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