On site we have a GE 9FA turbine. From AS BUILT manuals, GER 3620, there is a table for Turning. These are the generic operational guidelines for the time that the GT should be at turning after a shutdown or a trip and then restarted for normal operation. Verbally we also have been informed that GT can be restarted in less than 8 hours following a shut down if the wheelspace temperature is higher than 200 oC. Do you have any operation guidelines from GE on that issue? As you understand restart times are critical to avoid abnormalities on main GT parts but there is no automatic protection from MArk VI logic on that.

 

Your question isn't clear.

There is a minimum amount of time that a unit must be on cooldown ("turning gear") before it is taken off cooldown, or, when the shaft is allowed to coast down at zero RPM, and is considered to be "at rest." When the unit is off cooldown (off "turning gear"), there is no L.O. flowing to the bearings because the Aux. L.O. Pump is shut down.

And, the general industry practice is that the absolute earliest a GE0-design heavy duty gas turbine can be taken off cooldown ("turning gear") and allowed to remain "at rest" (at zero RPM) is when the highest wheelspace temperature is less than approximately 200 deg F. This is because the heat from the wheelspaces will make its way "down" the turbine shaft to the bearing journals and the bearing material that GE uses typically becomes soft at temperatures above 300 deg F. So, in order to prevent possible damage to the bearings the practice is to never take the unit off turning gear (which means there will be no L.O. flowing to cool the bearings) until the highest wheelspace temperature is more than 100 deg F less than the temperature at which the bearing material becomes soft.

As for how soon a turbine can be re-started after it reaches cooldown ("turning gear"), there's typically no restriction--presuming the unit is a typical GE-design heavy duty gas turbine, and is NOT a single-shaft STAG (Steam And Gas turbine and generator) unit. Sometimes, there are restrictions placed on single-shaft STAG units because of steam turbine temperatures or axial positioning or eccentricity considerations.

If there is no logic to prevent a re-start as soon as a READY TO START indication is achieved, then there's likely no restriction on how soon a turbine can be re-started after reaching cooldown ("turning gear").

Again, if the unit at your site is a single-shaft STAG unit (the gas turbine, steam turbine, and generator are all coupled together into a single unit) there might be other restrictions which might have to be considered.

Hope this helps.

 

Starting and restarting of GT

The only protection that is available in the mark VI logic is that you cannot turn off the cooldown cycle before 24 hours from the time the machine reaches 0 speed. But there is no restriction for the machine to go from cooldown to "ready to start".

The practice i have seen is as follows

once the machine trips , you can restart it within the 4-6 hour time interval when the wheel space temp is above 200 deg. If this time has passed and the wheelspace temp goes below 180 deg , you have to wait till the end of cooldown cycle (24 hours) or when the wheel space temp goes below 70 deg and remains

below the temp for 2 hours to again restart.
I asked a BHEL person why this is so. he gave me the following reply
" The reason is to differential expansion of the casing and the compressor and turbine blading. during the period when the wheel space temp is about 200-70 deg, the tip clearance is the min. If you start the machine at this time the turbine blades can expand and rub with the outer casing and cause "tip liberation" (??!!!). undoubtedly this will also liberate a lot of money from your company."

this was quite scary enough for the operational senior manager (and myself included) and my parent site follows this procedure adherently.

i have not read any supporting evidence or GE papers which discuss this , but this is what is followed in almost all the places. Perhaps CSA or Otised can give more info supporting/opposing this theory.

given that we are in this topic i would like to add this query. Does anyone know more about the side effects of forced cooling. I have seen this practiced in some sites. after the machine is shutdown they crank the machine fro 30 min , 2-3 times causing forced air flow which reduces the wheelspace cooling time from say 18-20 hours to less than 10 hours. the idea of the rotary maintenance is to shut down the machine at night at around 9 PM , given forced cooling / cranking at 12 Pm anf 4 am. the machine wheel space temp gets reduced by 50 deg at 7-8 am in the morning and they start their work (mostly burner replacement) ". I have not heard of any problems arising from this yet. But i want to know if this a healthy practice of not. is it followed in other sites also ? yes this does have the advantage of reducing the maintenance time from 24 - 30 hours to less than 15 hours , but are there any side effects to the forced cooling procedure ??