HP Bypass Station1. Overview (Diagram D)The HP Bypass System consists of the following parts:•Pressure setpoint generation (Diagrams DA/RA)•Pressure controller (Diagram DB/TB)•Temperature controller (Diagram DC)•Spraywater isolation control (Diagram SD)2. Pressure setpoint generation2.1 Startup2.1.1 Mode Selection (Diagram RA)The BP controller is set to “BP auto” by the operator.Operator sets “startup mode ” with “BP start mode on/off” display button.In “startup mode” the generator it is set to one of the three operating conditions:•Cold start up•Hot start up•restartThe “cold start up” is active whenever the main steam pressure is below “Pmin”.The “hot start up” is active when the main steam pressure is above “Pmin” and below“Psync”.The “restart” is active when the pressure is above “Pmin” and the BP valve is closed.The signal “cold/hot start up” is generated when the operating “cold start up” or “hotstart up” is on.The “startup mode” is deselected as soon as the pressure reaches “Psync” and the BPvalve is open. The “startup mode” is also deselected when the signal “HP Turbineloaded” is active or when the pressure controller is switched to manual.The “fixed pressure mode” is active when the “startup mode” is not on and the“follow mode“ is not on and the “shut down mode” is not on.The “follow mode” is active when the BP valve is closed and the DEH signal “HPTurbine loaded” is active. Further the “follow mode” is active when the signal “Boileron” is not on and the BP valve is closed or the BP is in manual control.2.1.2 Setpoint Generator (Diagram DA)At “startup mode ” with no pressure in the boiler the setpoint generator is in the “cold start up ” condition. The pressure setpoint “Ps actual” is set to “Pmin” value. And the BP valve opens to a minimum position value “Ymin” (see 3. Pressure Controller).As soon as the main steam pressure is above “Pmin” the “hot start up ” condition is activated. The pressure setpoint “Ps demand” is increased, whenever the BP valve position “Yset” exceed the position value “Ym”. The increase of the pressure set point is limited by the gradient limiter and only positive pressure gradient is allowed during “hot start up” operation, decrease of the pressure setpoint “Ps actual” is not permitted in this operation mode.As soon as the main steam pressure reaches “Psync” the “startup mode” is deselected and the “fixed pressure mode ” is activated. The pressure setpoint “Ps demand” is set at the current last pressure set point “Ps actual”. In “fixed pressure mode” the pressure set point can be adjusted by the operator.With increasing turbine load BP valve will close automatically due to pressure drop. As soon the BP valve is closed the set point generator will switch over to “follow mode ” (see: 2.2 Load Operation). In this mode pressure set point “Ps actual” will follow main steam pressure.In case of pressurized boiler at startup the “restart ” condition is selected while the BP valve is closed. The pressure set point “Ps actual” is following the main steam pressure but only negative pressure gradients are allowed. As soon as the boiler pressure starts to increase the bypass valve will start to open. When the BP valve open the setpoint generator will go to “hot start up ” condition if the pressure is still below “Psync” or to “fixed pressure mode ” if the pressure at startup is already above “Psync”.T im eB P v P re s s u re / L o a d / F lo w 100 %50 %20 %40 %60 %100 %80 %2.2 Load Operation (Diagram /DA/RA)During load operation, i.e. when the HP turbine is taking over all the produced steam,the bypass valves are closed. The “follow mode” is activated.The pressure set point “Ps actual” is following the actual main steam pressure. Themaximum pressure gradient is limited, depending on the actual main steam pressure. Adifferential pressure value “dP” is added to the pressure set point “Ps actual” to keepthe BP valve closed (see: 3. Pressure Controller).If the main steam pressure is increasing above the Bypass controller set point “Psactual + dP” the BP valve starts to open. The set point generator is switched over to“fixed Pressure mode”.2.3 Shut down2.3.1 Mode selection (Diagram RA)When a plant shut down is planned the “Shut down mode” can be selected by theoperator. The “Shut down mode” can only be selected when the DEH signal “HPTurbine loaded” is active and the BP controller is on auto. As soon as the DEH signal“HP Turbine loaded” is deactivated the “Shut down mode” is deselected and the setpoint generator change to “fixed pressure mode”.When the boiler shuts down, the BP valve will close and the set point generator isswitched to “follow mode”2.3.2 Setpoint Generator (Diagram DA)At “Shut down mode ” the pressure set point “Ps actual” is following the main steam pressure but only negative gradients are allowed. As soon as the boiler pressure starts to increase the BP valve start to open.After the “Shut down mode” is deselected the set point generator is on “fixed pressure mode ”. At “fixed pressure mode” the set point generator controls the actual pressure. As soon the boiler shuts down the BP valve is closing and the pressure generator is switched to “follow mode ”. In “follow mode” the pressure set point will again follow the actual main steam pressure.3. Pressure Controller (Diagram DB)The pressure controller is receiving the actual main steam pressure signal and the pressure set point “Ps actual” from the set point generator. The controller is generating the common valve position demand signal. The signal is send to four M/A station where the valves can also be operated individually.In “follow mode ” when the “dP on” is active a differential pressure is added to the pressure setpoint “Ps actual” to keep BP valve close.At “startup mode ” when the “cold start up ” condition is active the BP valve are opened to a minimum position “Ymin”. The “Ymin on“ is deselected when position is above the minimum position “Ymin”.B P v P re s s u re / L o a d / F lo w 100 %50 %20 %40 %60 %100 %80 %4. Fast open and trip close logic (Diagram TB)The “monitor trip” command for fast opening of the BP valves is generated from DEHsignals “Turbine trip” or when the main steam pressure has reached the quick openingpressure threshold “dp max” above the pressure setpoint “Ps actual”.The “monitor trip” is blocked when pressure is below the pressure value “Pfo” andduring the “startup sequence” is active.The HP bypass valve will emergency close in case of low spray water pressure or toohigh HP-BP outlet temperature.5. Temperature controller (Diagram DC)The temperature controller is receiving the actual signal “HP-BP outlet temperature”from the transmitters and the “Temp setpoint“ set by the operator. The controller isgenerating the spray water valve position demand signal.The feed forward signal is generated from “Main steam pressure” and BP valveposition and is added to the valve position demand signal. With the adjustable value“wpl” the feed forward signal can be adapted.The temperature controller has one M/A station to control the BPE spray valve.The temperature controller is set to automatic mode as soon the BP valve is opened.When the BP valve closes the spray water valve closes also, and the temperaturecontroller is set to manual.The steam flow is calculated out of the BP valve position, main steam pressure andtemperature.7. Spray water isolation valve control (Diagram SD)The spray water isolation valve is open as soon the BP valve is not closed. When theBP valve is closed the spray water isolation valve will close again.IP Bypass station1. Overview (Diagram /D)The IP-Bypass Controller consists of the following partsPressure set point generation (Diagrams /DE/RE)Pressure controller (Diagram /DF)Quick open and emergency close functions (Diagram TF)Temperature controller (Diagram /DG)Spray water block valve (Diagram /SH)2. Pressure set point generation (Diagrams /DE and /RE)2.1 Boiler start up2.1.1 Valve closedBoiler and steam production is started. IP-Bypass is kept closed until pressure reachesPmin.Automation:IP-Bypass A/M station set to auto by the operatorThe “startup mode” is set by operator with “Start up on/off” signal.The valve is kept closed until hot reheater pressure reaches Pmin2.1.2 Min pressureAs steam production increases, LP-Bypass is opening and controlling pressure at Pminset point. When valve opening reaches Ym value pressure set point will be increasedby the predefined gradient.Automation:The A/M stations are auto, the “startup mode” is set.The valve will open to keep pressure constant.When valve opening reaches Ym the operation mode change to pressure ramp2.1.3 Pressure rampAs steam production increase, the pressure increase according defined gradient. Thepressure is increased up to final startup pressure Psync.Automation:The A/M stations are auto, the “startup mode” is set.The pressure setpoint increase is limited by gradient limiter.Only positive gradient GRD1 is possible, the negative gradient is set to zeroWhen pressure reaches Psync operation mode will change to the “fixed pressuremode”. The “startup mode” is reseted.2.1.4 Changeover to Fixed pressure modeHot reheater pressure is controlled by IP-Bypass. Turbine is started, synchronized andloaded. As turbine takes steam, IP-Bypass will gradually close.Automation:The A/M stations are autoThe valves are in pressure control with setpoint Psync.If necessary the setpoint can be adjusted by operator.As the turbine takes steam, the IP-Bypass will closeWhen IP-Bypass is closed and signal “Turbine on” is on, the operating mode change tofollow mode2.1.5 Changeover to Follow modeThe pressure set point is sliding according to boiler load.Automation:The A/M stations are auto.The target hot reheater pressure setpoint is sum of sliding IP-BP inlet pressure, definedby gradient GRD2, and additional pressure bias dP. When IP-Bypass starts to open theoperating mode is changed to fixed pressure at actual pressure value.2.3 Boiler run downBoiler run down memory must be set to prepare set point generation for boiler …rundown“, so that the bypass opens when the boiler load is reduced.Automation:IP-Bypass A/M station set to auto by the operatorThe “run down mode” is set by operator with “Run down on/off” signal.The pressure setpoint decrease is limited by gradient limiter.Only negative gradient GRD is possible, the positive gradient is set to zeroAt some point the steam production is higher than the flow through turbine, thepressure will rise and IP-Bypass will open.Bypass changes to fixed pressure mode with set point at actual pressure value.Boiler is shut off. IP-Bypass will close to keep the boiler pressurized.3. Pressure controller (Diagram DF)IP-Bypass controls hot reheater pressure.Automation:IP-Bypass A/M station set to auto by operator.When in manual IP-Bypass controller output is tracking valve position.When the IP-Bypass outlet flow is higher than value “Fs max” the valves start to closeand limiting the maximal steam flow to condenser.4. Temperature controller (Diagram DG)The IP-Bypass outlet steam is cooled down by injection of necessary amount of spraywater.Automation:When IP-Bypass is closed the spray valve is closed.When in manual controller output is tracking valve position.At IP-Bypass pressure control valve opens the temperature controller A/M station isset to auto.The necessary amount of spray water “Fs water” is calculated as function of IP-Bypassinlet steam flow and enthalpy and wanted enthalpy value at cooler outlet and.5. Quick open and Emergency close (condenser protection) of IP-BP(Diagram TF)The fast opening and emergency closing functions are operating independently of thepositioning loop programmed in the pressure controller itself.A too high control deviation acts as the fast opening criteria. In the case of fastopening, the pressure controller is first internally switched to automatic operation, andonly if it can be switched to automatic operation, the signal …fast opening“ is given tothe fast opening device. During start up mode, the criteria …control deviation too high“is blocked. With the monitor …Pfo“, the fast opening must generally be released.In the case of fast opening, the pressure controller output signal Ys is set to a preadjusted fixed value …Yfo“. Thus, optimum preconditions are brought about forstarting again automatic pressure control operation.Externally generated fast opening commands can be connected to the controller input"fast opening criteria". Such signals have then the same effect as a signal from thedeviation monitor. A timer limits the length of external commands.The IP-bypass valve is emergency closed to protect condenser (pressure high,temperature high, level high)Automation:The trip close command is active during follow criteria:-Condenser pressure high-Condenser temperature high-Condenser level high-IP spray water pressure lowThe trip close will close LP-Bypass and set A/M stations to manual.The pressure setpoint is in follow mode.Operator needs to reset quick closing of bypass system and set A/M station to auto toenable restart of bypass system.Pcond max (Value by condenser supplier)Tcond max (Value by condenser supplier)Lcond max (Value by condenser supplier)6. Spray water isolation valve (Diagram /SH)The spray water isolation valve opens or closes depending on IP-BP valve position: IfIP-BP valve is not closed spray water isolation valve opens, if IP-BP valve is closedisolation valve closes.7. Curves 7.1Start up curveT im eL P B P re s s u re 100 %50 %20 %40 %60 %100 %80 %L P7.2 Shut down curveL P B P re s s u re 100 %50 %20 %40 %60 %100 %80 %LP Bypass station1. Overview (Diagram /D)The LP-Bypass Controller consists of the following partsPressure set point generation (Diagrams /DJ/RJ)Pressure controller (Diagram /DK)Quick open and emergency close functions (Diagram TK)Temperature controller (Diagram /DL)Spray water block valve (Diagram /SM)2. Pressure set point generation (Diagrams /DJ and /RJ)2.1 Boiler start up2.1.1 Valve closedBoiler and steam production is started. LP-Bypass is kept closed until pressure reachesPmin.Automation:LP-Bypass A/M station set to auto by the operatorThe “startup mode” is set by operator with “Start up on/off” signal.The valve is kept closed until hot reheater pressure reaches Pmin2.1.2 Min pressureAs steam production is increasing, LP-Bypass is opening and controlling pressure atPmin set point. When the valve opening reaches Ym value pressure set point will beincreased by the predefined gradient.Automation:The A/M stations are auto, the “startup mode” is set.The valve will open to keep pressure constant.When valve opening reaches Ym the operation mode change to pressure ramp2.1.3 Pressure rampAs steam production increase, the pressure increase according predefined gradient. Thepressure is increased up to final startup pressure Psync.Automation:The A/M stations are auto, the “startup mode” is set.The pressure setpoint increase is limited by gradient limiter.Only positive gradient GRD1 is possible, the negative gradient is set to zeroWhen pressure reaches Psync operation mode will change to the “fixed pressuremode”. The “startup mode” is reseted.2.1.4 Changeover to Fixed pressure modeLP bypass pressure is controlled by LP-Bypass valve. As LP turbine takes steam, LP-Bypass will gradually close.Automation:The A/M stations are autoThe valves are in pressure control with setpoint Psync.If necessary the setpoint can be adjusted by operator.As the turbine takes steam, the LP-Bypass will closeWhen LP-Bypass is closed and signal “Turbine on” is on, the operating mode changeto follow mode2.1.5 Changeover to Follow modeThe pressure set point is sliding according to boiler load.Automation:The A/M stations are auto.The target LP bypass pressure set point is sum of sliding LP-BP inlet pressure, definedby gradient GRD2, and additional pressure bias dP. When LP-Bypass starts to open theoperating mode is changed to fixed pressure at actual pressure value.2.3 Boiler run downBoiler run down memory must be set to prepare set point generation for boiler …rundown“, so that the bypass opens when the boiler load is reduced.Automation:LP-Bypass A/M station set to auto by the operatorThe “run down mode” is set by operator with “Run down on/off” signal.The pressure set point decrease is limited by gradient limiter.Only negative gradient GRD is possible, the positive gradient is set to zeroAt some point the steam production is higher than the flow through turbine, thepressure will rise and LP-Bypass will open.Bypass changes to fixed pressure mode with set point at actual pressure value.Boiler is shut off. LP-Bypass will close to keep the boiler pressurized.3. Pressure controller (Diagram DK)LP-Bypass controls LP BP pressure.Automation:LP-Bypass A/M station set to auto by operator.When in manual LP-Bypass controller output is tracking valve position.When the LP-Bypass outlet flow is higher than value “Fs max” the valves start to closeand limiting the maximal steam flow to condenser.4. Temperature controller (Diagram DL)The LP-Bypass outlet steam is cooled down by injection of necessary amount of spraywater.Automation:When LP-Bypass is closed the spray valve is closed.When in manual controller output is tracking valve position.At LP-Bypass pressure control valve opens the temperature controller A/M station isset to auto.The necessary amount of spray water “Fs water” is calculated as function of LP-Bypass inlet steam flow and enthalpy and wanted enthalpy value at cooler outlet and.5. Quick open and Emergency close (condenser protection) of LP-BP(Diagram TK)The fast opening and emergency closing functions are operating independently of thepositioning loop programmed in the pressure controller itself.A too high control deviation acts as the fast opening criteria. In the case of fastopening, the pressure controller is first internally switched to automatic operation, andonly if it can be switched to automatic operation, the signal …fast opening“ is given tothe fast opening device. During start up mode, the criteria …control deviation too high“is blocked. With the monitor …Pfo“, the fast opening must generally be released.In the case of fast opening, the pressure controller output signal Ys is set to a preadjusted fixed value …Yfo“. Thus, optimum preconditions are brought about forstarting again automatic pressure control operation.Externally generated fast opening commands can be connected to the controller input"fast opening criteria". Such signals have then the same effect as a signal from thedeviation monitor. A timer limits the length of external commands.The LP-bypass valve is emergency closed to protect condenser (pressure high,temperature high, level high)Automation:The trip close command is active during follow criteria:-Condenser pressure high-Condenser temperature high-Condenser level high-LP spray water pressure lowThe trip close will close LP-Bypass and set A/M stations to manual.The pressure setpoint is in follow mode.Operator needs to reset quick closing of bypass system and set A/M station to auto toenable restart of bypass system.Pcond max (Value by condenser supplier)Tcond max (Value by condenser supplier)Lcond max (Value by condenser supplier)6. Spray water isolation valve (Diagram /SM)The spray water isolation valve opens or closes depending on LP-BP valve position: IfLP-BP valve is not closed spray water isolation valve opens, if LP-BP valve is closedisolation valve closes.7. Curves 7.1Start up curveT im eL P B P re s s u re 100 %50 %20 %40 %60 %100 %80 %L P7.2 Shut down curveL P B P re s s u re 100 %50 %20 %40 %60 %100 %80 %。
