Pressure during transfer between 2 pumps 1

[Andy22]

We have lubrication oil system that depends on continuous pressure to protect a pump and turbine system with jounal bearings. When power to one pump is lost a second pump turns on immediately but we have found that the system still trips on low bearing oil pressure. Are there any guidelines anywhere about the output of a pump while it is coasting to a stop? I am talking about the 1 sec or so from the time that the first pump loses power to the time that the second pump starts and speeds up enough to build-up pressure again. Is the loss of pressure almost instanteous or is there a short period of time where the coasting impeller still generates some flow/pressure?

 

[BigInch]

Theoretically there is still some pressure generated, until the impeller slows to around 50% or so of running rpm, but with system resistance slowdown can be very fast, even for 1000 HP pumps with relatively massive impellers, so it would be reasonable to expect a very fast loss of pressure for a small lube oil pump, maybe 1/4 second or less.

Going the Big Inch!

http://virtualpipeline.spaces.msn.com

 

[Yorkman]

I would be think there is some type of lubrication resevour to protect the journel bearings from damage in the event of power failure, to allow the system to coast down. Would a simple "oil pressure safety" switch be a solution? Say a 3 second delay to allow the backup lube system to get established. We use a similiar safty device on a 4000 ton centrifugal compressor with journal type bearings. Granted we're only doing 6000 - 8000 RPM

I'm not a real engineer, but I play one on T.V.
A.J. Gest, York Int./JCI

 

[Artisi]

Seems you need some type of pressure accumulator to account for any momentary upsets during the change over phase during the start-up of the standby unit. I would ask the hydraulic control system suppliers for assistance.

Naresuan University
Phitsanulok
Thailand

 

[MikeHalloran]

What prevents backflow through the unpowered impeller?



Mike Halloran
Pembroke Pines, FL, USA

 

[bingopin]

As suggested, you could use either a very short time delay in the trip and/or a pressurized accumulator. In the rare cases where you run with only one pump and suffer a failure, the accumulator, if large enough, would get you down safely. We have lube oil acumulators on some large centifugal compressors.

 

[JJPellin]

A standard API requirement for a lube oil skid on a critical machine would include an accumulator sized to provided the needed flow and pressure to the bearings during a trip/auto-start scenario. The system must be stable or else the protection of the auto-start is useless. We test our systems when the compressor is down. You should be able to trip the main pump, allow the auto-start of the standby pump and not reach any trip point for low oil pressure. You should be able to start up the second lube oil pump with the main pump still running (false auto-start) with no alarms or trips. You should be able to switch coolers or filters, slowly or quickly and get no alarms (assuming they are properly vented and pressurized before switching). If the system cannot pass this test, then additional engineering is needed for the pressure controls, accumulators, PSV protection in order to make it stable through all of these events. An accumulator manufacture can help you determine the proper size and pre-charge for an accumulator on your oil system.

 

[JJPellin]

You also need to examine the set-points for your instrumentation. A typical system in our plant might run with 15 psi normal oil pressure to the bearings. They get a low oil pressure alarm at 12 psi, decreasing which also auto-starts the standby pump. The compressor trips off at 9 psi, decreasing. Make sure that there is a realistic gap between the auto-start and trip switch set-points.