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

[Wat]

Rated load vs. test load

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! )

Play, be safe, and have fun!!!

[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! )

Play, be safe, and have fun!!!

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

[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 .

[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]

hydro tests

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.