Screw compressor system losing oil differential pressure

[orm]

Hi,

Just come across this site and wondered if anyone could help. I have a Mycom 2520 two stage screw compressor core packaged by other company operating on natural gas.
The problem is on a start up it keeps losing the oil differential pressure, but this only seems to happen when the discharge pressure reaches 14bar (normal output 23 bar). The compressor has recently had a rebuild with new bearings and operated ok up until now.

Any ideas what could be causing the oil differential pressure to drop? (new filters installed)

Thanks in advance for any suggestions

[Andy]

Hi,

Just come across this site and wondered if anyone could help. I have a Mycom 2520 two stage screw compressor core packaged by other company operating on natural gas.
The problem is on a start up it keeps losing the oil differential pressure, but this only seems to happen when the discharge pressure reaches 14bar (normal output 23 bar). The compressor has recently had a rebuild with new bearings and operated ok up until now.

Any ideas what could be causing the oil differential pressure to drop? (new filters installed)

Thanks in advance for any suggestions

Hi orm

in refrigeration 23bar would be quite high but in your case the oil difference pressure is a product of the differance between the suction pressure and the discharge pressure, low discharge pressure will create low diff pressures.

From memory this package consists on a high and low compressor, both driven off the same shaft.

Does the oil pump run all the time or just prior and during startup

If the pump does not run all the time the low discharge is what is causing the problem, no doubt

If the oil pump runs all the time you have a separate problem possibly

cold oil with refrigerant in it during start up

warm oil, problem with the oil cooler or mechanical damage in the compressor.

Hope this helps.

Kind Regards. Andy

[Brads]

Check to oil injection valve into rotors of both stages, maybe to much oil be injected - only need suficient to maintain discharge tempeature at required level.

Brad

Hi,

Just come across this site and wondered if anyone could help. I have a Mycom 2520 two stage screw compressor core packaged by other company operating on natural gas.
The problem is on a start up it keeps losing the oil differential pressure, but this only seems to happen when the discharge pressure reaches 14bar (normal output 23 bar). The compressor has recently had a rebuild with new bearings and operated ok up until now.

Any ideas what could be causing the oil differential pressure to drop? (new filters installed)

Thanks in advance for any suggestions

[orm]

Thanks for your ideas

Yes, this compressor has a low and a high stage, the oil pump is running all the time when the compressor is running and up until aprox 14bar discharge pressure it will maintain a 2.5bar differential then it just seems to drop away - this also seems to coincide when the machine starts to suck gas. The oil is warmed up before running the compressor.

[US Iceman]

The compressor has recently had a rebuild with new bearings and operated OK up until now.

If the compressor operated with sufficient oil pressure before the rebuild, then the oil pressure problem developed sometime after the rebuild.

I would suspect the rebuild. It sounds like something has failed or is beginning to.

...this also seems to coincide when the machine starts to suck gas

If you mean the problem appears when the compressor starts to load then this sounds like an oil injection problem.

Check to oil injection valve into rotors of both stages, maybe to much oil be injected - only need sufficient to maintain discharge temperature at required level.

I think Brads is on the right path. The oil injection flow rate could be too high or an o-ring is leaking around the slide valve.

Is the discharge temperature in a normal range during operation?

[orm]

Took out the oil strainer today and found bits of white metal in there - bearings? Could this allow more oil to pass through them, increasing the flow and lowering the d.p

Discharge temp was fine when it was running before

The bearings were new with the rebuild - the question is - if it is them - is why they have failed. Any ideas guys?

[US Iceman]

There are two points to oil injection in screw compressors:

1) oil for bearing lubrication
2) injection for sealing the rotors and cooling the compression process

The majority of the oil pumped is used for oil injection. The bearings actually require only a small volume.

Discharge temp was fine when it was running before

Is the discharge temperature higher now?

What is the oil injection temperature?

We need to have more information to go on, otherwise this is like guessing.

Can you provide some of the operating data please?

[orm]

Hi

The temperature in the oil sump where the discharge feeds into is approximately 70C to 75C , this then gets pumped out of the oil sump, through the oil filter and thought the oil cooler, the thermostatic valve on the cooler controls its output to 50C. The oil cooler output then goes to the oil header pipe that feeds the first and second stage bearings, the injection oil into the screws and the slide valve control of the first and second stages. The oil header differential pressure is measured between the gas pressure in the oil sump (combine sump and separator) and the oil header pipe.

The temperatures mentioned are when the machine was running previously, but now the machine is tripping on low header diff pressure before these temperatures are reached, the oil sump gets to about 50C

On start up, the gas pressure reducing valve is 100% to recycle all gas, the oil pump runs first to build up the oil heater differential pressure to 3.0 bar (this is achieved at this point) then the main motor starts that drives the compressor. Approximately 5 seconds after this, the gas pressure reducing valve on the discharge output starts to close (to start building up the gas pressure). As the pressure reducing valve starts to close oil differential drops very slightly (0.2 bar). With the pressure reducing valve now less than 10% open, the first and second stage slide valves start to load up and the discharge gas pressure starts to build up. The slide valves are now at typically 20% each. (oil differential still dropping slightly – another 0.2 bar or so. When the gas discharge pressure reaches about 14bar (set o/p 23bar) the oil differential pressure drops rapidly (2.6 bar to 1.5 bar in about 3 seconds) the machine then trips as the low low set point is 1.5 bar.
As the oil differential pressure drops away so does the current drawn by the oil pump motor by about 1.5 amp i.e. 10.5amp down to 9amp.
Also the lube oil filter differential pressure starts to go high (1bar) about 1 second after the oil differential pressure drops away. The lube oil and filters are less than 2 months old.

The inlet pressure is 1.5bar, the first stage discharge is 4 bar and the second stage discharge is 23 bar.
I can supply more info if needed

Thank You

[RAVIKUMM]

Re: Screw compressor system losing oil differential pressure
Hi all,
I am a new user.
I am also facing this same problem with WRV (T) 255 Range single stage compressor .

Oil gets pumped out of the oil seperator sump, through the oil filter and thought the oil cooler. The oil cooler output then goes to the oil header pipe that feeds the bearings, the injection oil into the screws and the slide valve control of the first and second stages. The oil header differential pressure is measured between the gas pressure in the oil sump (combine sump and separator) and the oil header pipe.

Now the machine is tripping on low header diff pressure While loading the machine.i.e while reducing the suction pressure from 2 kg /cm2 to 0.75 kg/cm2.On start up, the gas pressure reducing valve is 100% to recycle all gas, the oil pump runs first to build up the oil heater differential pressure to 3.0 bar (this is achieved at this point) then the main motor starts that drives the compressor. Now
the inlet pressure is 1.5bar, discharge is 10 bar .Oil differential pr between filter is 2 kg at 20% load.While loading further we are facing problem.
.There is one differential pressure control valve in discharge line.It was also calibrated.Oil pump discharge pr is 14 kg but we are getting 12 kg after oil cooler and filters.Filters were changed and found no white metal traces.
I can supply more info if needed.
Please response.
Thanks,

Thank You

[Andy]

Re: Screw compressor system losing oil differential pressure
Hi all,
I am a new user.
I am also facing this same problem with WRV (T) 255 Range single stage compressor .

Oil gets pumped out of the oil seperator sump, through the oil filter and thought the oil cooler. The oil cooler output then goes to the oil header pipe that feeds the bearings, the injection oil into the screws and the slide valve control of the first and second stages. The oil header differential pressure is measured between the gas pressure in the oil sump (combine sump and separator) and the oil header pipe.

Now the machine is tripping on low header diff pressure While loading the machine.i.e while reducing the suction pressure from 2 kg /cm2 to 0.75 kg/cm2.On start up, the gas pressure reducing valve is 100% to recycle all gas, the oil pump runs first to build up the oil heater differential pressure to 3.0 bar (this is achieved at this point) then the main motor starts that drives the compressor. Now
the inlet pressure is 1.5bar, discharge is 10 bar .Oil differential pr between filter is 2 kg at 20% load.While loading further we are facing problem.
.There is one differential pressure control valve in discharge line.It was also calibrated.Oil pump discharge pr is 14 kg but we are getting 12 kg after oil cooler and filters.Filters were changed and found no white metal traces.
I can supply more info if needed.
Please response.
Thanks,

Thank You

RAVIKUMM

Cold oil would do this, does the tripping stop when the oil is up to temperature.

Check for cold oil in the oil separator.

Kind Regards Andy

[Mike W]

Sounds like the clearance between the rotors and the housing in the oil pump are excessive. Have you checked the condition of the oil pump bearings and the oil pump rotors. Best indication of the this condition is loss of oil pressure as the oil warms up. I hope this is useful.

Regards Mike W