Screw compressor element service life

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Screw compressor element service life

Postby Vegman » Fri Aug 16, 2013 12:43 am

Our vessel runs and Atlas Copco GA22 screw type air compressor as the "working air" compressor. Its one of those package types in a big blue box.
It runs 24 /7 although its unloaded probably about 70% of that time.
As part of an investigation into next years budget, I was looking through the manual trying to find the expected replacement time of the screw elements, and to my surprise ,there doesn't seem to be one. The highest service item listed was to replace the oil seperator at 8000 hrs.
I seem to remember on a previous vessel we removed the screw elements ( casing and all, as a unit) and replaced with a "reconditioned" at around 20 000 or 40,000 hrs- , I cant quite remember. We then sent the old unit to the Atlas Copco agent as an exchange. How they actual "recondition " one ,I dont know. Probably just replace some bearings and seals and take some measurements and throw it out if its out of spec.

Does anyone else have any experience of replacing these elements or any knowledge of expected service life?

Thanks / regards

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Re: Screw compressor element service life

Postby JollyJack » Fri Aug 16, 2013 6:27 pm

I found the manual here ... 315-02.pdf and you're right! Service schedule has the oil separator at 8000 hours as the longest interval period. The impellers are not even mentioned, except in the parts list. I sailed with one of these things, it ran forever, almost always unloaded, as you say, just humming away. We just followed the manual with filters etc, the big blue box was quite unremarkable and we had other fires to fight.

I suspect it's along the lines of the Rootes blower fitted to Screaming Jimmies, where the impellers don't actually touch anything and the roller bearings are oil lubricated. Relies entirely on mass flow to deliver the air.
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Big Pete
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Re: Screw compressor element service life

Postby Big Pete » Sat Aug 17, 2013 12:01 am

I agree with Jimmy, I dont think there is any metal to metal contact, the screws of the compressor are driven by timing gears and the oil film on the rotors makes the seal. They are pumping a mixture of oil and air which is why the oil seperator is so critical. So long as you use the correct oil and change the oil & filters on schedule there should be no wear.

I would take a condition monitoring approach. Maybe once a year drain down the air bottle completely and time how long it takes to fully pressurise it, when the time increases by 10% over the original (so long as there are no air leaks) it might be time to service exchange the unit.
Check the unit for vibration regularly and change the bearings if they start to vibrate.

No help in planning your budget going forward I am afraid.

You can always put it in your planned budget and then not do it, to free up some money for contingencies!

It is always better to ask a stupid question than to do a stupid thing.

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Re: Screw compressor element service life

Postby JK » Sat Aug 17, 2013 4:25 am

agree with above comments.
Bet the tech super catches it though, it when it is not bought. If not then, then the 2nd year when it shows up again. :P

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Re: Screw compressor element service life

Postby Vegman » Thu Aug 22, 2013 1:11 am

Thanks for your replies- Not too worried about the budget, it is only a "guide".
Yes ,I agree correct the oil is critical. I remember on the previous vessel , the lubes contract was with Shell, but the only non Shell oil we used was the "Atlas Copco Roto Inject Fluid". Even though the shell rep swore that their oil was just as good , we just seemed to get more problems if we didnt use the roto inject fluid. Also changing oil filters and cleaning out the reclaimer regularly was important.

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