# oil pressure problem

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• 11-20-2012, 09:57 PM
welling service
jpsmith1cm yes they are. I did the piston blow by test by putting a gauge on the CC and one on the suction port. I slowly closed the suction service valve and watch my gauges. They both fall at the same rate down to 0 PSI at which point I know I do not have an over pressurized CC. I guess it's possible to have an intermittent flood but haven't seen one. My rooftop condenser has 3 fans with one controlled by a fan cycle switch. The unit also has a 3-way head pressure control valve.
• 11-20-2012, 09:59 PM
welling service
gggg
• 11-20-2012, 10:02 PM
welling service
gggg
Quote:

Originally Posted by jayguy
low head pressure usually = low suction pressure. low suction pressure = low refrigerant flow rates. low refrigerant flow rates = poor oil return. poor oil return = no oil. no oil = 0 oil psid. < 10 psid = trip oil pressure switch. trip oil pressure switch = unhappy customer. unhappy customer = unhappy service tech.

math is fun!

• 11-20-2012, 10:10 PM
welling service
I like your math. I resemble that remark. I do not have low head nor do I have low suction therefor your great math doesn't solve my problem. When the unit trips it doesn't matter if it is 29 or 89 degrees. It trips when it takes a notion to no matter the ambient. I wonder if it is possible to have rod bearings worn enough to cause a oil pressure drop by letting too much oil escape too quickly. I say this because the oil pump pressure out put on start up is around 85 PSI and drops off to around 55 PSI in about 5 minutes.
Quote:

Originally Posted by jayguy
low head pressure usually = low suction pressure. low suction pressure = low refrigerant flow rates. low refrigerant flow rates = poor oil return. poor oil return = no oil. no oil = 0 oil psid. < 10 psid = trip oil pressure switch. trip oil pressure switch = unhappy customer. unhappy customer = unhappy service tech.

math is fun!

• 11-21-2012, 01:41 AM
jayguy
Quote:

Originally Posted by welling service
...the oil pump pressure out put on start up is around 85 PSI and drops off to around 55 PSI in about 5 minutes.

i think that this may be more of a key than first noted. this sounds like a compressor that is 'warming up' the oil/parts. crankcase heater working? does the net oil pressure stay the same after that?
• 11-22-2012, 02:19 PM
welling service
The NPOP stays around 15PSI and the CC heater is working. This unit has been running for a week now and hasn't tripped on anything. I hate this sneaky compressor.
• 11-22-2012, 05:52 PM
jpsmith1cm
Quote:

Originally Posted by welling service
The NPOP stays around 15PSI and the CC heater is working. This unit has been running for a week now and hasn't tripped on anything. I hate this sneaky compressor.

Do you have a rack controller or is it running on pressure controls?

What's the rest of the oil system look like? Is it a turbashed oil system or a standard separator/reservoir/OCV type system?

Could the oil float be sticking at the compressor? In the separator?

While, technically, 15# is 'good enough' I'm not happy with it at all.

If you've got 15# with a clean pickup screen, a proper oil level and a new oil pump that is properly installed, I'm quite suspicious of the bearing condition of that machine.
• 11-22-2012, 06:18 PM
welling service
This system has no controls except for a defrost timer for each LLSV. It supplies 6 different open face cases with with no stats. It never shuts off. This system has 2 comp piped in parallel and only runs one at a time. The other comp never trips. It has a new seprator/reservoir and each comp has oil level controller. The oil seprator return cycles from hot to ambient so I know the seperator is working. I agree, I like a little higher net oil pressure also. I really leaning toward a bearing problem. I have recently replaced the other comp. It had a broken rod and the crankshaft was in pretty bad shape. I pulled the suction end bell and found a lot of copper dust and just plain ole crud caked on the windings. I inherited this system just system. The previous tech solved problems by using jumped wired around controls and fuses.
• 11-25-2012, 12:26 AM
r404a
welling

you are really up against it here, but i admire your thoroughness. If that sump screen is ok, you have to have something internally wrong. you already tried an oil pump, and with a clean sump screen I have to say that you have a worn bearing there. I have seen a unit do this before, the oil was getting real hot and thinning out and causing this, by getting superheat undercontrol the problem went away. The unit had an old ragged out condenser coil as well. discharge temps were too high there as well. doesnt seem like you are having that issue though.
since you had a massive failure in a parallel compressor, could you have debris from the old compressor that got into your common lube sytem?
• 11-26-2012, 12:53 AM
welling service
very possible. I have changed the oil in both compressors twice. I really think it is worn bearings.
• 11-26-2012, 08:37 AM
timebuilder
I'm in the worn bearing camp as well.
• 11-26-2012, 09:57 AM
mccann
There are many reasons for oil fail. What is the superheat at the compressor? If it is low, below 20*, you may be getting flood back at times, which would explain broken rods. It could be intermittent as load changes. Liquid could feed to one compressor more than another because of piping. You will not always see foam. It will foam when suction pressure drops and boils off gas. If suction pressure is steady, no foam. I have seen many oil pumps changed; and in the process of pumping it down, they remove the liquid refrigerant from the crank case. They think they fixed it for awhile.
If Superheat is real high, crank case is hot, the compressor could be getting hot and going off on thermal overloads, because the contactor is pulled in, it's still looking for oil pressure on the control circuit. It depends on how it's wired of course.
The contactor or wire connections could be bad causing a thermal overload. Is it a mechanical or Sentronic oil safety? If Sentronic, the sensor could be bad, screen plugged, or the little o-ring could be bad that separates suction & oil pressure to get net oil pressure. It times out after 120 seconds below 9 psi, net, for Copeland.
Other questions not necessarily for your problem: is the sight glass dirty at the half way point so it looks half full? Is there an oil separator working ? Are there oil level controls sticking or plugged or stuck open feeding one compressor too much causing loss of differential pressure from the separator-reservior.
In a supermarket 60% of the time it's flood back, Sometimes it only floods coming out of defrost, sometimes only after product is real cold. Sometimes it drives me crazy. too late.
• 11-26-2012, 09:09 PM
welling service
mccann, the OPC is mechanical, compressor SH is around 25 to 30 degrees, sight glass is clean, oil level controls are working just fine and I have never seen any flood back but I am not there 24 hours a day so it could be a possibility. Oil separator is new. I recently installed it because the old one was a welded unit and it did not work. You cannot do any maintenance on these welded units. The oil pressure control is tripped so I'm pretty sure it is an oil pressure problem. For some unknown reason it has been on line for almost two weeks and hasn't tripped. I only run one of these compressors at a time and the other compressor maintains a NPOP around 65 PSI.
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