Oil color problem

Dear friends

one of my customers complained that one of his walk-in coolers is abnormal , when checking oil in sight glass it looked in grey color , so i took a sample of oil, it seemed to be contaminated with bearing or connecting rod material , i disassembled the compressor and checked i found every thing is normal,i cleaned all parts then assembled again , charged new oil and operated again but after 2 hours of operation the oil color again turned into same color , after repeating oil replacement for 5 times and operating compressor for same period of time the situation didn't change , i noticed that liquid is flooding back to compressor , i appreciate you help to find the possible causes of this phenomena , thanks in advance
helpful information:
compressor type : Bitzer 18hp
refrigerant : R404A
oil type : POE

Does it have an oil separator bad oil could be in there,grey oil points to metallic issues,did you change the filter drier I have seen them break down,what is the screen like in the expansion valve?

i have replaced the filter twice in 1st time the filter was not so bad , in 2nd time it was normal , the system has no oil seperator
thanks for your response

Hi khaled
2 questions
1) How old is the system .?
2) Has there been any new work on the system i.e. Pipe work ,brazing etc ?
Having carried out many warranty strip downs on compressors and also investigated many system problems ,some of the following may help you find the cause of your oil problem.

With the change from the old ***** gases to the new gases such as R404A,R134A, R407C etc meant also changing the type of lubricant from mineral oil to Polyoester oil.
Due to these "new" refrigerants having different operating pressure/temperature relationships from the old ones many compressor manufacturers use POE lubes with "additives" to enhance their lubricating qualities to overcome the various extra stresses compressors have to cope with.
These additives take some time to attach and bed themselves into the granular structure of the bearing surfaces and wearing parts of the compressor.
Unfortunately these "good" additives can be stripped from the POE oil by the desiccant in some driers before they can be adsorbed into the bearing surfaces into the machine. This leaves the oil somewhat less able to lubricate the machine safely.
It seems a combination of events can create this oil discolouring problem and severe bearing wear.
Such issues include:-
Poor installation practises i.e. Lack of ofn during brazing of the pipe work, use of to much brazing flux material, leaving the compressor open to atmosphere for long periods ( POE oil is highly hydroscopic ) adsorbs moisture.
Poor evacuation
Poor commissioning of the system ( poor condensing pressure control , poor superheat( expansion valve ) control
Most service filter driers have a combination desiccant type made from silica gel, molecular sieve and activated oxides in various percentages as deemed relevant for the job in hand.
Silica gel and molecular sieve are good moisture adsorbents whereas activated oxides are good for acid adsorbtion.
With a new plant there is No acid present as its a by product of the refrigerant,oil heat,electrical arcing etc and this comes after many run hours , so why have a drier that can absorb acid when there is none.
So in a new system you fit a filter drier to dry the system down to a level that the initial evacuation process cannot achieve. The best drier to do this will have a pure 100% molecular sieve desiccant .
By fitting this type of drier the additives are left intact to do their important task of protecting the machine.
Driers with silica gel and active oxides can and do absorb these additives ,so after the system has been operating for some time these multi desiccant driers can then be used as the additives will have been ingrained into the bearings etc and some weak acids will now be present in the system where the activated oxides desiccant comes into play.
POE oil and these new gases tend to clean the system by scouring flux and system debris back to the compressor sump.
So if any of the above issues occur and the oil has been degraded then the oil starts to discolour as metallic bearing particles slowly sludge to produce a grinding paste like material in the compressor sump

So fit some pure molecualar sieve driers and replace with fresh oil and sort out the liquid flooding problem and the problem should go away

Polyoester lubricant is made from acid and alcohol with the moisture removed in the process. So if the system evacuation process is not carried out correctly or the system is left open to the atmosphere for long periods during install and moisture is absorbed into the compressor lube charge , then this moisture will turn the polyoester back to its base product ie ACID
Acid in a dry system will have no detrimental effect on the running of the system, But the introduction of moisture into the system will reactivate the acid back into a corrosive substance that will ruin the lubrication and eat the motor insulation and scour the soft copper pipe work
So by fitting a pure molecular sieve drier in a new system , the drier will take the moisture level in the system down to a much lower level than the vacuum pump can , and ideally after running the new plant for a couple of days by replacing the drier you remove any captured moisture and debris and the new drier will give enhanced protection to the plant for many years. This can save a fortune in warranty costs

Dear Glenn
Thanks for your reply, I would like to inform you that the unit is 10 years old ,it was running normally for all that period, the problem began when the owner called somebody to repair a leak of refrigerant at the expansion valve distributor , after this guy finished the job and operated the unit the problem began, as I have mentioned in my thread , when checking the brazing job I found it very bad ,the owner told me that the brazing has taken long time of continuous heating , so I replaced the power element of the exp. valve , other details of the job are shown in the thread , I appreciate your efforts to find me the causes of this case and the suggested solutions , also I like to know why the refrigerant is flooding back to compressor.
So many thanks
Khaled Husain

Dear khaled
It's possible moisture has been drawn into the system via the leaking distributor. The oil discolouration has most likely occurred due to this ,plus due to the long brazing process ,flux, oxides etc, which have been washed back into the compressor sump.
Replacing the oil and fitting good quality filter driers should flush the system.
Regarding the flooding back issue ,you say you replaced the TEV power assembly ,but what about the orifice,as the seat may have become "wire drawn" due to dirt or flash gas when the unit had been running short of gas due to the leak.
Suggest fitting a replacement orifice and re setting the orifice spring to allow the TEV to operate with a reasonable superheat around 5k ,also check the tev bulb is fitted tightly to the suction line making a good contact, with a light wrap of insulation material over the bulb to prevent surrounding air movement affecting the bulb function. If possible a couple of pictures of the expansion valve and distributor and position of the bulb and equalising line will help us to advise you, also details of the TEV installed
.All the best Glenn