auxillary lube oil pump not started!!!

[mahmmoud30]

hi all
just before 2 days our booster compressor (2stages reciprocating compressor from neuman&Esser)was shutdown du to low low lube oil pressure signal,when we check, we found that auxillary oil pump has autostarted when lube oil pressure was low only but it run for 1 second only then stopped which trigger the low low lube oil pressure signal and shutdown the compressor. After that we tested the compressor again and the start sequance interupted because main oil pump can't maintain the desired lube oil pressure, we dismantle the main pump and found its link with the cranck shaft was damaged. My question why the auxillary pump run only for 1 sec then stop? has anyone see a cause like this?

 

[JJPellin]

I have seen this on two new machines. In both cases, it was found during testing before start-up. The case was an error with the set-up of the auto-start switch. These switches can be set up as latching or not latching. If the switch is set up as not latching, when the auxiliary oil pump starts up, it will increase the oil system pressure above the set-point and the switch will shut the pump back down. In some cases, the aux pump will start - stop - start - stop until the motor keeps it out. By resetting the switch to latching mode, it will keep the aux pump running until someone moves the hand-off-auto switch from auto to off.

If this is not the problem, then I would wonder if the motor driver on the aux pump is properly sized. Starting up on cold oil (I work in Minnesota) can overload the driver and cause it to trip off. But, if the motor tripped on overload, this should have been obvious since they would have had to reset the breaker to restart.


Johnny Pellin

 

[mahmmoud30]

thanks JJPellin for this rich response. we are in a very hot country and this case happed at midnoon.
we have local hand-off-auto swith near the aux pump which was in auto mode, is it the same as the auto-start switch that can be set up as latching or not latching or you mean something else? thanks again

 

[JJPellin]

I am not referring to the hand-off-auto switch. I am referring to the instrument switch that sends the signal to auto-start the aux pump. In many systems this is a simple pressure switch that causes a relay to close that starts the aux pump. In other more sophisticated installations, there may be a pressure transmitter rather than a switch. The signal from the transmitter could go to a controller or to a local PLC or DCS control system. Someone in your instrument craft will need to check to see if the pressure instrument that is used to auto-start the aux pump is configured as with a latching relay or not. I am not knowledgeable about electrical systems. But, I believe this is accomplished by wiring the output of the relay back to the connection that energizes the relay. That way, once energized, the relay will lock in and won't re-open if the original signal is lost. But, there are many, many ways to wire a control system like this.

Johnny Pellin

 

[mahmmoud30]

thanks again JJPellin, I'll be back to you after checking with instrument guys.