When you opened the drain, how did you manage to look inside the tank ??

The pressure driven explosion risk of these small compressed air tanks with an appropriately small compressor is vastly overstated. There just isn’t enough compressed air, and therefore energy, inside to blow it apart with bits flying everywhere.

internal oil – air mix explosions are a different matter and hugely dangerous. Fortunately ignition is very difficult to arrange and compressor oils are formulated to reduce the risk.

Usual failure is a small rust pin hole or partial seam split so the tank can’t hold air and the compressor will no longer bring it up to pressure. On anything where there is a weakness geometrically capable of creating a serious size hole or seam split the initial damage will be localised so air starts to be lost. As the damage spreads the pressure continues to fall rapidly until there isn’t enough left to drive the hole out or split larger.

Different story with a bigger tank holding lots more air as the pressure will hold up for much longer. In a practical world once the tank gets much bigger than the large propane / butane cylinders the amount of compressed air and energy inside is probably enough to blow small bits out before the drops the pressure fast enough. Say penny or 2 p piece size. The bigger the tank the worse it gets.

Personally I’d be more worried about the threaded socket for the gauge or air connector giving out and blowing the gauge, connector or plug across the room. Some are made looser than I’d ideally like.

Cheap regulator stems can be worryingly weak. Way back I bought  cheap a “customer returned faulty” compressor said to have a broken regulator. Turned out the wall thickness of the threaded stem where it emerged from the body casting was basically slightly more than nothing! I suspect it was broken off when the original purchaser screwed the regulator on by hand before reaching for the spanner.

Eeek!

That was SIP vertical, so called quiet, compressor that sits in the corner of the garage for tyre inflation and air over hydraulic bike ramp duties. No way on God earth would I use it for real workshop duties. Its loud, really loud, but for brief pump up maybe 10 times year it will do.

Less now I’ve got one of the Makita battery tyre pumper-uppers. Really good if you already use Makita and have the batteries.

But 4,000 psi plus really scares me. No short cutting there.

Clive

Sonic, you could always try this.

Chris

Incidentally small steam boilers below a certain bar litre size are considered intrinsically safe. It should be possible to use this as a basis for a don’t worry about it classification for compressed air tanks below a certain size based on stored energy.

Anyone fancy doing some numbers?

Martin

On 31 July 2024 at 06:34 Speedy Builder5 Said:

Does anyone know if these cylinders fail by leaking or rupture and explosion ?

There must be thousands of these around, anyone seen / had experience of a failure ?

Bob

Plenty of examples of air-compressor explosions on the web:

As can be seen from the example above, ancient reservoirs are far more likely to fail than new ones.

A common cause is failing to keep the tank dry by blowing off every so often.   A puddle of water left in the bottom creates a rust line running around the tank at surface level.   After ‘n’ years rusting digs a notch into the steel, which flexes every time the tank is pressurised or discharged.   Instead of  distributing pressure uniformly over the whole tank, force concentrates on the notch, gradually levering it open, and encouraging more corrosion, until one day the steel tears open along the notch, concentrating enough force at the ends to rip open good steel.

Dings in the tank are also potential stress-raisers.

Difficult to predict how long a particular tank will last before failing catastrophically, because much depends on maintenance and how often the tank is used – metal fatigue.  In normal use, where the owner replaces the whole compressor as soon as anything major like the motor fails, they are safe.  Very unwise to repair tanks by any means.  The problem arises when Mr Fixer keeps an ancient tank in service for decades by constantly repairing it without checking what the insides are like!    Or by Mr Innocent only using a compressor once in a blue moon and leaving the tank with water inside, giving it plenty of time to corrode a notch.

How dangerous depends on how big the tank is, the pressure when it fails (usually high), being in a confined space, and whether or not the operator is hit by flying objects.   Although a fair amount of energy is released, the pressures are relatively low, so mostly survivable;  loud bang, probably not enough to rupture an ear-drum.   Although fatalities seem fairly rare, I wouldn’t risk using an old compressor.  I see old kit as being “clapped out” rather than “tried and tested” until proven otherwise!

Dave

Valid comments on tyres Nick but the energy stored in the rubber has to be a factor in making tyre explosions more hazardous I would think.

regards Martin

For what it’s worth, I added extended drains to both of my compressors.  It gives the condensation somewhere to go instead of sitting in the tank causing rust, but also makes it easier to regularly drain it.

Mike

If I had an inexhaustible supply of round tuits, I’d fit a solenoid valve to my compressor tank so that when I unplug it it opens and drains. I always try to remember to open the manual valve, but no-one is perfect

Robert Ackinson’s comment on 35 litres being the limit of intrinsically safe is helpful.

<li style=”text-align: left;”>Its easy to get misled by comparisons with Steam Boilers. When a Steam boiler fractures the pressure begins to fall and so does the boiling point. So huge amounts of steam is released from the water which holds the pressure up and rapidly enlarges the fracture. The process is highly non linear which is why boilers explode. Air receivers merely decompress and so need a much larger volume of compressed gas to create an explosive failure. I looks like 250 bar litres is a good guide to how much to worry. I suspect most of us will have small compressors well under this size. All compressed air needs respect and good workshop practice even if our small compressors are not ‘scary’.regards Martin