Air tank safety: Discharge rate and outlet size

I have a couple questions about air receiver safety. I have a 20 gallon ASME carry tank (Grainger 4TK10). It has two 1/2" air outlets,
but I want to use it to honk an old fire horn which is not working when I plumb from just one of the 1/2" outlets. I need more flow to maintain proper blowing pressure at the horn. So my questions are these:
1. Is there a known maximum safe discharge rate for a 20 gallon ASME tank? The tank has a plate rating of MAWP 175 PSI @ 475 F. I need to meter out half-second blasts at a rate of approximately 80 CFM. Is this safe to do from a 20 gallon tank? What about from a pair of 20 gallon tanks?
2. Can I plumb the two 1/2" outlets into a common 3/4" or 1" tee? Is there any safety reason not to?
3. Lastly, the tank has a 1-1/2" NPT inspection port that has a 1/4" hole tapped in it for a guage. Can I replace this plug with a 1.5"-to-1" hex bushing and plumb right from there, or is this a no-no.
I want to get full flow to the horn, but I don't want to do anything that is likely to cause the tank to lose integrity or fail explosively/implosively. I've found lots of useful information about maximum tank pressures, but I can't find anything on the web about maximum discharge rates or air tank implosion.
Thanks,
Adam adam at airraidsirens dot com
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I don't think you have to worry about the tank. Truck shops use a similar tank with a 1&1/4"+ outlet size line and a dump valve. They point the discharge tube at the rim / tire bead area ofa newly mounted tire and 'dump' the contents of the tank in about a half second. The wave of high pressure air blows the tire up far enough that it catches the bead so the tire will stay mounted for the rest of the inflation process through the valve stem. What you are doing sounds tame by comparison. Some more info is at: <http://www.uniquetruck.com/products/index.cfm?fuseaction=subcategory&catego ryid=9&subcategoryid> Hope it helps.... Jeff

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Indeed! Thank you for the reference. It's particularly interesting to see that some of those carry tanks they're selling for that purpose are aluminum to boot. This is reassuring.
I should have noted too that what I called my "carry tank" is better called "portable". It's 50 pounds and ASME rated, the same tank Campbell Hausfeld uses on their portable contractor compressors, not one of those non-code tire inflator cans they sell at hardware stores.
Thanks,
Adam adam ATsymbol airraidsirens DOT com
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I wonder what they use in those potato and pumpkin cannons?
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The only one I ever saw used a 2" iron pipe with a cap on the end tapped for a spark plug. Some butane was sprayed into the pipe from a lighter refill canister, which mixed with the air of course, then the potato was jammed in almost to the bottom to compress the mix. Then, a 9V battery was applied to an ignition coil hooked to the spark plug. Next time I saw the potato it was hundreds of feet in the air. I didn't do this myself BTW, and I didn't want to be anywere near it when it was fired--I'm not crazy. I saw this experiment at an all-engineering-students frat house back in my college days.
-Adam adam at airraidsirens dot com
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Bah... You make it sound like all spud-guns are an accident waiting to happen.
Well, they are... But mainly if you can't AIM.
Best one I ever encountered was made out of nothing but PVC pipe and a flint sparker out of an old coleman lantern. Barrel was 1.5" pipe about 3 feet long, with a fiarly long taper cut on the inside edge of the muzzle end to make a pretty decent "knife edge" for cutting out good tight-fitting chunks of potato. The "breech" was made out of a 4x8 inch "cleanout" section with a screw-in cover, and the barrel was threaded into a 4-inch to 1.5 inch cover/cap that fit the cleanout tube. Punched through the side of the breech was the sparker. Firing sequence: Unscrew breech plug, spray a 5-7 second shot of rite-gard, final-net hairspray, or similar into it and slap the cap on. Grab big tater, and punch down onto muzzle end, cutting out a tight-fitting chunk of spud, punch spud-slug down with a carefully measured chunk of broom handle for compression, point, and spin the sparker. T-whunk! Tater gone bye-bye! At a rather impressive rate of speed. :)
Worked OK for watermelon, too, although that was more of a "shotgun" sort of ammo - The "slug" tended to break up on its way through the barrel, and generally came out as a splatter of red pulp, closely followed by the rind. Apples worked at least as well as potatoes. Tomatoes didn't do so good... The one experiment with those was classed a dismal failure - It didn't shoot the tomato - just turned it into ketchup, with a sound not all that much unlike a half-liquid fart.
We messed with that thing on and off for the better part of a year before one of the crew mis-aimed just enough to send the slug over the fence to shatter against the neighbor's house - only about 10 inches to the left of his 6x10 sliding glass door. We decided that things could be a bit "messy" if we kept using that area asa target, and wisely moved along to other pursuits. The guy's intended target was the fence itself, but he aimed just a touch high, it seems. I was able to watch the spud-slug miss the top of the fence by MAYBE 2 inches. The THWAP! as the spud hit the vinyl siding was quite impressive. At least to us. Neighbor was less than properly enthused. Can't really say I blame 'im... :)
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wrote:

Ours had a 4 or 6 inch PVC barrel, a 1 1/2 inch busing inside, then a cleanout plug at the end. Also used the flint igniter. Fuel was starting fluid or pentane. The 4 incher would shoot a well packed snowball quite nicely.
Always wanted to refine it for a more rapid fire and a larger barrel flame at night...
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adam smith wrote:

letting air out of an air tank that has pressure will not make it implode.. the only way it will implode is if there is a vacuum inside the tank and the pressure outside the tank is higher than the vacuum..
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A short 1/2" line with full-port fittings and valves will dump 80 CFM at an insigificant pressure drop, just a few psi at 175 psi.
Backing up here for a moment, the 20 gal tank holds 33 standard cubic feet of air at 175 psi. That's less than a single half-second blast. If it's taking more than a half second to dump the tank thru a 1/2" port, there must be a restriction somewhere.
Bob
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snip

You forgot that the tank pressure is dropping (flow proportinal to pressure)pressure at the port is dropping more rapidly than the average pressure in the tank, and the very rapid cooling going on. But it's still fast. Unless you are trying to drive a spud gun, then it is not near fast enough.

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