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  1. #1
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    Default compressor cutout pressure

    A thread in Marketplace got me thinking about the operating pressure of my old Clisby compressor. I have no idea what the condition of the tank is and it seems it is quite expensive to have the tank tested. Currently the compressor cuts out when the tank pressure is 130psi. I rarely use more than 40 psi for spray painting and tyre inflation. I use a thin 10m hose for painting and I need about 60psi at the regulator to achieve 40psi at the gun.

    Would it prudent to drop the cutout pressure to something lower than 130psi to reduce the stress on the tank? Obviously I will lose air capacity by doing this, but I doubt I need the full 130psi for my purposes. I'm assuming my cutout switch is adjustable.

    What pressure do your compressor tanks run at?
    Any thoughts on my idea?
    Chris

  2. #2
    BobL is offline Member: Blue and white apron brigade
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    Can't see why not.

    FWIW My Clisby is set to trigger at 135psi.

    Unless you run your compressor continuously, most of the wear happens at the higher pressures/temperatures which happens near the end of the recharge cycle so dropping the max P will ease that a bit. Of course if the compressor has to cycle more often then you will lose some of that advantage

    I know a chap that has his cycle between 80 and 105 psi

  3. #3
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    Quote Originally Posted by BobL View Post
    I know a chap that has his cycle between 80 and 105 psi
    Thanks, that's good to know.
    Chris

  4. #4
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    I would not be concerned about a bit of rust in your compressor air receiver 99.9% of the domestic compressors in this country would have rust on the inner surface of the receiver
    even if the tank does let go they pretty much never explode they just leak at the weak spot

  5. #5
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    Hi Chris, Guys,

    Normally a pressure vessel is tested at twice the working pressure.

    The way it is done is to seal off all the holes bar the one that is going to be used for the testing. The tank is filled with water and then more water is pumped in whilst monitoring the water pressure. Should the tank burst while testing there is very little pressure left to cause any damage or injury. Usually you just get a leak which is immediately noticeable on the gauge as a drop in water pressure. Normally you would let the tank stand under pressure for a while and monitor for any pressure drop, which would suggest a leak.
    Best Regards:
    Baron J.

  6. #6
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    Thanks gents. TBH I've never heard of a domestic compressor exploding, so I'm probably worrying about nothing. A quick Google tells me that to test it properly you need to measure the expansion of the tank under pressure to be sure of the tank's integrity.
    Chris

  7. #7
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    I have a Clisby compressor that is about 30 years old with a good size air tank. The pressure cutout is set to around 100psi.
    I do not set the pressure higher, not because of the air tank, but to reduce wear on the compressor.

  8. #8
    BobL is offline Member: Blue and white apron brigade
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    Quote Originally Posted by jack620 View Post
    Thanks gents. TBH I've never heard of a domestic compressor exploding, so I'm probably worrying about nothing. A quick Google tells me that to test it properly you need to measure the expansion of the tank under pressure to be sure of the tank's integrity.
    A few years ago I did an extensive search on the web about compressor explosions and any aftermath and found one on you tube showing a bloke getting badly hurt. The fact that there are so many compressors out there and only one video about it shows the relatively low risk. As others have said, the tanks that rust will just leak.

    The cost of getting a tank tested has to include covering the cost of the testers insurance. I suspect that given there are so few explosions the insurers will not have a way to adequately assess risk so they will charge premiums to cover their backsides multiple times over.

  9. #9
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    I adjusted mine to cycle between 70PSI and 100PSI.
    Chris

  10. #10
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    Here is video of a domestic tank explosion caught on cctv https://www.youtube.com/watch?v=KVP_A7eGYxw and some photos of others https://sites.google.com/site/metrop...mpressor-tanks

    The tanks I threw away measured with an ultrasonic thickness tester at less than the the design thickness minus their designed provision for rust. (Got the Clisby data from VIC and the Peerless was from NSW) These tanks are only 3mm thick with a 0.75 mm rust allowance. It doesn't take much to get pitting that deep with enough negligence.

    AS1210 has a maximum test pressure for hazard level D pressure vessels (ie the pressure in Kpa x the volume in litres is less than 1000000) of only 1.5 times the design pressure with many tanks only needing to be tested to 1.25 times. The original wall thickness was only 3mm 0.75mm is a 25% reduction in thickness but the testing on these tanks was only to a pressure 25% greater than the design pressure.

    Some compressors have reservoirs designed to AS2971 and these are not pressure tested before delivery. It seems a misuse of that standard to manufacture this type of air receiver using it.

    The main issue isn't that the tank might leak or even that it might burst it is the fashion in which it does this. If I could be sure a few pin holes would have signalled a problem well in advance they would have stayed.

    Also not being the sole user of these tanks was an issue too. Someone had removed the safety relief valve from the Peerless at one stage so I could never be sure it hadn't been taken past its design pressure. The Clisby had been on long term loan too.

  11. #11
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    Quote Originally Posted by jack620 View Post
    Thanks gents. TBH I've never heard of a domestic compressor exploding, so I'm probably worrying about nothing. A quick Google tells me that to test it properly you need to measure the expansion of the tank under pressure to be sure of the tank's integrity.
    Hi Chris, Guys,

    My understanding is that a properly designed pressure vessel will not expand measurably at its designed working pressure !
    Best Regards:
    Baron J.

  12. #12
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    Hi John,
    it certainly will. Small vessels like scuba tanks and firefighter's tanks can be tested in a water jacket. The expanding vessel displaces the water and the expansion is read on a calibrated tube. If it exceeds the specification the vessel is condemned. For bigger vessels (if you have lots of money):

    Pressure Vessel Test Service - Maximator GmbH
    Chris

  13. #13
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    Hi Chris, thank you for your reply.

    I don't dispute what you say ! Indeed in the case that you mention, very high pressure vessels, the continual pressure variance from empty to full will cause flexure and in turn fatigue.

    However I said

    My understanding is that a properly designed pressure vessel will not expand measurably at its designed working pressure !
    The conditions under test can be quite different, particularly if you are investigating the elastic properties of a tank.
    Best Regards:
    Baron J.

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