Tank Question

[1stdeadeye]

I just had an old 68/3k compressed air tank hyrdro-tested. Picked it up today and found something very interesting. They epoxyed the certification to the outside of the tank.

Here is the question: According to the testing, the tank is tested to 5k psi. Even though the bottle states 3k, does this mean I can swap out regs and make it a 4500 or is that just the standard test?

Thanks for your help!

[speeddemon]

To hydro tanks, they pressurize it a higher pressure than its rating (can't remember now the exact number now), and then measure the permanent expansion of the tank. If its under a certain number, then it is still safe. The 5k psi means they pressurized it to 5k to test its ability to safely hold 3k psi. Its still a 3k tank, don't fill it any higher.

[Automaggin2]

Scott, on most 3k tanks, the burst disk will blow at 5000 psi. The tank itself is made to hold well up to 15000 psi, but its extremly dangerous.

[FalconGuy016]

Its like overclocking your CPU, or speeding on a road. You'll most likely not have problems, but, do you want the higher risk?

[TJ03A]

Originally posted by FalconGuy016
Its like overclocking your CPU, or speeding on a road. You'll most likely not have problems, but, do you want the higher risk?

Umm, yeah....but if my overclocked cpu goes pop I'm out $150 for the cpu, if my overpressurized tank goes pop I'm going to out quite a bit more.

Now as for speeding, in the past month I've driven from NY/LI to Key West and back and then to the Shatner event. Lots of 65 up to 75 mph speed limits, well my perceptions on whats a normal driving speed on the highway had changed and a Suffolk county cop just gave me a ticket for doing 76 in a 55 to help me shift my perceptions back to 65 tops for LI :mad:

[Wat]

Just because a tank can survive a 5K load does not mean that its operating pressure can be 5K. The cycling of the tank from 0-5000 psi repeatedly will wear down the tank a lot faster than to its 3000psi operating rating. 5000psi loads can fatigue a 3000psi tank pretty quickly and failure will be immenant.

When they hydro test, they test the elasticity of the tank. The measure how much it expands when its filled to 5/3rds rated capacity and more importantly they measure how much it returns to its previous state. If it returns exactly to its previous state, than its 100% elastic and ok. If it expands 1ci but only contracts .8 ci when degassed, than the plastic deformation is .2ci (its usually measured in % of total volume). There's some rule somewhere on how much plastic deformation can occur before the tank fails the test.

Anyways, plastic deformation, unlike elastic deformation, never goes away. Fatigue is the accumulated plastic deformation. There will always be at least a tiny bit of plastic deformation regardless how light the load is. As the the tank gets filled and unfilled the plastic deformation accumlates until the tank fails. Imagine bending a paperclip back and forth a little bit until it breaks. If its a low load, it could be trillions of cycles to failure. If its a high load it come be just hundreds of cycles.

The kicker is that the amount of plastic deformation is usually exponential with the load. So twice the load can cause 10 x the fatigue and maybe thousands of times less service life.

So to sum up, don't fill your tank above its 3000psi rating. You may think its ok to break DOT laws, but you can't break the laws of physics.

[1stdeadeye]

Thanks everyone, I'll leave well enough alone.

[manike]

Originally posted by 1stdeadeye
Thanks everyone, I'll leave well enough alone.

Very sensible. :) Everytime I work out how much energy is in one of these tanks it scares me. It's definitely best to use things within their original design intents.

Due to some accidents over here lately I'm currently re-thinking all the elbow connections and macro lines I have on my guns... might be time to look for something better (higher rated).

Ultimately it's not worth mis-using something that relates to the safety of you and the people around you. You wouldn't use shop goggles to play paintball (they aren't rated for it), so don't put 4500psi in a 3000 psi tank which isn't rated for it (or worse a co2 tank... which people also seem to want to do lately! :rolleyes: )

Play, be safe, and have fun!!! :D

[AcemanPB]

I think I read somewhere that when a tank is hydro'd it is filled 5/3's past the recomended level, this would explain why your tank was filled to 5k PSI.

[1stdeadeye]

Originally posted by manike


(or worse a co2 tank... which people also seem to want to do lately! :rolleyes: )

Play, be safe, and have fun!!! :D

Okay Simon,
You peaked my curiosity. How can you fill a CO2 tank with compressed air? There is no fill nipple.:confused:

[Lohman446]

I will tell you there is a LOT of energy in a tank - my gauge was leaking the other day so I got a new one. I went to change the gauge. Here is where I made my mistake, I never stopped to think that the gauge is before the regulator (duh - IT WAS A STUPID MISTAKE). Now the 91CI tank only had about 2800PSI in it (it was a very small gauge leak - more annoyance than anything). Not thinking I unscrewed the gauge, it blew out of the tank (luckily not stripping the threads and not hurting anyone) - the tank then proceeded to bounce onto my leg (OW) then the floor - of course by now Im out of the room holding the door shut - the tank ended up about ten feet from where it started.

[manike]

Trust me, it's possible. You don't have to have the feed nipple on the tank... as long as it is on the system and the tank is open you 'could' get it to work. I've seen someone in the old days try to do it by putting the fill nipple on the other side of a twin port bottom line.

I'm not sure how people would try to do it now, but I've seen the question come up a few times on the net lately.

Either way it's definitely not a good idea though :D .

[speeddemon]

Originally posted by Lohman446
I will tell you there is a LOT of energy in a tank - my gauge was leaking the other day so I got a new one. I went to change the gauge. Here is where I made my mistake, I never stopped to think that the gauge is before the regulator (duh - IT WAS A STUPID MISTAKE). Now the 91CI tank only had about 2800PSI in it (it was a very small gauge leak - more annoyance than anything). Not thinking I unscrewed the gauge, it blew out of the tank (luckily not stripping the threads and not hurting anyone) - the tank then proceeded to bounce onto my leg (OW) then the floor - of course by now Im out of the room holding the door shut - the tank ended up about ten feet from where it started.

Hehe, my neighbor was telling me stories once about some of the industrial 6k psi cylinders that he had at work a long time ago. I believe it was ones that didn't pass inspections, so he and his coworkers would fill it up one last time, and then go out the the river (I think it was the Mississippi) and take an axe to the valve. They would shoot off into the air over the river and land quite a ways away.

I believe I've heard a story or 2 on here where reg's broke off or something, and sent the paintball tanks through walls.

[f3rr3+]

what kind of precautions do the people who hydro tanks use? i mean isnt there a chance, so be it a small one, that the tank could explode?

[Wat]

The tanks are placed in metal tub thats filled with water and then a lid is put on. I don't know the size of the tub but i imagine its pretty hefty. Then the tank is pumped with water (or is it air?) and a tube on the outside shows the tub water level rising and thus showing how much expansion is occuring.

Fibre wrapped tanks when they fail, shouldn't do so in a very spectacular manner (though still a big bang). The fibres should shred and rip open but not so much in the way of huge shrapnel, though i wouldn't want to be near any of them. Something that would be dangerous also is if the valve or neck failed and then you get a nice rocket that would probably punch through cinder block walls and stuff.

For our useage, i think the most dangerous aspects is all the lines, guages, hoses and fittings etc. People get lazy and casual at fill stations. I've noticed a few fill operators not check certification dates or fill ratings before. They really should do something like dig a hole or get a huge metal tub, hook up the lines, secure the tank and then put a lid on it all when you fill it. Chances are if anything fails it will be during the fill and i'm not just talking about the tank.

[manike]

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]

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. :D;)

[1stdeadeye]

Originally posted by aaron_mag


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

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!!!:D

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

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!;) :p

[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... :cool:

[Skoad]

my 4500 tank was tested up to 7000 psi.

[manike]

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!!!! :D

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 :D - 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]

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....:D ;)