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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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Sam Longley 127/01/2021 08:42:42
832 forum posts
30 photos
Posted by Gary Wooding on 27/01/2021 08:10:35:

My new compressor arrived yesterday and obviously I want it to last a long time. It will seldom be used continuously for any length of time - mostly for just a few minutes, barely long enough for the pump to start up.

The instructions say to release the pressure and drain it every day after use, and this got me thinking. Suppose the tank is empty and I fill it. The pressure stretches the tank metal and water in the air enters the tank. Suppose 1cc of water is included for every tankful of air. I use, maybe, 1/4 of the tank contents, so 0.25cc more water gets in. Suppose now, that no more air is used for a week, say. What's best? To release the pressure and drain the water, then wait a week and refill with air, or to leave the tank full and empty it every 4 weeks, say?

If I drain each time, then 4cc of water passes through the tank, but if I drain every 4 weeks, then only 2cc of water gets in. Each time the tank is emptied and refilled, it goes through a stretch/release cycle, which can't be beneficial, and just draining the tank doesn't dry the surfaces - which remain damp. So what's best?

Have you considered removing the inspection cap & pouring in some thick oil, shaking the tank about a bit & letting that settle in the bottom? especially if the drain was not exactly at the bottom of the tank. Might help the air tools as well. It probably would not do any harm if a speck of oil blew out onto your lathe when blowing down either. no worse than moisture. But then I have a water trap unit for spraying.

Mike Poole27/01/2021 09:16:30
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2893 forum posts
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The oil will tend to float on the water so may not be as helpful as you would hope for.

Mike

not done it yet27/01/2021 10:14:27
5626 forum posts
20 photos

pouring in some thick oil

The thick oil will be blown out when the tank is drained? Best would be to arrange the tank vertically with the drain at the centre of the bottom dome - but beware of any drain sticking up inside the receiver.

SillyOldDuffer27/01/2021 10:17:30
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6877 forum posts
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Posted by Sam Longley 1 on 27/01/2021 08:42:42:
Posted by Gary Wooding on 27/01/2021 08:10:35:
...

Have you considered removing the inspection cap & pouring in some thick oil, shaking the tank about a bit & letting that settle in the bottom?...

Definitely wouldn't put oil of any sort in a compressor tank! It assumes 'thick oil' will never catch fire on it's own or slowly degrade into a vapour that might diesel or ignite due to static electricity. There's a lot of energy in oil. Rather than going off with a giant scary pop as being discussed in this thread, burning oil in the tank would result in a real explosion, more violent with flame.

Another problem, assuming the oil sticks at the bottom, is water would tend to concentrate at the oil/tank edge, causing a line of rust to consistently weaken the tank along a line.

And compressed air usually needs to be clean. Oil in the output is certainly bad for paint!

Dave

Sam Longley 127/01/2021 12:19:19
832 forum posts
30 photos
Posted by SillyOldDuffer on 27/01/2021 10:17:30:
Posted by Sam Longley 1 on 27/01/2021 08:42:42:
Posted by Gary Wooding on 27/01/2021 08:10:35:
...

Have you considered removing the inspection cap & pouring in some thick oil, shaking the tank about a bit & letting that settle in the bottom?...

Definitely wouldn't put oil of any sort in a compressor tank! It assumes 'thick oil' will never catch fire on it's own or slowly degrade into a vapour that might diesel or ignite due to static electricity. There's a lot of energy in oil. Rather than going off with a giant scary pop as being discussed in this thread, burning oil in the tank would result in a real explosion, more violent with flame.

Another problem, assuming the oil sticks at the bottom, is water would tend to concentrate at the oil/tank edge, causing a line of rust to consistently weaken the tank along a line.

And compressed air usually needs to be clean. Oil in the output is certainly bad for paint!

Dave

Oil seems to work OK in a car without exploding in the sump & steering etc!! Albeit at lower pressure, but quite hot, one assumes. We used to put it in my site compressors & it did no harm to them. I would have thought that it would have got flung round the inside of the tank & lined the inside. Only a suggestion for consideration.

Anyone doing paint jobs needs to use a condensation trap for starters, so that should not be an issue.

As for air being clean-- Is there not a unit for injecting oil into the air line for tool lubrication? I am sure that my air bench presses had one on my window manufacturing line. Long time ago, so memory a bit faded.

Still have the water trap paint filter though.

 

Edited By Sam Longley 1 on 27/01/2021 12:29:09

Nicholas Wheeler 127/01/2021 14:32:46
504 forum posts
28 photos

I wouldn't want oil in the compressor tank for all the reasons stated.

Oil in a sump isn't pressurised much, and doesn't have much oxygen to allow it to burn. That isn't the case in a tank of compressed air!

If used air tool oilers are fitted to the tool, after the airline connector. From what I've seen, most light users don't have them and rarely oil the tools manually.

Jeff Dayman27/01/2021 14:45:54
2069 forum posts
45 photos

Many pressure tanks for domestic water supply for wells, with a jet pump, here in Canada were hot dip galvanized in and out. These were rated to 75 or 100 psi continuous service. It would be a nice option to be able to buy a hot dip galvanized heavy gauge tank or a stainless heavy gauge tank air compressor. I would gladly pay a little more for those options. As well, automatic water drain valves as use on railway locomotive air brake air tanks here would be useful, and not a lot extra. (these are just a pneumatic time delay valve that exhausts a short burst at regular intervals.)

old mart27/01/2021 16:08:30
2686 forum posts
176 photos

I would not recommend adding oil, that is one of the things that you want the air to be free of. Oiling rotary tools is a separate thing, downstream from the reciever. A shield between the compressor and the working area would direct the blast away from the operator if it was strong enough, but keeping the reciever in a separate area would be best, if possible.

Edited By old mart on 27/01/2021 16:10:19

Nicholas Farr27/01/2021 16:12:12
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2624 forum posts
1225 photos

Hi, I don't know about an explosion risk of oil in your compressor, but if you want clean air, you won't put it in your tank. Air service units are very often used in industry, which very often include a regulator, a water trap and an oiler, most of them with a regulator that I've seen, have the regulator first, but some have it second, however the water trap is always before the oiler. It is quite likely that if you put oil in the tank, it will migrate out with the air flow over time and if you put a water trap in the air line, over time the oil will probably gunge the water trap up and then you may find a significant drop in air pressure and most likely volume available for your usage. Draining the water should be on a regular basis and automatic ones relieve you of having to do it.

On a job a workmate and myself, had to drain down a diesel tank and pipework. The tank was easy, but most of the pipework was in a narrow gulley in the floor of the building and there were also areas where the gulley was just between pipes under the floor. We could not just undo the pipework and let it into the gulley, as it was an open ended one that drained into a river and there was electric cabling in the gulley as well, (we were doing the job for the Environment Agency) so we came up with the idea of blowing the diesel out with compressed air and catching it in a container, but had a passing thought of combustion like in an engine as it was a very long pipe run of 1" black pipe, but we went for it anyway. The result was! the pipework was cleared of 20L or so of nice clean red diesel and no mess and no bangs.

Regards Nick.

Gary Wooding28/01/2021 15:35:47
802 forum posts
205 photos
The instructions say to release the pressure and drain it every day after use, and this got me thinking. Suppose the tank is empty and I fill it. The pressure stretches the tank metal and water in the air enters the tank. Suppose 1cc of water is included for every tankful of air. I use, maybe, 1/4 of the tank contents, so 0.25cc more water gets in. Suppose now, that no more air is used for a week, say. What's best? To release the pressure and drain the water, then wait a week and refill with air, or to leave the tank full and empty it every 4 weeks, say?

If I drain each time, then 4cc of water passes through the tank, but if I drain every 4 weeks, then only 2cc of water gets in. Each time the tank is emptied and refilled, it goes through a stretch/release cycle, which can't be beneficial, and just draining the tank doesn't dry the surfaces - which remain damp. So what's best?

Doesn't anybody know the answer? Is it best to empty the tank after use, or just blow out the water with minimal loss of presure?

Robert Atkinson 228/01/2021 15:52:37
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904 forum posts
17 photos

From fatique perspective it is always best to reduce the number of cycles (the very high pressure pure air bottles that were used for early thermal imagers had limited number of refills before being scrap). Most bursting failures of cheap tanks seem to be fatique related rather than corrosion which results in leaks.

As has been mentioned more water is added every stroke of the compressor so minimising that reduces how much water enters the tank

Overall best solution for a compressor used reguarly (more often than once or twice a week or the pressure does not leak away beween uses) is just drain the water and leave the tank pressurised. If it is only used infrequenty, drain it down and leave it empty. If in a warm dry location leave th drain valve open so it acually dries out.

Robert G8RPI

Nicholas Farr28/01/2021 18:50:43
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2624 forum posts
1225 photos
Posted by Gary Wooding on 28/01/2021 15:35:47:
The instructions say to release the pressure and drain it every day after use, and this got me thinking. Suppose the tank is empty and I fill it. The pressure stretches the tank metal and water in the air enters the tank. Suppose 1cc of water is included for every tankful of air. I use, maybe, 1/4 of the tank contents, so 0.25cc more water gets in. Suppose now, that no more air is used for a week, say. What's best? To release the pressure and drain the water, then wait a week and refill with air, or to leave the tank full and empty it every 4 weeks, say?

If I drain each time, then 4cc of water passes through the tank, but if I drain every 4 weeks, then only 2cc of water gets in. Each time the tank is emptied and refilled, it goes through a stretch/release cycle, which can't be beneficial, and just draining the tank doesn't dry the surfaces - which remain damp. So what's best?

Doesn't anybody know the answer? Is it best to empty the tank after use, or just blow out the water with minimal loss of presure?

Hi Gary, well every thing you use has a stretch/release cycle one way or another, but everything is, or should be, designed to do this within the elastic range of the materials used, so you have to ask yourself if the manufacturers have got their sums right and do they leave any safety margin. Do you worry about the stretching/releasing cycles of your car fuel tank, (assuming you have a car that uses liquid fuel) OK it might not be as extreme, but fuel tanks aren't that thick, and you probably have emptied and filled it many more times, plus all the forces holding the fuel in, when the car is in use, or have done before the current situation we are all in. In the long run, everything has a finite useable life, but the hardest bit is to decide when you feel the need to replace or just don't need the things you have anymore.

Regards Nick.

not done it yet28/01/2021 19:38:33
5626 forum posts
20 photos

Looks more like GCHQ had been taking too much interest in this thread? The title has been changed to something less dramatic.🙂

Edited By not done it yet on 28/01/2021 19:39:31

Martin Kyte28/01/2021 21:33:02
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2216 forum posts
38 photos

I had a compressor tank fail at work albeit a small bambi type. It failed at a weld where the tank brackets attatch to the frame. I estimate at around 60PSI. All I got was a loud pop and a hiss as it deflated. I suspect many are overthinking this and seeing disasters where there are none. I guess for large air receivers some more serious thinking needs to be done but compressed air stores a lot less energy than steam. If you are under 250 bar litres it doesn't officlally need inspection so I would assume the powers that be deem it intrinsically safe when it fails much like the lowest category steam boilers.

regards Martin

Paul Lousick28/01/2021 22:02:29
1692 forum posts
625 photos

Another reason for releasing the pressure in the tank is relieve the stresses on the springs in the pressure relief valve and gauge. Leaving the springs in a compressed state can cause a permanent set and they will not return to their normal zero pressure position and could give a false reading. Releasing the pressure is also a recommended procedure when using welding gas regulators.

Paul.

Chris Crew28/01/2021 23:26:24
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32 forum posts

I have had a small Machine Mart 'Tiger' compressor in my workshop for the last thirty years, it is constantly switched on and fires up to replenish the reservoir as and when if I am blowing swarf off the machines or I occasionally drag it down the drive to reflate a car tyre. A few weeks ago it occurred to me to remove the drain plug on the bottom of the cylinder and, guess what? After thirty years constant use absolutely nothing came out, not a drop of water or anything else for that matter, so I replaced the drain plug and expect the compressor to be still going strong long after me!

Mike Poole28/01/2021 23:47:20
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Moderator
2893 forum posts
68 photos

Have you checked there is no blockage of the vent?

Mike

Gary Wooding29/01/2021 07:31:43
802 forum posts
205 photos
Posted by Chris Crew on 28/01/2021 23:26:24:

I have had a small Machine Mart 'Tiger' compressor in my workshop for the last thirty years, it is constantly switched on and fires up to replenish the reservoir as and when if I am blowing swarf off the machines or I occasionally drag it down the drive to reflate a car tyre. A few weeks ago it occurred to me to remove the drain plug on the bottom of the cylinder and, guess what? After thirty years constant use absolutely nothing came out, not a drop of water or anything else for that matter, so I replaced the drain plug and expect the compressor to be still going strong long after me!

Chris, read the first post in this thread.

SillyOldDuffer29/01/2021 10:35:52
Moderator
6877 forum posts
1539 photos
Posted by Chris Crew on 28/01/2021 23:26:24:

... After thirty years constant use absolutely nothing came out, not a drop of water or anything else for that matter, so I replaced the drain plug and expect the compressor to be still going strong long after me!

So, what are we to make of this? The thread opens with a burst compressor tank:

burst.jpg

And then Chris says his compressor is fine after 30 years of neglect and he expects it to last longer than him!

What's going on?

The problem is human beings are bad at assessing risk. Unless the consequences are bleeding obvious we don't get it! We understand it's unwise to sit in the bath with an electric fire balanced on our knees, but might reject the seriousness of Covid because the streets aren't littered with corpses. What virus, what problem?

And there are many risks where personal experience is almost irrelevant because the risk is statistical, only hurting a percentage. At least a thousand cases have to be analysed to understand what's going on. Smoking is a good example: comparing smokers with non-smokers by the million clearly shows - on average - that non-smokers live 10 years longer, while smokers are about 20 times more likely to die of cancer, bronchitis and emphysema, and 4 times more likely to die of heart disease. Whilst the statistics tell the awful truth, individual smokers don't see it. It takes years to do the damage, not everyone is affected, and we foolishly trust personal experience. This allows addicts to argue, 'My granny smoked 80 a day until she was run over by a tram aged 95' without realising the evidence is valueless unless compared with millions of other life experiences.

The range of possibilities often follows the bell curve of a normal distribution. This example is of academic performance. Given a year's worth of school-children, their exam results will look like this:

bell-curve.jpgThe same shaped curve likely covers the burst compressor and Chris's experience. The probability of a neglected compressor lasting 30 years is low and so also is the probability of tank bursting. Both are possible, but neither is likely. They are rare events on opposite sides of the curve, special cases. Most compressors sit in the middle: they aren't in good nick after 30 years, nor are they likely to spectacularly go pop during normal usage.

Owners can change their position under the curve for good and bad. Regular maintenance is good, but I suggest extending the life of an elderly compressor is asking for trouble. Designers know a great deal more than us, such as the type of steel used, calculated and actual burst pressures, the expected life of the equipment (in years and operating cycles), safety factors, corrosion resistance, and much else. Big difference between confidence and knowing. When it comes to safety it ain't smart to rely on luck!

Dave

 

Edited By SillyOldDuffer on 29/01/2021 10:38:38

noel shelley29/01/2021 11:41:57
381 forum posts
9 photos

Gentlemen Please. This tread has on the one hand attracted far to much theory and on the other very little. It would appear that we nearly all seem to have a compressor of one size or other - how many have had a pin hole leak let alone a catastrophic shell failure ? Most safety is based on historic practical experience tempered with some theory and good practice(common sense). The issue of explosion due to THICK oil in the tank. The ignition of even a light oil at the pressures and temeratures we are talking about is not easy. Diesel fuel will not ignite until the temperature in the combustion camber is raised to about 800*C and the fuel finely atomised. When cold this process will often need the help of a glow plug to the get things started, so thick oil in a even warm tank is so unlikely to explode or ignite as to be on a par with a pig with wings !

There are many things that CAN go wrong, a blocked drain, a failed non return or unloader valve, a seized or stuck safety valve, the pressure control switch operating at the correct pressure. Then there's the motor?

The idea of a galvanized tank is good but one thought would be that it might instigate stress cracking due to the brittleness of the zinc/iron intermediate layer.

If you are reading this you have so far survived living with tis dangerous device ! Noel.

Edited By noel shelley on 29/01/2021 11:44:02

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