power turbine lubrication system failure during start up 1

[rotw]

Hello,

On one plant, we have a GE aero derivative gas turbine driving a generator. On the lubrication system of the gas turbine (power turbine) the oil pumps are failing to deliver the required supply pressure to the turbine during start up and system is not stable.
The relief valves at the discharge of the oil pumps with return to reservoir do not have a by pass. Could this be the reason that the system is not able to ensure the needed pressure level for supply of oil during turbine start up ?
What kind of functional checks can be done to troubleshoot this problem?

Thank you for any help.

 

[MikeHalloran]

Uh, read the pressure gages once in a while?
Uh, check the dipstick on the oil tank?

Consider the possibility that the lube oil pumps are worn out, and not capable of maintaining the required pressure.
I'd expect the turbine controls to have sensors for lube oil pressure, that would be arranged to shut down the turbine when the pressure falls off.
... which could also happen if there's not enough oil in the supply tank; i.e., start the pump, draw down the level, and the pressure falls off, etc.
How does the lube oil get to the tank once it's gone through the turbine bearings? Is there a pump that could be off, or a filter that could be clogged?





Mike Halloran
Pembroke Pines, FL, USA

 

[byrdj]

I have not seen a GT so I am not intimate with your oil system. Just from the comment of "relief valves" I'll assume the oil pumps are positive displacement driven my electrical motor. the reliefs would then be set to dump the excessive oil volume when on turning gear while limiting the pressure. It could be the reliefs are stuck or failed and as oil volume needed increases as the rotor speed increases the reliefs are not closing back to maintain pressure.

as Mike H mentioned I have also seen a low level go lower as the volume of oil needed increases (oil in transient in the drains) and the suctions become exposed.

the "funniest" problem I had was contiminated oil resulted in foaming during rolling and "liquid" oil level dropped below suction

could you provide your control diagram showing lubrication?

 

[racookpe1978]

What has YOUR GE turbine representative said?
How long has the problem been occurring?
Once running, what are lube oil levels?


TALK TO YOUR GE REP!!!!

 

[rotw]

Mike,
"I'd expect the turbine controls to have sensors for lube oil pressure, that would be arranged to shut down the turbine when the pressure falls off." correct.
I will check the return line to reservoir as you suggested.

I need to look for a control diagram.

 

[MikeHalloran]

I think racookpe1978 is correct; your first call should be to GE.


Mike Halloran
Pembroke Pines, FL, USA

 

[ccfowler]

It wouldn't hurt to verify the actual condition and composition of the lube oil. Contamination, non-conforming make-up additions, etc. may result in property changes just enough to cause or magnify troubles while the oil appears to be in good condition.

Valuable advice from a professor many years ago: First, design for graceful failure. Everything we build will eventually fail, so we must strive to avoid injuries or secondary damage when that failure occurs. Only then can practicality and economics be properly considered.

 

[JJPellin]

I have no experience with Gas Turbines, but I believe that the oil systems are probably very similar to the systems we used on our large steam turbines. A number of possibilities occurred to me:

• The relief valves are most likely full-flow valves. These can be built with cuts in the seats to allow a certain amount of flow to keep the lines flooded and warm. If your system was designed with valves that do not have cut seats, and valves with cut seats have been installed, this would reduce the available capacity.
• I assume you have already tested the relief valves. If they are full-flow valves that have to be tested and adjusted in service on the actual oil system where they will operate. These should be tested and adjusted with the same oil they will see in service and at normal operating temperature. If they were adjusted with cold oil, they may pass more oil at full temperature.
• I assume that the pressure control for the system uses a combination of spill-back control valves and in-line pressure control valves. They valves should be checked to make sure that they are operating correctly and they the pressure references used to control those valve are properly located.
• The spill-back pressure control valve probably has a bypass. That bypass could be partially open or leaking by.
• On initial start-up, there may be additional oil demand as lines are filled, overhead lube oil run-down tanks are filled and automatic shut-down valves are pressurized and actuated. It might be possible to run both lube oil pumps during this transition to provide additional oil until normal, steady-state conditions are reached.
• The lube oil pumps probably have basket strainers in the suction lines. They should be checked and cleaned.
• There is probably a pressure equalization line between the two oil filters and oil coolers. Before start-up, all of the spare filters and coolers should be flooded, vented and pressurized. Otherwise, a small stream of oil would be diverted through this equalization line.
• The main lube oil filters could be restricting the flow if there has been any change made such as going to finer filters elements to improve filtration.
• Some of the systems use a “kidney loop” filter which diverts a small stream of oil and passes it through a much finer filter and then dumps it back into the reservoir. If this filter was blown out or assembled incorrectly, it could be diverting a stream of oil back to the tank.
• In addition the full-flow relief valves, some systems also have traditional PSV (pop valves). These could be leaking by.
• If the tank is equipped with a breather vent or flame arrestor, it could be plugged up. As the system draws down the tank level to fill all of the needed lines and equipment, the tank needs to draw in air. If the breathers or other vents were plugged up, this could result in loss of pump performance as the tank developed a vacuum with the drop in oil level.
• As noted above, the lube oil pumps could be worn out. But, this is very unusual in clean lube oil service. If any work has been done to the pumps, they could be assembled incorrectly or otherwise defective.

Without a system diagram, it is difficult to be more specific. Some time has passed since your original post. Have you determined the problem yet?


Johnny Pellin

 

[chief]

Check the rotation direction of your oil pumps and motor amps as a preliminary, before the obvious suction and discharge strainers and possible jammed relief valves

Offshore Engineering&Design

 

[NCTHAI]

For GE supplied frame machines, there are 2 valves in series with the pump and in outlet. They are for air release and for pressure control - separate devices. Please check if any of these devices are leaking. Another possibility is leak in pipe - quite difficult to investigate as all piping is inside tank. Simple way to check is - your pump discharge pressure should be higher than your bearing header pressure. If Pump discharge pressure > Header pressure, then you need to look somewhere in header for leak. If not, before the header and it can be anything - from pump suction to piping up to discharge.