Lube Oil System Aux pump delayed

[Ahmed Elzawawi]

I have problem to analyze in petrochemical ethylene plant ,the problem with ethylene compressor RT-10 , when the operator turn main oil pump off MOLP or the pump goes off for any reasons the aux. ALOP pump delayed to takeover Feeding oil to the bearings in other words to pressurized the line to 9.5 kg/cms ,which dramatically caused total S/D to the RT-10 compressor , although the pump in auto- mode and pressure switch PZA1059-L activated normally at 8.4 kg/cms, my question how that happened and how I can start to investigate TO OVERCOME THE PROBLEM.

 

[georgeverghese]

One way to give the aux LO pump time to build up pressure will be to install a gas filled LO accumulator (or bladder type accumulator) sized to run down its oil into the LO circuit long enough to ride out this switchover transient. It may be best to get the compressor manufacturer to get this done for you, or at some minimal risk, raise an in house engineering request. The latter may be quicker.

Is the main oil pump driven off the compressor shaft? So if this pump goes off, the compressor is also shutting down anyway?

 

[Ahmed Elzawawi]

hi georgeverghese

thank you for response ,
the main pump driving by steam turbine
Ahmed

 

[georgeverghese]

Well, in this case, a LO bladder accumulator or pressurised LO rundown tank is one option. Another option is to keep the ALOP running during normal operation also. Another option will be to check with electrical engineers if there is another type of electric motor that will accelerate to full speed in a much shorter time for the ALOP - the existing electric motor for this ALOP is probably not accelerating to full speed fast enough.

 

[Ahmed Elzawawi]

HI .. Thank you for swift reply I will check with electrical staff and reply to you.
Ahmed

 

[rotw]

Typically you need an accumulator (unless main pump is shaft driven) and this needs to be sized properly for the transition time to restore pressure via aux. pump. Looking at systems specified to API 614 for example, I know some companies by internal practices use 2 sec for sizing the accumulator but some others use much bigger values (which translates into bigger acumulator capacity). Motor startup time should not be an issue unless we talk relatively big size and/or limited power margin available. One possible cause could be degradation of performance of aux. pump or higher lube oil duty than the nominal/designed for duty, which ultimately could translate into higher startup torque requirement (say for a centrifugal pump). But to my opinion, this should not deviate "too much". Or your system design and sizing may have been stretched and with poor contingencies/margin. This is all speculative.
One thought: Can the pressure switch (PZA1059-L) made such that is is activated at higher pressure than 8.4 kg/cm2, this as first attempt mitigation?

 

[Ahmed Elzawawi]

to set activation of pressure SW at higher pressure than 8.4 kg/cm2 , that will made very close to 9.5 fg/cm2 normal pressure of MOP on other words diff between MOP & AOP Setting i.e 0.5 kg/cms

Ahmed

 

[LittleInch]

You could put a 5 second delay on the low lube oil pressure trip?? Surely a few seconds of low oil pressure wouldn't do any real harm?

A process trip doesn't do the compressor any good either so might be am option?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[don1980]

It sounds like the piping system isn't liquid-full which this switch is made. There could be air pockets forming in the ALOP piping system, or an air pocket is created immediately after the MLOP stops. If everything is liquid-full, you'll immediately be see the desired pressure when the ALOP is started. Go out in the field and investigate. See where the ALOP discharge line connects to the main line. Is there a vertical rise, above the elevation of the lube oil reservoir? Are there any high points above the elevation of the reservoir?

Another possibility is that the hydraulic bypass valve is opening when the ALOP starts, and it's slow to re-seat. This should be a truly modulating valve (e.g. Fulflo valve) that's specifically for lube oil skid service and not a conventional safety valve. A conventional PSV can cause this type of problem.

 

[rotw]

to set activation of pressure SW at higher pressure than 8.4 kg/cm2 , that will made very close to 9.5 fg/cm2 normal pressure of MOP on other words diff between MOP & AOP Setting i.e 0.5 kg/cms

Set it then at 8.9 kg/cm2, not big change but may buy you one or two seconds for auxiliary pump to come online. Maybe you could combine this with LittleInch's suggestion, all in all incremental improvements (and not very costly) ...