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:D 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:confused:

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

[Josip]

Hello, Orm :)


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.

maybe you answer to your problem ;) diff oil pressure arise always when oil filter is clogged (white metal you found there is maybe from previous bearings) but now coming back with oil from oil separator and oil cooler and stop at oil filter.

BTW, did you wash (clean) complete oil system (with oil) before installation of renewed compressor;)

Remain debris can cause another overhaul :(

Best regards,

Josip :)

[orm]

Hello Josip:) ,

Some more information, the machine was running fine and the filer d.p was 0.16bar constant. The compressor was then stopped to allow maintenance on another part of the plant. This fault happened the first time the compressor was restarted and it tripped on low oil d.p:(

The oil system was cleaned the best we could – not all parts were accessible, the oil was refilled and pump round the system for 1 day with a new filter element and checked ok

The system has the ability to change over the oil filter in service– (this one never being used) and the same trend happened:confused:

Cheers
Orm

[US Iceman]

HI orm,


The compressor was then stopped to allow maintenance on another part of the plant.

Was the maintenance connected to a part of the system that was connected to the compressor suction?


The oil system was cleaned the best we could – not all parts were accessible, the oil was refilled and pump round the system for 1 day with a new filter element and checked ok

Was the pump around cycle only on the screw compressor package? If so, the lube circuit for the package was probably clean to start the compressor.

If the system was serviced upstream of the compressor suction, the debris that you are dealing with now is probably the result of the service work.


The system has the ability to change over the oil filter in service– (this one never being used) and the same trend happened.

I've seen this first hand on some start-ups after the system has been taken down for service or system modifications. On one project we went through more than 60 oil filters in a two week period.

The gas being drawn through the system will pull all of the junk back to the compressor. Since the oil filters are doing their job, they fill very quickly. Hence, low oil dp.

[orm]

Hi US Iceman:)

The maintenance was unrelated to the compressor system.

Yes the pump around cycle was on the compressor system only.

The compressor was rebuilt due to oil compression in the screws causing scoring on the casing – this was identified as a faulty drain down valve not allowing the oil to drain out on a start up. The compressor was fully opened up and rebuilt with a new part of the casing and a complete new set of bearings. The bearings that were removed looked discoloured some had black carbon deposits (cause unknown – these were aprox 5 years old from new). The compressor was then run for approximately 25 days up till this stop and then the fault on restart

When the oil d.p goes up, it then also starts to fall with the oil header differential pressure until the trip. Also the current going down on the oil pump motor:confused:
The oil pump has its own internal 4bar differential and then there is a differential pressure set valve after the cooler set at 3 bar – both have been checked and found to be ok

Cheers

[Andy]

The oil pump has its own internal 4bar differential and then there is a differential pressure set valve after the cooler set at 3 bar – both have been checked and found to be ok

The oil pump reduces in amps:confused:

Why, is the pump still getting a good oil feed:confused:

Did you clean the pump suction filter and check the feed line to the pump:confused:

Is the pump ok:confused:

Kind Regards. Andy:)

[Josip]

Hi, orm


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.

How long time after rebuilding? Maybe you have stucked oil pressure regulator!?



The gas being drawn through the system will pull all of the junk back to the compressor. Since the oil filters are doing their job, they fill very quickly. Hence, low oil dp.

this is possible cause especially if you have a very fine or paper oil filters.

Another question: how long after start your compressor is running = 5, 10 min or?

I had a similar problem here with one reciprocating compressor, start with cold oil was ok but with rising oil temperature oil DP falls and compressor stops before reaching normal oil temeperature. Cause was worn slide bearings but the same is if oil pump is not ok.

But if you have too high Oil DP on oil filter it means your filter is clogged (for screws we have warning at 0,5 bar and cut out at 0,9-1,0 bar)

It is not easy to determine cause on this way, sorry for so many questions, just want to help ;)

Best regards,

Josip :)

NB

Due to my english I need a longer time for writing, this post must be after US Iceman and before your and Andy's posts;)

[US Iceman]

But if you have too high Oil DP on oil filter it means your filter is clogged (for screws we have warning at 0,5 bar and cut out at 0,9-1,0 bar)

That is a good point. When the oil DP is decreasing, is the filter DP increasing?

PS to Josip: Don't worry about taking longer to write in English. Sometimes it takes longer for me to think about the problem! :)

[orm]

Hi Andy

Yes the oil pump reduces in amps – as if the oil flow is getting easier!

It seems strage that everything is normal up until the system pressure reaches 14bar even the filter d.p, so assume the oil pump is ok and getting a good feed.

[orm]

Hi Josip

The compressor has rum for about 25 days after the rebuild – all ok, then compressor stopped for maintenance on another part of the plant (unrelated to compressor) and on starting, this problem occurred (never seen before). We dismantled the pressure regulator and it looked ok, even tried blanking the output to check that it was not passing – still the same problem.

Machine run time depends on the system pressure. If the system is flat (all gas vented out) the machine runs for about 2 minutes until it reaches 14 bar – then trips. If you try and run again from this pressure it runs about 45 seconds before the trip. Before starting the compressor, the oil system is warmed and circulated so it is all up to 40C.

The more questions the better – need all the help I can get!

[orm]

Hi US Iceman

The system has the ability to trend some of the parameters, the filter d.p starts to increase just after the oil dp is decreasing, but then the filter d.p also then starts to fall. The motor current also starts to fall as if the flow is getting easier?

Is it possible to post the trend? Would than help?

Cheers

[Josip]

Hi, orm

please, can you rotate your compressor by hand to see if there is a big resistance or compressor rotate smothly? Of course make your system flat (empty).

Please, debris in oil filter you found after trip when you were not able to start compressor or before?

Best regards,

Josip :)

[US Iceman]

HI orm,


Is it possible to post the trend?

Yes, but the file size has to be less than 100 kb. This is a size limit placed on the site for attachments.

Please... If you can post the data trend with time stamps that should help us to "see" what is happening.

[Josip]

Hi, orm

if you have a dbf. file make a ZIP then we can open it in excel and make a graph (trend)

Best regards,

Josip :)

[orm]

This is a print screen from the first start.

The trends are:

Pink – lube oil motor current
Green – oil filter d.p.
Light blue – oil header d.p.
Black – discharge pressure
Yellow – oil sump temp
Red – oil header temp (after cooler)
Purple – main drive motor current
Blue - % open of gas pressure control / recirculation valve

The two black vertical lines give the values – shown at the side

[Andy]

This is a print screen from the first start.

The trends are:

Pink – lube oil motor current
Green – oil filter d.p.
Light blue – oil header d.p.
Black – discharge pressure
Yellow – oil sump temp
Red – oil header temp (after cooler)
Purple – main drive motor current

Blue - % open of gas pressure control / recirculation valve

The two black vertical lines give the values – shown at the side

Please explain the gas recirc valve set up, as it all goes pear shape when it starts to rise and when the discharge pressure rises.

Kind Regards. Andy:)

[orm]

When the gas leaves the second stage of the compressor, it goes into the primary separator / oil sump (combined). The gas then leaves the primary separator and goies through a fin fan air blast gas cooler to lower the gas temperature to 50C. This then goes through a smaller secondary separator and then though a non return valve into a receiver / accumulator vessel.
The gas pressure control valve is between the secondary separator and the non return valve. It is used to control the output pressure by diverting anything above 23bar back to the suction intake. On a start up this is 100% open, and then closes to build the pressure up, in normal running this is less than 10% open. If the discharge pressure goes above 23bar, the valve will open slightly to reduce the output pressure.
Hope that’s clear :confused:

[orm]

Hi Josip

I will try rotating the compressor when I am next there (Monday now)
Compressor had operated fine up until this point – the debris was found after this start-up

I have a later trend (few days after the first start) that I don’t have access to at the moment – but this shows more d.p on the oil filters


Cheers

[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