I am a user of two GE 6B turbines. I wanted to ask about your experiences regarding the criteria for restarting turbine after scheduled shutdown or trip (Wheelspace temperatures, ratchet duration, etc.). I heard recomendation that: Turbine may be restart within 2 hours after stop or trip. In case not restarted within 2 hours after shutdown or trip, turbine shouldn't be restarted before reaching full cool down.

In GER-3620M (Heavy-Duty Gas Turbine Operating and Maintenance Considerations) - page 45 - Turning Gear/Ratchet Running Guidelines
there are only recommendations for ratchet. There isn't information about Wheelspace temperatures or other parameters before startup.

https://www.ge.com/content/dam/gepower-pgdp/global/en_US/documents/technical/unused assets/hdgt-operating-maintenance-considerations-report.pdf

 

There isn't wheelspace or other parameter recommendations because as long as the unit goes on and stays on Cooldown when it gets down to zero speed, it can be restarted at any time without any restrictions.

GE-design Frame 6B heavy duty gas turbines have always been robust machines, that's one of many reasons there are so many of them in service for decades now.

 

After submitting this topic, a similar topic from 2011 was displayed under my topic: https://control.com/forums/threads/restart-times-of-gt-9fa-after-stop-trip.37135/

The topic concerns GE 9FA turbine. In this topic there is an information about expansion of casing and and turbine blading (tip clearance) after shutdown. I have heard opinion that if certain regimes for Wheelspace temperatures are not followed honeycomb (second and third stage) seals can be damaged?

 

F-class machines are like beautiful women; they require a LOT of special attention and tender love and care. They were not designed for the kinds of service they frequently see (load following; cycling)--they run best at Base Load.

6B machines, on the other hand, are sturdy, robust machines that can withstand a good deal of abuse and just keep running. They can be cycled repeatedly with almost no ill effects.

A 9FA is physically much larger than a 6B. However, the design of the casings is not the same. F-class machines have less metal in the casings and are more prone to retain heat (that sounds counterintuitive but the residual heat in the rotor (compressor AND turbine) adds to the heat retention). As things begin to cool after several hours the casings will begin to contract but the turbine rotor can still be very warm. And, it's also been shown that the compressor rotor will bow in an upwards direction possibly causing a problem if the unit were to be started.

6B machines have smaller rotors and casings but they all cool at a very similar rate.

 

CSA, thanks for your answers. Despite your strong arguments, I have some doubts. So how to understand guidelines of GER-3620M (Heavy-Duty Gas Turbine Operating and Maintenance Considerations) - page 45 - Turning Gear/Ratchet Running Guidelines?

Before Startup:

Case C

• Hot rotor, <20 minutes after rotor stop - Ratchet duration: 0–1 hour

Case D

• Warm rotor, >20 minutes & <6 hours after rotor stop - Ratchet duration: 4 hours

Case E.1

• Cold rotor, unbowed, off TG <48 hours Ratchet duration: 4 hours

For me especially interesting are case D (Warm rotor) and E.1. (Cold rotor). Why 4 hours? And not, for example 2 or 3 hours?

 

You asked for experiences regarding Frame 6B restarts. That's what I offered--my experiences.

GER-3620 is an extremely difficult document to read and understand. Why? Because they try to cover every nuance of every Frame size and do it with as few words as possible. It would have been better to do separate publications for each group of machines (B & E class: F class; H class). If I recall correctly, GER-3620 was originally written for F-class machines, and later modified to include the other Frame sizes.

I'll add further to your doubts (a word I loathe and despise) by telling you that a peaking power plant I worked at ran the ratchet every morning for four hours (on two 7EA machines). The engineering manager wanted to reduce the electrical consumption of running the ratchet (Aux. L.O. Pump; Aux. Hud. Pump; L.O. Mist Eliminator; Gen. L.O. Mist Eliminator), but kept going back to GER-3620 and it's 4 hour ratchet recommendation. We did some testing over a couple of months and reduced the 4 hours to 1 hour, and there were never any problems with high vibration during start-up in more than 20 years.

GER-3620 is mostly recommendations, but, honestly, GE doesn't have a lot of operational experience. And, they're the OEM (Original Equipment Manufacturer), so their recommendations should be expected to be conservative. For the last time, 6B machines are sturdy and robust workhorses designed and built for rough service. They can handle just about anything you can do to them (within reason).

Good day, sir.