Hi we have 7FB gas turbine on our site

But We have issues with 88QA lube oil pump

We have two lube oil pumps

When we change 88QA-1A to 88QA-1B

lube oil pump

Lube oil pressure switch low(L63QA1AL)

Happens for 3 seconds

So we test lube oil pressure switch

But it's working properly at rated lube oil pressure

Have you ever seen this event when lube

oil Pump changes

I need your help

 

Use the L.O. system P&ID to look at the piping and components. There should be check valves on each pump to prevent flowing backwards through the pump(s). It sounds like one or both of the check valves is not working properly.

ALSO, check all of the filter fill/transfer and drain valves to make sure they are all in the proper positions.

 

Our mechanical team replaced new check valve but the problem isnt solved
Should we check the drain valves?

 

I don't have any P&IDS to review. You are going to have to review the P&IDs and look at all possible components. Usually there is a foot valve at the submerged suction of each of the Aux. L.O. pumps. There is usually a small hole drilled in the discharge check valve to allow a small flow of oil to enter the pump and flow down to the foot valve, to keep the suction line full in anticipation of quick service. This is another possibility, and if the new check valve doesn't have the small orifice it may be allowing the suction line to drain and causing a delay in establishing pressure. Without being able to see the P&ID for the system at your site it's not possible to say exactly what the configuration is, but you can and you need to use that drawing to make a plan and work through the various possibilities. This is called a process of elimination, checking various components to see if they are working properly and moving to the next possibility/component. Proper testing/checking is key to this, as is keeping good written record of tests AND responses. When the problem isn't obvious this is the best way of troubleshooting, the only way. It can be slow going, but by making educated guesses--and keeping good records of test/results--this will eventually result in resolution of the problem.

Best of luck! Please let us know what you find.

 

Did this happen also when you switch from 88QB to 88QA?

 

Yes. On all the units I worked on with 88QA-1A and 88QA-1B they were interchangeable--have the same operating philosophy. To have a "bumpless" transfer, it's necessary to have a similar configuration. It doesn't--shouldn't--have any difference when switching from one redundant pump to another redundant pump; the pressure should NOT drop when transferring redundant pumps.

You can--should--read the L.O. System tab of the Operations & Maintenance Manuals to see what it says about the system and operation.

This all should have been tested during commissioning.

The L.O. P&ID should be helpful.

 

Of course, switching pump must be bumpless

I'm asking the author, to help diagnose his problem

the meaning of this question is to help focusing on part that fails

for example few years ago, we have pressure drop when switching from pump a to b , but didn't occur from b to a.
It was pump a discharge check valve that gets stuck open

but it also could be check valve b getting stuck close

but it can't be common line failure

 

Not at all. it always happened switching from a to b

 

How can you solve the problem?

You just replaced the discharge check

valve which is located on pump b ?

 

We have waited an long outage, when shaft line was cold enough to stop turning gear and lube oil system.
So that there is no risk to conduct test

then we tested change over, with pump discharge a close, and with pump discharge b close.
with pump discharge a close there was no pressure drop when switching on pump b

(when change over from a to b, pressure drop and control start both pump)

As the shaftline was cold we could stop lube oil system and disassemble the check valve

to keep unit available, we immediatly change it for a new one
And the faulty check valve was refitted later in our workshop

 

I hope you're saying the problem has been solved.?.?.?

Because it's unclear (to me) how the test was conducted, what the results were, and if the problem was resolved.

 

problem has been solved

field technicians statement was: when switching from lube oil pump A to lube oil pump B, pump B start, then pump A stop, lube oil pressure drop, and then pump A automaticly start (at this point both pump are running)

we thought about 3 possibilities:
check valve A stuck open, check valve B stuck close, pump B impeller has broke down (very unlikely)

we waited an outage where the unit was stopped long enough, that the shaft line was cold
because we didn't want to conduct test on lube oil system if turning gear and lube oil are still required (wheel temp and bearing temp)

so when both pump are running we close the pump A discharge valve, and ask to run on pump B.
When pump A stop there was no pressure drop.
wich confirm that there was an issue with check valve A

 

Thanks for the clarification!

Happy to hear you were able to solve the problem.

P&IDs are a wonderful thing, aren't they?

 

yes sir !

PID are crucial for operation, LOTO, and maintenance

 

We just changed the period when two pumps are running. So we solved the problem i really appreciate your help

 

Thank you for the feedback.