HPBP & LPBP

HP Bypass Station
1:- HP/LP bypass system can handel 60% of boiler load.
2:- HPBP valve is a combined pressure reducing and desuperheating station valve
3:- Spray water is tapped from BFP discharge  and spray controled by spray control valve.
4:- HPBP dumps the depressurised and desuperheated steam with suitable parameter into CRH line.
5:-Can operate without coordinate with turbine.
6:- Allows quick raising of steam parameters at a level acceptable to turbine for rolling operqtion during start up.
7:-Oil supply unit is installed for operation of HPBP & LPBP valves 
8:-Supply oil pressure is maintained at 160 kg/cm2 by the pump and accumulator unit.

Modes of HP Bypass Operation
1:- Boiler start-up with TG set at stand still.
2:- Rising of steam parameters to a level acceptable to turbine for rolling.
3:- Turbine start-up and loading while steam flow gets transferred to the turbine
4:- Parallel operation with turbine on load rejection.
5:- Allowing boiler operation while turbine tripped.

Interlocks: (safety interlocks)
1:-If HP bypass valve opening below 2%, then spray water isolating valve (BD)automatically close.
2:-If HP bypass valve opening is above 2%, the control of spray water isolating valve (BD)opens and the spray water control valves (BPE) changed to "Auto" mode.
3:-If the downstream steam temperature of HP bypass valve (BP)becomes high (380 C), then HPBP valve will closed.
4:-If condenser vacuum is very low (less than 0.6 kg/cm2  abs), the HP bypass valves close.
5:- Bypass station has a fast acting channel for full opening of the valves in about 3 sec (normal timing 8 sec) after following conditions:
  Generator circuit breaker opens
  Turbine trips
  Turbine load shedding relay operates
  Control deviations greater than a preset value

  Manual operation
6:- HPBP warm up valve will auto closed if LPBP valve is opened more than 2%.
7:- HPBP pr CV (BP valve )  auto blocked if 
BP valve closed condition is appear.
Oil supply pressure is very low.
BP valve position transmitter is failed.
7:- Pressure devition b/w MS pressure & HPT first stage pressure high.

8:- Pressing of fast opening push button.

LP BYPASS STATION
1:-LPBP maintain required pressure in the reheater, bypassing the extra steam through steam dumping device into condenser.
2:-The desuperheating and pressure reduction takes place in the dumping device.
3:-The spray water for desuperheating in dumping device is taken from CEP discharge.
4:-LPBP dump the excess steam present in the reheater circuit due to its non-steady state operation.

5:- LPBP dump steam to condenser, such that to maintain the reheater pressure at a desired value at instant load changes.
6:- Control of LPBP maintain the required flow maintained through the IP & LP turbines.
7:- LP bypass system is controlled (opening & closing) by control oil supply unit (seprate system from HPBP & Governing System).
8:- LPBP has an independent control system, works in conjunction with HPBP system.
9:-Emergency stop valves and control valves of the LP bypass are combined in a common  body (called as a dumping system).

INTERLOCK AND PROTECTION OF LPBP SYSTEM

1:-LPBP is closed if 
Condenser vacuum is > 0.6 kg/cm2.
Water injection pressure < 10 kg/cm2.
2:- Low vacuum trip device will close LPBP if condenser pressure > 0.6 kg/cm2.
3:- Exhaust hoot temperature; alarm > 90 °C  & trip at 110 °C.
4:- LPBP drain  valve is opened if condenser pressure is not high <0.3 kg/cm2.
5:- HRH drain is closed if LPBP > 2% open.
6:- HPSU Pump emergency off.
7:- Oil tank level  < 150 mm.
8:- Oil temp > 80 °C
9:- Colling fan
On : oil temp > 65 °C.
Off: Oil temperature < 50 °C.
9:- Both pump on &  oil pressure < 120 kg/cm2 with time delay 2 min.
10:- Fire protection chaneel-1 & channel-2 operated.