View Full Version : oil pressure

aaron crimmins
06-04-2002, 01:57 AM
can someone please explain to me how a oil pump on a compresser actually pumps oil? also would liquid refrigerant slugging cause a loss of oil pressure? i know it washes the oil off of bearings and other parts but would it trip on oil pressure saftey?

Mike Hopkins
06-04-2002, 03:42 AM
You've opened yourself up to some questions. What kind of compressor will ultimately dictate the type of oil pump used.
Some compressors have dedicated positive displacement pumps,
some use pressure differential and have no pump at all, some use a vortex type principal to deliver the oil to the bearings.
Liquid slugging would cause the oil pressure to drop and resulting oil failure trip, but maybe not before other bad things happen, like valve and or piston and rod damage (recip comp).
Continuos floodback would be the more appropriate culprit. A good check would be to measure the compressor oil sump temp.
Should be warm or at least 20-30*F warmer than compressor refrigerant entering temp. Lower temps. will indicate floodback.

Mike Hopkins

10-04-2002, 12:01 PM
Just to add to the very correct and concise answers from Mike and Marc, it is not allways liquid slugging that can cause the refrigerant induced oil failure problems but also a build up of liquid in the crankcase due to condensation of the refrigerant when the compressor (recip) stands for a period of time in a cold enviroment without crankcase heaters. This can cause oil failure trips without the "normal" signs of slugging (excessive valve noise etc) but will show a cold crankcase. Just to add to the oil pressure relief comment, a number of oil pumps have pressure reliefs in the pump as well as they run on two different pressures, one pressure for the unloaders and another (lower pressure) for the lubrication, the Grasso is a case in point.

Dale Robberts

aaron crimmins
10-04-2002, 09:53 PM
thanks for the replies. this was a carlyle six piston compressor 06d with two unloaders. i was running a 5psi net oil pressure after adjusting the txv's to keep from flooding back. oil level, crank case heater, were all good. i had the unloaders running fully loaded, so we changed the oil pump and that didn't work, after looking at the cut-out drawing of this compressor i think the oil pressure regulator is bad, any thoughts? by the way this is a brand new compressor we just installed but it hasn't ran for more than 10min. the old compressor had a busted crank from flooding back.

11-04-2002, 08:18 PM
Aaron, what about the oil, would there be dirt in it blocking the oil pump suction strainer, and is this the correct oil for the aplication?
As for the op relief I have seen bitzers break the spring in this causing as you describe, but not after 10mins, has this thing pumped liquid?
Regards. Andy.

aaron crimmins
12-04-2002, 01:47 AM
okay, carlyle sent us a new compressor and we installed it today. first we ran it without switcing the heads ,valve plates, etc. and it ran fine so we put on the other heads with the two suction cut off unloaders and we had 5psi of oil pressue again!! so we put back on the other heads without the unloaders and we had 0psi oil pressure!!! I down loaded the carlyle service guide and everything we are doing looks correct. we need 12-18 psi oil pressure, saftey trips after 120sec. below 9psi. carlyle tech service is stumped and they are checking into where and who has rebuilt these two compressors. i'll let everyone know what we find out.

Mike Hopkins
12-04-2002, 03:37 AM
Sounds like you have my luck! One of my next questions was to be if it was a "new'" new compressor or a rebuild by a local shop.
It almost sounds as though somehow, with the head or gasket configuration you are using the crankcase is getting pressurized with discharge pressure and negating the output of the oil pump.
Carlyles (carrier) can be quite confusing with all the head and gasket configurations they have. You can have different head gaskets for suction cutoff or hot gas bypass unloading and the same as for the heads. Maybe not fully bypassing head pressure to the sump but enough to cause the oil pressure problem. Here lies the importance of figuring net oil pressure from oil pump outlet - sump pressure and not compressor entering pressure. Curious now what kind of pressures you are running on this compressor... discharge, entering and sump, oil pump outlet?
One compressor with a new pump maybe I'll buy but not 2 in a row with little run time. Agree, maybe you're getting a bad lot and something has changed in the reman. process good to look into. I have had MANY problems with the generic rebuilders, mostly leaks but also reuse of valve reeds, plates, etc. Even had one where they put in a used oil pump and when it failed to make oil pressure on startup the rep. admitted it, yeah we sometimes use the old pump if it looks OK! Pisses me off when you spend the money for something that is supposed to be rebuilt and you pull the heads to fix their leaks and the valves have wear on them. That compressor never turned a lick by me and they paid for some complete new valve plates and gaskets, should have charged labor too, but they did send a man out to help. One thing I have found is to avoid the fiber head gaskets, go with the teflon coated metal ones, and torque them per spec. The cheaper fiber gaskets will leak and soon. Also makes service a lot easier.
Mike Hopkins

aaron crimmins
21-04-2002, 07:45 PM
well carlyle sent us a third compressor same model but it was slightly different and it ran just great! 18psi of oil pressure. we sent the other two bad compressors back to them and they are looking at them on tuesday. i hope they let us know what was wrong with them but i dought it. they are paying for the xtra lobor to install the two bad ones which was really suprising. thanks for the help!

25-04-2002, 03:20 AM
Just as a side-note.... what was/is the compressor superheat? Was any oil recovered?

aaron crimmins
28-04-2002, 07:04 PM
the superheat before we got a good compressor and finished charging it was 25f after was 14f. we did return oil from the evap and lines after starting the new comp. so we changed oil. i talked to the carlyle rep. who looked at the two comp. that would not put out any oil pressure and he could not figure out why so they shipped them to carlyle research to look at, he said they would send a report of there findings. they are being suprisingly honest and helpful.

29-04-2002, 08:38 PM
Good for you! I hope you keep us posted on what the report says. :D

29-04-2002, 11:32 PM
I have a 7.5 HP DW Copeland compressor that keeps tripping on oil pressure switch. This is the scenario. Customer shuts down system each morning and restarts evening and runs it all night.
It a a freezer room.

Unit has no crankcase heater. On start up, the oil pressure switch trips.

No sign of liquid slugging. I think refrigerant condensing in the oil, ie: cold start, thick oil,

I normally press the "reset" button. A notice on the Danfoss oil pressure switch says, " Do not press reset button in case of compressor damage" I do anyway.

Should I check the pressure of the oil, theres a schraeder access port, what should the reading be?

I cant hear any knocking noises, but then even it it was I wouldnt be able to tell anyway

Any suggestions??

29-04-2002, 11:35 PM
Does the unit have a pumpdown?

aaron crimmins
30-04-2002, 12:59 AM
sounds like you better sell your customer a crank case heater, if it's not a pump down or your customer does not shut it off correctly so it does pump down, liquid is probably migrating and when you start it up ,it trips on oil saftey.

01-05-2002, 11:10 PM
Ok, got it..........system has pump down, but each time he shuts down he pulls the main switch.......ie: plenty of refrigerant hanging around everywhere

Thats another one sorted..........is this forum GREAT or what!!!