oil pressure differntial

[philjd26]

hi

i am having oil trips on a system particulary in cold ambient conditions...the delay on the switch is 120 secs, i was wondering could this be extended by a further ninute or 2 as any time i seem to arrive on site everything seems to be ok...superheat,crankcase heaters,are all ok working wit a pump down system.

After its satisfied temp the crankcase sight glass foams up.

any thoughts

rgrds phil

[sedgy]

hi philjd26,
tell me what we are working on ? water chiller?

[al]

Phil,

there's most likely a problem somewhere, what differential have you on start up and while running, is the oil level ok while running, is the oil filter clean(if fitted), is the solenoid passing?

It could be liquid sitting in the crankcase which pumps out and then when you arrive all appears ok (btw check that customer isn't taking power off unit overnight for "energy saving!!")

al

[lowcool]

gday phil
liquid presence in the sump is the logical answer for an oil failure trip as you described.if its a new instal the l/p cut out point may be set to low causing oil to foam due to sudden reduction of surface pressure but i have not seen this empty a sump enough to cause oil falure on a correctly designed system.

if everything is set and running correctly i would go for higher wattage sump heater

[cool101530]

It's rare, but if there are worn connecting rod bearings, the heat they generate is absorbed by the oil. Eventually, it gets so hot, it thins out too much for the pump to create enough pressure. Sometimes takes hours for it to show up, and by the time you get there, everything is fine.

[chemi-cool]

Hi Phil,

Time can not be extended on oil pressure controls.

You can check if they work OK and the oil pressure simply by connecting your gauges to the oil schreder valve.

Liquid refrigerant in the sump Will have the same symptom. [lack of pressure] But you have mentioned that the system is pumped down so its not likely to happened.

Check the oil filter and the oil pump operation.

One more thing you can do is change the pressure differential to a lower one. It will give you more time and protect it from tripping in cold ambient start up.

[old gas bottle]

sounds like the switch is doing its job so its normaly one thing or the other, to much oil , if its foaming in the glass its overfull at least some of the time,have you given it a change ? or if its done a lot of hours,maybe compressor wear

[Grizzly]

Hi Phil.
Just a thought but what type of system is it?
Does it have air cooled condensers?
If they are of the large oversized type the below may be relevant.
Only I have known on some of our sites that are more exposed to the elements.
Where the situation occurs when the ambient temps are low enough and or there is a particularly strong cold wind
blowing through the condenser/s.
Causing a large drop in condenser standing pressure (a type of over condensing).
Which in turn creates the situation upon compressor start-up where there is not enough "Lift across the compressor to sufficiently raise the internal temps and pressures.
Resulting in the comp control system failing on several different faults.
On of them being Low oil Pressure Diff.
Just a thought, I have seen the above often enough to add to the clever and plausible, answers that have gone before.
Good Luck and please let us know how you get on?
Grizzly

[R. skiffington]

I would check the control circuit wiring, remember the oil safety needs to be last in line. I've seen cases where the oil safety is before another safety like the lp switch, the unit trips on lp and there still power on the oil safety, if the Lp doesn't reset in time it will trip on oil pressure. I realise the system is on pump down so it's not the lp switch in your case but it's worth checking. Sometimes faults are too simple and we spend too much time trying to think outside the square when the answer is smack bang in the middle of the square!

[philjd26]

not as much trips as the cold snap has passed, however this system is a dx saerving a server room so it is constantly on...

the solinoid passing could be one thing to check but this would eleminate cold conditions trip, it would be constantly tripping....

opertaing on r22, also found another system wit low oil level in sump...!! where does oil normally migrate and whats the solution to return this oil??

thank for answers so far lads.

tgds phil

[al]

Phil

Try pumping the system down using the receiver, do it 2 or 3 times and this should drive the oil back to the sump, keeping the unit running at full load would also help.

What brand compressor is it? Is the unit repeatedly cycling on low load as many dx units do at this time of year?

Al

[philjd26]

no reciever on these systems al....what about head pressure what would you like to see on the head?? iv got 16 brg!!!

rgrds phil

[bill1983]

hi phil
how is the oil returned to the compressor? is it through natural flow with the refrigerant or a separator/solenoid/float system.
at 16 bar discharge, there may be too large a pressure differential between the high and low sides causing the oil to foam on startup. would prefer to see less than 15barg running pressure on the high side. and no more than 10 - 11 barg diff, so no less than 4 barg on the low. discharge pressure seems high for the time of year. fan speed control, fouled condenser fault with condenser fan etc??

[al]

Phil

My own view would be to cycle on at 18 off at 15, esp in these low ambients, is there an oil seperator?

I think you answered this but is the crank heater ok?

Pm me if you want

alec

[oldesky]

hi philjd,This problem is sometimes extremely hard to diagnose. If an oil separator is fitted I would suggest insulating the oil sep and installing a check valve between oil sep and condenser. This will help to prevent refrigerant migrating back from the condenser to the oil sep and diluting the oil. I would also look at cleaning the oil pump to make sure none of the oil holes are semi blocked or restricted. If the oil isnt clean I would change the oil and fit an oil filter.

[Pykester1]

Hi.
Try checking the superheat. If the expansion valves are allowing liquid refrigerant to return to the compressor crankcase this may cause oil foaming. Also check the loading on the evaporator is sufficient. I have known faulty evaporator fans to cause oil foaming and oil pressure trips. Also some compressors have a high pressure relief valve fitted to the inside of the oil pump. Check that this is not passing.
Hope this helps.

[jgroberson]

I see allot of good people are trying to help with good suggestions
I will try to add more oil safety trip fixes
#1 Assuming this is a single comp without oil separator and floats (not a rack system)
If the comp is a suction cooled semi hermetic with an internal oil check valve and it looses oil level while running but level raises back to normal as it sits idle to correct level in sightglass when turned off then you have severe cylinder blowby causing the oil to hang up in the motor cavity as it can't drain through the check valve while running as the high crankcase pressure keeps the check valve closed. Comps are designed to have a lower pressure in the crankcase than the motor cavity pressure to make the oil drain through the check valve while running but you can't read this difference on two gauges as it's very small
If the oil level rises after the foam effect stops when you pull the disconnect I would call the manufacturer support dept and discuss blowby toubleshooting methods and or comp replacement
#2 R-22 comps if run with high ambient as in dirty condensors etc for a long time will have very dirty oil visable only after an oil change and can cause extreme foaming and may require several oil changes to stop foaming
#3 You state you have no receivers with a pump down system I have never seen that before and you might have refrigerant flood back without receivers (check your suction superheat)and if you also have headmaster valves for winter pressure control usually set at 190 PSIG on R-22 you have to have receivers to hold the extra winter charge in the summer here in the USA
I hope I have helped someone on the other side of the globe
John Roberson