compressor rebuilds

[scuba]

Just wondering what everybody bases their screw compressor rebuilds on,run-time or vibration,oil and end float testing. Our company runs mycom screws and they rebuild the compressors after 25,000 hrs run time even know all our tests our well within the manufacturers specs. I think this is throwing money down the drain. And if the rebuilds are based on run time what is a more reasonable amount of hours.

[CHIEF DELPAC]

We try to rebuild our MYCOMS at 40000 hours.We rebuilt a 320 su at 43000 hrs last Feb. This compresser would not have run much longer without sustaining damage to the rotors.

[US Iceman]

Some time ago I listened to a presentation by a well known Frick person. He stated we should be expecting 100,000 hours or more from the compressor life. I happen to agree.

Compressors are treated so poorly it is a wonder they last as long as they do. I think the issue with short time rebuilds is that we are still treating screw compressors as if they were recip.'s.

The use of oil and vibration analysis and other long term preventative procedures can allow you to monitor the condition of the compressor and decide when the compressor should be rebuilt, rather than an arbitrary run-time value.

My opinion is that we should spend more time keeping the inside of the system clean and use high filtration performance filters. Once a bearing is damaged due to debris it is already in a failure mode.

[IceMan_4000]

Some time ago I listened to a presentation by a well known Frick person. He stated we should be expecting 100,000 hours or more from the compressor life. I happen to agree.

Compressors are treated so poorly it is a wonder they last as long as they do. I think the issue with short time rebuilds is that we are still treating screw compressors as if they were recip.'s.

The use of oil and vibration analysis and other long term preventative procedures can allow you to monitor the condition of the compressor and decide when the compressor should be rebuilt, rather than an arbitrary run-time value.

My opinion is that we should spend more time keeping the inside of the system clean and use high filtration performance filters. Once a bearing is damaged due to debris it is already in a failure mode.

So perfectly put US Iceman,

I get asked questions like this all the time. I think Hrs are just to satisfy the consumer requirement for an estimated life expectancy, and the manufactures need to sell machines.

The only true way to tell the how the inside of machine is, oil and vibration analysis.

If you want to push a machine to its limits then you better listen to what it is telling you.

[nh3wizard]

Just wondering what everybody bases their screw compressor rebuilds on,run-time or vibration,oil and end float testing. Our company runs mycom screws and they rebuild the compressors after 25,000 hrs run time even know all our tests our well within the manufacturers specs. I think this is throwing money down the drain. And if the rebuilds are based on run time what is a more reasonable amount of hours.

Sometimes this depends on if its a owned or leased facility, I had work for a company 15 years ago that had leased the facility from someone else and that was the part of the contract to have the screw compressors rebuilt after the set amount of hours.

I agree it is a waste of money if everything is still within manufacturers specs.

[josef]

I agree with the US Iceman

[US Iceman]

I am a big believer in synthetic oil, oil & vibration analysis, and high performance (low micron) rated filters.

Synthetic oil is more stable and can easily checked for breakdown by using the oil analysis methods. Sure this oil is more expensive than mineral oil, however...what is the cost of the oil program (and compressor rebuilds) over a period of time. This is first cost mentality versus long term benefits & costs.

The logic of rebuilding compressors every XX hours is the same as changing oil every XX hours. When we do not use good analysis methods we end up resorting to old time methods that are based on rules-of-thumb or anecdotal evidence, which may not present a clear picture of anything.

[RANGER1]

I find a few other factor can effect rebuild intervals.

Is slide valve at 100 % load all the time . Any screw this is weakest point if constantly adjusting causing slide valve/housing and guide block or guide wear.
Older as well as new plants can be less than perfect . Usually the cheapest price wins.
How much redundancy if any has plant got . Clients ( accountants ) dont like to waste money on equipment thats not running .
If it does break down , how much money will client loose because of not doing preventative maintenance.
If they can all do 100,000 hrs i probably wont have much to do .
I think 30-40 000 hrs high side operation and 50-60 000 hrs booster .It also depends on quality of previous rebuild as in Australia there is no changeover service like in USA and Europe.

[brian_chapin]

I am a big believer in synthetic oil, oil & vibration analysis, and high performance (low micron) rated filters.

How about a recommendation on synthetic oil?

[IceMan_4000]

No matter what oil you are going to switch to, I would make sure that is is sutible for the o-rings in the seal of the machine. It could be a buna or neoprene or HSN which can have compatibilty issues with the oil you are changing to.

always a good idea to ask the manufacture

[josef]

Answer many questions
www.irc.wisc.edu/file.php?id=214

[Segei]

I think that this is good article. Just one remark. Total energy use to overcome friction is 3-6%. I remember that one company promised to save 10% of energy use by improving friction. How can you save 10% if you use only 3-6%?

[Mongrel]

There are many factors which contribute to Screw compressor longevity. The main consideration of an overhaul is the Thrust bearings which are typically of the angular contact type. Compression ratio, Operating temperatures, Oil type and qualities, Oil cleanliness (read here filtration quality), Oil temperature, Oil pressure (not to be excessive, which is usually the case, and as well not to have suffered oil dilution from liquid flood backs etc) Load profile (as in load variation causing slide valve adjustment), Stop/Start frequency, can all be a major contribution to thrust bearing life. Manufacturers' recommended rebuild frequency is generally an arbitary figure based on experience. Equipment not adequately maintained will deteriorate accordingly. Conversely, equipment with absolutely ideal conditions will perform more than admirably. I have personally seen Mycom 320 Compressors stripped after 100,000 hours and still look like they had just arrived from the factory. It should be remembered that White metal Main bearings are a Non contact bearing (designed to run on an oil film) and as such could last, who knows how long???? Where as, a roller bearing has a designed LMR ("Life in Millions of Revs"). This is the main reason that Thrust Bearings do require changing periodically, as do ball roller mains. As for oil type? Cost being a major consideration, needs to be weighed up against proven performance, not just sales stated or anticipated performance. Synthetic oils are expensive and costly to change when required. Recommended Mineral oils can be just as effective if all other parameters are adequately controlled. As for Vibration monitoring? VA is limited to the monitoring of Roller bearing (Thrust or Main), coupling allignment, and rotor meshing. It cannot monitor, Slide valve wear, Balance piston and sleeve wear, White metal main bearing wear and journal wear, Guide block wear. Oil particulate analysis can indicate component wear to a great degree, and as such, is most helpful. Operational Log trending is also another great tool. Computerised data logging is very popular for analysis recently. Variations in operating conditions can be readily determined. Sorry about being so long winded. regards Mongrel.

[US Iceman]

How about a recommendation on synthetic oil?

Sorry Brian. I must have missed these last few posts.

I would talk to CPI Engineering in Midland, MI.

web link: http://www.cpieng.com/

They are the experts and who Frick, FES, Howden, and Vilter all use.

[US Iceman]

I remember that one company promised to save 10% of energy use by improving friction. How can you save 10% if you use only 3-6%?

Perhaps it is 10% of the 3-6%, which is a lot less of the total?

[josef]

US Iceman,
web link, good explanation

[RANGER1]

Each refrig plant is different and you can also judge overhaul after first rebuild .
If it looks good maybe extend run hrs a bit longer and re-judge next time .
If they run 100,000 hrs you may only see it three times in your career (in my case anyway ) .
I have heard of a mycom 320 L booster running 160 000 hrs and is still going , thats got to be some sort of record but no one is making any money from it .

[chilldis]

synthetic oil,regular oil analysis,vibration analysis and good old fashioned observation of each compressor. trust your impressions of a unit. if something seems to change over a short time period there is a very strong possibility that a overhaul is indicated.