(OT) Compressed Air Tank Questions

[Ken_McE]

I have come across a little compressed air tank that I'd like to clean up and overhaul. I don't know anything about good compressed air practices and wondered if anyone might have some advice?

The steel tank has an expiration date stamped in it. If the tank looks good, can I safely ignore this?

There is a drift of fine black particles inside the bottom of the tank. They are as small as sand, but are shiny and black and rounded. Anyone know what these are? Should I leave or dump them?

Thanks in advance~

 

[gcbryan]

The practices for scuba tanks are that they be hydo'd every 5 years. The date stamped in your tank may be a hydo inspection date.

Inside the tank is probably flash rust or pitting. Again, with a scuba tank this is fairly common and the coarse of action would be to tumble the tank.

I assume others who work with compressed air in a shop environment can more specifically answer your questions but I thought I'd throw in my 2 cents in case any of it is applicable to your situation.

 

[gadget_lover]

There are lots of "compressed air" cylinders available. Some were actually used for air (like scuba tanks) and others were for gasses like oxygen, propane, argon, etc. Compressed gasses are high pressure. thousands of PSI.

The markings on the tank have meanings, such as when it was tested. and (generaly) to what pressure. Try finding the marking on the web, or take it to a local welding supply shop.

Are you going to use it for an auxiliary tank for your shop compressor, or are you going to have it filled for welding, oxygen supply, etc?

Shop compressors seldom get to 200 PSI, so I would guess you are probably safe using a tank (formerly rated to 1800 psi) as an auxiliary compressed air tank.

Dan

 

[65535]

Post up some pictures!

Pressure vessels don't expire, but their hydrostatic certification does.

The stamp indicates a passed hydrostatic test date, different vessels require testing done on fixed intervals.

Depending on the size of the tank it could be a lot of things, and a small tank to one person may be huge to another, and vice-versa.

Since it has particulates that don't appear to be rust, and the fact that there is an opening large enough to allow you to inspect the tank, I get the feeling it's likely an old pressurized blast media storage tank, which would be pressure rate. Though I can't recall a requirement for a unit like that to have a hydro cert.

You can get it hydrostatically tested, which would be the quick way to find out if it is safe to use, because the test is exactly designed for that.

A DIY method is to find a small hydraulic hand pump completely fill the cleaned tank with water allowing not air, attaching the pump and filling it to 3/2 service pressure and check for leaks and bulging. Also before and after the test check for rust, pitting, and any other damage to the inside and outside of the cylinder.

This is a flashlight forum, so grab your edc light, take some pictures and post them up.

 

[gadget_lover]

A note about home testing:

When they test them at the shop, they do it in such a way that it contains the blast if it blows up. A rutured tank at 3/2 * 1000 psi can be exciting, maybe even deadly.

Let the experts test it.

Dan

 

[PapaLumen]

Yep, have you seen that Mythbusters where they knock the valve off a big tank and fire it through a wall they built in the workshop. It went through the wall and then half way through the wall of the building :eek:

 

[gadget_lover]

Places I know that will pressure test tanks (for a fee)

Welding supply
Scuba shops
Medical gas supply.

Dan

 

[precisionworks]

will pressure test tanks (for a fee)

Welding supply
Scuba shops
Medical gas supply.

Fire extinguisher service companies may also be able to test the tank.

 

[KC2IXE]

Doing a pressure vessel test with gases DOES require amazing safety gear, but as the poster said, doing it with WATER, and a small hydraulic hand pump, is amazingly safe.

The reason an air tank (or any other tank with a compressable medium in it is so bad is that well, there is a LOT of energy stored in the compressed medium. By using WATER/Hydraulic fluid etc, the ONLY energy that is there is the amound of force you have in the hand pump, PLUS whatever stretch there is in the system (rubber hoses, the steel actually flexing etc) - aka a small amount

Live steam modelers use these hydro tests all the time, and they are testing something a LOT more dangerous than an air tank at the same pressures, due to the energy in the phase state transition

 

[gadget_lover]

If I'm interpreting this correctly;

The reason a gas is dangerous is that you have a lot of cubic feet of it compressed into a few cubic feet of space, and when it lets go the gas expands to it's original volume? It's not the 1000 psi of pressure?

But a liquid that does not compress is OK, because that 1000 psi is only impacting the walls of the container, and when that lets loose, the 1000 psi is less of an issue?

I thought the energy in 1000psi was the same whether it was stressing gas or steel. Is that wrong?

Daniel

 

[65535]

If I'm interpreting this correctly;

The reason a gas is dangerous is that you have a lot of cubic feet of it compressed into a few cubic feet of space, and when it lets go the gas expands to it's original volume? It's not the 1000 psi of pressure?

But a liquid that does not compress is OK, because that 1000 psi is only impacting the walls of the container, and when that lets loose, the 1000 psi is less of an issue?

I thought the energy in 1000psi was the same whether it was stressing gas or steel. Is that wrong?

Daniel

Yep, think of balloons, a normal balloon filled with air explodes when ruptured, causing a pressure wave and accelerating the balloon shrapnel outwards at a fair clip. A water balloon is under the same pressure when it ruptures give or take 1-2 psi and the water just falls to the ground nothing rapidly accelerates outward.


Water does not expand or compress very much at all, air can be compressed or expanded many times, when you release water pressure from a steel pipe the pressure drops of almost instantly (note a rubber hose will expand like a bladder and when pressure is released it maintains pressure until the rubber is at it's neutral state) where as air takes time to decompress in the pipes before the flow stops.

Most hydrostatic tests are done with water, if a vessel does burst the water decompresses nearly instantly and the tank is relieved of pressure, with air, if the tank ruptures it quite literally explodes.

The only caveat of the test is to get 99% or more of the air out, having a tank rupture at 150psi-300psi isn't much in the grand scheme of compressed gases, but a 5gallon tank exploding in the same room will likely knock you unconscious from percussive pressure waves, then you have to deal with the fragments of the vessel doing amazing speeds.

This is also why SCH40 PVC pipe is water rated and not air rated, and NEVER EVER to be used with air.

 

[KC2IXE]

65535 got it right - Assuming there is no elasticity in the containing vessel (water ballons ARE elastic, so) and figuring water as inelastic (well, it actually DOES compress - but in amounts that are below measurement levels except in the finest labs, ALL the energy in the system is by whatever is supplying the pressure - so say you have a hand pump with a a 2 oz capacity - the whole amount of liquid that can come out under pressure is 2 oz

 

[Torque1st]

A good hand pump for pressure testing is a common lever operated grease gun. Just use a grease zerk with a T between the pressure gauge and the tank.

 

[Wilmette]

[snip] This is also why SCH40 PVC pipe is water rated and not air rated, and NEVER EVER to be used with air.

What 65535 said! Google "PVC compressed air accident" or similar for an evening of sobering reading.