I'm curious about the reason for the difference in lube oil tank differential pressure during gas turbine startup and shutdown. During startup, the lube oil tank differential pressure is around 8 inches, but it decreases to 6 inches when stopped. I don't quite understand why there is a difference. During both startup and shutdown, the lube oil pump is continuously running, and the LUBE MIST SEPARATOR fan (88QV) is also continuously running. For reference, the role of the LUBE MIST SEPARATOR fan is to create a negative pressure (-) in the tank to extract insoluble gases and facilitate the discharge of lubricating oil from each bearing to the Oil Tank. In my opinion, it might be related to the decrease in tank oil level as lifting oil is supplied when the RPM drops below 50%, and the decrease in oil temperature when stopped, but I'm not sure. The differential pressure I'm referring to is the difference between the internal pressure of the tank and the external pressure (atmospheric pressure).

 

@hohoim,

Without being able to see the P&IDs for the Lube Oil System and the Cooling & Sealing Air, and without knowing what frame size machine you are referring to, it's difficult to say what might be causing the difference in L.O. tank "pressures" during STARTing and shutdown.

Usually, during shutdown the L.O. tank vacuum is a little lower than during STARTing and normal operation. And, for some machines the L.O. tank vacuum decreases as the machine is loaded, but in my experience it should always be a very slight vacuum, even at Base Load.

Some machines with hydrogen-cooled generators have more "evacuation" fans to ensure no hydrogen collects in the nooks and crannies of the top ("ceiling") of the L.O. tank where it might cause a problem if there were ever a spark or excessive temperature to ignite the hydrogen and L.O. vapors.

But, again, without knowing more about the machine and without being able to view the P&IDs for the machine, it's difficult to say anything more for certain.

I will add that if this is a Frame 9E or 9F/FA machine then GE Belfort will probably have added some instrumentation to monitor L.O. tank pressure/vacuum. For decades there was not even a L.O. tank pressure/vacuum gauge (there probably should have been, but there wasn't) on GE-design heavy duty gas turbines. GE Belfort has unilaterally decided to add instrumentation to many machines now under their purview and in many cases they haven't provided any documentation to help with understanding the readings under various circumstances--because if it's obvious to them, well, it's obvious to everyone else, right? And, the French always have a better (in some cases--but not all!) way of doing things than what's worked well for decades for the majority of GE machines in service today. (I have seen this happen when the turbine control system is upgraded from older designs and includes the new GE Mark* HMIs--which can display a LOT of information, about which there is little or no documentation about how to interpret the readings. People start insisting this or that isn't working properly on machines they've been operating for years, or even decades, but didn't have access to the instrument readings via the operator interface.) Too much information without any documentation can lead to a lot of confusion and misunderstandings and even mis-operation.

And a lot of myths and falsehoods and misinformation.

 

@hohoim,

Without being able to see the P&IDs for the Lube Oil System and the Cooling & Sealing Air, and without knowing what frame size machine you are referring to, it's difficult to say what might be causing the difference in L.O. tank "pressures" during STARTing and shutdown.

Usually, during shutdown the L.O. tank vacuum is a little lower than during STARTing and normal operation. And, for some machines the L.O. tank vacuum decreases as the machine is loaded, but in my experience it should always be a very slight vacuum, even at Base Load.

Some machines with hydrogen-cooled generators have more "evacuation" fans to ensure no hydrogen collects in the nooks and crannies of the top ("ceiling") of the L.O. tank where it might cause a problem if there were ever a spark or excessive temperature to ignite the hydrogen and L.O. vapors.

But, again, without knowing more about the machine and without being able to view the P&IDs for the machine, it's difficult to say anything more for certain.

I will add that if this is a Frame 9E or 9F/FA machine then GE Belfort will probably have added some instrumentation to monitor L.O. tank pressure/vacuum. For decades there was not even a L.O. tank pressure/vacuum gauge (there probably should have been, but there wasn't) on GE-design heavy duty gas turbines. GE Belfort has unilaterally decided to add instrumentation to many machines now under their purview and in many cases they haven't provided any documentation to help with understanding the readings under various circumstances--because if it's obvious to them, well, it's obvious to everyone else, right? And, the French always have a better (in some cases--but not all!) way of doing things than what's worked well for decades for the majority of GE machines in service today. (I have seen this happen when the turbine control system is upgraded from older designs and includes the new GE Mark* HMIs--which can display a LOT of information, about which there is little or no documentation about how to interpret the readings. People start insisting this or that isn't working properly on machines they've been operating for years, or even decades, but didn't have access to the instrument readings via the operator interface.) Too much information without any documentation can lead to a lot of confusion and misunderstandings and even mis-operation.

And a lot of myths and falsehoods and misinformation.

The gas turbine type at our power plant is 7FA, and the L.O Tank vacuum pressure(D.P) cannot be seen on the CCR (HMI) and can only be confirmed by a local gauge on site. I was wondering why there is a difference in the L.O Tank differential pressure when it is normally started up and shut down. Thank you for your response!