Slide valve wear on Grasso screw after just 54000 Hrs

[Lodiev]

I need some clearance on a major issue.
We purchaged a Z52 Grasso screw compressor 11 in 1998, and it ran nicely.
A cupple of month back i noticed that the compressor is not running at full amps anny more when it is 100% loaded.
When we opend it we saw that the slide valve housing inside the casing was worn so badly that the slide valve moved upwords and cutted away the rotor tips.
This was the firs time in my life of working on screws(16jrs) that i saw this happening to a compressor.
It had onley 54000 running hrs, and its stufed.

The sad thing is that we have 4 of these compressors on site and im worrying about them.
I have worked on all tipes of screw compressors but Grasso's, never liked them. the sullair's,stahl's,sabroe's and howdens just keep on running with minor problems.

Dit anny one out there come acros this before and can u maby explain why this will happen to a screw
or is Grasso just not wurth bying?
i'm considering to replace it with a Howden WRVi 255/1.93 screw, do anny one have one for me at a good price?

Thanks

[TXiceman]

I would suspect that at some time, the machine was run with an escessively high oil pressure to the injection oil. Lifts the slide valve.

Another issue could be that at 54,000 hours it is due for a rebuild. If the slide valve controls were not set correctly, it would let the machine hunt and work the slide valve enough to cause excessive wear. The the slide bore or valve is worn, it will let the valve rock side to side and can get up into the rotors. You should have been able to hear that occuring.

Do you have a the machines under a PM program and monitor the vibration analysis? If not, they should be.

The Howden can experience the same wear if not maintained.

Ken

[colindawson]

I think 54000 hours is way over the top.depending on use an overhaul should be carried out 30000 to 40000 hours.excessive slide valve wear is as your comment is do to the slide cont. loading and unloading.Have you considered a repair.

[Lodiev]

Thats just the thing, this must have happend over a period of time.
there were no unusual noise or vibration in the machine.
The only thing i noticed is that the motor amps dropped over time.
The oil pressure is always constandly running at 250 Kpa. The micro is a Micro logic panel from Grasso self.
The load and unloading controlls of the micro ar hunting but it is not becaurse of oil pressure or loading solenoids,its the programming of the micro self.
As i said i have worked on other machines with 80 000 hrs on and there were no wear on the slide valve or slide bore.
The machine were regularly serviced, and end float were checked every 6000 hrs.
The thrust bearings and white metal bushes were stil in superb condition when opend.
Is it not possible that there could be a design fault with the Z series?
This is the second fault of this tipe i'v heard of.

[Lodiev]

Acording to screw compressor manufacturers it can last 25 years if properly serviced.
This screw only lasted 11yrs.
The casing can not be repaird and must be replaced,the rotors is also not repairebl.
so its only the two end sheilds and covers thats reuseabl, thus cheaper to replace.
A new screw from Grasso is ZAR 432,000.00 deliverd in south africa wher a Howden is ZAR 382,000.00
I tend to agree with Ken about the hunting of the slide that caurst ecsesive wear.

[Lodiev]

Ken/Colin

I normally use the M&M micro prosessors.
Can the problem be with the programing of the controller?

[PaulZ]

Hi Lodiev
We service a lot of screws and the problem of a worn slide valve is common on compressors that load and unload a lot. This causes wear in the bore as well as the valve itself. If there is still some of the rotor tips left it can be repaired but will have some reduction in capacity, about 10 to 15%. If the tips are gone completely a new compressor is required.
I have seen this on Howdens, Stals and Mycoms so it is not just a problem on one make of compressor. Grasso's are generally fairly reliable and good build quality.
If you decide to repair the compressor you will need to get the bore line bored to true it up then get the slide built up and ground back so the clearance between the OD of the slide and the ID of the bore is between .0015" and .002". We have done this on numerous occasions with success.
A worn slide valve can be detected when the compressor is unloaded, it will rattle up to about 50 or 60 %.
When the thrust bearings are checked the slide valve should be checked at the same time.
Hope this helps
Paul

[Lodiev]

Thanks Paul
What can be i do to make sure this do not happen to other compressors?
Are there a way to stop this wear?
Do u know the Micro processor's that Grasso use?
I have noticed that it do load and unload quite a lot.

[PaulZ]

Hi Lodiev
I am not familiar with the Grasso micro processor we don't use factory controllers we use Allen Bradly PLC and get our programs written to our specs.
This wear can't be stopped only reduced.TXiceman's suggestion of high oil pressure could have been the problem and I have to agree with him about the hours. I wouldsuggest you contact Grasso and get a copy of their recommended service schedule and follow it.
The only way of not having this happen again is regular inspections and making sure Grasso's operating parameters are followed.
Have you checked all the others compressors for rattling at full unload?
If you notice any unusual noise I would inspect the compressor asap.
As stated by TXiceman get a vibration analysis done as part of a routine service schedule.
Screws have parts that wear and need replacing or repairing after certain amount of run hours.
I have seen screws that have done 60,000 hours and have very little wear, but these compressors run fully loaded all the time and run under design conditions.
One other thing to consider is the type and grade of oil. Is the oil used recommended by Grasso.
Hope this helps.
Paul

[Lodiev]

All running conditions is within grasso specs.
I use Petro Canada's Reflo 68A oil on all 23 screw packages and takes oil samples on a regular base for analising.
All i can make out from all the posts that could have been the problem is oil pressure that could have been to hi.
Do u know what the oil pressure shoeld be on a grasso Z52 oil ports to discarge difference?

[PaulZ]

Hi Lodiev
This is what I have regarding oil pressures, I am not familiar with a Z52 machine.
Paul

[Lodiev]

Thanks Paul