HP BYPASS & LP BYPASS SYSTEM

INTRODUCTION

• A system provided in order to bypass steam from HP turbine termed as HP BYPASS system.
• A system provided in order to bypass steam from IP&LP turbine termed as LP BYPASS system.
• Work in parallel with Turbine during load rejection in a coordinated manner with steam generator.
• During start up of Boiler to build matching steam parameters required for turbine and also when load transfer to turbine.
• The bypass system would function in a non-coordinated manner as per the operator demand.
• Establishes a parallel path of HP and IP/LP turbine through reheater.

Requirements for various modes of operation

• Constant pressure.
• Sliding pressure.
• Modified sliding pressure.

HP BYPASS SYSTEM

FUNCTIONS

• Quick start up of the steam generator from cold and warm start up conditions.
• Raise and match steam parameters quickly in cold, warm and hot conditions.
• Protects the reheater tubes by ensuring continuous flow through it.
• Parallel operation of boiler & turbine.
• House load operation.
• Parallel operation of the bypass with turbine in the event of large load throw-off.
• Limited safety valves operation during trip-outs.
• The most important feature of this system is to allow operation of the boiler independent of the availability of the turbine.
• In the event of turbine trip, the boiler can be kept in operation and the bypass system provides an alternative path for flow of steam from boiler to condenser.
• Thus the boiler need not be relighted during restarting of the unit and a considerable time & oil is saved.

HIGH PRESSURE BYPASS SYSTEM

• Diverts the steam before main steam stop valve to the cold reheat line.
• Reduces the rated steam parameters of the incoming steam from the super heater outlet to the expected parameters in the CRH line.
• During parallel operation with turbine when latter is operating at no load, the turbine exhaust temperature could rise.
• The HP bypass station shall be used to bring down the temperature sufficiently so that the mixed steam (HP bypass downstream plus HPT exhaust) temperature should not exceed allowable inlet temperature limits for reheater.
• For a part load rejection, if bypass operates on its pressure controller, the downstream temperature shall be maintained with minimum overshooting or undershooting of the controlled variable from the set value.

SYSTEM DESCRIPTION

• Two parallel branches that divert steam from MS line to CRH line.
• BP-1 and BP-2 are pressure reducing valves with mounted electro hydraulic actuator for each main steam lines.
• Steam pressure on valve upstream side can be maintained at desired level
• Steam temperature downstream maintained by BD, BPE-1 and BPE-2 valve mounted electro hydraulic actuators.

CONDITIONS FOR OPENING OF HPBYPASS

• Generator Circuit Breaker opens.
• Turbine trips.
• Turbine Load Shedding Relay operates.
• Pressure deviation between MS pressure & HP turbine 1^st stage pressure high.
• Pressing of “Fast Opening “ push button.

TEMPERATURE CONTROL OF HIGH PRESSURE BYPASS SYSTEM

• The downstream temperature shall be maintained to the set value with minimum undershooting or overshooting of controlled value.
• The temperature shall be controlled within the range of ± 5 degree C.
• The bypass pressure controller shall maintain the set pressure within ± 1%.
• If the deviation between the set point and controlled variable exceeds an adjustable preset limit for HP bypass pressure controller, the system shall transfer the control from auto mode to manual mode with an alarm.

CONDITIONS FOR CLOSING OF HIGH PRESSURE BYPASS

• Condenser vacuum becomes low (0.6 kg per squire cm).
• Downstream temperature becomes very high (380 deg C).

CONDITIONS FOR OPENING OF HIGH PRESSURE BYPASS

• During large load rejections, the bypass shall open in about 3 seconds after initiation of signal from the following:
  • Generator breaker open.
  • Turbine trip.
  • Turbine load shedding relay operates.
  • Control deviation greater than preset value.
  • Manual action.

INTERLOCKS

• The spray water isolation valve (BD) automatically closes if the openings of HP bypass valve are below 2%.
• If opening of the HP Bypass valve is above 2% the control of spray water control valves shall be changed to auto mode notwithstanding their initial conditions.
• If the steam temperature at the downstream of HP bypass station becomes very high, the closing signal to HP bypass valve initiated and the positioning loop changed to manual mode.
• If opening of the HP Bypass valve is less than 2 % a Force Close command is given to it to close the valve.
• HP Bypass shall be closed if Condenser vacuum falls below 0.6 ata.

LOW PRESSURE BYPASS SYSTEM

• Enables to establish an alternative path for dumping the excess steam from the reheater outlet directly into the condenser at suitable parameters bypassing the IP and LP turbine.
• Protects condenser from excessive temperature and pressure that could result from sudden injection of superheated steam.

SYSTEM DESCRIPTION

• Consists of a combined system of stop valve and control valve Each stop valve and control valve is provided with its own Electrohydraulic actuator(EHA).
• Water injection system.
• EHA of each LPBP STOP valve is provided with a pilot valve and 2 Trip solenoid valves.
• The opening and closing of LPBP stop valve is done over the pilot valve.
• The EHA of each LPBP CV is provided with a proportional valve and an interlocking valve.
• The proportional valve is controlled over a positioner by the pressure controller.
• As a result the proportional valve positions the EHA, which in turn operates the LPBP CV.
• The interlocking valve is provided in the LPBP CV actuator to prevent the proportional valve from acting on the power cylinder.
• It holds the CV in the last position when HPSU system header pressure becomes very low.

INTERLOCKS & PROTECTIONS OF LPBP SYSTEM

• Condenser wall temperature >90 deg c
• Condenser pressure>0.6 kg per sq cm(abs)
• HPSU pump emergency off by:
  • EPB at HPSU operated (both recirculation pump & Main pump off)
  • Hydraulic oil tank level<170 mm. Trip (value as per OEM)
  • Hydraulic oil temp.>65 deg c Trip (Value as per OEM)
  • Both CF pumps(HPSU) 1&2 on and pressure <120 kg per sq cm (time delay of 2 min) {valuse as per OEM}.

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