Lube Oil for Flooded Oil Screw Compressor broken after 7200 operating hours 3

[nangkring]

Hello, I am new in the Eng-Tips forums and my background is not sufficient to determine the cause of the problems regarding the Oil Flooded Screw Compressor that I was dealing with.

I found conditions in the screw compressor oil in milk white condition after operation for 7200 hours. Conditions compressed gas composition has about 89 mole% Methane, Ethane: 3.5 Mol%, Propane: 1.5 Mol%, Butane: 0.5 Mol%, C6 +: 0.25 Mol% and CO2: 5 Mol%.

Upstream pressure of 24 bar with a temperature of 28 deg C and 28 bar pressure compressor downstream with a temperature of 60 deg C.

My question is, whether the damage to the gas compressor oil is due to the existing condensate in the gas will be compressed and whether the upstream temperature 28 deg C is still too low to cause the emulsion between gas condensate with oil?

Thanks for anyone who can help.

 

[zdas04]

Two issues. First, all screw oil is hydrophillic, it likes water vapor. If you run the oil too cool then you get milky colors, terrible lubricity, very high viscosity, and very high surface tension (i.e., small drops will not coalesce into large drops) all because you are not cooking the water back out of the oil. Virtually every problem I see with oil flooded screws is due to running the oil outlet temperature too cool. Discharge temperature should be right around the boiling point of water (205-215 F or 96-102 C). You get there by a combination of reducing cooling, reducing oil flow rate, and increasing compression ratios (usually with a backpressure valve).

The second issue is that many screw oils will absorb heavier hydrocarbons (like a lean oil NGL plant). You need to send your gas analysis to the compressor manufacturer and follow their recommendation on oil selection. With your analysis you'll probably be directed to a very expensive synthetic oil, but none of the less expensive mineral oils will work with that analysis.

The good news is that when I had exactly the problem you described, someone told me to raise the discharge temperature and all of my problems went away within a few days and never came back. Fixing the first problem just requires vigilance and persistence.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.
The plural of anecdote is not "data"

 

[nangkring]

Thanks to zdas04 to answer pretty clear.
I also did a test to degas the sample gas oil compressor oil which can be recovered visual along with my sample viscosity when heated at temperatures of 160 C. If this show is so great that damage portion of C6+ gas compressor oil?
Indeed, there is an opportunity for me to raise the temperature to be higher downstream by first heating the gas at the upstream side with Dew Point Heater. Is the upstream temperature will approach 45 C is still safe enough to go to the gas compressor?

 

[zdas04]

45 C is a pretty cool temperature, shouldn't hurt anything. The big problem is that the thermal mass of the oil is many times the thermal mass of the gas so adding heat to the gas is rarely a winning strategy. It makes more sense to replace the pill in the Constant temperature valve with one with a higher temperature (I hate those valves, but if that is the temperature control you have, I want to see a 180 F (82 C) minimum set point).

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.
The plural of anecdote is not "data"

 

[rconnor]

I agree with zdas04 comment about temperature being an/the issue.

I just wanted to add that sometimes this points to the compressor being oversized and not working hard enough. If you have multiple compresors, one solution would be to shift some load to this compressor by turning another compressor off. You could solve your oil issue and reduce energy costs.

 

[nangkring]

Unfortunately deluge valve to regulate the flow of oil to the cooler to be set at 50 C.
Gas compressor only 1 unit available as a gas booster to the gas turbine.

 

[bcs5274]

brand type specs of oil in use??

might not be correct for application

 

[zdas04]

I don't know how to say this more strongly--set the oil out of the screw at 50 C and you are assured of (1) excessive oil loss; (2) failed bearings; and (3) very short life for rotors and casing. I guess policy can be more important that what the machine needs.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.
The plural of anecdote is not "data"

 

[nangkring]

Oil used in accordance with manufacturer's recommendations screw gas compressor, KOBELCO, KB-105-68.
After 7200 hours of operation the viscosity dropped from 68 to 41 Cst and followed with oil color changes to white milk.
There is one odd thing about the gas separator upstream side of the compressor. Is the gas separator inlet higher than the outlet can effectively eliminate the existing liquid in the gas?

 

[zdas04]

What you are describing is the oil dehydrating the gas. Separation will not remove water vapor. You can put a dehydrator upstream of the compressor, but your reboiler costs will be very high.

If you dig into the Kobelco specs you'll find that the maximum allowable water vapor content in the inlet gas of 7 lbm/MMSCF (110 mg/Sm^3). Your gas content is closer to 400 lbm/MMSCF. Your exit water content is probably something like 50 lbm/MMSCF. The difference stays in the oil until the oil is fully saturated. That is why it is milky and the viscosity is high. You can't afford the fuel for the reboiler on an upstream dehydrator. Especially when you can actually fix the problem for a few hundred dollars. Oh yeah, you have a policy. Good luck with that.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.
The plural of anecdote is not "data"

 

[nangkring]

If I can take the conclusion, that the type of gas compressor with oil injected screw will allow the oil to absorb water and hydrocarbon gases contained within. If these conditions continue to occur, it will cause loss of quality gas compressor oil. There are several possible solution that is offered is to make the temperature of the gas discharge side of the compressor as high as possible or at least 205 F. The second action is to remove water and hydrocarbons on the upstream side of the compressor gas. What is the more effective to prevent the decline in gas compressor oil? I'll tell you later my options after I test at my workplace.

 

[zdas04]

You have 3 options: (1) do nothing and expect considerable lost oil for the 2-3 months it takes to reach the point of catastrophic failure; (2) remove all water vapor and condensible hydrocarbons upstream of compression which is possible but very expensive; or (3) accept that the oil is hydrophillic and make certain that the oil is hot enough to cook the absorbed gases back out of the oil. The last one can be done by increasing compression ratios (either lower suction with a suction controller or raise discharge with a backpressure controller), reduce cooling, reduce oil flow, and/or raise the temperature into the process.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.
The plural of anecdote is not "data"