Trimming rotors on screw compressors

[passandscore]

Hi Guys,

I have to replace a Mycom i160SS-M ammonia screw compressor. The compressor makes such a loud constant noise when is starts up and runs. You need ear protection. It is awful. We are fairly certain it is a bearing. All pressures and temperatures are normal when it is operating.

We have had this problem in other facilities (Ice Rinks) with this model of compressor. Mycom has trimmed the rotor to allow it to suit our application prior to initial installation. Can anyone tell me if trimming the rotor could lead to premature failure. Is it common to trim rotors?

[RANGER1]

passandscore,
When you say trim rotors, do you mean length?
How much?
Suction end?
any pictures, diagrams?
is it to match a capacity or something.
I don’t think it’s common, but with repairs etc it happens.
Howden WRVI compressors they trim probably 10-15 mm of discharge end to install plates for Vi.
No real problem.

Can you describe plant with operating pressures & temps.
Oil cooling type
single or several compressors on one base or system.

Is is there possibility suction check valve or oil solenoid valve if fitted leaks filling compressor with oil or refrigerant.

Lots of questions!

[Grizzly]

Hi Passandscore.
Like Ranger I am not a hundred percent what you mean by trim.
But as an educated guess, could you possibly be talking about Skip Steps.
Where on commissioning the Engineer will load the compressor 1% at a time.
listening and watching for Refrigerant Gas resonance through the machine.
Most screws can growl quite badly at certain running speeds / gas flow conditions.
These speeds are then skipped within the software.
That's from new.
How old are your comps?
Grizzly

[passandscore]

Apparently they trimmed the suction. Not sure how much. I will have to find that out.
The compressor is 4 years old. I have simply been asked to replace the compressor. This may be a cause of a bad bearing but of course I have to do my due diligence to ensure that is indeed the case. Attached are the links to the diagrams. System operates at 150psi discharge and 20-25psi suction.

Package Diagram: https://drive.google.com/open?id=1uR...sCFkens5i-KBem

Field Flow Diagram: https://drive.google.com/open?id=1Kc...--RqPjcv1iqifu

[RANGER1]

Passandscore,
Looks like a well designed system.
As far as trimming rotors, which is slightly unusual in normal refrigeration, can’t really see an issue unless not put back together 100%.
Manual recommends 30-40,000 hrs between major service, really need to pull apart & inspect.
Manual recommends
- maximum 30 seconds for oil pressure to be above suction by 500 kpa , so check differential valve after oil separator. Really should be set minimum 600 kpa differential, as oil filter differential trip recommend set 100kpa.
- suction check valve bypass isolation valve should be kept closed.
- lube oil solenoid should be ball valve type, as standard solenoids in oil lines can take many seconds to close causing compressor to be flooded with oil.
- for some reason Mycom recommend check valve after oil lube solenoid valve, but if ball valve would think no problem, check out.


I think a lot of issues COULD occur with compressor potentially flooding with oil when turned off/on.
When turning on, oil compression will reduce thrust bearing life considerably.
A machine like this could have a oil sweep mode when turned on/off.
If turn machine off, first oil lube solenoid closes, say 5 seconds later compressor stops.
Same on start up, maybe when run signal achieved maximum 15 seconds.
I know you can’t really achieve this without proof, but at least you can look for oil flooding of compressor as first, then report.
If machine is full of oil, it cannot free drain when off due to check valve between compressor & oil separator.
Compressor rotors should always be above oil level in oil separator, so can have possibly of free drain.
Also check operation of auto oil drain, make sure it does not flood back to much when operated.

More than what you ask for, but can’t help myself.

[passandscore]

RANGER1,

Thank you for the detailed reply. I will check out everything you mentioned and report back.

[mbc]

*Hi Ranger

for some reason Mycom recommend check valve after oil lube solenoid valve, but if ball valve would think no problem,?

I think .
This check valve ,Prevent oil and gas of refrigeration inside oil goes back through oil pipe and solenoid to the pump
And during start pump can work and pressurize our system

[passandscore]

Update:
I have sent the compressor back to Mycom. They will inspect and rebuild it. I suspect they will tell me what I already know. Bearing failure due to either high oil temps or poor oil flow. I suspect that the issue is due to a poor oil flow rate. Mycom recommends 17GPM at the inlet to the compressor. At this time I am not sure what we have. I am hoping to rent an ultrasonic flow meter to confirm. Unfortunately, our engineers are convinced that the flow is adequate based on calculation alone. I am skeptical because we are only using a 5/8" line to supply the oil to this compressor. It seems too small for this model. Other than that we have clean, clear oil with proper oil temperature. Oil analysis came back in good order.

I will keep you posted.

[Tycho]

Update:
I have sent the compressor back to Mycom. They will inspect and rebuild it. I suspect they will tell me what I already know. Bearing failure due to either high oil temps or poor oil flow. I suspect that the issue is due to a poor oil flow rate. Mycom recommends 17GPM at the inlet to the compressor. At this time I am not sure what we have. I am hoping to rent an ultrasonic flow meter to confirm. Unfortunately, our engineers are convinced that the flow is adequate based on calculation alone. I am skeptical because we are only using a 5/8" line to supply the oil to this compressor. It seems too small for this model. Other than that we have clean, clear oil with proper oil temperature. Oil analysis came back in good order.

I will keep you posted.

5/8" inch pipe seems to be a bit on the narrow side, considering that the connection on the compressor is 1" if I was reading from the correct datasheets.

Also from your diagrams it seems like there is a lot of changing up and down in dimensions on the oil line.

oil line.jpg

I did a quick calculation, and to squeeze 17 gpm through a 5/8" pipe, the velocity in the pipe is 19 feet/second, I like to see the velocity in oil lines at no more than 7 feet/second.

I'm not familiar with this compressor package, so how is the oil pressure measured? is it with a flow switch or with differential pressure?

[RANGER1]

Tycho,
You could be on to something there.
Mycom programme indicates 56 l/min 30 degC sct -15deg sst
68 l/min 35 deg C sct

Fittings all over the place, some from Tycho exploded view show 1/2' threaded, if I read correctly, which is a big restriction.
It has a pressure switch connected to oil line & discharge, how does that work, what is it set too?

[passandscore]

Hi guys,

The oil pressure relies on the discharge pressure to create a differential pressure. Initially there was a differential control that would act as the safety control. The setpoint was 40psi. There has been a lot of issues with that setup because during low ambient when the compressor was offline for a while it would take a bit of time to establish proper pressures and this would cause the control to trip. The other techs were simply jumpering out the control! So no oil protection! Since then it has been set up like a reciprocating compressor where the oil pressure needs to be higher than the suction pressure. Setpoint is 40psi. I am not too sure that this is ideal for screw compressors. Perhaps you have thoughts on this.

Mycom got back to me. They reported that the thrust bearing was pitted and nearly disintegrated. Their words, not mine. Apparently the worst they have never seen for a compressor with only 15,000hrs. The rotors had knicks and chips in them. I wasn't expecting that news. Apparently not cost effective to rebuild. So they figure that foreign matter has gotten inside the machine but there wasn't any visible that didn't get picked up in the suction strainer. I assume I will be getting more pieces to this puzzle as I begin to dig a bit deeper. Mycom is still not happy about the lines sizes.

Any chance that high velocities could cause damage like this? It may be a stretch. Thanks for your feedback.

[RANGER1]

Passandscore,
Can you verify what fittings are in main oil supply, is there some 1/2” threaded fittings.
Compressor has 1” connection, so that may say something!
Are you getting true oil pressure at machine connection? There is a fitting to connect directly into main oil gallery.
Also a lot machines with differential oil feed are set with minimum pressure differential across machine.
Mycom manual for this machine recommends 500kpa (75psi) differential for max 30 seconds, as 40 psi (280 kpa)sounds a little low.

Also what type of oil solenoid, ball valve, conventional solenoid?

Did you find a lot of oil in machine when you swapped out.
Wondering if full of oil on start up.

[Josip]

Hi, all ... quite a guiz this thread ...

I do not recognize this type of screw i160SS-M so I took a parallel with
FM160L-M ... I believe must be quite close if not equal, but I can be wrong...

Anyhow ... I agree with Tycho ... oil lubricating pipes are quite weird ...
up and down with dimensions of reducers, valves, filter etc... too many
obstacles causing oil pressure drop...

According to my old Mycom data ... main oil pipe after oil cooler for this compressor
is DN50 and reduced close to compressor at DN20 ... without any reducer only
stop valves and oil filter size DN50 ...

Also according to data I have ...
Lube oil pressure should be
Max ... Pd+4kg/mm2 (G)
Min ... Pd+1kg/mm2 (G)

So 40psi is about 2,7 bar what seems quite ok as a pressure, but I am not sure
if there is enoug oil flow ... through pipe of 5/8" ... sounds to small

... in screw compressor we need higher oil pressure then is our discharge pressure
and surplus oil flow is used to lubricate and cool down all bearings what was probably
reduced in this compressor causing that big damage ... sounds thare were some
lubrication and cooling but not as required ... sorry for damaged machine and problems caused

... another problem was ... jumpering out lube oil pressure safety control ... what was not, is not and never will be alowed ... that is maybe main cause of that big damage of beraings and whole machine ...

You need to use 3-way lube oil distribution valve so even in harsh condition (low ambient temperature i.e. cold oil) there is warm oil at inlet into compressor means no oil pressure control trip ...


Best regards,
Josip