Vacuum Pulling

INTRODUCTION:

• Turbine glands seal the shaft where it passes through the casing against atmosphere. 
• The turbine glands are self-seal type beyond approx 30-40% load.
•  For initial sealing purpose steam from APRDS (HTPRDS / LTPRDS) source through the gland steam control valve is taken to seal all the HP / IP / LP turbine glands.
•  At starting the vacuum is raised in the condenser by using vacuum pumps and sealing all the HP / IP / LP turbine glands by supplying seal steam. 
• During normal operation the vacuum is created by condensation of steam and is maintained by vacuum pump.
• At the start up stage both the vacuum pumps remains in service (hogging operation) and one vacuum pump remains in service during normal operation of the turbine (holding operation). These vacuum pumps are liquid ring type of pumps.

OPERATION PROCEDURE

The requisites / plant Conditions:

• No PTW is pending under the associated system such as APRDS / CEP/ Turbine / Vacuum pumps / condenser / GSC / ACW / control fluid etc.
• TG set is on barring.
• One CEP is in service & DM MAKE UP pump in service.
• One CW PUMP is in service & both condenser passes are charged and flow is normal.
• One control fluid pump is in service.
• Turbine side flash tanks are lined up (SDFT, UFT, LPDFT, HPDFT, steam flash box - 1 & 2). 
• TDBFP A/B exhaust valves are closed with IBVs and seal steam lines are isolated.
• APRDS is charged and adequate pressure and temperature are available .HT / LT pressure 11kg/cm2 and HT temperature should be greater than 300ºC, LT temperature should be greater than 230ºC.
• All steam drain valve to UFT and MS, CRH, HRH atmospheric line drain manual valves should be in open condition.
• Ensure vacuum breaker is in open condition.
• Valve gland sealing is to be charged from gland seal header (pr >6 kg/cm2). Gland seal header can be charged either from DM make up line or from CEP discharge header.
• Seal steam controllers are working properly and are available.
• DM make up pumps / CEPs are available for make up of separating tank of vacuum pump.
• LP exhaust hood spray is lined up.
• Two ACW pumps are in service and ECW system in service

OPERATING PROCEDURE

WARMING UP PROCEDURE FOR AUXILLARY STEAM LINE TO SEAL STEAM STATION:

• Ensure that the turbine seal steam supply control valve and it’s by pass motorized valve are in closed condition.
• Ensure that seal steam manual valve at 8.5m in closed condition.
• Operation of the seal steam controller is ok.
• Seal steam station (supply and leak off system) is lined up.
• Operation of bypass control valve is ok.
• Open the main turbine seal steam supply line (from auxiliary PRDS) manual drain valve to UFT at 8.5m.
• Now charge the steam line by opening the manual isolating valve 10 to 15 % at 32m preferably from unit HT header at APRDS station to warm up the system. 
• After warm up open the same manual-isolating valve 100% and maintain the seal steam temperature before the supply control valve greater than 270ºC.

LINE UP PROCEDURE FOR VACUUM PUMPS

• Flush the vacuum pumps by opening the drains and then fill fresh make up water. Maintain normal level in the separator tank greater than 75%. The source for the separator tank can be lined up either from DM make up or from CEP discharge through gland seal header.
• Ensure that all the drain valves in vacuum pump and separator tank, heat exchanger of vacuum pump are in closed condition.
• Open the individual condenser pass manual isolating valve (At 8.5 meter near LP bypass CVs) and the individual suction manual valves of the vacuum pumps at 0m.
• Check the individual air operated suction solenoid valves IV-1 are closed in both vacuum pumps.
• Charge the heat exchanger from ACW side and check the filter DP. If found chocked take immediate actions to clean it.
• Release the vacuum pumps and Seal water recirculation pumps EPBs.
• Now the pumps are ready to start.

LINE UP PROCEDURE FOR GLAND STEAM CONDENSER:

• Ensure GSC charged from waterside.
• GSC drain is lined up to LPDFT.
• In case of non-availability of GSC the GSC bypass valve in water side and the safety valve in steam side are to be kept open.
• GSC siphon must be filled with water.
• Note: At seal steam temperature before seal steam supply control valve less than 210 degree “seal steam interlock” alarm appears and at temperature less than 190 degree seal steam supply interlock becomes active and closes the supply control valve and its bypass motorised valve fully.

PROCEDURE FOR VACUUM RAISING:
PRELIMINARY CHECKS:

• Seal steam is lineup up to the Seal steam station at 8.5 metre and temperature before seal steam supply control valve is greater than 270º (if seal steam is charged from Unit LT PRDS then this should be greater than 210ºC).
• Ensure the valve gland sealing system is in charged condition.
• Vacuum pumps are lined up.
• GSC is lined up and start any one GSC exhauster fan.
• Close vacuum breaker.
• Start both vacuum pumps one by one along with its seal water recirculation pumps.
• Observe the followings (by sending operator in local)

1. Vacuum Pumps vibration and sound.
2. Working fluid temperature.
3. Separator tank level is normal.

• Observe that the condenser vacuum is rising slowly.
• Once the condenser pressure reaches -0.2 kg/cm2 (vacuum), then slowly charge the seal steam by opening the APRDS seal steam supply line manual isolating valve at 8.5 metre and by opening seal steam controller. Maintain the seal steam header pressure and temperature (For maintaining seal steam header temperature open MAW 11, and MAL 81).
• Observe the vacuum is further improving.
• Once the vacuum is reached upto rated level fill the vacuum breaker siphon with water from GS header.
• If vacuum is not obtained upto the rated value, Then thoroughly check for any air ingress and attended it.
• Monitor and maintain rated seal steam pressure and temperature continuously.
• Observe the turbine barring speed, It may raise approx from 72 rpm to 120rpm with rated vacuum.