Controlled Fast Cooling Procedure

Controlled Fast Cooling Procedure

  • Normally Rihand-II and III 500 MW units take 8-9 days to stop Barring
  • Using CFCP, it takes roughy 75 Hours after Desynchronisation.
  • It is done in 2 stages.
  • First Stage: Unit Parameters Lowered By Reducing Load and steam Parameters.
  • Second stage: After Desynchronisation, till 350 Deg( HP Casing Temp), it is to be cooled thru Natural Cooling. thereafter by Modulating HPCV and Vacuum( Forced Cooling) maintaining Drop rate 5-6 Deg/Hour.
  • During this : Top/Bottom DT, Differential Expansions, Axial shift, Bearing Temperatures, Bearing & shaft Vibrations need to be Observed and Logged in the sheet and if any Deviation Found, should be rectified.And if these deviations are beyond rectification, then abort force Cooling.
Stage I : Parameters Reduction on Load:
  • Ensure all Auto Loops and SLCs in service and MS Temp <480 deg to be bypassed in order to lower Temperature. (Desk, Control Desk, SCE  & C&I with Proper logging in Register).
  • Start Parameters Reduction Before 6 Hours to shut-down
  • First Reduce Load by 5% (25 MW of 500 MW), then stabilise Load and Steam parametrs.
  • Then Reduce MS Set point to 525 Deg .( Rate of Reduction of MS Temp :1.5~3 Deg/min)
  • Further Reduce Load by 25 MW and stabilise parameters, and after that reduce MS Temp to 515 Deg.(If SH Attemperation in Manual if not maintaning )
  • Repeat Above steps, till 300 MW and HP Casing Temp 480 Deg.
  • Check HP Exhaust Temp with HP Exhaust Pressure, it should be above Saturation Temp value.
  • Do not reduce load and MS Temp together because of DE, Transient vibration, Lower Margin of TSE.
  • Open all MAL drains, HP SV/CV, Extraction drains as there may be consensate caused during parameters reduction.
  • After HP Casing temp reaches 480 Deg, Desynchronise Machine.
  • Ensure AOP, JOP, Barring gear starts as per logic.
  • Note down the Coasting down time from 3000 rpm to 510 rpm,210rpm,120 rpm.

Stage II : Controlled Fast Cooling of Turbine
  • After Depressurisation of Ms Line( Thr Pr=0ksc), Ensure BSVs and their IBVs are Closed & Isolated.
  • Ensure all MAL drains, Extraction NRV drains are Open as per SLC.
  • Ensure HP/LP Heaters Extraction Valves are closed and Alternative Drip valve OPEN.
  • Kill Vacuum and Isolate Seal steam and GSC Ex Fan in On condition for intial cooling.
  • Isolate Aux PRDS from Unit and Other Interconnection with other Units.
  • Ensure HPBP Spray v/v closed
  • CW charged, one CEP in service and monitor LP Exh Hood temp
  • In 24 Hours, HP casing Temp reduces to 350 Deg


    • Controlled Fast Cooling by admitting air :
  • Ensure Turbine on barring gear.All stop & control valve closed
  • Ensure Casing Temp reached 350 deg by natural cooling
  • Open air inlet sockets b/w main stop & control valve and cover opening thru mesh
  • Set Test valve of all stop valves in Closed Condition.
  • Open HP control v/v. IP CV remain closed using Test motor(closing test oil signal)
  • Ensure gland area is clean, Pull Vacuum without seal steam charging
  • Ensure all MAL valve Open & Their isolating manual valve Open, SLC drain OFF
  • Stop GSC Ext Fan, allow air to be sucked thru Socket and gland area
  • Maintain vacuum -0.2 ksc initially, monitor TSI, TSE(lower margin in limit)
  • Maintain 5-6 deg/hour Temp drop by  by regulating HP CVs and vacuum pump.
  • After 48 Hours, HP Casing temp drops to 110-125 Deg. from 350 Deg
  • If drop rate is less than 5 deg/hr increase vacuum -0.4ksc
  • at 110 deg, Stop Barring

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