Expansion tank

[D.M. Procida]

A few weeks ago my children proudly announced that they'd made an air rifle from some plumbing parts and a plastic bottle and a Schrader valve.

I was annoyed because they were supposed to be studying for their exams, but then both impressed and a bit horrified to discover that this thing can fire a triple-A battery about 100m.

"What you really need is some sort of metal tank instead of that bottle" I pointed out to them, and someone must have heard, because yesterday evening in the lane behind our house I found a 10-litre expansion tank.

So, a couple of purely theoretical questions - firstly, the tank has a screw fitting - what could we^they use to attach the trigger - - to it?

Secondly, the water side of the expansion vessel of course has the rubber membrane attached to it, which is going to need to be ruptured in such a way that air pumped into the valve on the air side can exit rapidly from the other end when required - any suggestions on how to do that?

Thanks,

Daniele

[Harry Bloomfield]

D.M. Procida brought next idea :

Just an idea to throw in the pot (or expansion tank)....

If you could heat up the tank, without damage to the valve, surely that would wreck the membrane? Maybe pressurise the membrane, then apply heat to the end opposite the valve you will then hear it burst.

[Onetap]

[Onetap]

Plumbing fittings, any plumbers merchants would sort you out.

Secondly, anything sharp poked in the pipe connection, possibly already per ished, which may be why it was dumped.

You should be aware that what you're making is legally a firearm, requiring a Firearms Certificate (airgun of more than 12 ft/lbs energy ISTR). I kid you not I'm afraid; Mr Plod's sense of fun about these things was surgicall y removed some years ago. I'm not bothered, don't get caught with it.

The tank may burst if overpressurized, especially if it's rusted, so don' t let them get a compressor of any sort on it.

[harryagain]

The membrane is also the seal bewtween the two halves so heating is a bad idea.

It's not a good idea to encourage this project. The tank could be overpressured and burst. ( I note it has never crossed your mind to fit a safety valve.)

Also these sort of projects are wonderful until some kid loses an eye. It will occur to them at some point to shoot one another. This is why we don't let kids have air rifels too young in this country.

[dennis]

What is the maximum safe pressure for the tank? It may well be less than the bottle and may not be safe.

[D.M. Procida]

Surely a welded steel tank is going to hold a higher pressure than an old plastic fizzy drinks bottle?

Daniele

[Onetap]

Yes, when new. You don't know how rusted the insides are. You'd usually test something like that by filling it with water and pressurising to 2x the working pressure. If it fails under air pressure, bits of metal shrapnel can fly around.

Even the plastic drinks bottle can be very dangerous if it bursts under pressure.

[Onetap]

When new, yes; the problem is that you don't know how much the inside has rusted. You'd normally fill a tank like that with water and pressure test it at 2x the working pressure, annually to prove it was safe with air.

If it bursts under air pressure, bits of metal shrapnel can fly around.

Even the plastic drink bottle can be dangerous if it bursts under air pressure.

[D.M. Procida]

Good idea. I can fill it with water, and pressurise using air to test.

The bottle seems to be happy with 100 PSI, but I'm not that keen on it. They've covered it in gaffer tape to help keep it together in the case of failure, but I am not sure how well that will work in practice.

Daniele

[Onetap]

100 psi? Are you serious?

The idea is to get all the air out, so pressurizing with air won't do that.

The problem is that the air compresses and can store loads of energy in being compressed.

So, if you pump up a tyre with a foot pump, you're pumping away for 10 minutes and a large part of your effort is stored in the compressed air. If something bursts, there's a sudden and explosive release of that energy.

Water doesn't compress. If you pressure test a pipework system, you can bring it up to the test pressure, say 10 or 12 bars, with a few strokes of a hand pump, if you've got all the air out. There's very little stored energy.

I once hydraulically pressure tested a pipework system using a car foot pump immersed in a tank of water. I may have looked like a loony pumping it up with one shoe and sock off and my trouser leg rolled up, but there is no way on God's earth I'd pressure test anything with air.

If you're leak testing (not the same as pressure testing) on an unproved system, you shouldn't go above 0.5 bar (7 psi) and, even then, you should evacuate the area, tape down hoses, board windows, etc., etc..

I think you need to step back and consider what you're doing with this.

The spud gun thing is potentially lethal, the bottle at 100 psi could shatter and injure someone, the tank is really getting into the realms of anti-personnel devices.

[D.M. Procida]

Yeah, but if it's nearly completely filled with water, there'll only be a tiny amount of compressed air.

I know, I've seen (and heard) a bicycle tyre explode.

OK, I like my kids to have a bit of dangerous fun, but not that dangerous.

I admit though I don't really understand how a bicycle pump and its rubber hose, and a bicycle wheel and tyre can handle those pressures and much more, safely, if a steel tank can't.

Daniele

[Onetap]

They all can, when new. You don't know how much of the steel is left, what pressure it will fail at or how it will fail if you did over-stress it. You just don't want to find those things out when it's got air in it.

You also don't know how accurate your pressure gauge is. I've seen some cheap ones with huge errors. If you were pressure testing, you'd have one with a serial number and a dated calibration certificate.

I'd just fill it with water, pressurize it with the mains ( if you've got a decent pressure) and use it at a pressure below that. The mains pressure usually goes up overnight (less demand). Or a bicycle pump in a water tank.

If you want to get your kids interested in why you'd avoid pneumatic pressure testing in favour of hyhdraulic, get them to look up the stress/pressure hydraulic testing of the de Havilland Comet fuselage, by immersing it in a huge tank at RAE.

The failures of the pressurized Comet fuselages at (ISTR) 6 psi was reckoned to have an equivalent energy release of about 500lbs of TNT (or was it 50?); it was a large volume of air.

One piece of the wreckage recovered that had the imprint of a flying coin clearly impressed into it.

Even 6 psi is something to be wary of.

[Onetap]

[Tim+]

Except that he's filling it with water first so the stored energy will be very small.

Tim

[Onetap]

Yes, I understood that.

That would reduce the risk.

Whilst there's air in it, there is still a forseeable and avoidable risk.

This is what can happen in a hydraulic test, if you fail to remove all the air or other compressible gases.

Second item here;

Very high test pressure in that case though.

[Tim+]

An insignificant risk. Blowing up my bike tyres is far more dangerous. Much bigger volume and routinely filled to 100psi.

Do you think that the sky is going to fall too? Seriously, you can find risk anywhere but pressuring a container that is already full of water with a tiny quantity of air is not going to be that dangerous.

Tim

[dennis]

They are both engineered to keep the cost down, there may not be much margin in either these days. An exploding plastic bottle may blind you, the tank may kill a few of you.

An expansion tank only needs to work at about 3 bar, they are probably tested to about 10 bars, you have no idea what has been done to the tank in question.

The plastic bottle may well work to a higher pressure, AIUI a bottle of coke is about 60 psi (4 bar) at room temp and higher in a hot car but they are nowhere near exploding.

[Onetap]

How can you know the magnitude of the risk?

How can you know what pressure it might fail at?

You don't. You're deluding yourself, thinking you can assess the risk from afar.

You haven't seen the test rig and don't know how much air may be trapped in it.

I know that I don't know the magnitude of the risk, I am better informed about the nature of the risks and so I will avoid them if I can.

Tyres are designed to work at that pressure, there are steel cord embedded in the tyre to withstand the pressure, ISTR. The inner surface of the tyre doesn't corrode, you can get to it to inspect it, you know where your tyres have been and replace them regularly when the outer tread has worn away; all unlike that tank.

So far this thread has covered kids, firing AA batteries from a prohibited firearm, pressurising a fizzy pop bottle to 100psi, replacing that with a scrapped steel tank of unknown condition and using an uncalibrated gauge.

I don't know the magnitude of the risk, but I'm surprised this hasn't already ended in tears.

[Onetap]

PS She said it's a 10 litre tank. Is your 'bike' a motorbike or how do the tyres have a much bigger volume.