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Got a 8 hp semi . Had a call out over the weekend. Unit had tripped on oil pressure switch. I reset it despite a warning on the control that do not reset in case of compressor damage.
A previous engineer had changed the oil. System runs on R22. Trips out at regular intervals.
Why is it doing this and what are the tests that can be carried out to ascertain if the oil pump is not doing its job
Thank you
measure to actual oil pressure and compare to value in manual, mostly it's switch's problem
please check following things.
1. Check oil brand & viscosity of changed oil.
2. Check if the oil filter is clean or not.
3. Is there some plating action occuring in the compressor?
4. Check oil level in crank case. colour of oil?
5. If your oil pressure is ok but still it is triping you have to check your op cutout. The procedure of checking op cut out must be given in companys catlogs.
6. There can be an electrical or mechanical problem causing the compressor to trip on overload. The OP ctrl then times out and trips.
Is there an oil seperator fitted?
If so check to see if the gravity oil return is o/k. During the run time if the oil doesn't return the unit trips out but by the time you get to it the system has been off cycle and the oil has returned. Very difficult to diagnose this type of problem unless you are in the right place at the right time:(
I'm with you there, Frank!
Once had exactly this problem on an LT pack. It eventually turned out that the mesh in the oil separator had broken up, clogging the oil line to give exactly the situation you describe.
Can you imagine what it was like trying to braze in the new separator with everything in the room vibrating like a V12 engine?!
(No, they hadn't got the vibration damping correct. Not by a long, long way!)
If I remember right, Copelands should have something like 30-50 PSIg oil pressure on top of the suction pressure. Does anyone out there agree / disagree?
It's also worth checking the oil strainer. Silly, I know, but you'd never believe the number of times it gets forgotten. It became our first check on oil faults after a while, on a certain large supermarket chain. (Before we realised how badly the previous company had maintained them, of course! ;) )
all my be true w/ out more info I like to start simple dose it have a fan control on the condensor motor or a headmaster. If it does look to see if the unit is in a area w/ winds that rip through the condensor had this prob. many time on a/c uint you do not say if its a frig or a/c. if it has that new coppland oil control change the sencer @ the botom of the coperssor they clog up
Hello Tony
Compressor is on a freezer unit. Has a fan control which comes in at very intermittent periods, constantly kicking in and out, runs for maybe 30 seconds and cuts out again.
Sensor on the bottom of the compressor. Please elaborate slightly what this sensor is.
I could be wrong regarding what Tony is referring to, but this might be worth a mention. Parallel racks have an oil separator, an oil filter, oil floats, and with Copeland compressors, Sentronic oil failure controls.
The Sentronic control uses a single sensor with small orifices to measure the differential pressure between the inlet and outlet of the oil pump. These areas can clog up with debris and render the sensor useless.
One of my mechanics made an interesting observation regarding oil filters. He noted that some manufacturers are using filters that will bypass the oil when they become clogged. "Seems like a good idea" he said, "but it isn't."
He would much rather the compressors shut down on a legitimate lack of oil pressure as opposed to fouling up the sensors by letting dirty oil get through.
He makes a good point.
In your particular case, Aiyub, I would look for some sort of pattern. Does the failure seem to occur before or after defrost? You could be washing the oil out with liquid refrigerant as a result of TEV overfeed with a frosting coil...... or liquid migration during defrost.
Also, keep the sacred rule of service in mind: Always go back to the last repair.
Why did a previous mechanic change the oil? Did he put a synthetic oil in place of the mineral oil? If so, you are cleansing the oxidation from the interior of all welds and solder joints, and clogging could be a culprit, whether in the compressor sump screen or in the oil failure sensor.
Let us know when you finally figure this one out, Aiyub.:)
Aiyub,
Have you come to a conclusion on this issue?
Fan cycle? The unit doesn't also have some head pressure control valve in the circuit also, does it ?
How long has the compressor been in commission? sometimes the vanes on the pump will wear, and if 3-phase, you can change the rotation of the motor. In that regard, there is a slide valve of sorts in the pump that will "switch" according to motor direction to assure proper oil flow direction. Perhaps if rotation was changed, operation of the "switching wasn't 100%.
Okay, a little far-fetched, but some ideas anyway.
I'm curious as to what you find.
I have a 10hp semi (Copeland) that I employed in a freezer app. Erroneously obtained C.U. w/o accumulator. Nuisance trips occured coming out of defrost. The linset is only 3 meters long! I increased TEV superheat to 20+degF at compressor inlet. Nuisance trips stopped!
Hey! Maybe you might consider checking the superheat and discharge temperature and take notes if this goes on much longer.
I've also had trips related to blow-by (worn compressor). Compare your crankcase pressure to your suction presure during a pump-down. This one really drove me crazy. Intermittent oil trip. during normal operation, the crankcase pressure vs. suction pressure was good, but when the unit went into a pump down, my crankcase pressure would rise astronomically and cause oil failure on pump down.
All other test relating to compressor operation proved negative for failure.
see ya'
Im taking a very keen note of each and every possible causes posted, all very appreciated so thank you.
At the moment I have not been called out to this "dastardly" unit yet....... but I must make a point of going out and allocating some time to it, before the whole system fails
But rest assured, I will post my findings very soon.
Im working on a big restaurant refurb project at the moment and the schedule is very tight. It is Leicesters first , Cuban Bar
So you get transported to Havana and into the den where , ( whats that authors name??? ) used to drown his sorrows
Anyway , Ive been putting in Air conditioners, walk ins. bottle coolers, ice makers....etc etc
Everyday Im covered in builders muck, sand, cement, and getting used to all these guys , sparkies, chippies, whats the slang for plumbers?? anyway all these guys working together, borrowing each others tools, having my pieces of stuff go missing
For our American freinds, spoarkies are electricians, chippies are carpenters
Its been an intresting project for me, Ive also designed the bar and the kitchen.
Who says a fridge mans life is a boring one?????
The oil pump raises the pressure of the oil in the crankcase to a desired pressure in order to push it through the elements of the machinery operating at suction or crankcase pressure. I don't follow the "crankcase pressure vs suction pressure" phrase. Perhaps you mean "inlet and outlet" of the oil pump?Quote:
This one really drove me crazy. Intermittent oil trip. during normal operation, the crankcase pressure vs. suction pressure was good, but when the unit went into a pump down, my crankcase pressure would rise astronomically and cause oil failure on pump down.
What causes an oil failure event is the loss of differential pressure across the oil pump.
I am interested about how crankcase pressure rises astronomically during a pumpdown, Herefishy. What could be going on?
I would like to react to Dan's note:
"One of my mechanics made an interesting observation regarding oil filters. He noted that some manufacturers are using filters that will bypass the oil when they become clogged. "Seems like a good idea" he said, "but it isn't."
He would much rather the compressors shut down on a legitimate lack of oil pressure as opposed to fouling up the sensors by letting dirty oil get through."
I think that the idea of by-passing the oil is to prevent too high oil pressure between the oil pump and the filter. Filter would even be destroyed if there is no by-pass.
Nico.
The oil filter is between the separator and the compressor. How can you have too high an oil pressure between the oil pump and the filter?Quote:
think that the idea of by-passing the oil is to prevent too high oil pressure between the oil pump and the filter. Filter would even be destroyed if there is no by-pass.
There is no opportunity for the pressure differentials you mention in any of the setups that I have seen.
A clogged oil filter simply stops oil feed from the separator. Oil systems with reservoirs have a 20 psig check valve to suction to prevent oil overfeed.
High pressure oil feeds accomplish the same thing with differential valves.
In either case, an oil system whether working or not will only have a 20-to-30 psig differential across the filter.
A clogged filter in an oil feed system does not see high pressure differentials. It simply stops feeding oil.
Coalescing oil separators have an internal filter which does dual duty: filtering the oil and precipitating the droplets into the separator. If that filter becomes clogged, then high pressure differential will require a bypass..... such as blowing out an O-ring..... but then the separator becomes ineffective. It cannot separate oil when the filter is bypassed because it loses the precipitating surfaces.
Since I am on a ramble, Nico, the concept of a filter becoming destroyed is no worse than a filter that no longer is filtering. Air filters come to mind. If they clog up, they get sucked into the intake or collapse..... whatever.
Why build in a mechanism that senses the filter cannot do its job anymore and choose to not filter anymore?
End of ramble.
OK Dan, I got your point and agree. I was not aware of the position of the oil filter you mentioned.
"There is no opportunity for the pressure differentials you mention in any of the setups that I have seen."
In the bigger industrial machines it is not unusual to have no oil filter between the separator and the crankcase, but instead an oil filter between the crankcase and the oilpump (external or internal). That would be a filter without by-pass grade 40micron. However, after the oil pump there is a fine filter (10 - 20 micron) and that filter should be able to by-pass to prevent high pressures when gloged.
In larger screw compressor units you often have the "coarse filter, pump, fine filter" setup between the oil separator and the compressor. Again you need a by-pass on the fine filter.
Just a related oil story:
Just two weeks ago I had a $15,000 repair on a Grasso RC4211 compressor because the local technicians replaced the coare filter (suction oil pump) for a fine filter. When the compressor reached about ambient pressure (R22 blast freezer) they lost oil pressure due to the high resistance (= pressure loss) in the pump feed. Disolved refrigerant start to produce gas and gone is the oil pressure. Damage was because they kept resetting and eventualy one smartass by-passed the oil pressure switch. That was the second time that month that I found an oil pressure switch by-passed. The other time it resulted in a srewed srew compressor.
Nico.
Quote:
Originally posted by Dan
I am interested about how crankcase pressure rises astronomically during a pumpdown, Herefishy. What could be going on?
The cylinder(s) had worn to the extent, that blow-by gas would pressurize the crankcase. The Copeland Discus compressors that I refer to have a check valve in the oil return. Normally your crankcase pressure is lower than your suction pressure.
In the scenario that I refer, when the PD between the crankcase and the discharge reached a particular point, the blow-by increased. Because the check valve in the oil return circuit prevented oil (pressure) from returning to the suction, the crankcase pressure would rise to a point that would near the discharge pressure during the pump down.
There may have been some PD at the point of trip, but I'm not certain that the crankcase pressure may have exceeded the range of the oil pressure safety on the low pressure bellows.
:)
One other thing, perhaps that may be worth ovserving, is I know that there are several variations of low pressure safeties. Johnson-Penn produces two common controls. One is specified for say Copeland (as an OEM), another for Carlysle. The difference pertains to the delay characteristics of the control. One may want to be sure that the control applied is appropriate for the machine, or meets mfgr. requirements/specifications.
Well, Nico, I was thinking inside my box. :) Thanks for bringing up the industrial applications. Probably a differential alarm would complement the bypass filter setup?Quote:
In the bigger industrial machines it is not unusual to have no oil filter between the separator and the crankcase, but instead an oil filter between the crankcase and the oilpump (external or internal). That would be a filter without by-pass grade 40micron. However, after the oil pump there is a fine filter (10 - 20 micron) and that filter should be able to by-pass to prevent high pressures when gloged.
Good point Herefishy. Also the differential settings are significantly different between Copeland and Carlyle compressors. Carlyle operates at a much lower differential pressure than Copeland.Quote:
The difference pertains to the delay characteristics of the control. One may want to be sure that the control applied is appropriate for the machine, or meets mfgr. requirements/specifications.
Would that be the check valve between the motor and compressor chambers?Quote:
In the scenario that I refer, when the PD between the crankcase and the discharge reached a particular point, the blow-by increased. Because the check valve in the oil return circuit prevented oil (pressure) from returning to the suction, the crankcase pressure would rise to a point that would near the discharge pressure during the pump down.
Hi:)
just a thought. Some compressors have their oil filter on the pump suction, others on the pump discharge and in others they have both.
Also in Industrial aplications it is a good idea to fit an inline oil filter between the separator on the compressor, by doing so the oil controls (level) are protected and the baked oil in your separtor is strained of carbon before it enters the compressor.
Regards. Andy.
Quote:
Would that be the check valve between the motor and compressor chambers?
.......yes......:)
If you are evaluating oil pressure, you cannot "hook up" to the suction access to troubleshoot. You must connect to crankcase pressure. The crankcase can be doing some very different things than would be observed by monitoring the suction inlet pressure.;)