decreasing of lube oil supply after compressor start up

[Jack Nicholson]

Hello all. I haven't seen any compressor topic!. So i post my question here.

After ethane compressor start up, lube oil supply pressure to compressor and electric motor bearings all decrease suddenly (from 1.2 barg to 0.5 barg), so we have to increase them up to the normal values.
Likewise after compressor trip, all lube oil supply pressure increase. How can we overcome this issue?

 

[JJPellin]

What type of compressor? What are the drivers for the main and auxiliary lube oil pumps? Is either oil pump shaft driven? Does the auxiliary oil pump auto-start and auto-stop?

If this happened to one of my machines, I would assume that the main oil pump was shaft driven. I would assume that the auxiliary pump shuts off once the main pump builds pressure. I would assume that the relief valve (probably a full-flow relief) was set too low on the main lube oil pump but set properly on the auxiliary lube oil pump. I would verify the settings and function of the auto-start / auto-stop instrumentation. I would verify the settings of all relief valves. But, without the answers to my questions above, this is all just guessing.

Johnny Pellin

 

[LittleInch]

Agree with JJ as usual. Check out how the lube oil is being supplied as this looks like a main / auxialry pump supply switch over issue.

Your main oil pump might be worn or badly set wrt relief valve pressures, filters blocked etc whereas the aux pump only runs for short periods and hence is much better.

How do you "increase" the supply pressure?

More questions than answers here.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Jack Nicholson]

Oil pump driver is independent of main compressor. It's electeical motor. Oil pump performance is ok during main compressor start.

 

[JJPellin]

You did not answer my questions, so I cannot answer yours.

Johnny Pellin

 

[LittleInch]

So what else is going on when the pressure suddenly falls?

But some answers tot he questions would be useful to eliminate some issues.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Jack Nicholson]

You did not answer my questions???!!!! Bro, i have already answered your question.


What type of compressor? Centrifugal What are the drivers for the main and auxiliary lube oil pumps? I don't know what you mean by shaft driven, but it's centrifugal pumps. The pump shaft is not coupled with main compressor shaft Is either oil pump shaft driven? Does the auxiliary oil pump auto-start and auto-stop?
[highlight #729FCF]Yes, it's auto-start[/highlight][highlight #FCAF3E][/highlight]

 

[JJPellin]

You finally answered the most important question. It is a centrifugal compressor. Are you sure your lube oil pumps are centrifugal? That is very unusual. Is the lube oil system also used for seal oil?

I have a new guess based on the tiny bit of information provided. I assume the lube oil system is common with seal oil. When compressor builds gas pressure, the seal oil pressure increases.This increases the flow of clean seal oil return. This moves centrifugal oil pump further out on its curve, causing the pressure to drop.

Johnny Pellin

 

[JJPellin]

I can continue guessing. Compressing ethane with a centrifugal compressor should require high rpm. There is probably a gear increaser between the motor and compressor. We really need a schematic of your oil system to have any chance of helping you.

Johnny Pellin

 

[Jack Nicholson]

Dear JJPellin.
I had a mistake! our lube oil pump is screw type...
compressor sealing system is dry gas seal (not oil).

 

[JJPellin]

So, what can cause the pressure to drop downstream of a screw pump? The pump could be worn out. But, that would not be reversible. It is much more likely that the system resistance to flow is reduced. What can cause that? A relief valve (PSV or full-flow) could be diverting flow. But, why would that change when the compressor is running or not running? The pressure controls (assuming a spill-back, pressure control valve) could open to divert flow. But, why would that change? The resistance of oil flow through the coolers could change. Unlikely. The resistance to flow through the filters could change. That one seems believable. We have one system where the pump capacity is barely enough and a relatively small change in oil temperature changes the system pressure because of pressure drop through the filters. When the compressor is not running, the oil temperature drops down to the setting of the oil heater (let's guess 90 °F). When the compressor runs, the temperature increases to the setting of the temperature controller (let's guess 120 °F). Less resistance through the filters and the oil supply pressure to the bearings goes up, not down. That is not it. The last option that comes to mind is a drop in resistance to oil flow to the bearings. I am not sure what would cause that. Perhaps something with the flow to the overhead lube oil rundown tank (if you have one). We really need a drawing of the oil system configuration to guess any more.

Johnny Pellin

 

[LittleInch]

Please post the schematic or P&ID of the lube oil system which might give us a chance.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.