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Author: Subject: Just a warning about air compressors
David Jenkins

posted on 18/2/22 at 02:47 PM Reply With Quote
Just a warning about air compressors

Came across this while browsing YouTube today... scary...






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coyoteboy

posted on 18/2/22 at 03:56 PM Reply With Quote
My compressor makes me wince every time it charges. My seam is off the bottom, but I really dislike a pressure vessel with a seam aligned with the worst case stress.

I came very close to building an external house for it. I really will now.

Imagine the barotrauma from that, even if you didn't get taken out by shrapnel.





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adithorp

posted on 18/2/22 at 03:56 PM Reply With Quote
And that's why my insurance insist on engineers reports. Including ultrasonic thickness checks on the receiver incase of internal corrosion.





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David Jenkins

posted on 18/2/22 at 04:19 PM Reply With Quote
He did a follow-up...



He admits that he made mistakes, but he also says that the replacement isn't going back in his garage!





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Mr Whippy

posted on 18/2/22 at 05:03 PM Reply With Quote
hmm does look like it got very thin due to rust inside. Yeah I have no idea what the condition of mine are like inside or ever drained water out of them so think I'll have a look this weekend. That sure looked like a huge explosion

Thanks for posting.



[Edited on 18/2/22 by Mr Whippy]

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mcerd1

posted on 18/2/22 at 06:07 PM Reply With Quote
quote:
Originally posted by adithorp
And that's why my insurance insist on engineers reports. Including ultrasonic thickness checks on the receiver incase of internal corrosion.

same here - insurance company won't touch it without an annual inspection





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Oddified

posted on 18/2/22 at 07:43 PM Reply With Quote
My compressor lives outside in a B+Q garden storage box, wired and piped inside. Best thing i ever did from noise and freeing up space in my workshop....and of course if it should go 'pop' it's highly unlikely anyone will be anywhere near it.
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JoelP

posted on 18/2/22 at 08:11 PM Reply With Quote
Mine is very old now and full of rusty water. Time to bin it I reckon.





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coyoteboy

posted on 18/2/22 at 09:09 PM Reply With Quote
quote:
Originally posted by adithorp
And that's why my insurance insist on engineers reports. Including ultrasonic thickness checks on the receiver incase of internal corrosion.


House insurance?





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perksy

posted on 18/2/22 at 09:10 PM Reply With Quote
You see things like this and makes you wonder if buying a secondhand one is really worth it as you'll never really know what your buying?
I want a bigger one and was thinking of buying used, but not now...

There's some serious rust inside that tank

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mcerd1

posted on 18/2/22 at 10:49 PM Reply With Quote
quote:
Originally posted by perksy
There's some serious rust inside that tank

You'd be amazed how quickly they can rust internally - they just get thinner and thinner until one day they go bang
(I'm not going to break out the thin wall pressure vessel calculations though - it'll give me nightmares, I always hated those calcs...)

after a few years all of them will be thinner at the bottom than the top - the only way to know when its got too thin is to get them tested - plenty of places will do testing for you so why risk it



My dad's big machine mart one gets tested every year or two (business insurance send there own contractor we get no say)
basically its drained and visually inspected, then thickness checked with UT, then hydro-static pressure test to check the relief valve (filling with water makes it much safer during the test) - its ~20 years old now and might only have a few more years according to the reports we get.
(you can normally just replace the tank when it comes to that though)

but this compressor was bought to replace an old second hand one that had been made in the 1930's
we did scrap that one cause there was no way we could trust it, but on inspection it was surprisingly good - I guess they went for the brute force engineering solution back then (ie. just make it really thick then add some more thickness for luck) - but on the other hand it was that heavy that if it had gone I wouldn't want to even imagine the shrapnel damage it could have done
besides steel grades/strengths were a bit more haphazard back then too....



[Edited on 19/2/2022 by mcerd1]





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indykid

posted on 19/2/22 at 11:00 AM Reply With Quote
My compressor came out of the skip at work and the receiver had been cut up (scrapped due to a bad thermal overload switch (3) on the motor that nobody diagnosed). I now have the compressor on a small skid and a 230bar oxygen bottle stood next to it, inverted with a stainless pipework sump at the bottom. If I need extra capacity, I have a 47kg propane bottle that I pipe into it.

Using a compressor at this time of year, it collects a phenomenal amount of water. I'd say one of the best mods you can make to a standard compressor would be making some pipework up to make it convenient to drain down the receiver. If it's a fiddly screw underneath, you're never going to bother. With an elbow, a short pipe and a ball valve, it's dead easy. If you stick a sintered silencer on the end, it's not even a noisy or messy affair, just stick a little pot under it to catch the water.





me? ambivalent? well, yes and no

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nick205

posted on 20/2/22 at 09:11 AM Reply With Quote
Scary!

Posted on here before - a place I worked years ago had a big industrial scale compressor with pipes air round the building.

The compressor itself was housed outside the building in a soundproof shed. Still would have been a bang, but well away from people!

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MikeR

posted on 20/2/22 at 03:33 PM Reply With Quote
so how do you know if your compressor is safe?

I've got an aldi one that is probably 15+ years old. I drained the water a week or two ago and it was orange but only a few drops worth - 10 or 20ml maybe (i'm guessing at a volume). Ran it with the drain bung to try and get anything else out, pressurised it to a couple of psi and opened the drain again, shook it etc and nothing extra came.

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David Jenkins

posted on 20/2/22 at 04:13 PM Reply With Quote
quote:
Originally posted by MikeR
so how do you know if your compressor is safe?

I've got an aldi one that is probably 15+ years old. I drained the water a week or two ago and it was orange but only a few drops worth - 10 or 20ml maybe (i'm guessing at a volume). Ran it with the drain bung to try and get anything else out, pressurised it to a couple of psi and opened the drain again, shook it etc and nothing extra came.


It's tricky for an amateur - people who run these things for a living have to get them checked periodically for insurance purposes - and for safety and occupational health inspector reasons of course. The air tank would be scanned with an ultrasonic probe to check the thickness of the steel - the inspector would know how thick it should be, and how much corrosion is permissible. There may be an internal inspection with a borescope as well.

After that, all but one of the holes would be plugged, the air tank filled with water, then more water pumped in to a set pressure - the figure of 150% of working max pressure rings a bell. If it survives that without fracturing then the tank is good - until the next inspection. Don't rely on my guess - it may be 200% for a steel vessel. At that point the safety over-pressure valve would be checked, together with a check that the pressure gauge is at least in the correct ball park. These would be tested after the max pressure was tested.

Only the hydraulic test is practical in the home workshop, as long as you can get hold of a decently accurate pressure gauge and a small hand-operated water pump. When I used to have a 5" gauge model steam locomotive, the club's boiler inspectors used to get their gauge properly checked for accuracy at regular intervals. Mind you, steam boiler explosions are 100 times more nasty than compressed air tank failures, as the volume of high-pressure steam at say, 100psi, increases by around 500% when the failure happens...

The advantage of hydraulic testing is that, if there is a failure, all that happens is you get wet - as water is near-enough incompressible then it's only the elasticity of the tank and fittings that increases the pressure. If it fails then all you usually get is a thin stream of water piddling out somewhere. Usually, when you're testing, it's one of the things that you've made to plug the holes that leaks - very annoying! I used to have a box of boiler plugs ready for the 2-yearly inspection (2 years for copper boilers, 1 year for steel) and it was always them that failed during tests.



[Edited on 20/2/22 by David Jenkins]





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David Jenkins

posted on 20/2/22 at 04:19 PM Reply With Quote
Out of random curiosity - is there any reason why they don't make air compressor tanks out of stainless steel?

Is it a structural thing? For example, work hardening over time, weld fractures, or similar. Or is it simply that mild steel is cheaper and the manufacturers are penny-pinching? I guess the latter, but there may be a technical reason.

Do industrial air compressors use stainless steel? You know, the places where cost isn't an issue!





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David Jenkins

posted on 20/2/22 at 04:34 PM Reply With Quote
BTW: If anyone's wondering I have a 10-year-old large-ish compressor in the garage that only gets drained when I remember and, to be perfectly honest, gets neglected.

Clearly it's time to give it a proper and thorough inspection...





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mcerd1

posted on 20/2/22 at 07:35 PM Reply With Quote
quote:
Originally posted by David Jenkins
Out of random curiosity - is there any reason why they don't make air compressor tanks out of stainless steel?

Cost I'd imagine, but partly because normal grades stainless can be a pretty crap structural material - they just don't have the near perfect elastic behavior with a clear yield point like mild steel typical has (and other structural steels too)

same reason that most stainless (and aluminium alloys) make a poor choice as structural bolt material - there is a reason all the production cars use 8.8 or 10.9 bolts and its not cost...
(Also why none of us should be using stainless bolts on critical bits of our cars like suspension, steering, brakes, roll hoops etc...)


that said I'm sure it would be possible to design a stainless tank with suitable margins - but if your going that far then why not go all the way to COPV's
https://en.wikipedia.org/wiki/Composite_overwrapped_pressure_vessel



I'm definitely no expert on pressure vessels but in other more general structural use mild steel really only has the corrosion issue to deal with, if you keep the design within its elastic limit fatigue / creep etc.. just aren't issues for most structures
and as a bonus its easy to form and weld too
In my industry we deal with the corrosion problem by galvanising nearly everything - but I would imagine that could cause a number of issues on a pressure vessel even if you could add enough bung holes for the venting....

[Edited on 20/2/2022 by mcerd1]





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Sanzomat

posted on 20/2/22 at 07:42 PM Reply With Quote
quote:
Originally posted by David Jenkins
quote:
Originally posted by MikeR

The advantage of hydraulic testing is that, if there is a failure, all that happens is you get wet - as water is near-enough incompressible then it's only the elasticity of the tank and fittings that increases the pressure. If it fails then all you usually get is a thin stream of water piddling out somewhere.


[Edited on 20/2/22 by David Jenkins]


Similar to hydraulic systems with hydraulic fluid rather than water. A few years back I was part of the team building a new universal test facility for Airbus. The hydraulic system was really interesting. The operating pressure was 280 bar or over 4000psi and it was tested to over 6000psi. It passed first time but I asked what would happen if there was a leak and they basically said, not much - as it is tested hydrostatically hardly any fluid comes out for a big drop in pressure. Apparently flexible high pressure hydraulic hoses are quite dangerous if they fail though as they can puncture with small holes that give a very fine needle of spray that can inject itself through people!

[Edited on 20/2/22 by Sanzomat]

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MikeR

posted on 20/2/22 at 07:54 PM Reply With Quote
quote:
Originally posted by David Jenkins
BTW: If anyone's wondering I have a 10-year-old large-ish compressor in the garage that only gets drained when I remember and, to be perfectly honest, gets neglected.

Clearly it's time to give it a proper and thorough inspection...


Mine had been neglected for a bare minimum 6 years - ie as long as we'd been in the current house surprised how little water came out, however its only used once in a blue moon.

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mcerd1

posted on 20/2/22 at 07:57 PM Reply With Quote
quote:
Originally posted by MikeR
...surprised how little water came out, however its only used once in a blue moon.


If you've not had it running much of that time you'll not have been adding moisture into it

but still if reading all this makes you worried enough then maybe best to see if you can find someone to test it for you


I've got another very small machine mart one that I really should get looked at myself - but as it hardly gets used it tends to get forgotten about

[Edited on 20/2/2022 by mcerd1]





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SteveWalker

posted on 21/2/22 at 12:10 AM Reply With Quote
Fully filled with water, a small, water hand-pump and a pressure gauge are all you need to test - I've done that for a boiler at the local model engineering society. You would need to replace the safety valve with a solid replacement or at least hold it shut.
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jester
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posted on 24/2/22 at 10:07 AM Reply With Quote
I can see why they are banned from buliding sites then
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907

posted on 19/3/22 at 04:38 AM Reply With Quote
For those that are wondering they do make compressor tanks from stainless steel and probably
most of you have experienced one, as for hygiene reasons they supply air to your dentists drill.
Ive made loads in my time and are used in conjunction with oil free compressors that have PTFE
bearings. The ones I made were made from 10 NB sch10 pipe and a couple of pressed end caps.
Welds argon backed, full penetration and x-rayed, and insured and annually tested by Eagle Star.

Paul G

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