Bearing Sealing Air Pressure

  • Thread starter Oluwafemi Ogundaisi
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Thread Starter

Oluwafemi Ogundaisi

What is the Bearing Sealing Air Pressure Value for a Gas Turbine Frame 6B? Either expected or Actual Value to achieve a proper (ideal) sealing on the bearing(s) and if there is any supportive mathematical calculations involved please.
It varies depending on vintage of machine and type of combustor, as well as with IGV angle and compressor discharge pressure.

Unfortunately, GE doesn't publish anything about this, and if the unit is equipped with a Bearing Sealing Air Valve and low pressure switch, there is only the pressure switch setting to judge when the pressure is actually low enough to alarm.

And, I don't recall ever seeing a Bearing Sealing Air pressure gauge, either, provided by GE or one of their turbine packagers.
Dear Oluwafemi,

I do not know if there is an optimal or ideal pressure for bearing sealing air on your Frame 6B. I will give you my experience on the 7EA that I maintain.

The general idea I think is to provide sealing and cooling air for the bearing seals to prevent leaks. The bearing seal air valve has source air from either the 5th stage of the compressor, or the 11th stage. When the unit is started the sealing air valve is in a position to extract air from the 11 stage of the compressor, because the pressure from the 5th stage is very low. The bearing seal air valve actuator is fed air from the 5th stage, such that as the unit is loaded and 5th stage pressure rises to approximately 5 psi, the sealing air valve will shift and begin supplying air to the bearings from the 5th stage. This pressure will rise as the unit is loaded. The valve will stay in the position supplying bearing seal air from the 5th stage until the unit is unloaded and 5th stage pressure begins to fall below 5psi.

There is typically always a switch to indicate the position of the bearing seal air valve. It is there to generate an alarm if the valve is not in the correct or intended position. If the valve is stuck in the position of the 5th stage then bearing seal air pressure will be too low during startup or low load. If the valve is stuck in the 11th stage position then bearing seal air pressure and temperature with be too high at higher load conditions.

I have noted that the alarm that signals the valve out of position, comes in at times as a nuisance. The logic for valve out of position alarm is driven by the digital input of valve position, and logic that looks at CDP pressure. Since 5th stage pressure and CDP pressure can vary slightly based on ambient conditions and compressor efficiency the logic values sometimes indicate the valve out of position, even when it is correct.

I hope this is helpful, if you have a more direct question or problem you are experiencing please provide further information.