How long a GE gas turbine can be kept at fire speed before selecting auto, and how long can the GT be kept at FSNL before synchronising? If there are limits, what are they?

 

@DSVPrasad,

Neither have definitive time limits in my experience. Having said that, FIRE Mode is operating with the IGVs at the minimum mechanical position and thus restricting air flow (which is already restricted because the axial compressor speed is much less than rated). Something that also happens--especially if running on gas fuel--is that as the machine warms up the speed will increase and the increase in speed will also cause the P2 pressure to increase which will increase the fuel flow-rate into the combustors. I've seen people select FIRE and "walk away" from the machine only to return 25 or 30 minutes later to find it above 40% speed and increasing and they are shocked and screaming, "The Mark* is not working properly!!!" (which isn't correct). Personally, I view FIRE mode as a test mode and occasionally use it to dry out a machine after an off-line water wash if the machine isn't going to be re-started within 24 hours or so after the rinse is complete, and I never leave the machine unattended while warming up my biryani and getting a cuppa when it's in FIRE Mode. And, two minutes is about as long as I've ever left it running when not drying out a machine. And one last thing to remember about FIRE mode--all the time the machine is running (burning fuel) it's using a torque assist from the starting means, too--and some starting means aren't rated for "long-term" operation (think diesel engine starting means with a minimal cooling water system that can easily overheat!).

AUTO or REMote operation at FSNL is a little different. The IGVs are at minimum operating angle, and the machine is self-sustaining (meaning no torque assist from the starting means); often the compressor bleed valves are closed at FSNL and the machine (if it's a B or E class machine, including Frame 5s) will have stopped the Aux. L.O. and Aux. Hyd. pumps and are running on the Main pumps. The limiting factor for this mode of operation for some machines is the exhaust--especially if the machine exhausts into an HRSG (boiler)--some boilers can't be operated for very long at this "low" air flow (even thought the IGVs are at minimum operating angle some boilers need more air flow than is available at FSNL). Again, my personal limit for FSNL is about 30 minutes (often the amount of time required to warm up the machine minimally before performing a running overspeed test).

 

@DSVPrasad,

Neither have definitive time limits in my experience. Having said that, FIRE Mode is operating with the IGVs at the minimum mechanical position and thus restricting air flow (which is already restricted because the axial compressor speed is much less than rated). Something that also happens--especially if running on gas fuel--is that as the machine warms up the speed will increase and the increase in speed will also cause the P2 pressure to increase which will increase the fuel flow-rate into the combustors. I've seen people select FIRE and "walk away" from the machine only to return 25 or 30 minutes later to find it above 40% speed and increasing and they are shocked and screaming, "The Mark* is not working properly!!!" (which isn't correct). Personally, I view FIRE mode as a test mode and occasionally use it to dry out a machine after an off-line water wash if the machine isn't going to be re-started within 24 hours or so after the rinse is complete, and I never leave the machine unattended while warming up my biryani and getting a cuppa when it's in FIRE Mode. And, two minutes is about as long as I've ever left it running when not drying out a machine. And one last thing to remember about FIRE mode--all the time the machine is running (burning fuel) it's using a torque assist from the starting means, too--and some starting means aren't rated for "long-term" operation (think diesel engine starting means with a minimal cooling water system that can easily overheat!).

AUTO or REMote operation at FSNL is a little different. The IGVs are at minimum operating angle, and the machine is self-sustaining (meaning no torque assist from the starting means); often the compressor bleed valves are closed at FSNL and the machine (if it's a B or E class machine, including Frame 5s) will have stopped the Aux. L.O. and Aux. Hyd. pumps and are running on the Main pumps. The limiting factor for this mode of operation for some machines is the exhaust--especially if the machine exhausts into an HRSG (boiler)--some boilers can't be operated for very long at this "low" air flow (even thought the IGVs are at minimum operating angle some boilers need more air flow than is available at FSNL). Again, my personal limit for FSNL is about 30 minutes (often the amount of time required to warm up the machine minimally before performing a running overspeed test).

Great response. Very clear. Thanks a lot.
The idea is to warm up the HRSG gently at firing speed and then also at FSNL with the diverter damper fully open. This was already tested, and the rate of heating of HRSG was within limits. Everything went fine. Just wanted to know if there is any serious low side to GT integrity short term or long term. We will be watching the parameters of concern as per your advice. Thanks again.