow signal which is the feed water flow demand signal (set point forfeed water flow). 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 watercontroller. This controller output will be the desired speed signal for the individual BoilerFeedwater Pump(BFP) speed control system. Auto/manual station is provided forauto/manual selection and operation. Position indicator is provided for the motor drivenBFP hydraulic coupling scoop position indication and speed indicator is provided forturbine drive BFP speed indication.

02. FURNACE DRAFT CONTROL

The main objective of the control is to maintain the furnace pressure constant at the

desired 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 frequency drive system. Furnace pressure is measured by three transmitters.One signal is selected by mid value auto selection circuit for control. Excessivefurnace pressure is monitored for directional block on Induced Draft(ID) and ForcedDraft(FD) fans. Furnace pressure is compared with set point and error, will haveproportional and integral action. Fuel demand signal is added as a feed forwardfeature. Master Fuel Trip(MFT) feed forward feature is provided to minimise negativefurnace pressure excursion. Separate auto/manual station, and the position indicatorfor each ID fan regulating device 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 fortaking corrective action. The corrected signal is used to position the ID fan inletdamper. The ID fan inlet damper positions between a maximum and a minimumposition limit for optimised control action. If ID fan position goes outside these limits, an

error signal goes to a controller, whose output is used to vary fan speed to bring backthe inlet damper within 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

than that 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 withthe selected feeder speed signal through a high signal selector to maintain theminimum header pressure. Deviation if any will have proportional and integral action.Separate auto/manual station and position indicator are provided for each Primary Air(PA) fan regulating 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. Thisair flow signal is compared with variable air flow set point as a function of feeder speed. Any error between these two signals will have proportional plus integral action. Rate ofchange of fuel demand signal is added to provide feed forward feature. Anauto/manual station 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 set

point and error will have proportional, integral and derivative action. An auto/manualstation with position indicator, is provided.





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 iscompared with the set point and any error will have proportional and integral action. Aproportional value of total steam flow and derivative of drum pressure signal are takenas feed forward feature for the control. An auto/manual station is provided. The outputof A/M station 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. Theflow is linearised by means of square root extractors. The total PA flow measured foreach mill in service, is added to obtain total air flow to the boiler. This signal iscompared with the developed set point. The air flow demand from coordinated controland actual fuel flow whichever is high (lead lag system) is selected to ensureenriched combustion air. The oxygen in the flue gas at the inlet of AH is measured asprimary or redundant. Transfer switch can be selected for either average value orindividual value. This signal is compared with excess air set point and any error will

have proportional and integral action to have better combustion efficiency. High/lowlimiters are used to limit the value in case the oxygen analyser is out of service. Underany circumstance the air flow should not be less than 30% MCR flow. This signal isthe developed set point and the air flow signal will have proportional and integral actionin 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 fromFSSS, Boiler auxiliaries interlock system Maximum Deviation Limit (MDL) etc. are

provided. Separate auto/manual station and position indicator for each FD fanregulating device are 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 ofthe fuel flow controller. (lead-lag system). This is to ensure that under anycircumstance the fuel 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 heavy oil flow corrected for calorific value. The feeder speed/rate measured signalis hooked up to the control after a delay to suit the process lag. The actual fuel flowsignal is compared with the developed set point signal above and any deviation willhave proportional and integral action. The controller's output signal is the position

demand signal for feeder speed regulating device. Bias unit is provided to modify thesignal whenever 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 thefuel flow and increase air flow to a safe value and both the air flow and fuel flow controlis transferred to manual.


08. AIRHEATER TEMPERATURE CONTROL

The control basically consists of the following two interrelated control systems.

A. Cold end temperature control .

B. Hot gas damper control.

SCAPH steam flow control

The purpose of this control loop is to maintain the cold end temperature above the dewpoint thus preventing condensation and resultant corrosion of airheater surfaces. Thecold end temperature of AH. i.e. AH air inlet and AH flue gas outlet temperature ismeasured and averaged. The average temperature is compared with the set point andany deviation will have proportional and integral action. An auto/manual station withposition indicator is provided. The operation of this control may be required whilelighting up the boiler. The steam valves are automatically slowly closed if the measuredcold end temperature is approximately at setpoint value and the associated hot gasdamper is not fully open. This improves the overall efficiency by allowing the hot gasdamper instead of the steam valve to raise the cold end temperature when possible.


Hot gas damper control

This loop is used to equalise primary and secondary airheater gas outlet temperatures.This serves to reduce gas temperature imbalances (stratification) at ID fan andincreases unit efficiency. In the automatic mode, average primary and secondary air-heater gas outlet temperatures are compared and the resulting signal is received by aPID controller. The controller output is received by two function generators, whose

output determine damper position. A limit is provided to maintain damper operatorbetween a specified minimum and full open position. An alarm is activated if dampers





reach their minimum position. When a temperature difference is sensed, thecontroller output will change and the damper controlling the hotter gas outlet will beginto close while the damper controlling the colder gas outlet remain open. (see functiongenerator on the reference drawing) the resultant redistribution of hot gas will serve toequalise the outlet temperatures. With this scheme one damper (that controlling the

colder outlet) will always be fully open while the other damper modulates between itsminimum and full open position. In manual control, dampers are to be initialised by theoperator at 100 % open position. A signal indicating that both dampers are fully openis to be interlocked into the ID fan start logic as a start permissive. Transfers aredesigned to be bumpless. If control power is lost dampers are to lock in position or tobe forced fully open.

Refer notes in control scheme for interlocks.

09. 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 beprovided for modulating the following

1 SH spray water valve - left

2 SH spray water valve - right

3 RH spray water valve - left

4 RH spray water valve - right

5 Burner tilt power cylinder.

Each measurement is manually selectable between a primary and redundant element.Each pair of elements is compared and excessive deviation between the primary andredundant is alarmed, with control transferred to manual.

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 anerror signal equal to the average deviation of Tsho set point and each measured final





SH (left and right) outlet temperature (Tshol and Tshor). The Tsho set point isprogrammed as a function of unit steam flow for constant pressure operation (or slidingpressure operation). The outer loop controller establishes a set point for the SH DESHoutlet temp. This set point is compared to measured Tshdso and the resulting error isused to position the SH spray valves.

If the deviation between Tsho set point and measured Tshol or Tshor exceeds a presethigh limit, control of the outer cascade loops is transferred from average Tsho to theindividual final SH outlet temperature, Tshoa and Tshob. Strong feed forward to the SHtemperature control which interact with each other are provided. These feed forwardsare needed to compensate for over firing and under firing which may be requiredinitially. When changing unit load, feedforward signals are utilised as follows (using unitload increase as an example):

1. The rate of change of fuel demand increase (an indication of over firing) is used to

open the SH spray valves 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 temp. todrop after item-1 above occurs, and is used to close the SH spray valves to counter thesubsequent 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 outlettemperature (Trhdso), an index of the immediate effect of spray valve operation, is inthe inner loop.

The outer loop PID controller receives an error signal final RH (left and right) outlettemperature (Trhol and Trhor). The Trho set point is programmed against the mainsteam flow curve, for constant pressure operation (or sliding pressure operation). The

outer loop controller output is modified with tilt position to establish a set point for theRH DESH outlet temperature. This set point is compared to measured Trhdso, and theresulting error is used to position the associated RH DESH spray water valve.

If the deviation between the Trho set point and the individual measured final RH outlettemperature exceeds a preset high limit, control of the outer cascade loops istransferred from average Trho to the individual measured final RH outlet temperature(Trhol and Trhor) to allow spraying of 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 twoerrors (i.e., the lower of the two temperatures) is selected for nozzle tilt control. Aninterlock is provided 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 disallowfurther raising of the tilts, and thus prevent possible over heating of the SH and RHoutlet lead.

Refer notes in control scheme for interlocks.

10. 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 haveproportional & integral action in the controller. The controller output is the positiondemand signal for light oil pressure control valve.

11. 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 for

heavy fuel oil pressure control valve. One motorized inching bypass valve is provided.When the main valve is not in operation the bypass valve can be manually operated.

12. ATOMISING STEAM PRESSURE CONTROL

Atomising steam pressure is controlled by maintaining the pressure at the header. Twopressure transmitters are used to measure the pressure and the healthy signal isapplied to the controller where it is compared with the set point. The error signal willhave proportional & integral action in the controller. The controller output is the positiondemand signal for the Atomising steam pressure control valve.

13. LIGHT OIL PRESSURE CONTROL (PUMP HOUSE)

Light oil pump outlet pressure is measured and compared with the setpoint. Thedeviation is subjected to proportional and integral action. The output of the controlleris used to position the Light oil pump outlet pressure control valve to maintain thepressure at the set value.

14. HEAVY OIL PRESSURE CONTROL (PUMP HOUSE)





Heavy oil heater outlet pressure is measured and compared with the setpoint. Thedeviation is subjected to proportional and integral action. The output of the controlleris used to position the Heavy oil pressure control valve to maintain the pressure atthe set value.

15. HEAVY OIL TEMPERATURE CONTROL (PUMP HOUSE)

Heavy oil heater outlet temperature is measured and compared with the setpoint.The deviation is subjected to proportional and integral action. The output of thecontroller is used to position the HFO temperature control valve to maintain thetemperature at the set value.

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monnet - recommended scheme and write up of auto controls

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