Bop+Controls

ABB

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Balance of Plant Control

BALANCE OF PLANT CONTROLS

1. DEAERATOR PRESSURE CONTROL PAGE : 1-3 2. DEAERATOR LEVEL CONTROL PAGE : 4 3. CEP RECIRCULATION CONTROL PAGE : 5 4. HOT WELL LEVEL CONTROL PAGE : 6 5. SCAPH TEMP. CONTROL PAGE : 7 6. BFP RECIRCULATION CONTROL PAGE : 8 7. HP HEATER – 5&6 LEVEL CONTROL PAGE : 9-10 8. LP HEATER – 2&3 LEVEL CONTROL PAGE : 11-12

ABB Balance of Plant Control

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DEAERATOR PRESSURE CONTROL

The pressure of steam inside the Deaerator is controlled by modulating two control valves :

a) the pressure control valve in Auxiliary Steam line, when adequate pressure in the CRH line is not available,

b) the pressure control valve in Cold Reheat Steam line when extraction steam is not available during bypass operation ,warm and hot start up.

Depending upon the unit loading , the operation of the Deaerator pegging can be divided in three parts as under :

1. Normal Operation 2. Operation on Turbine Trip with Boiler survived 3. Operation on Turbine Trip with Boiler also tripped

1) NORMAL OPERATION : During starting / low load or turbine bypass operation of the Unit, Deaerator is pegged with Aux. Steam, when CRH pressure is less than 4 Kg/cm2. The set value is a max. selection of operator adjustable fixed set point or 1.5 Kg/cm2 preset value. The set value is compared with Deaerator pressure and the error signal is applied to PI controller whose output acts upon a aux. Steam control valve. When CRH Steam Pr. is more than 8 Kg/cm2, the control of Deaerator Pr. is switched over to CRH Steam Control Valve, and Aux. Steam Control Valve is gradually closed to min. The effective set point of 3.5 Kg/cm2 acts on PI controller whose output modulates the CRH steam control valve during bypass, warm and hot start up operation. When the load rises to more than about 45 % of the full load, the turbine extraction steam pressure exceeds 3.5 ata and the NRV’s in the extraction line allows extraction of steam to the Deaerator, thereby raising the Deaerator Pressure further and beyond 3.5 ata ( as per extraction steam pressure ). The CRH steam supply valve is then closed down automatically, as the prevailing Deaerator pressure is more than the set point to it’s controller. When ever extraction steam pressure is less than 3.5 ata because of load rejection or HP / LP Bypass operation, the CRH steam supply valve controller maintains the Deaerator pressure at 3.5 ata . In case of sudden load rejection, the pressure in Deaerator may reduce at a faster rate than is permissible and may cause serious damage to the equipment.


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To arrest the pressure decay rate under such circumstances, a variable / sliding set point is generated from the actual Deaerator pressure signal reduced by 0.5 ata through controlled tracking integrator GNI. Set Point Deaerator Pressure 1.5 Kg/cm2 + - -dy SE CRH Pr. > 8 Kg/cm2 Auto CRH Pr.<4 Kg/cm2 Low Load By Pass in operation Aux. Steam Control Valve

The GNI output slides down at a rate not more than 0.5 ata per min. The biased and sliding set point is compared with the min. pegging pressure of 3.5 ata in maximum selector, and greater of the two is given as set point to CRH steam valve controller with PI control structure. Hence, in case of sudden load rejection, when pressure of turbine extraction steam falls at a steep rate, the pressure decay rate in the Deaerator is arrested to not more than 0.5 ata per min. by steam supply from CRH steam line. 2) OPERATION ON TURBINE TRIP WITH BOILER SURVIVED :

Under the circumstances , when the turbine has tripped and the boiler has been survived through automatic / manual opening of HP / LP Bypass Valves and with proportionate reduction of Fuel & Air input to Boiler; the control of Deaerator Pr. is taken over by CRH Steam Control Valve with sliding operation similar to that as explained above for sudden load rejection. A controlled tracking integrator (GNI) tracks the actual Deaerator pressure less by 0.5 Kg/cm2. The output of the tracking integrator slides down at the rate not more than 0.5 Kg/ min. and is compared with the operator adjusted manual set point in a minimum selector. The output of the min. selector or a preset value of 3.5 Kg/cm2,

PI+P

MAX

& I


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whichever is more is passed on to the controller as set value and is compared with the Deaerator pressure for the control of CRH steam control valve.

Deaerator Set Point Pressure

0.5 Kg/cm2 - + - Gd = 0.5 Kg/min. Turbine Tripped 3.5 Kg/cm2 + - CRH Pr.>8 Kg/cm2 Auto -dy SE CRH Pr.<4 Kg/cm2 Low Load Extrn. Pr.>min. Hot start Bypass not in operation Warm start Bypass in operation

CRH STEAM CONTROL VALVE 3) OPERATION ON TURBINE TRIPPED WITH BOILER ALSO TRIPPED : Under the circumstances when Boiler is also tripped on tripping of the Turbine, the control of Deaerator Pressure is switched over to Aux. Steam Control Valve, as there will not be any steam in CRH line.

GNI

MIN

MAX

I

& I

I &

PI+P


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DEAERATOR LEVEL CONTROL

Deaerator level control loop maintains the water level in the Deaerator within the permissible limits by regulating the condensate flow to Deaerator. The measured Deaerator level duly compensated for density changes is compared with the fixed operator selected set point. The resultant error signal is applied to a PI controller ( used as master controller ). The output of this controller is added to the deviation between total condensate inflow into the Deaerator and total feed water out flow from Deaerator. The Deaerator inflow signal is computed by adding pressure & temperature compensated condensate flow with HP Heater – 5 & 6 drain flow . These three element signals are applied to the slave PI controller, the output of which acts upon the condensate flow control valve for the regulation of condensate flow to Deaerator.

Total FW Set Deaerator Condensate HPH-5 HPH-6 Flow Point Level Flow Drain Drain

Flow Flow Condensate Flow Control Valve

PI+P

PI+P


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CEP RECIRCULATION CONTROL The minimum condensate flow through gland steam condenser specially during start up and low load condition is controlled by regulating the flow control valve in CEP Re-circulation line to condenser. The measured condensate flow at the outlet of gland steam condenser is compared with a manually set desired value and the error signal is applied to a PI controller , the output of which modulates the re-circulation valve, so as to maintain the condensate flow through gland steam condenser above the desired set value .

Set Condensate Point Flow

Condensate Re-circulation Flow Control Valve

PI+P


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CONDENSER HOT WELL LEVEL CONTROL a) LOW LEVEL CONTROL Measured value of Hot Well level is compared with a manually set low level desired value, the error signal is applied to a PI controller, the output of which modulates an emergency make up water control valve . b) HIGH LEVEL CONTROL Measured value of Hot Well level is compared with a manually set high level desired value, the error signal is applied to a PI controller, the output of which modulates condensate spill valve, so as to regulate condensate spill to condensate surge tank. Set Point Condenser Set Point Condenser Low Level Hot Well High Level Hot Well Level Level Emergency Make up Condensate Spill to Surge Tank

Control Valve Control Valve

PI+P

PI+P


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AIR HEATER COLD END TEMP. CONTROL (SCAPH CONTROL)

The Air Heater cold end temperature is maintained by modulating each of the heating steam control valves in the Aux. Steam line to Steam Coil Air Pre Heater ( SCAPH) at the air inlet end of Re-generative Air Pre Heater (left & right). The flue gas temperature at the outlet of Re-generative Air Pre Heater outlet ( called as cold end temperature ) is compared with a operator selected set point. The error signal is applied to a PI controller ( used as a master controller ). The output of the master controller form a variable set point and is compared with a measured air temperature at the SCAPH outlet. The resultant error signal is applied to a slave PI controller, output of which regulates the Aux. Steam control valve as mentioned above for the purpose.

Set Flue Gas Temp. Air Temp. Value RAPH outlet SCAPH outlet

Auxiliary Steam Control Valve to SCAPH

PI+P

PI+P


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BFP RECIRCUALTION CONTROL In order to avoid the risk of BFPs to run dry in case of closing of the Feed Water Control Valve , the re-circulation control is fore seen . This control loop maintains the feed water flow through each operating BFP above a pre-specified minimum value as per recommendations of BFP manufacturers, especially during startup and low load conditions. The set point is computed as a function of the pump speed or scoop position through a function generator, and is compared with the BFP suction flow . The error signal is applied to a PI controller , whose output regulates the respective re-circulation control valve in order to maintain the desired minimum FW flow through each BFP.

SET VALUE BFP SUCTION FLOW RECICULATION CONTROL VALVE

PI


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HP HEATER-5&6 LEVEL CONTROL

A) HP HEATER-6 NORMAL / EMERGENCY LEVEL CONTROLS

The condensate level of HP Heater-6 is controlled by modulating the following three control valves : 1. Normal Low Level control to HP Heater-5 2. Normal High Level control to Deaerator and 3. Emergency Very High Level control to Condenser .

Three separate control loops are envisaged for the purpose. The measured HP Heater-6 level is compared with the manually set desired value. The error signal is applied to a PI controller, whose output modulates the respective drain control valves. The set points are selected in such away so as to avoid interaction among these controllers. Extraction Steam Normal Normal Emergency Low High Very High Level Level Level FEED WATER FROM HPH-5 TO HPH-5 TO DEAERATOR TO CONDENSER

HP HTR-6

PI

PI

PI


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B) HP HEATER-5 NORMAL / EMERGENCY LEVEL CONTROL

The condensate level in HP Heater-5 is controlled by modulating the following two drain control valves : 1. Normal Level control to Deaerator, and 2. Emergency Very High Level control to Condenser. Two separate control loops are envisaged for the purpose. The measured HP Heater-5 level is compared with the manually set desired value. The error signal is applied to a PI controller, whose output modulates the respective drain control valves. The set points are selected in such away so as to avoid interaction among these controllers. Extraction Steam Normal Emergency Level Very High Level FEED WATER FROM BFP TO DEAERATOR TO CONDENSER

HP HTR-5

PI

PI


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LP HEATER-2&3 LEVEL CONTROL

A) LP HEATER-3 NORMAL / EMERGENCY LEVEL CONTROL

The condensate level in LP Heater-3 is controlled by modulating the following two drain control valves : 1. Normal Level control to LP HEATER-2, and 2. Emergency Very High Level control to Condenser. Two separate control loops are envisaged for the purpose. The measured LP Heater-3 level is compared with the manually set desired value. The error signal is applied to a PI controller, whose output modulates the respective drain control valves. The set points are selected in such away so as to avoid interaction among these controllers. Extraction Steam Normal Emergency Level Very High Level CONDENSATE FROM LPH-2 TO LP HTR-2 TO CONDENSER

LP HTR-3

PI

PI


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B) LP HEATER-2 NORMAL / EMERGENCY LEVEL CONTROL

The condensate level in LP Heater-2 is controlled by modulating the following two drain control valves : 1. Normal Level control to LP HEATER-1, and 2. Emergency Very High Level control to Condenser. Two separate control loops are envisaged for the purpose. The measured LP Heater-2 level is compared with the manually set desired value. The error signal is applied to a PI controller, whose output modulates the respective drain control valves. The set points are selected in such away so as to avoid interaction among these controllers. Extraction Steam Normal Emergency Level Very High Level CONDENSATE FROM LPH-1 TO LP HTR-1 TO CONDENSER

LP HTR-2

PI

PI

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