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Air Compressor Warning

The bomb in the workshop (retitled to encourage air compressor owners to read this thread)

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Oldiron24/01/2021 12:46:56
975 forum posts
40 photos

The tank drain valve on my small compressor is left just cracked open all the time. I very rarely use it for more than a few minutes at a time. In 10 years it most probably has less than 15 hours running on it. When I switch it off I always listen to make sure the air is bleeding off and hopefully any water that has accumulated with it. Maybe a waste of £ leaking air every time I use it but I am sure that it has been drained. I also made sure when I installed it that the drain was at the very bottom of the tank. When you look at some tanks the drain is not actually at the very bottom.


Chris Evans 624/01/2021 13:45:01
2056 forum posts

Reading this lot has convinced me to replace my 20 year old compressor, I am very lax at draining down but when I was at work I drained daily. Home discipline has wained.

Phil Whitley24/01/2021 14:08:05
1442 forum posts
147 photos

Looking at pics and videos of compressor failure all round the net, one sees the same thing over and over again! there is little or no rust in the tank, at least not deep penetrating rust, and the tanks always fail at the side of the long weld, not the weld itself. This is a classic fatigue failure and is caused by the fact that the metal from which the tanks are made is far too thin, and when they fill with air,they expand like a balloon and when the air is used, they contract, which causes metal fatigue, and the weakest part of the tank is the metal next to the weld!

All these tanks have safety valves, and yet they are still failing, which means either a faulty valve, or that the tank is failing below the set pressure on the safety valve, which once again indicates that it is fatigue failure. It is not caused by poor maintenance, although you should drain at least weekly for other very good reasons, it is caused by poor manufacture and low quality steel. I have a 1949 broom wade tank which is made from rolled boiler plate 1/2" thick. It is showing no signs of failure, and probably never will . As someone has mentioned above pressure vessels used to be tested at 100% over pressure to find the SWP, today they are tested at 50% overpressure, so ask yourself why the standard has been lowered!


not done it yet24/01/2021 14:08:45
6806 forum posts
20 photos

When you look at some tanks the drain is not actually at the very bottom.

And even if it is, the drain (on some of the real cheapies) protrudes into the tank - thereby never allowing all water to drain! Cheap compressors are not designed and built to last. I will stick to my slow revving two stage cast iron compressor pump for as long as I can keep it going. It was built to last 10 times as long as these cheap things that need to over-hype the real capability of the machine ( by quoting air displacement instead of free air delivery).

Colin Whittaker24/01/2021 14:16:53
142 forum posts
17 photos

I read somewhere, maybe on this site, that a mains power to close solenoid valve on the drain would automate the draining of the tank. I've done it and everytime I unplug the compressor there's a noisy automatic purge. If I ever replace my compressor (I'm only 64) then I'll do the same again.

Cheers all, Colin

noel shelley24/01/2021 14:28:59
1339 forum posts
21 photos

It's always a good idea to lift the safety valve by hand periodically. If done on a full tank it should need little effort indicating that it is set not far out. If it won't lift or needs force, then it needs repair or replacement and resetting before the compressor is used again. noel

Sam Longley 124/01/2021 14:39:55
941 forum posts
34 photos

Personally, I think you are over thinking this. If the pressure relief valve fails then a build up in pressure will cause catastrophic failure. My compressor cuts off at 120PSI & i know that the safety valve goes off at just over that because I do test it from time to time. As for the tank going rusty, All that is going to happen is that a section will rust through and a pin hole will let air out. It may spread to a hole of a few mm. It is not going to explode.

If, however, one starts to try repairs etc or alter the working pressure to a higher than recommended, then that is another matter. For the relatively small cost of a typical hobby machine it is not worth keeping them. Get a new one.

As for gas cylinders- now that is a different matter. We had a fire in one of my site huts & there were a large No of bituthene- rolls of felt- next to some propane bottles. The heat bent the barrel of a cartridge nailing gun. As the cylinders exploded they went through the ply shed sides & uncoiled like long springs. They were recovered yards away.

Rik Shaw24/01/2021 15:26:03
1482 forum posts
398 photos

I am very wary of big bangs having been injured in one as a kid so I must admit to being a bit windy watching these vids especially as my 5/6 year old 25 litre PowerCraft budget job lives under a bench within a few feet of my carcase.

Since I replaced all the connectors on the compressor, hoses and tools with PCL standard connectors I am no longer cursed with air leaks. Since then my normal pattern of usage is to bring the compressor up to pressure then switch it of at the mains socket. I mainly use it for blowing out machined work and using it thus a full reservoir can remain usable for a week or two. Repeat as necessary.

The tank collects very little water – most times I undo the drain I get air only. I wonder if this is because the workshop is heated, insulated and double glazed? I have just been up there and unscrewed the drain and only got air – if memory serves right the last time I undid the drain would have been maybe three or four months ago.

I am not offering any advice or recommendations as to the safe and proper way of using a “bomb” merely my way of doing things.

This is the first time I have ever seen a report of one of these budget compressors going bang. Hopefully it’s a bit of a “one off”.

One final thing and call me a “wuz” if you will but when I DO power the thing up I always retire safely to the house while it is inflating . As soon as that deafening “BLAAAAAR” stops I resume normal service.

Rik (still wincing at the split reservoir in the vid)

old mart24/01/2021 16:07:22
3771 forum posts
233 photos

Regular draining may or may not help with internal corrosion, but always helps with reducing the water in the lines which can make spray painting difficult. The compressor at the museum is in a lean to outside the building, and to make the job of draining the reciever easy without getting too technical, I removed the drain cock and connected a length of hose. The hose passes through the wall to an inline valve on the wall and then back out to discharge on the ground. Being easy to reach, the water is drained each time the compressor is shut down. Otherwise somebody would have to go outside, unlock the compressor house door and reach underneath to drain it. I don't see that happening very often. Funny how these threads bounce back and forth across the pond.

mechman4824/01/2021 16:29:07
2947 forum posts
468 photos

I have a Clarke 'Ranger' compressor plus a small paint sprayer compressor, both of which I use rarely but even so I leave the drain valve on both of them open after use. So far no sign of moisture/ water from the either so i don't think I have a lot to worry about at the mo'.


old mart24/01/2021 16:36:18
3771 forum posts
233 photos

Many of the compressors which don't get used much could have their working pressure reduced to give a bit more peace of mind. You probably don't need 150psi if most of the use is paint spraying, 50psi might be adequate.

not done it yet24/01/2021 17:03:18
6806 forum posts
20 photos

Posted by Sam Longley 1 on 24/01/2021 14:39:55:


All that is going to happen is that a section will rust through and a pin hole will let air out. It may spread to a hole of a few mm. It is not going to explode.



That makes it very clear that you do not read through the thread before posting. Please go back to Gary’s first post and just look at one of the videos he linked. Then, perhaps you will not try to deny the obvious.

Neil Wyatt24/01/2021 17:37:48
19032 forum posts
734 photos
80 articles

A few people have been in touch today as a result of this thread.

Current advice is NOT to use water for testing air receivers.

Please see this link and the downloadable best practice guide:

Be safe, we don't want to lose any of you...


Nicholas Farr24/01/2021 17:39:41
3360 forum posts
1542 photos

Hi, I don't know what grade of steel is used in these compressor tanks, but some steels can suffer sudden catastrophic failure in very low ambient temperatures when being highly stressed, by brittle fracture, Brittle Fracture I'm not saying this is the case in the O/P's situation, but one should take such things into considerations before blaming poor quality steel or jumping to other conclusions. Of course any welding on such steels can change the structure in the HAZ (heat affected zone) so unless you know the grade and can perform any heat treatment that may be required after welding, then any welding on them should be a no-no. Of course, there may be a minimum and maximum working temperature in the manufacturers users operating manual, which should always be observed.

Regards Nick.

Edited By Nicholas Farr on 24/01/2021 17:50:36

Neil Wyatt24/01/2021 17:41:41
19032 forum posts
734 photos
80 articles

For avoidance of doubt:

"Hydraulic/hydrostatic testing (i.e. pressurising the system with water) is no longer a recommended test method. Significant problems arise in removing the water after the test, and there are potential safety aspects of any residual water left in the system being ejected under force when compressed air is reintroduced. Contact BCAS if details of the hydraulic method are required."

jimmy b24/01/2021 18:41:08
786 forum posts
42 photos

My Aldi compressor is about 16 years old, gets drained now and then.

Always left full of air.

Having read this thread, I dragged it out and took the drain fitting out, so I could have a look inside, with my bore scope.

Rusty as hell at the top and bottom, but the sides looked pretty good,

I'm undecided if I'll replace this or not....


norman valentine24/01/2021 19:02:14
280 forum posts
40 photos

I have been left almost shaking with disbelief at this thread. Some years ago I bought a small compressor at an autojumble. When I got it home I placed it on my bench and switched it on. I was a little surprised at how long it took to come up to pressure when suddenly the pressure relief valve blew off. I was standing beside it at chest level, imagine if that had exploded!!!!

My next compressor will be new.

Speedy Builder524/01/2021 19:08:08
2613 forum posts
212 photos

Neil's comment is interesting in that the testing information is all about supply lines and equipment excluding air receivers. It is straight forward. As model engineers, most of us have a compressor, most of us are sitting on a potential bomb yet no one seems to know how to safely test these air receivers. We pride ourselves in pressure testing piddly little steam boilers yet sit on top of a far more lethal system inside our workshops in regular use. Inspection by borescope doesn't really allow us to inspect under 30 years of rusty sludge to determine shell strength.

Has anyone found approved reliable methods of testing, and not the U tube videos that proliferate the net ?


old mart24/01/2021 19:26:00
3771 forum posts
233 photos

The museum has regular mandatory reciever inspections and I have never been able to unscrew the plugs at either end for a visual check. The inspector does an ultrasonic thickness check of the lowest point where the corrosion is most likely. He told me that fortunately, explosions are rare and the usual cause of leaks is the partial failure along a weld.

Nicholas Wheeler 124/01/2021 19:26:42
928 forum posts
87 photos
Posted by old mart on 24/01/2021 16:36:18:

Many of the compressors which don't get used much could have their working pressure reduced to give a bit more peace of mind. You probably don't need 150psi if most of the use is paint spraying, 50psi might be adequate.

Ever tried it with a small(ish) compressor? Makes for a much less consistent air supply at the gun, which is the last thing you want when spraying. And the compressor runs all the time even when painting small panels.

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