I have a 4,000 hp motor driving a generator through a variable speed fluid coupling. There is 18 psi of oil pressure supplied to the 4 sleeve bearings for the motor and generator. There are sight glasses on the bearing drains. all 4 bearings are supplied by the same feed line and all the bearing drains go into a common header that drains into the fluid coupling (the oil reservoir) The vent fan on the fluid coupling is working, we do not see andy pressure fluctuation in the oil supply. Every 10 minutes or so the oil draining from the generator outboard bearing stops for 1-2 seconds and then starts again. Has anyone seen this before or have an idea what is causing it?

thank you

Joe Durushia
Monticello Nuclear Plant.

Joe,

Do the shaft vibration levels change coincidently with the oil stop? I have not seen or heard of the oil flow stop, so I am trying to understand it while you live with it.

Walt

I have never seen that. I will talk through my thoughts on the subject.

If it is like machines I am familiar with, their is a reservoir below each bearing (oil ring grabs from the reservoir and pulls up onto the bearing).

You have a continuous feed into the reservoir from your oil supply through an orifice.

The oil drains from the reservoir when it overflows a dam.

In steady state the quantity into the reservoir should equal quantity overflowing the reservoir dam into the drain.

Now why would it stop draining? A few ideas (in order most likely to least likely):

1 - Maybe the oil rings are moving erratically. They may spin in an area that picks up little oil. Then when they move into an area that picks up a lot of oil there will be more splashed around the bearing and the level will go down (until fills up again). Look at oil rings through viewer to see if motion is erratic.

2 - Maybe supply pressure is oscillating. Look at oil supply pressure gage.

3 - Does the quantity draining appear very small? If so it could be a matter of surface tension or other small effects enough to disrupt the overflow.

There is often an oil level indicator in the reserovir (should never change... always at the overflow point). What does it show?

If bearing metal temperatures remain normal and level is normal I personally wouldn't be concerned but would investigate it a little to try to understand it like you are doing.