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
