Propane compressor troubleshooting

We have a natural gas refrigeration system with propane in our company.
Two oil flooded MYCOM screw compressors 6oo kW each, one operating and one stand-by.
When natural gas flow increases and thus the load on the refrigerating system, the oil temperature starts decreasing and after a while the compressor shuts down due to low oil pressure. The oil is a synthetic brand produced by CPI Engineering Services Inc. and delivered by the packager as the first charge. There are some doubts that the oil/propane separator does not perform well, or that the oil itself is not appropriate, but no final proof has been found.:mad:

Hi, dear ppersic :)

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Originally Posted by ppersic

We have a natural gas refrigeration system with propane in our company.
Two oil flooded MYCOM screw compressors 6oo kW each, one operating and one stand-by.
When natural gas flow increases and thus the load on the refrigerating system, the oil temperature starts decreasing and after a while the compressor shuts down due to low oil pressure. The oil is a synthetic brand produced by CPI Engineering Services Inc. and delivered by the packager as the first charge. There are some doubts that the oil/propane separator does not perform well, or that the oil itself is not appropriate, but no final proof has been found.:mad:

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.... what happen when compressor is running at min capacity .... oil is at normal temperature and oil pressure is OK .... what is discharge gas temperature ....

type of oil cooler? type of propane condenser?

which kind of control for compressor you have ... Mypro or something else ... do you have some history of your problem ...

sorry so many questions, but that is necessary ...;)


btw this message should be in Forum: Industrial - and Sub-forums: industrial compressors or hydrocarbons ...

Best regards, Josip :)

Oil temp must be higher than saturated condensing pressure otherwise oil dilution and foaming will definately occur .

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Originally Posted by Josip

Hi, dear ppersic :)




.... what happen when compressor is running at min capacity .... oil is at normal temperature and oil pressure is OK .... what is discharge gas temperature ....

type of oil cooler? type of propane condenser?

which kind of control for compressor you have ... Mypro or something else ... do you have some history of your problem ...

sorry so many questions, but that is necessary ...;)


btw this message should be in Forum: Industrial - and Sub-forums: industrial compressors or hydrocarbons ...

Best regards, Josip :)

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Thanks for the input,
The oil cooler is plate type with a branch of propane flow used as the coolant. Three various control valves are involved on oil and propane side to control oil cooling (quite a sophisticated feature indeed).
The condenser is air cooled, finned pipe bundle type. This is a PLC controlled process which includes Three Ethylene Glycol (TEG) dehydration unit upstream, i.e. dehydration and refrigeration unit are thermodynamically related. There is a possibility that this dehydration process upstream has some thermodynamic influence as well.
I mean, natural gas should enter the refrigeration unit quite dry already with water being absorbed by TEG.

Regards,

Predrag :D

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Originally Posted by RANGER1

Oil temp must be higher than saturated condensing pressure otherwise oil dilution and foaming will definately occur .

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Thanks for the input.
That's exactly what occurs when the natural gas flow increases above certain limit.
The gas flow is thus approximately 40% below the design flow what is desastrous regarding the production output, and that happens at quite convenient ambient conditions. In summertime we could expect 45C amb. temperature what will make the life even more difficult.
Yes, we know basically that the symptom is oil dilution, but what is the root cause ? I mean, which part of the thermodynamic circuit could cause all that ? Wrongly sized equipment, or some control element failure ? As I said, there is a TEG dehydration unit upstream which is part of the same process.

Regards,

Predrag :D

As you said that there are e control valves for cooling oil through plate heat exchanger. how control valve are being operated ? From where they are getting the signals for closing and opening ?? One of the control valves can be faulty because oil is over cooled due to more flow of liquid propane on HEX side.what is the oil grade ?? may be CPI -1516-150 or what ?? You can get the dilution curve from the mycom or supplier. presently compressor is running at part load operations (The gas flow is thus approximately 40% below the design flow what is desastrous regarding the production output) therfore discharge temperature/ oil temperature can be high and as you increase the load, the temperature goes down. Dilution can be one of the problems but we have already used above oil grade succesfully for different propane refrigerant application. . did u employ two pumps ( 1w+1S) and a by pass valve controled by Oil / gas differential pressure?? seperator mesh pad can be check for any kind of entrainments because that will lower the efficiencies of seperation.Is oil level building in chillers also?? because that can give an idea about oil seperator efficency. how many secondary separators....are employed??

Is it a new sysyem that has worked and now problem ?

Is oil in oil seperator heated before start up , say 45 deg C ?

Does standby unit have all its own oil cooler , oil seperator etc , does it work OK ?

Generally propane (R290) does not require much oil cooling. You need to go back to Mycom or the selling distributor and have them provide you with a detailed compressor rum with data including discharge temperature, oil temperature and and oil cooler heat of rejection in 10% increments from 100% load to 10% load. You may find that the oil cooling is not properly designed for the system.

As a test, valve off the propane flow to the plate oil cooler and watch the oil temp and discharge temperature.

Also look at the design data at reduced condensing pressures.

Also, is the evaporator a flooded design and you are running a saturated suction to the compressor? It must be dry and no carry over from the evaporator. Just a little bit of carry over will kill a propane screw.

Where is you liquid level for the evaporator during these problems. You only need to have the level controller set to control about 2 rows below the top of the exchanger bundle. The splashing from the boiling action will keep the top rows wetted during heavy loads. During light loads, the surface area is not required.

Since you are air cooled, you need to hold the condensing temperature a minimum of 5dC above the oil temperature to prevent condensing of refrigerant in the oil system.

Ken

Since you are air cooled, you need to hold the condensing temperature a minimum of 5dC above the oil temperature to prevent condensing of refrigerant in the oil system.

Ken[/quote]


As per my experience , Oil temperature shall be at least 15 deg c above the condensing temperature.Please confirm