3 replies to this topic

[A mukherjee]

Hi all,
We have a refrigeration unit in our air separation plant.The plant is of vapor compression type where air is cooled by R-22a refrigerant.There are 3 local gauges for measurement of oil pressure,suction pressure & discharge pressure (of reciprocating compressor) respectively.
Now I would like to know:
1.Why the oil DP is measured by subtracting the suction pressure from oil pressure shown at local gauge board?
2.Why trip is required at low oil DP?
3.What is the danger of operating compressor at low suction pressure(around 0.5 kg/cm2(g))?
Regards,
Arpan

[Art Montemayor]

Arpan:

The queries you bring out should all be fully explained and detailed in your compressor’s operating manual – which should be part of all the manuals issued with your installation of the air separation plant.

Have you lost these manuals, or have you purchased a used unit without any operating instructions? Nevertheless, all reciprocating compressors operate with the same basic oil lubricating designs within their power frames (crank ends).

You state there are “3 local gauges for measurement of oil pressure, suction pressure & discharge pressure”. I know what suction pressure means. I also know what discharge pressure means. But I don’t know what you mean by the generic term, “oil pressure”. If there are really 3 oil measurements, where is the third one being taken at? Normally there are only two (2) oil measurements – suction and discharge.

• The oil DP is measured by subtracting the suction pressure from oil pressure shown at the local gauge board because THAT IS THE DEFINITION OF DP (DELTA PRESSURE – PRESSURE DIFFERENCE). This difference is the DRIVING FORCE existing for there to be positive oil flow from the oil pump’s discharge port, through the oil filter, the crankshaft, the crankshaft main bearings, the connecting rod bearings, the connecting rod, the crosshead, and other vital lubrication sites within the power frame (or crankcase) of your refrigeration compressor.
• If a low delta pressure is detected, it must mean that the oil pressure discharged by the pump is very low. This is basic since we have to presume that all the above restrictions and resistances detailed in #1 above are essentially constant. If the oil’s driving force is low, then that means that the oil flow is also low. This spells mechanical trouble – a lot of potential mechanical trouble! If this happens, alarms and bells should go off immediately and the machine should be stopped and the cause investigated and resolved.
• The design pressure drop for the oil system should be clearly spelled out in your Operating and Maintenance Manuals. You should know this by heart. If you are continuing to operate below the prescribed minimum oil pressure drop, then you run the risk of what I describe in #2.

I hope this personal field and operating experience helps you out.

[A mukherjee]

Thnxx a lot ART for ur detailed reply.

But I just have a little doubt,
Why to measure the oil DP the oil pressure developed by oil pump has to be subtracted from suction pressure of refrigerant?
Is suction pressure of refrigerant is equal to the final pressure reached by oil after all the pressure drop?
Regards,
Arpan

[Art Montemayor]

Arpan:

Why is it that you don't reply to MY questions?

If you resort to finding and reading ALL the literature and manuals that the fabricator of your compressor has furnished you or your company, you will discover that the oil that is pumped by your oil pump is part of the oil inventory kept in the refrigeration system. The bulk of this oil inventory is to be found in the crankcase sump of the compressor, where it is introduced into the suction of the oil pump. All this oil is pumped to the different oil "targets" I previously mentioned and the oil is returned back to the sump of the crankcase.

If you study and understand this basic lubrication system, you will then understand why the pressure difference between the discharge and suction of the oil pump is an indication of positive oil flow to the main bearing surface targets that are important lubrication sites in the compressor.

Please read your compressor manual. This is an important and prerequisite job requirement in order to operate your machinery in an efficient and safe manner.

I hope this explanation is understood.