Good Evening Fellows,
Mulling over the idea of using a 100lb. Propane tank (empty of course)… as a extra “storage” tank for my compressor. Wondering what the normal pressure rating is when used for propane, and wether it would suitable at all for holding 100 -135 psi of air.
Planned to install it upside down in order to install a drain
.
Your feedback would be appreciated as I am not interested in blowing the side out of my newly constructed boat shed, oops..
thanks..Wilson
Replies
Might try calling local propane co. and asking. Seems like it wouldn't be a problem, but I would surely ask first on this one.
ASME specs for propane tanks include normal 172# @ 100ºF and max @ 250#. NB, I don't think propane tanks can be hydro tested for recertification, so once it's past its date, it's may be a junker (sorry, I don't know the dates for the US).
* just for comparison, here a tank is good for 12 years, then can be revalved and recertified for 5 years at a time; but, I don't know the recert procedure - we have to turn the small tanks in.
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Phill Giles
The Unionville Woodwright
Unionville, Ontario
Edited 4/15/2002 9:37:27 PM ET by Phill Giles
I've used the 5 and 10 gal tanks for years as air pressure tanks, rated 300 psi as I recall.
Biggest worry is the definition of "is" it "empty". From fear of liability, you local dealer will likely tell you NO!!, as a little remaining propane can really blow. Don't recall the mixture range of propane/air ignition, but that is the biggest worry, which would blow the boat shed out, fer sur!. To be positive its empty, my technique has always been to store the tank upside down (valve down and open) for a week in open air, fill with soap and water and agitate, flush with water and then drain for another few days, flush with air a few times with no esd spark potential nearby, all with valve down. Now the tank is empty and ready to use for air. Paint it green so you know it's air and not propane.
300 psi comes to my mind too and that would make sense (or even be a bit low) because the vapor pressure of propane is 300 psi at 58C=137F. And on a 115F day in Phoenix, a tank in the sun (much less in a closed vehicle) would get about that hot. And yet you don't hear of propane tanks blowing up during a heat wave.
If you vent the tank fully, there will be 15 psi (absolute) of propane remaining inside. Filling it up with air to 75 psi gauge (=90 psi absolute) would put give 1 part of propane and 4 parts of air inside (20% propane). 20% is above propane's "upper explosive limit" or UEL - i.e. it is too rich to burn. Venting again and refilling to 75 psi (gauge) would leave 4% propane which is definitely within the explosive range. DO NOT shoot the tank with an incendiary, armor-piercing round while it has 4% propane in it. Just don't do it, even though it would be interesting. Another time and it's down to 0.8% and you are (just) below the lower explosive limit = LEL = too lean to burn. I'd do it one more time, for a total of four fills & vents, before putting it in service. Plus 2 or 3 more times if you want to get rid of the smell of mercaptans.
While venting the propane or the first few propane/air mixtures, I'd observe the same precautions as propane tank workers do - no metal on metal unless non-sparking alloys are used. So loosen with a normal wrench to hand tight, but then put that wrench away and open by hand. No Smoking. (Duh.) Ground yourself to a water pipe or metal fence pole. You could go one step beyond what propane workers do by avoiding any synthetic clothing and working in high humidity, such as early in the morning. Think about who and what is downwind because the first two vents will hug the ground since it is colder and denser than air.
David Thomas Overlooking Cook Inlet in Kenai, Alaska
Edited 4/15/2002 10:46:12 PM ET by David Thomas
Please be careful.
I have read about accident that occur when people use tanks designed to be non-refillable. Such tanks have no margin of safety to cover the metal fatigue cause by repeated filling, emptying and thermal shock a compressor tank is exposed to. When a tank fails it sometimes does so with great violence. A bomb is a close analog.
You propane tank is refillable, I assume, and so this may not be a problem.
Interesting if that is the logic of the "$ 25,000 fine and/or 6 onths imprisonment" for transporting 1-pound propane cylinders, if refilled. (Those little ones for camping stoves, laterns, and soldering torches). Because it would seem that they flex a lot with temperature changes. From at least 200 psi on hot days to LESS than atmospheric pressure during my winter. So should there be a "use by date" on them? Or is it just a way to sell 0.25 gallons of propane for $3.99 or, at $16/gallon, about 12 times the price of bulk propane.
Not that I would ever refill one. Oh, no. No me. I wouldn't pop one in the freezer to lower its pressure and then connect it to a bulk tank. Not me.
But if I were going to, I'd weigh it accurately before and after. Because the one pound of steel should only have 1.0 pounds of propane in it. Not the 20 ounces that will fit. Not that I've tried that, you know.
Could that be the reason that the unskilled shouldn't refill those containers? If there's "headspace" in there, it can only go to the propane's vapor pressure which increases as it heats up. But if there is only liquid in there - watch out! The liquid propane expands much faster than the steel container and there would be no limit to high a pressure could be reached. Well, obviously, the limit would be when the explosion occurs.
David Thomas Overlooking Cook Inlet in Kenai, Alaska
Edited 4/16/2002 12:49:09 AM ET by David Thomas
Another caveat: We're talking propane tanks here - those fairly thick and heavy ones. Not butane, freon, or helium for ballons tanks. Those are all much thinner steel. I have seen them used on the job site for air tools, but the margin of safety just isn't there for me to consider it.
David Thomas Overlooking Cook Inlet in Kenai, Alaska
Thanks for the calculations & reminder of explosive mixture range.
Ah, the fun we've had with propane. I have tried to "blow" a propane tank in much younger days, but 5 gal tank only flames out the hole if the valve is off, but did succeed in blowing one with 1/2 lb. powdered zinc/sulfur mix added inside (man, did Mom scream at me for scaring her, ignited it using a paper clip as the ignitor, plugged cord into dryer outlet 50 feet away, lights dimmed across the neighborhood due to paperclip short, then boom and fireball, luckily the cops didn't come - 1950's!)
The way I fill the "disposable" cylinders (I do weigh them and only put in 10 oz.) is to loosen the side schrader valve (had to make a special tool) so small amount of evaporation cools the small tank, fills real fast, but sure don't have any ignition source around as it's done. Wife won't let me put stuff in freezer anymore since she almost put some mixed epoxy stored in tupperware into a cake by mistake once. Have not bought a small tank for probably 20 years. BTW, the threatened fine only applies to interstate transport.
For added fun (like David, I've never really done this<G>) I usually do it in a wheelbarrow outside on a calm day, then when done, throw a match into the wheelbarrow, nice benign dancing flames work their way to the bottom of the wheelbarrow.
Junk: I put epoxy in the freezer too. I guess I've labeled it well enough because my wife has never subsituted epoxy for honey in a recipe.
Saw a moron toss a 1-pound in a campfire once. The clever people jumped behind rocks. The dumb ones jumped behind tents. It seemed like a long time before it blew. Cinders all over the place required some impromptu firefighting.
A plumber co-worker had an old (14 oz? - the tall one) propane tank that had the valve much more accessible. So it was easy to vent during filling with just a pair of plier. But freezing the recieving cylinder has the advantage of not venting anything till the very end, when you got the valves all closed.
I write the tare (empty) weight on all my cylinders in case I'm topping off a partially used one. And if I get more than 16 ounces in one, I just run an 8,000 BTU/hour camping stove head on it for 10 mintues per extra ounce. Unless I'm going to use it immediately and fairly continuously - then the evaporative cooling will keep the pressure down.
You've had propane cylinders ice up? I had a 500-gallon tank ice up (below the liquid level) when running a 1,200,000 BTU/hour burner off of it. And for my backpacking hot tub, I have to toss the 20-pound cylinder in a bucket of the heated water to keep the pressure up.
"A am a professional. Do not attempt this at home."
David Thomas Overlooking Cook Inlet in Kenai, Alaska
Have had the 14 oz ones ice up camping at 7500 ft. Once resorted to putting the tank over the flame for a few seconds trying to avoid hypothermia.
Like you say, don't try this at home.
Art B.
Yeah, because of all those "No Smoking for 50 feet" signs, people really freak out when you direct a flame onto the side of a propane tank. I take it as a sign of a confident plumber who is sure he has no leaks.
MSR/REI now have propane/butane mixes that work better at altitude than butane only without the need for as much steel as propane. But all of them have a lower temp limit that isn't very low at high heating rates.
David Thomas Overlooking Cook Inlet in Kenai, Alaska
I seem to recall something from the fire academy called a "bleve". From the acronym for boiling liquid expanding vapor explosion. Direct flames on a pressure vessel cause the liquid to boil. The physics of liquid to gas expansion progress so quickly that even pressure valved tanks choose to rupture. The term for a propane grill was "backyard bleve" and bic lighters were called "pocket bleves" Back in the 1960's, if memory serves, a town in Oklahoma was leveled when a rail tank bleve'd. But then I run my grill on natural gas through a garden hose.
I just read that about 20million 4 to 40 lb propane tanks built before 1998 just became obsolete in the USA by a law that just took effect. Tanks without an overfill prevention valve can no longer be refilled. If I remember correctly, all tanks with the proper valve have a triangular valve handle instead of a round one. Someone else probably knows more about this than I do - since all of my tanks are larger than 40lb, I didn't pay that much attention.