Air Compressor Warning

[Colin WhittakerParticipant @colinwhittaker20544]

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 shelleyParticipant @noelshelley55608]

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 1Participant @samlongley1]

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 ShawParticipant @rikshaw]

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 martParticipant @oldmart]

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.

[mechman48Participant @mechman48]

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'.

George.

[old martParticipant @oldmart]

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 yetParticipant @notdoneityet]

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.

……

Sam,

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 WyattModerator @neilwyatt]

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:

http://www.bcas.org.uk/article/pressure-and-leak-testing-of-compressed-air-systems-best-practic-50.aspx

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

Neil

[Nicholas FarrParticipant @nicholasfarr14254]

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 WyattModerator @neilwyatt]

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 bParticipant @jimmyb]

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….

Jim

[norman valentineParticipant @normanvalentine78682]

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 Builder5Participant @speedybuilder5]

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 ?

Bob

[old martParticipant @oldmart]

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.

[Nick WheelerParticipant @nickwheeler]

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.

[not done it yetParticipant @notdoneityet]

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

Is there any reason why the methods appropriate to small steam boilers should not apply to larger vessels?

 

Edited By not done it yet on 24/01/2021 19:29:07

[Sam Longley 1Participant @samlongley1]

Posted by not done it yet on 24/01/2021 17:03:18:

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.

……

Sam,

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.

Fair comment, but I had a number of large compressors on building sites throughout my working life as well as workshop compressors & i am only repeating a comment from one of the testers, when we asked him about the small cheepo ones in the joinery shop. They were kept in cages. More to prevent people damaging the motors rather than prevent explosive damage. I was worried about how often they should be renewed.. I was quite careful with the ones used in the working environment.

As for the video, i might still be unsure about the safety valve. I would imagine a bit of dirt could easily make it stick & a bang like that blow the grit free. That is probably why model steam engines have to have 2 safety valves.

[Mark RandParticipant @markrand96270]

Hydro testing is a perfectly acceptable method of air receiver testing.

Air lines, as in Neil's link, are another thing entirely. Works both ways, I've been in a power station where a gate valve was cracked open in an auxiliary cooling water system that had not been filled. Valve and flange left the 18" pipe when the slug of water from the pumps hit it. That was only two bar, but the water was traveling pretty fast when it hit the valve…

Hydro testing as per boiler code is totally appropriate in a club or home capacity.

 

My opinion only!

 

As to the original post. I also suspect that the safety valve and the pressure switch failed, rather than the tank failing. Metallurgical examination would be required to show how it could have failed that way at normal operating pressure even with a 20% loss of wall thickness.

Edited By Mark Rand on 24/01/2021 20:27:57

[Paul LousickParticipant @paullousick59116]

Commercial air receivers in Australia have to be registered (AS 43434:2014) and externally inspected every 2 years and internally every 4 years. Home compressors do not have to comply with this design code but recommended for your own safety.

Opening the drain valve will remove the water but not the moisture inside of the receiver. The moisture will condense and will cause rusting. This is why on a steam engine, the boiler is emptied (blown down when the pressure is about 5-10 psi)) while it is still hot. The manhole and inspection bungs are removed and the residual heat in the boiler evaporates the remaining moisture. This will not happen in a air compressor receiver because it is cold and a sealed vessel.

Paul

(PS. When did you check your compressor tank ??? )

[SillyOldDufferModerator @sillyoldduffer]

Posted by Mark Rand on 24/01/2021 20:23:53:

…

As to the original post. I also suspect that the safety valve and the pressure switch failed, rather than the tank failing. Metallurgical examination would be required to show how it could have failed that way at normal operating pressure even with a 20% loss of wall thickness.

Agreed a metallurgical exam is needed to prove it, but the way fatigue cracks propagate is well known.

Looking at a cross-section through the tank, it can be seen once started a fatigue crack crowbars itself into the metal every time the tank flexes. Expansion due to pressure opens the crack and contraction due to emptying closes it. Both actions concentrate force at the point. At first the crack gets deeper very slowly, but, as the crowbar gets longer, the rate of growth accelerates. When the crack is deep enough, the metal can't resist the pressure and tears like a sheet of paper along the flaw. Nothing obvious until the tank rips open with a bang.

Fatigue cracks can be started with with a ding, scratch, or corrosion. When an item is subject to fatigue cracking, the best way to avoid catastrophic failure is to take the item out of service at the end of it's design life, i.e before a slow moving crack has had enough time to become dangerously deep.

Highly stressed components like aircraft wheel struts are often polished to increase their strength by removing the tiny surface imperfections that act as stress raisers. It's not just for show!

Dave

[bernard towersParticipant @bernardtowers37738]

You could fit an adv as they do on trucks!

[Nick WheelerParticipant @nickwheeler]

Dave, your representation of a defect in a round tank is the wrong way round; the wide part of the V should be on the inside. Which would make the pressure pushing outwards even more effective at enlarging it.

[Ramon WilsonParticipant @ramonwilson3]

Posted by not done it yet on 24/01/2021 19:28:43:

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

Is there any reason why the methods appropriate to small steam boilers should not apply to larger vessels?

Edited By not done it yet on 24/01/2021 19:29:07

Not at all NDIY – indeed that's exactly how decompression chambers were tested in my day – wouldn't think it would be any different today. Most were about 4'6" dia and around 12' long, the doors each end were shut and the entire chamber filled with water and the inspector would couple up his pump just as an inspector would for a boiler. The vent would be capped off once the water came out of the vent on top. Believe it or not given the size difference the hand pump was really not that much bigger than we would use. Pressure would be taken up and held and the circumference measured before and after pressurising.

As said a decompression chamber is just a big volume tank with doors on – I would imagine that that is still how it's done.

Tug

[not done it yetParticipant @notdoneityet]

That is probably why model steam engines have to have 2 safety valves.

Sam,

So do compressors – one electrical switch (probably not too applicable to steam engines🙂 ) and the mechanical over pressure valve. They don’t blow up more than once! It was why the compressor in the workshop (I mentioned earlier) was caged – not to prevent damage to the unit by the workers.

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