Tank Question

[manike]

Re: hydro tests

Originally posted by Wat
Then the tank is pumped with water (or is it air?)

I believe it is water (or some fluid), because the fluid isn't as compressible as air and there are reduced risks if it does rupture.

I agree the biggest risk is in fittings, we had a nasty incident over here just recently where a fitting blew and piece went through someone's eye. Thankfully after a week or so of hospital it looks like he will be ok.

[Lohman446]

Here's me being hypocritical again... I just don't want to sound like I am doing something I am not.

There is a good reason to where your paintball goggles at the fill station.

[f3rr3+]

yeah, ive had a gauge blow up at me on a fill station never went back to that place. (i had numerous other problems, them screwing that up was the straw that broke the camels back)

[aaron_mag]

Re: Re: hydro tests

Originally posted by manike

I believe it is water (or some fluid), because the fluid isn't as compressible as air and there are reduced risks if it does rupture.

If I remember my physics correctly liquid CANNOT be compressed at all. Only a gas can be compressed and if it is compressed enough it becomes a liquid. I think that they compress it under water and check how much it changes in volume (all thanks to Archimedes). If it does not fall within certain tolerances then it is no longer usable (safely).

Scott- quit being such a greedy bastard and just be happy that it rehydroed properly.

[1stdeadeye]

Re: Re: Re: hydro tests

Originally posted by aaron_mag


Scott- quit being such a greedy bastard and just be happy that it rehydroed properly.

LOL!

That is my loaner tank. I let all of my buddies use it and the R/T the first time they go out to play with me. I am like a crack dealer...the first one is free! Then they are hooked!!!

Just get tired of them running back for air so much!

I was suprised that it passed too. All the times I beat it against a rock while cursing the liberal dogma and it still works!

[Wat]

Liquids can be compressed but by only a miniscule amount. Its so small, that for just about every purpose outside the classroom it can be considered incompressable. However, i don't remember if it was all liquids or just most liquids.

I'm pretty sure Manike is right that the tank is filled with water. If the tank fails you don't have all this gas expanding. The water may be incompressible but that is not the same as being unpressurizeable.

[dinger]

this thread makes me wanna put 4500psi into a 3Kpsi tank...

i mean i know better, but im just a dumb cucumber like that...

[Skoad]

my 4500 tank was tested up to 7000 psi.

[manike]

Re: Re: Re: hydro tests

Originally posted by aaron_mag
If I remember my physics correctly liquid CANNOT be compressed at all.

Lies to children (anyone read disc world books?).

Some liquids can be compressed as Wat points out. At school you learn to start with that they cannot but as you learn more you learn that some can and that initially what you were told when you started education were 'lies to children'. It's not much in relation to gases but it is still possible.

It's why you DON'T put water in the brake system of your car, but use brake fluid (a type of oil maybe?) instead.

[Doc Nickel]

Actually, the two reasons you don't use water in your brake system are rust and the boiling point.

Rust is obvious, and the boiling point is important because brakes, after all, have to convert a whole cars' worth of momentum into heat.

Liquids are, for all intents and purposes, incomprsssible, but if the liquid boils, the resultant "steam" IS compressible, which would lead to loss of braking power.

Those are far more important reasons than water's .000001% compressibility.

Doc.

[manike]

Here's a table of water volumes at different temperatuers and pressures. Yes, water does in fact compress as you raise the pressure (no surprise here), but it doesn't compress by much.


temp F(C) 0 atm 500 a 1000 a 2000 a 3000 a
32 (0) 1.0000 0.9769 0.9566 0.9223 0.8954
68 (20) 1.0016 0.9804 0.9619 0.9312 0.9065
122 (50) 1.0128 0.9915 0.9732 0.9428 0.9193


A mile under water gives about 150 atmospheres of pressure, so that should be less than 1 percent compression, according to the table.

- If Lake Superior were compressed to its maximum its water level would drop around 9 inches.

i.e. The compression of water depends on more than just one factor, but for all intents and purposes you can pretend it conforms to ideal conditions.

[aaron_mag]

Next thing I know you will be telling me my third grade American history class was not 100% accurate!!!!

Now I am curious on how they actually hydro test. I thought I knew but.......

[Doc Nickel]

I wrote an article for Paintball Magazine a few years back, but I lost track of the original files, so until I get a copy of the magazine, I haven't been able to put up a version on my site.

But, heavilty simplified:

The valve or reg is removed from the tank to be tested, and the tank is filled with water.

The tank is then attached with an adapter to the lid of the hydrotest vessel, which is a large, heavy-wall pipe with a locking flange at the top to secure the lid. This vessel is also filled to the top with water.

The lid is dropped into place (usually spilling at least some water, to make sure the vessel is as full as possible) and locked down. The equipment is set, and a powerful compressor connected to the fitting in the lid, pressurizes the tank inside.

As Wat noted, the tank expands, literally like a balloon (on a much smaller scale) when it's pressurized. The slightly larger tank displaces some of the water in the test vessel, water which is moved to something very much like a very accurate scale and/or very finely-graduated tube (like a thermometer; the fluid rises and indicates a mark on the tube.)

This displaced water is calculated to determine how much the tank has expanded.

Now, the pressure in the tank is released, and it shrinks back down to normal. Again, the water is drawn back into the test vessel by the contraction of the tank.

However, sometimes not all the water is drawn back in- this indicates the tank has expanded and stayed slightly expanded. This means the shell of the tank has literally stretched (and weakened slightly.)

The hydrotester (hydro = water, of course) has reams of books and reports from the makers of the tanks themselves, as well as the DOT (Department of Transportation) and the CGA (Compressed Gas Association) and others who oversee the production and use of pressurized-gas bottles. This information tells the tester how much initial expansion isto be expected, and how much permanent stretch can be allowed.

If the tank expands too much, this indicates the shell is weak- a weak wall expands too much, which means it moves more when filled, and that leads to metal fatigue as the tank is cycled (filled and emptied.)

Metal fatigue weakens the wall further- as in Wat's paperclip analogy, one I used in my original article, as I recall - and once it weakens to a certain point, it can rupture (that means fail catastrophically- it can explode.)

Some permanent stretch is to be expected, which is why the fiber-wrapped tanks have been limited to a fifteen-year/five-test lifespan- the testing itself stresses the tank, and after five tests they expect the tank to have weakened slightly just from that.

Newer fiber-wraps have changed the spec somewhat, almost entirely due to the demands of paintball players, but they still have a fixed lifespan.

Doc.

[Muzikman]

Re: Re: hydro tests

Originally posted by manike


I believe it is water (or some fluid), because the fluid isn't as compressible as air and there are reduced risks if it does rupture.

I agree the biggest risk is in fittings, we had a nasty incident over here just recently where a fitting blew and piece went through someone's eye. Thankfully after a week or so of hospital it looks like he will be ok.

This is the reason why I use NO macro (or Micro) line and also do not use those swivel 90 fittings (like KAPP and other companies make). It's steel braid and solid fittings for me!!!!

[aaron_mag]

Fascinating. I figured since water could not be significantly compressed you couldn't use it to hydrotest. I figured the hydrotest referred to the displacement of water test. As Wat pointed out, however, there is a difference between compression and pressurized.

Interesting.....

Limited life span works fine for me. Won't holographic paintballs in the future make compressed air unneeded anyway....