MYCOM 3225 Oil Pressure Problems

[dwinter411]

Hey everyone,

I could really use some help on this issue. Here is the situation. The compressor keeps shutting down on low oil pressure differential on the high stage. What I see happen most of the time is when we slug the compressor with a lot of feed (Nitrogen, propylene, ethylene mixture), the compressor loads up, the discharge of the compressor goes from about 300 psig to 325 psig (this doesn't always happen), and then the oil pressure starts swinging and eventually shuts the compressor down on low oil differential. What is weird is that this doesn't happen every time. I've been out there and watched the oil pump (IMO 3 screw pump) swing between 315 pisg and 450 psig before it shuts down.

A little history on this compressor is we wiped out the high stage thrust bearings and the rotors hit the case. The compressor was rebuilt and the shop told us that everything was in manufacturer tolerance. Ever since we had the compressor rebuilt, we have struggled with this problem. Sometimes it happens constantly over a week. Sometimes it will run fine for a few months. The thought is that maybe the second stage has too much clearance in it, which prevents us from being able to control the oil pressure as well.

The oil pump has been replaced many times and we have found no issues with it. There is still some concern that maybe we are having issues with the oil separator and that is giving us issues with suction of the oil pump.

Has anyone had a similar problem with controlling the oil pressure or does anyone have any ideas?

[Grizzly]

Hi dwinter.
Welcome to the forum.
Are you a fridge Engineer? Only I ask because there is lots of info about what has happened.
But little about how your system works etc?

Your "feed" is a new one to me, so I cannot comment too far on the subject.
But with the little you have said, I would be looking at the quality of the oil and the contamination within?

Also yours sounds like a classic case of liquid slugging. Especially given the history you explain. What type of system are you operating? I am interested in the liquid control!
Grizzly

[RANGER1]

Dwinter,
More info would be helpful
type of oil cooling
oil & discharge temp
oil pressure & trip point
oil level in relationship to pump imagine vertical oil seperator is installed
signs of oil foaming
has anything else changed like filters mesh size etc on oil pump suction?

I think Grizzly is asking type of level control on vessel on suction of compressor.
If level incorrect possible liquid can enter compressor which can cause oil to be diluted effecting oil pressure.
signs might be foaming or oil/discharge pressure may reduce below normal.

can you also explain oil pressure regulation control, is it in pump or seperate or both.

[dwinter411]

I am a mechanical engineer working on a recover/refrig system. This first compressor takes in the vent gas from a process unit and increases the pressure. Then it is sent to a refrig unit to separate the propylene and ethlyene from the nitrogen. After that it is sent back to the unit. I saw there was some knowledge of MYCOM compressors on this forum, so I thought this was the place to be.

We have had some issues with oil contamination. We have had some fine polymer make into into the unit which carried over from our process unit. Also, we suspect that the propylene and ethylene is diluting the oil and could be causing foam issues.

I originally thought that maybe there was some liquid slugging, but the temperature and pressure that we see show that the feed should all be in gas phase. Our system consist of a suction knock-out drum (with level indication) going to the compressor with an aftercooler hairpin heat exchanger. From there that gas goes to the refrig unit to be chilled from a gas to a liquid and recovered. The oil to the compressor has a three way valve that diverts the oil through a heat exchanger as needed to maintain a temperature of about 130F. The pump suction pipe goes to the bottom of the oil separator vessel, where the pressure corresponds to the compressor discharge pressure (about 300 psig). There is a manual pressure regulator valve on the discharge of the oil pump, basically just a spring and a piston, that is set to maintain a differential pressure between the compressor discharge and pump discharge of about 60 psig.

I'm not sure how much detail you are asking for, but I hope this helps.

[dwinter411]

Shell and tube heat exchanger with water for oil cooling
Oil temp ~ 130F
Compressor discharge temp ~ 220F
Oil pressure ~ 40 psig with a shutdown at 15 psig
Vertical oil separator is installed. Oil level remains at about 70%, which is slightly above the pump. But the separator is pressurized with compressor discharge.
We recently took out the pump suction screen and found some fine plastic on it. Cleaned it and reinstalled it, but it didn't help the issue. The screens in the oil separator have not been changed for 3 years.
No signs of foaming in the local level gauge, but it is possible.

We haven't seen any levels in the suction knock-out drum. The pressure and temperatures on the suction side indicate that propylene should be in gas phase. The level could be incorrect though.

How would liquid cause a drop in oil pressure?

[RANGER1]

Shell and tube heat exchanger with water for oil cooling
Oil temp ~ 130F
Compressor discharge temp ~ 220F
Oil pressure ~ 40 psig with a shutdown at 15 psig
Vertical oil separator is installed. Oil level remains at about 70%, which is slightly above the pump. But the separator is pressurized with compressor discharge.
We recently took out the pump suction screen and found some fine plastic on it. Cleaned it and reinstalled it, but it didn't help the issue. The screens in the oil separator have not been changed for 3 years.
No signs of foaming in the local level gauge, but it is possible.

We haven't seen any levels in the suction knock-out drum. The pressure and temperatures on the suction side indicate that propylene should be in gas phase. The level could be incorrect though.

How would liquid cause a drop in oil pressure?

If liquid enters compressor it may dilute oil & because oil is hot the liquid refrigerant will flash off, possibly causing foaming.
If you check oil pressure reserve it can also tell us how much extra oil is going through oil pressure regulator on pump.
Basically try to increase oil pressure when running to see what you can get it up to.
If you can increase it to say max of 100psi then you have oil reserve, then reset it back to normal.
This tells us when conditions change like increase in discharge pressure, that there should be enough capacity from oil pump to cover it.
If not either oil pump is not supplying enough oil or compressor requires to much.
As the differential pressure across the machine increases, so does oil flow.

Is there any visual signs in knock out pot of a level?
Can the level transmitter be verified
Do any trends show sudden decrease in discharge temp or suction inlet temp indicating liquid refrigerant entering compressor?

Discharge temp looks high, is this normal, max is usually 90 deg C (194 F) Trip point.

How long since the machine was serviced, i would suggest 25000 hrs is overhaul time on Mycom compound compressor.

Is gauge glass for oil level on oil separator clean, accurate & easy to read?

What type of oil pump is it model etc Mycom, Viking ?

[dwinter411]

Thanks for the explanation. I will try some of these things and see how they work out.

There is no visual signs of level in the knock out pot, but we will verify the level gauge. There are no trends showing a sudden drop in temperature.

220F discharge temp is normal of this machine.

The compressor was last overhauled in Jan 2011. The oil pump (IMO) was replaced with a spare two months ago.

Yes, the oil gauge shows that the oil is clean and it is fairly easy to read.