INTRODUCTION:
This bypass system has been providing to allow the steam generator to build up during start up, matching steam parameters with the turbine. The steam generated is dumping into condenser. Thus avoiding the loss of boiler water. This system enable starting of the unit of sliding parameters & also facilitates hot restarting of the unit in the event of losses of load on turbine, the bypass system disposes the steam produce by the boiler automatically to the condenser without affecting the boiler operation.
The bypass has two sections: HP & LP:
The HP Bypass system diverted the steam before main steam valve (MSV) to cold reheat line CRH. HP bypass system also reduces the rated steam parameters of the incoming steam from the super heater. SH) to the steam condition expected in the CRH line.
The LP bypass system divert the incoming steam from Hot reheat line(HRH ) line before intercepting valve(IV) to the condenser After reducing HRH steam parameters to the condition approximately LP steam turbine Exhaust steam
The purpose of HP bypass system is to allow the steam pressure at the HP turbine inlet controlled before & during turbine start-up, as well as during trip & shut down conditions the HP bypass (BP) valves. In addition the dump steam is attemperater (de-superheated) by spray water via temperature control valves (BPE valves) to the temperature in accordance with the cold reheat inlet the design capacity of HP bypass is 70 %.
For matching of live steam & turbine metal temperature for a quick start up, initially HP bypass valves dump the steam to the re-heater circuit finally to condenser via LP bypass system then the steam parameters at the boiler outlet are being raised. Hence the use of turbine bypass station, it is possible to build up the matching steam parameters at the boiler outlet during regime of starting independent of steam flow through turbine.
The steam generated by boiler & not utilized by turbine during startup or shutdown conserved within the power cycle & the losses of steam in to the atmosphere is cut down to the barest minimum.
SCHEMATIC ARRANGEMENT OF HP & LP BYPASS SYSTEM
HP-bypass station
HP LP bypass will open 100% with fast opening device when;
1 Generator circuit breaker open.
2 Turbine trip.
3 Load rejection relay will operated i.e.
• Load throw off > 50%
• Balance load < 20%
• Frequency > 49 Hz
HP bypass valves will close when;
1 Condenser vacuum very low< 540 mm of Hg.
2 HP bypass down stream temp > 380 degree cel.
3 BD valve open/close in following cases
• BD valve opens when BP1,BP2 valves actual position >2 % or the controller demand >2%.
• BD valves closes when BP1,BP2 valves actual position < 2% and controller demand < 2%.
LP bypass station
LP bypass valves will close when;
1 Condenser vacuum is very low < 0.7 kg/cm2
2 Injection water pressure low
3 Condenser exhaust temperature > 90 degree cel.
DESIGN CAPACITY OF HP BYPASS STATION 210 MW (LMW) UNIT
Total capacity of both bypass valves = 2 x 100 = 200 T/hr.
Max M.S. Temp = 540°c
Max. M.S. Pressure = 140 ata.
Down steam temperature = 380 °c
DESIGN CAPACITY OF LP BYPASS STATION 210 MW (LMW) UNIT
Total capacity of both bypass valves = 2 x 112 = 224 T/hr
Design temp = 540°c
Steam temperature after desuperheated = 200°c
Steam pressure at upstream of bypass valve = 6 ata (max)
OPERATION OF BYPASS SYSTEM
OPERATION OF HP LP BYPASS SYSTEM:
When ever short trip outs takes place, the steam supply to turbine is totally out off from the main steam (high) pressure line. Now certain quantity of the high pressure & high temperature is passed through HP by pass system where the pressure of the steam is reduced & then de-superheated to the down stream conditions The comparatively cold steam at blow pressure ( cold reheat steam ) is passed through the re-heater to cool the re-heater coils.
The hot but blow pressure steam (hot reheat steam ) is once again isolated from the turbine and bypassed through the low pressure system to the condenser after pressure reduction & de-superheating: Therefore with HP & LP bypass system, a minimum circulation is maintained in the boiler tubes ,super-heaters & re-heaters with low firing rate during short trip out .When the short trip outs are over, the bypass system can be closed & high temperature, high pressure steam in main steam line & low pressure high temperature steam in re-heater outlet are readily available with a matching temperature of turbine for quick restart & subsequent reloading.
The bypass system shall be in operation during the following conditions.
1 Cold start
2 Warm start
3 Hot start
4 Load rejection & house load operation
5 Quick shut down of turbine
6 Turbine trip
SCHEMATIC ARRANGEMENT OF HP & LP BYPASS SYSTEM
Pre-start conditions for charging of HP-LP Bypass:
• Turbine oil system is started & turbine is running on barring gear.
• Condensate extraction pumps are started & condensate discharge header pressure is normal.
• CW system is established.
• Vacuum in the condenser is greater than 0.85 kg/cm2
• Boiler feed pump is started & supplying adequate discharge header pressure.
Warming of main steam line & HP Bypass valves;
• During cold startup the warming up of main steam line can be started after boiler blow down at 10 kg/cm2 pressure
• Ensure that drain valves of main steam lines ,CRH & HRH line are open
• Charge the main steam line by opening integral bypass valves (MS 301A,302A)
• Open the warm up valves of HP bypass valves locally
• The warming up operation of MS line is completed when steam temperature before HP bypass is more than the mid-wall temperature of ESVs.
• Open the main boiler outlet valves (MS301 &302 & close their integral bypass valves)
Preparation of LP bypass system;
• Open the warm up valves of LPBP1 &LPBP2
• Open drains of LP bypass line
• Set the fixed set point at 3 kg/cm2
• Put the LP bypass valve controller on “ AUTO “
• Switch ON the automatic control interface
Preparation of HP bypass system;
• Observe that MS Pr ( R ) & MS Pr ( M ) indicators show the correct reading
• Set the pressure master set point at 35 kg/cm2
• Set the temperature set point at 230 degree c Note- This point is required to be increased in accordance with HP turbine exhaust temperature after loading
• Ensure that BP valves control is on manual & valves are closed
• Check that BD valves & BPE spray valves are closed
• Check that normal pressure is available in oil supply unit
Charging of HP-LP bypass system;
• Open manually both BP1& BP2 valve by 5% simultaneously
• Check that BD valve opens when BP 1,2 position is greater than 2%
• Check that spray valves BPE1, BPE2 go to “ AUTO “ & adequately open
• After awaiting for 15 minutes open BP1 & BP2 further to 10 % for charging of re-heater & warming of LP bypass valves
• When the actual HRH pressure crosses the set value 3 kg/cm2, check that LP bypass injection water valves ( first stage v1 & v3) opens
• Check that LPBP1 & LPBP2 stop valves open
• With increase of control deviation, LPBP1 & LPBP2 control valves open
• Keeping the HP bypass valves on “ AUTO “
• Initially HP bypass are opened manually in step of 55 up to 50 % position.
• With increase of BP1,BP2,valves positions, LPBP1,LPBP2 valve position will increase.
HP BYPASS SYSTEM FOR KWU TURBINE :
The purpose of HP bypass system is to allow the steam pressure at the HP turbine inlet controlled before & during turbine start-up, as well as during trip & shut down conditions the HP bypass (BP) valves. In addition the dump steam is attemperated (de-superheated) by spray water via temperature control valves (BPE valves) to the temperature in accordance with the cold reheat inlet The design capacity of HP bypass is 70 %.
For matching of live steam & turbine metal temperature for a quick start up, initially HP bypass valves dump the steam to the re-heater circuit finally to condenser via LP bypass system then the steam parameters at the boiler outlet are being raised.
Hence the use of turbine by pass station , it is possible to build up the matching steam parameters at the boiler outlet during regime of starting independent of steam flow through turbine.
The steam generated by boiler & not utilized by turbine during startup or shutdown conserved within the power cycle & the losses of steam in to the atmosphere is cut down to the barest minimum.
LP BYPASS SYSTEM FOR KWU TURBINE :
The purpose of LP bypass system is to allow the steam pressure at the LP turbine inlet controlled before & during turbine start-up, as well as during trip & shut down conditions the LP bypass (BP) valves .
During startup & shut down & at operation below minimum boiler load, the volume of steam not utilized by the IP & LP cylinders of the turbine must be passed to the condenser via LP bypass valves. This requires the bypass control system to maintain the pressure in the re-heater constant in accordance with the preset reference value. In the event of disturbances ( transient conditions), e.g. load shedding or trip-out, the amount of excess steam passed to the condenser depends on the capacity of condenser.
The LP bypass station is utilized for the following tasks.
• Control of steam pressure after re-heater
• Dump the excess steam to condenser after de-superheating via LP bypass valves in dump tube.
• Establish flow of steam from reheat lines & build up steam parameters at re-heater outlet (HRH parameters) during unit start-up.
• Release of steam entrapped in HPT & re-heater circuit in case generator circuit breaker opens.
• Prevent the boiler tripping on re-heater protection after turbine trip-out by maintaining minimum flow through re-heater.
Main advantages of HP-LP bypass system are:
• To establish required steam parameters at super-heater & re-heater outlets in short time for best matching of steam & turbine temperatures.
• To handle the difference between the steam generated & that consumed by the turbine under transient conditions. The difference is dumped in the condenser.
• To unable house load operation of the machine.
• To achieve quick start ups, faster loading & reduced startup shut down losses.
• To save considerable amount of D.M. water
• Enables quicker warming up of steam lines.
• Minimize operation of safety valves
• Improvement of boiler stability under transient conditions.
• Boiler can be kept in operation with normal steam parameters even after substantial load rejection.
• Independent operation of boiler & turbine is possible, thus tapping full utilization of turbine flexibility
CONCLUSION :
The presence of HP-LP bypass system has been seen greatly benefits the plant. Beside improving the performances of plant. HP-LP bypass system enhances the life of the plant. Required steam parameters at super-heater & re-heater outlets in short time for best matching of steam & turbine temperatures. The difference between the steam generated & that consumed by the turbine under transient conditions. The difference is dumped in the condenser
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