Propane Tank to Air Storage

The rating is on the original valve - usually 175 or 375 - they are test to 3x that - more then your compressor can dream of

Thanks, Im not good with writing so that means a lot to me

I don't know any people who have a compressor capable of reaching pressures high enough to set off the pressure release and this tank stays with my 100PSI MAX compressor i don't see a need for a pressure release on the tank.

Thanks

Matthew

Their is nothing to build - just get a soft pol from an old grill and plug it it - lotsa wasted effort here - no need to take the valve off - it has a built in pressure release

This seems overkill. If the tank is vented to atmospheric pressure and the cylinder wall temperature is room temperature then there cannot be enough remaining propane gas in a 20 pound tank to amount to more than a pop if you deliberately lit it. On the other hand, the ethyl mercaptan which is added to make propane smell bad is itself highly flammable and the vapors explosive. This residue lingers for a long time even in a vented cylinder. I'm not sure how much damage this could cause.

This "vented to atmosphere" shouldn't be considered adequate for purging a tank of propane. A mixture of propane and atmosphere (oxygen/nitrogen) can burn if there is around 10% to 25% propane gas. It will burn very fast and explosively. Above this level and there isn't enough oxygen to support a flame - test carefully! Below 10% and there isn't enough gas to reliably light. Again, if you want to test it, test safely.

I should have noted that the tank must be both vented to atmosphere and be at ambient temperature. If you accidentally vent a full tank of propane it will spray propane at moderate pressure for a period of time. Slowly the pressure will drop until the propane comes sputtering out at very low pressure. Someone might inadvertently think the tank is near empty, but it could still be near full of liquid propane at atmospheric pressure. You would notice that the sides of the canister will be wet or frozen with condensation. The tank will be very cold. What happens is that the pressure drop causes the liquid propane to boil off as a gas. This cools the propane. The propane temperature also drops due to adiabatic cooling. I'm not sure if the temperature drop is mostly due to the boiling or due to the adiabatic cooling. I think it's mostly adiabatic. As the propane temperature drops the pressure at which it is a liquid drops. In other words, at atmospheric pressure the propane can easily remain a liquid as long as it's cold enough. Looking at a phase diagram for propane it looks like this is -44 degrees fahrenheit. So obviously you could only be sure no propane is left if the tank warms up to ambient temperature -- assuming your ambient temperature is over -44 degrees (so don't try this in Antarctica).

my take is same with you...It's an overkill. if those guys Wanna take the few extra miles to be safe...Let them be.

You'd be amazed how much LPG penetrates the surface of the metal, the only safe way to remove all traces of the gas is to steam the interior for about an hour. When inspecting the interior of LPG road tankers we would take steam the interiors for two hours, allow to cool, gas test, then steam for another two hours. If the tankers were not inspected straight away after the second steam/cooling they always failed the gas test as the gas leached from the metal surface.

Using an old propane tank MAY be ok 99.9% of the time, but its like cutting 44 gallon drums that have only contained oil or grease - its a recipe for someone to be killed sooner or later.

My claim is that the porosity of the metal walls of a 20 pound propane tank is not sufficient to adsorb a meaningful (explosive) amount of propane.

The rules requiring the interior of purged tankers to test negative for gas can't be designed to prevent explosions at the levels you test for. My guess is that the rules were established to allow for subsequent safe operations inside the tank. Workers can easily determine if the tank had been properly purged or not simply by checking that there is absolutely no gas left inside the tank. If you are going to ask someone to crawl inside the tank and start welding in there, then you owe it to that person to be sure there is no doubt about safe conditions. Otherwise you get into this situation:

"Hey, my gas detector shows there's still gas in this tank. I'm not going in there."

"Don't worry. There's always a little gas left."

"Oh, OK. That's good because I left my flashlight in my truck so I was just going to use this book of matches for light."

It may also be that you were actually purging and testing for ethyl mercaptan. Unlike propane ethyl mercaptan is a liquid at atmospheric pressure. Lots of it likely remains inside the tanker long after the propane is gone.

Gas testers detect gas. They don't in particular detect "dangerous" or "explosive" gas conditions. They just detect gas. Most don't quantitatively measure how much gas. They give relative readings. I have a combustible gas detector sensitive to 5 ppm (in methane, but likely similar for propane). The lower explosive threshold of propane is 2.1% -- that's about 20000 parts per million. So if your gas detector is as sensitive as mine then it will alert you to the presence of gas at a level four thousand times less than the minimum explosive threshold. If I was a welder then I would trust the tank had been well and truly purged if my gas detector couldn't detect any gas at this level.

I'm not advocating against caution, but some of the precautions I've read here strike me as absurd. If a tank has been purged and left vented to the atmosphere for 24 hours then I don't see what the concern is. The residue ethyl mercaptan is more of a concern to me. I don't know of any physical or chemical process by which a significant amount of propane could have been adsorbed into the walls of the tank under pressure and then slowly released at atmospheric pressure. I am aware that metallic hydrides can store significant amounts of hydrogen, but I don't see how this would apply here. Perhaps nickel in stainless steels might pull hydrogen out of the propane molecule. The walls of the tank would have to be spongy and porous to store any significant amount of hydrogen. It's an interesting thought, but surely one would have found this phenomena described somewhere.

That leads to my final point. If this actually were an issue then shouldn't it be possible to find this phenomena described somewhere in a government safety warning, OSHA manual, ISO guideline, research report, or scientific journal abstract? While Google and the internet are not infallible, I can't find anything at all to support these claims of residual propane dangers. I would be delighted if someone can point me toward a reference to support these claims because I would find the phenomena fascinating and I would like to learn more.

One last note of caution, most valves on modern consumer propane tanks today DO NOT vent if the value is simply turned open. The valve requires that the proper hose be attached before the it will allow gas to flow. If you just open the valve and come back 24 hours later then the tank is still full of propane.

I agree that the gas does leach into the metal but what's the difference in surface area between a road tanker and a 20 Lb. bottle? In a word, significant

I also agree with the statement about oil drum cutting so I use either a cold chisel or a barrel head removal tool

Surface area is not significant, its the explosive limits of butane/propane that are critical here. You only need about 10% (varies slightly) gas to 90% air mixture inside the bottle to form an explosive mix. I know you still need a source of ignition, but that can come from all types of sources such as static electricity that comes from the small particles of oil in the compressor air. 99.9% of the time no problems, its the .1% that will kill or maim.

Surface area is not significant, its the explosive limits of butane/propane that are critical here. You only need about 10% (varies slightly) gas to 90% air mixture inside the bottle to form an explosive mix. I know you still need a source of ignition, but that can come from all types of sources such as static electricity that comes from the small particles of oil in the compressor air. 99.9% of the time no problems, its the .1% that will kill or maim.

I agree that the gas does leach into the metal but what's the difference in surface area between a road tanker and a 20 Lb. bottle? In a word, significant

I also agree with the statement about oil drum cutting so I use either a cold chisel or a barrel head removal tool

I saying "safety first, last and always".

Thats a bit overkill.

Just leave it upside down in the sun for a week and your good to go (for air).

The smell left behind is just mercaptin that has leeched into the steel not propane.

If you're going to cut one you should fill it with CO2 from a cars tail pipe.

Thanks for the advice.

I found this one at my uncle's farm with the valve off and full of water. This tank was certified in 1975 so 10 years for it to be outdated gives us 1985. The valve must have been removed sometime between 1985-2000 so it has been sitting outside for at least 14 years. All the propane smell was gone and i looked inside and their was no rust and their still is the black coating on the inside.

Once again thanks for the advice, im thinking of changing another tank into one i can take with me and ill use that method to clean out the tank.

Thanks

Matthew