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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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Gary Wooding24/01/2021 08:09:59
982 forum posts
254 photos

A friend at my club sent out the following YouTube links as a 'head's-up' to all members.

*Air Compressor Warning*

*His View on What Went Wrong*

My compressor doesn't get a lot of use - mostly for a monthly car tyre pressure check, but it got me worried 'cos I haven't drained it for several years. I lowered the pressure to about 5psi and opened the drain plug - nothing came out! I removed the drain plug and managed to clean out some gooey stuff from it - rather like black poster paint. But still nothing came out of the tank. I then started to poke around the plug opening with a short bit of stiff wire and eventually some rusty coloured water came out. Not much, about 100ml I suppose. I've replaced the drain plug but haven't filled the tank yet.

What do you think?

not done it yet24/01/2021 09:09:43
6806 forum posts
20 photos

Hi Gary,

That is why I test my 150psi compressor tank at 300psi! It is a vertical receiver and is always drained at end of use. It is old but of heavy construction (unlike the modern offerings). It is also at least twice the size of the one in that video.

I think you likely should change the receiver - there are loads of cheap compressors dumped after the pump has failed. Cheap, as in purchase price of 3000rpm buzz boxes.

I bought a small receiver to sit outside my workshop a couple of years ago. It cost me £2.20 and looks like it is in perfect condition.🙂 It will be tested, it will be mounted vertically and it will have a remotely powered drain valve and it will not be highly pressurised. I will also check it with my endoscope (‘cos I have one!) before testing it.

Where I worked, over 30 years ago, there were literally dozens of compressors dotted around. Most were not overly guarded (but safety tested regularly for insurance purposes). Only the one in the instrument department, fitted under a bench, was protected, to a certain extent, by a robust frame with heavy gauge expanded steel mesh, to prevent the receiver from expanding too far, if it did let go.

I’ve seen the results of explosions first hand - some controlled events and a few which were not. One such uncontrolled explosion was of a fan, fitted as ‘an extra’ which blew coal dust to a burner pipe. It should never have been positioned where it had been installed. Workers used to sit on it at times (nice and warm). It exploded leaving a shape like a tulip, while running down after a plant shutdown/trip. The air to fuel ratio obviously passed through the explosive range and up it went. The replacement was protected by a robust guard!

DiogenesII24/01/2021 09:21:19
557 forum posts
221 photos

'Black poster paint' doesn't sound good, because it'll be iron from the steel walls of the tank.. ..if there's a larger bung that you can remove to inspect the inside it would be worth a visual check - the trouble with leaving standing water in it is that pitting tends to be worst along the water-line - obviously a linear stress-raiser is not good..

The options depend much on the age, size quality etc., of your compressor - you could blank the bungs and hydraulically test (to say 1 1/2 X WP? any advice welcome) if the thing is a professional / quality bit of kit and will be expensive or difficult to replace with something of similar quality..

..if it was a small tool-shop cheapy then it's pretty much a no-brainer - anything that was built-down-to-a-price in the first place is most likely to have been of the minimum thickness to provide an adequate safety margin when it left the shop all those years ago..

I have a nice industrial compressor that spent the first half of it's 40-year life in a workshop on a seafront, but having seen the pits inside it two years ago when I recognised my subconscious habit of making sure that I was never in the same room with it under pressure, it's in complete retirement until a decent receiver comes up at a suitable price..

Martin Connelly24/01/2021 09:35:06
2137 forum posts
222 photos

I fitted a timed relay operated vent to the base of a large receiver once. It opened once a day for about 5 seconds to release any water and was to avoid someone having to remember go out and do it manually. Even that operating if you were not expecting it would make you jump, and that's with a sintered exhaust muffler on it.

I don't think his idea of cooling the air going into the tank would help, it is not removing the water content from the air. Hot air will hold more moisture but cooling it down does not remove it in a closed pipe.

Looking at his tank it has a large plugged boss with the drain at the centre, I would think that is for internal inspection and is probably of more use than tapping it to see if it is weak at any point. I suspect that the weld was a stress raiser and that repeated pressure and release cycles had flexed it. The requirements for weld caps to avoid stress raisers are quite clear, more weld is not a better weld and sharp transitions are a no-no. Corrosion may have contributed but without proper examination of the exposed surface where the failure originated it is impossible to be sure what the root cause was.

Martin C

Nick Clarke 324/01/2021 09:51:25
1422 forum posts
63 photos

The drain plug seized in a 'high street' compressor I had had for 20 years or so. After ignoring it for a few years, like the OP, I ended up turning it on its back (it was oil-less) cutting the drain plug off and using an easy out and a bit of local heat to get it out. Turning it right way up only a dribble of very rusty water came out but that was sufficient to make me think and so it went out to the bottom of the drive where within 10 minutes an Eastern European gentleman in a white Transit took it away.

I bought a replacement with a slightly higher spec to that of the old one from the same high street supplier for a lower actual price than the original. Quality seems about the same, but the peace of mind from scrapping the old one, which after 20 years owed me little, was immense.

Edited By Nick Clarke 3 on 24/01/2021 09:52:35

Chris Evans 624/01/2021 09:51:47
2056 forum posts

I have a cheap compressor that cost around £50 about 20 years ago. I will put my hand up and admit to not draining it daily, more like monthly when I think about it. Time I thought about a replacement to ensure safety. Like a lot of tools when you reach age 70 plus, the how long will I be using it for question comes up.

Brian H24/01/2021 10:03:45
2312 forum posts
112 photos

I'm just off out to the workshop to check mine. I'll also make sure it starts at a very low pressure.


Nicholas Farr24/01/2021 10:06:10
3360 forum posts
1542 photos

Hi, quoting Martin, "I don't think his idea of cooling the air going into the tank would help, it is not removing the water content from the air. Hot air will hold more moisture but cooling it down does not remove it in a closed pipe."

I couldn't work that out either, only a drier will remove water before it get to the receiver, which can be a unit as big as the compressor, where I worked years ago the they had one as the plants used a very large volume of compressed air as they had multiple air rams operating all sorts of slides and doors etc. In industry all air compressors are normally inspected internally annually, by independent qualified persons, before they get an insures certificate. One time in my last job while working at a customers site, a work mate and myself had to drain down a nitrogen receiver and remove the inspection doors and some pipework, but the first part of the job was to put up exclusion barriers and no entry signs before we opened the draining valve or removing the cap on the draining valve and then we had to wait a half an hour or more after the receiver had drained down before we could go back and remove the inspection doors, the receiver being about 3M high and about 1.2M in diameter.

Regards Nick.

Andy_G24/01/2021 10:09:25
173 forum posts
The tank in the original has signs of previous repairs around the bung in the bottom of it,

Edited By Andy_G on 24/01/2021 10:10:12

Nick Clarke 324/01/2021 10:30:24
1422 forum posts
63 photos

And another


MichaelR24/01/2021 10:33:08
474 forum posts
76 photos

Not knowing much about workshop compressors and the mention of water being in the air tank brings back memories of my National service days as a MT driver in Berlin, in winter we had to put Antifreeze in the braking system air tank for obvious reasons.

So in a cold home workshop would the same apply to the home workshop compressor tank, could anything affecting safety freeze up ?

Just me having a rambling thought.


Mike Poole24/01/2021 10:42:53
3335 forum posts
73 photos

Considering a pop bottle can take 6-10 bar I would suspect the tank ruptured due to a failure of the pressure switch and surely there will be a safety release valve as well? Having a safety valve is no good unless you know it will operate when it is needed. One of my work tasks was to do the electrical safety inspection on machinery, it is not surprising that we nearly always found something that required attention. Maintenance and testing of a compressor should be a task that is understood well and performed regularly. A compressor is on my get one one day list but it was always going to live in an external sound insulated box as they are generally a bit noisy. As safety usually involves some redundancy then an ultimate pressure switch and a mechanical over pressure safety valve would be essential I would have thought.


Howard Lewis24/01/2021 11:00:08
6104 forum posts
14 photos

When I bought my compressor the supplier, (A local compressed air specialist ) advised draining the compressor after every use, certainly at least once a week. The other advice was to leave the drain open when the compressor was not in use. In this way, and condensation is able to drain out, even if it is a film on the walls which gradually runs down

On the bus company, I drained the big garage compressor 200 gallon reservoir every week. It always froze the moisture coming out, so that draining stopped until the ice thawed, sometimes several times.

The air brake reservoirs on the buses were drained every time that they came over a pit, for any reason..

Even my little 50 litre reservoir freezes in the same way from time to time.

A faulty pressure vessel can be dangerous (A friend lost the end from two fingers when a compressed air tank, thankfully a small one, one exploded close to him! )

There is picture of, admittedly a steam boiler, from a post World War 2 Baltimore and Ohio loco that exploded and tore itself from the frames and finished up inverted on the damaged rails.

I would suggest running the compressor with the drain plug out, so that anyhing obstructing the drain will, hopefully be blown out.

Based on what you say, it ought to be hydraulically pressure tested, before further use, to ensure that you are safe.


Ramon Wilson24/01/2021 11:08:30
1319 forum posts
382 photos

For quite a few years I had a cheaper SIP compressor with volume tank inside my workshop. It was very noisy so boxed it in though it was accessible. I would drain it when I thought about it and loads of rusty water would fill the top of a P38 lid at least once. Only switched on when required it would make me jump every time it kicked in - it was right beside my lathe! - which it would do about every twenty mins or so - never could trace the leak that caused the drop in pressure???

Any way one day went for a cuppa and stood outside in the garden suddenly aware that the compressor had kicked in but was constantly running. Went to look - workshop filled with the noise of escaping compressed air. A hole about the size of a finger nail had blown through at the side of the weld on one end cap about halfway up the tank. Poking it the metal was quite thin over about 30mm or so but a friend requested it to see if he could replace the tank. He then thought he would give it a try and welded it!! and yes it's still in use but not with the regularity of use I gave it.

Yes I know and quite agree - not good practice at all I'd say and certainly not to be recommended but that's what he did.

I now have a much quieter one resident inside the garage - peace in the workshop reigns


Regards - Tug

Ramon Wilson24/01/2021 11:18:14
1319 forum posts
382 photos

As an aside to the above.

Just to put the potential into perspective, in my diving days I once had a diver on my crew - Phil P - who in previous years had been in close proximity to a air filter that had exploded on a HP compressor. One side of his upper body was peppered with small black pock marks where the charcoal filler inside the filter had penetrated his skin to some depth.

A different type of compressor in psi output I agree but the potential is always there.


not done it yet24/01/2021 11:18:26
6806 forum posts
20 photos

So in a cold home workshop would the same apply to the home workshop compressor tank, could anything affecting safety freeze up ?

It should not! There should not be any accumulation of water in the receiver, if the proper maintenance instructions are followed. Generally the warm/hot compressed air would sufficiently warm the safety valve gear while in use Which should be properly lubricated, as well, at intervals. The risk of these explosions would not be increased unless working in really cold conditions - and we would unlikely be in our workshops if that cold!

Nicholas Wheeler 124/01/2021 11:22:28
928 forum posts
87 photos
Posted by Mike Poole on 24/01/2021 10:42:53:

Considering a pop bottle can take 6-10 bar I would suspect the tank ruptured due to a failure of the pressure switch and surely there will be a safety release valve as well? Having a safety valve is no good unless you know it will operate when it is needed.

I wonder how just effective a compressor would need to be to rupture a typical welded steel tank in good condition? Mine's driven by a claimed 2Hp motor, and a fairly small compressor head; I doubt that could do it.

These failures are not caused by the compressor operating outside its design, but by the material of the tank failing. That's largely due to the tank rusting internally due to the water in the compressed air, and is why any compressor pressure vessel should be drained frequently and inspected regularly. That often doesn't happen in professional workshops and hobbyists are even more lax.


My 30 year old SIP compressor needs a new starter, is even noisier than it should be, and takes far longer than it used to fill the tank. I've been considering stripping it and replacing all the worn parts, but this recent spate of videos and posts across several forums have convinced me that I'd be better spending a little more cash on a new one, and saving the time for something more productive. That's a chance to get one more suited to the paint spraying which is my main use - I only use air tools when a decent electric(battery for preference) equivalent isn't available

Edited By Nicholas Wheeler 1 on 24/01/2021 11:24:19

Howard Lewis24/01/2021 11:23:24
6104 forum posts
14 photos

Direct drive compressors are noisy!

A brand new SIP compressor kept cutting in for no obvious reason. Eventually, we found a pin hole in the weld on one end. A coded welder vee'd the weld and rewelded the area.


SillyOldDuffer24/01/2021 11:38:35
8664 forum posts
1960 photos

Um, I'd like to see a second opinion before assuming the weld failed due to rust. Could be, but...

In the second video, this looks good:


while the first video suggests it's been mended and is in much worse condition:


Looks like the tank has been mended. Have to say patching up safety critical items isn't smart unless you really know what you're doing. And most of us don't! There's a big gap between really knowing and confidence based on limited personal experience. On a job like this years of hands-on experience is low value compared with x-ray and ultra-sound testing!

I'd like to know how often the compressor was used. Looked like a busy work shop, so maybe every day for 10 years. A problem with pressure vessels is they have to cope with more than just pressure. They also have to be strong enough to resist metal fatigue caused by repeated expansions and contractions. These caused the de Havilland Comet to break up in flight, even though the pressure changes were low. Micro-cracking is invisible, it can't be detected by simple means, and it's accelerated by corrosion, physical damage and any other stress-raiser. In this example, the weld may have failed due to fatigue, or a ding, not corrosion. Without a proper examination no one knows.

Metal fatigue means pressure vessels have a limited lifetime after which they are expected to be scrapped. Manufacturers assume that customers won't extend tool life by repair, and most don't. The whole unit is replaced when the motor eventually breaks, or compressor fails, or the gauges misbehave. Home-workshop guy is probably the main exception, because we have ways and means of fixing stuff. In our world keeping old gear going is a good thing, but there are always exceptions. Probably not wise to keep a big air compressor going past it's use by date!

I suggest at least 3 of the improvements suggested in the video are wrong:

  • Running the compressor outside. Unsafe air compressors shouldn't be used at all! It has to be approached to operate it, and what about passing neighbours or wandering children? With H&S it's more important to protect others and their property than yourself.
  • Hitting the tank with a hammer is more likely to cause problems than detect them! Pointy hammer even worse.
  • The longer pipe intended to dry the air won't remove water.

Instead, I suggest reading the instructions and applying a full maintenance regime. In addition to draining water and replacing consumables, proper maintenance includes checking the compressor's general condition and age. I suggest a well used air compressor should be replaced immediately the manufacturer's recommendation lifespan is reached, and whenever a major repair is required.

Age isn't simply measured in years, just as important is how frequently and for how long the compressor is run.

Sad but true, all tools have limited lifetimes. Nothing is meant to last for ever. If irreversible wear and tear makes a tool dangerous, get rid... Good news, exploding compressors are unusual. This hasn't put me off mine!


Ramon Wilson24/01/2021 12:41:17
1319 forum posts
382 photos

A couple of things I think are worth further comment.

NDIY says early on "That is why I test my 150psi compressor tank at 300psi!" - well I hope that's done hydraulically as that's bad practice if not. Forgive me if I'm wrong 'ND' but I've always thought that a pressure vessel is a pressure vessel and testing should always be done hydraulically.

The second matter is one of 'water' being produced. In simplified terms it is a result of the cycling of air pressure within the receiver and a direct product of increase and decrease in pressure and temperature. As a volume of contained air is pressurised it increases quite significantly in heat - when it decreases it rapidly cools and moisture within the air condenses. I can describe this based on my own personal experience of the situation. As anyone on here who has sat inside an active decompression chamber will know that is exactly what happens in use. A decompression chamber is nothing more than a large volume tank.

As the pressure is decreased - to rise to the surface - the chamber atmosphere begins to cool rapidly and the moisture content in the air increases to such an extent it can be seen s a mist. That moisture condenses immediately on the walls and rolls to the bottom. The slightest increase in pressure sees the reverse - the air clears and warms immediately indeed it was used as a method to clear the moisture for those in the chamber - rise past the stop point by 1 foot of sea water then increase the pressure to go back down one FSW. That moisture on the walls however remains - clear to see and wet indeed. 

 The moisture content of the air incoming to the compressor obviously plays a part but it is the action as above that creates the moisture build up in a tank of any size - the physics remain the same. In other words a compressor constantly cycling even within the warmest of environments will do like wise

Quite some time back I bought one of those small cheap airbrush compressors without a reservoir. It was fitted with a moisture trap/regulator directly on the outlet of the compressor. Within just a few minutes of use water was coming out of the airbrush and on the model. The temperature increase (and decrease) was within the hose from between the water trap to the airbrush and caused a rapid build up of moisture. The water trap was at the wrong end of the system if you like. Fitting a volume tank with the trap on it's outlet cured the problem instantly - the moisture remained in the volume tank the trap virtually redundant.

I guess in short - 'drain regularly' is the best dictum to adhere to

As always no granny in sight - just a comment smiley


Edited By Ramon Wilson on 24/01/2021 12:42:11

Edited By Ramon Wilson on 24/01/2021 12:45:20

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