Gas Turbine minimum time on turning gear after shutdown

[nangkring]

Hai all, I operate Gas Turbine with power output 186 MVA, 16 stages compressor and 4 stages turbine. Because off network request the gas turbine often start and stop in same day. When gas turbine shutdown, it will brought to turning gear operation at speed about 90 rpm for 24 hours and then turn to standstill. If I want to make gas turbine in standstill because certain of urgent maintenance on supporting system (Lube Oil) in less than 24 hours, what kind operation parameter should I checked? Is it need to consider operation parameter such as Exhaust Temperature, Temperature Outlet Compressor, Stator Slot Winding Temperature (Operation Parameter available on HMI)?

 

[ScottyUK]

90 rpm is pretty fast for barring gear, are you sure that's correct?

Generally the turbine will cool faster than the generator rotor, and the generator rotor becomes the most significant time constant when determining how soon the unit can come off barring. The upper stator slot RTDs will give you some idea what the rotor temperature is doing, but not with any degree of accuracy. We used to keep a similar-sized unit on barring for 12 hours post-shutdown if it might need to re-start during that period. After 12 hours it could be shut down ready for restart.

 

[krisys]

The barring is a precautionary measure to de-stress the mass. As you are turning off as a special case. May be a best compromise is to wait for 12 hrs and then bring it to standstill. This is an exceptional case which should prevail.
A small stress accumulation should not impact on the overall performance.

 

[ScottyUK]

It's not just a precautionary measure, it's an operational requirement. If a thermal bend is allowed to develop in the generator rotor then the resultant imbalance will prevent the machine being run until the rotor has cooled and the rotor straightened. That can take days, which equates to a lot of lost generation. If kept on barring the rotor doesn't need to cool down because it maintains a uniform temperature and therefore remains straight.

It might be tempting to bring the machine up to cranking speed soon after shutdown to help remove the heat from the generator, but this will also result in rapid cooling of the thinner turbine components such as blades and vanes which will definitely cause stress within them.

 

[racookpe1978]

The GT rotor "springs" (bends) due to the very uneven heating/cooling differences between the top and the bottom of the compressor casing.

It is this springing (bending) that causes compressor wear and rubbing. And, yes, it is catastrophic if you startup when the bending is out-of-allowed limits. It is the blades rubbing that is the failure.

 

[Hoxton]

It is usually built into the shutdown / restart sequence

So, you cannot restart until the turbine has cooled down

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[byrdj]

I was curious about this discussion given I only have steam turbine experience.

Nangkring concern of “…urgent maintenance on supporting system (Lube Oil)…” was a similar consideration for many of my assignments. In all those events I never thought about the generator, BUT with the temperature the turbine’s bearing babbit would reach from turbine casing heat through the rotor.

The primary rule of thumb was when turbine casing temperature was under 500F, the bearing babbit should stay under 300F. Thus, for urgent repairs the unit was cooled as much as possible during power reduction and it still was 12 to 24 hours before the barring stopped and lube oil secured.

The return from such urgent maintenance was very critical. The lube oil was restored and ran until bearing babbit was under 200F before any attempts to place on barring. In the very unlucky event the rotor’s thermal bow was excessive, it might require additional time to cool (worse case 2 to 3 days from initial shut down). Once the rotor could be turned with barring gear, positioning it 180 degrees for 4 hours to help straighten the thermal bow. Once on turning gear, even if measured rotor eccentricity indicated thermal bow resolved, the normal 4 hours barring gear would be increased to 8 prior to attempting to roll to speed.