Anyone ever build a pipe bomb... I mean compressed air tank?

[NathanStewart]

Long story short, I'm building a car that has air bags in the rear suspension.  Looking for a small air tank.  Don't need much capacity; only 1-1.5 gallons but don't have much room to work with.  Can do long and kinda skinny but not short and fat.  Need to do the math but something in the 3.5"-4" diameter range and however long to get me my desired capacity.  Max pressure is 150 psi.

So why build my own?  Because the supply of small 1 gallon tanks is pretty small (at least from what I can find).  Also, it seems like a lot of cheap, poorly made crap has infiltrated the market and I don't trust a $40 tank from eBay.  Seems like a lot of tanks are being made out of some very thin material these days.  In fact, I've seen more than a few pictures of air tanks that have exploded and it looks to be from poor construction quality or thin material (16 gauge sheet maybe?) or both.

I'd like to do stainless or aluminum for corrosion resistance but steel is also doable.  I've found 3" sch10 stainless pipe and pipe caps from a local place.  Is sch10 (~.110 wall thickness) enough for 150 psi or would sch40 be better or would it be overkill for 150 psi?  Or is my pipe dream going to give me a pipe bomb that'll blow off the back of my car?

Any suggestions or advice would be greatly appreciated.   

[John Burk]

Tanks are safe to test if they're filled with water .

[bearingburner]

I think sch. 10 is a little thin especialy in that dia. Sch.40 is better but as I remember it is only rated for 125 PSI

[McRat]

If money isn't an issue (usually is though), you can use a CO2 shifter assy like:

http://www.biondoracing.com/CO2Bottles.shtml

This already has a regulator, is very small, and tested to a high pressure, and can be remotely activated.

[Interested Observer]

Your 3” stainless pipe should be OK.  The nominal wall of  sch 10 pipe is .120”.  Assuming 30,000 psi yield strength of the material, the pressure to yield the wall would be about 2000 psi  (sig=PD/2t).  

It would be wise with this light wall to use caps, as you mention, or other externally engaging end fittings to keep the threads from dilating and going out of engagement.  Assure the closures are also of comparable capacity.  Then hydrotest, as mentioned above, to 2 or 3 times the operating pressure in a safe situation--secondary containment, under water, etc.  

In stead of all that it may be worth the effort to do more looking for commercially available solutions--air tanks, air cylinders, hydraulic cylinders etc.  You could probably get a commercial air cylinder assembled with caps on both ends (no rod or rod hole) which would have ports and such already included, i.e. Bimba.

[Glen]

Look into using a hydraulic accumulator, they come in all sizes and use a bladder. I have seen them as small as 2" dia and 12" long. All are safe and high pressure housings.

[NathanStewart]

Your 3” stainless pipe should be OK.  The nominal wall of  sch 10 pipe is .120”.  Assuming 30,000 psi yield strength of the material, the pressure to yield the wall would be about 2000 psi  (sig=PD/2t).  

It would be wise with this light wall to use caps, as you mention, or other externally engaging end fittings to keep the threads from dilating and going out of engagement.  Assure the closures are also of comparable capacity.  Then hydrotest, as mentioned above, to 2 or 3 times the operating pressure in a safe situation--secondary containment, under water, etc.  

In stead of all that it may be worth the effort to do more looking for commercially available solutions--air tanks, air cylinders, hydraulic cylinders etc.  You could probably get a commercial air cylinder assembled with caps on both ends (no rod or rod hole) which would have ports and such already included, i.e. Bimba.

IO, thank you for the info but I see I've left out one important piece of info.  The caps I'm thinking of using would be butt-weld caps, not threaded.  So I guess at that point the question becomes do I trust my welding enough to build a pressure vessel that will live in my car?

Knowing that the caps would be welded on, does this change your thinking?

[Interested Observer]

My thinking is that a properly welded formed cap would be preferred to a threaded connection, and is probably the best solution.  However, just a solid piece welded in may be problematic, depending on its design and the nature of the weld.  Still may be better than threaded, but depends.  A formed cap will flex with the barrel, but a stiff plug will likely increase the localized loading on the weld where the barrel wants to "inflate" when loaded but the plug doesn't want to follow it.  This doesn't preclude this method, but some thought should go into it.  And successful proof testing covers a lot of sins.

[Glen]

Nathan. (Google pressure vessels )there is a ton of information to he had. Includes a lot of calcs. on design and pressure testing etc.

[Ron Gibson]

Aluminum fire extinguishers are pretty thin walled and are pressurized to 195 lbs for use. Maybe use one of them

Ron

[Tman]

Nate, I have seen your welds, they are so nice I would have no qualms using a tank you built. Just test it for piece of mind. I like the stainless idea. 

[Stainless1]

Nathan, there are a lot of medical oxygen bottles and airplane oxygen storage bottles that would work in your situation.   Airplane bottles go out of date and must be replaced, inspection not allowed on most, the fire extinguisher is also a great idea...
 Also a lot of compressors that have twin tube air tanks.  Premade parts with fittings are often easier and less expensive than ones you make, be very careful when testing any tank
Check craigslist and ebay ...

[Peter Jack]

Nathan, if you're going to test the tank use hydro testing procedures. The tank is filled with water or another fluid and pressurized to a multiple of the pressure you're planning on using. In you're case I'd be tempted to just use hydraulic fluid and a hydraulic pump with a gauge. Because the fluid isn't compressed a failure doesn't result in a violent explosion. It still doesn't hurt to put the piece being tested inside a larger piece of pipe during the test and staying clear of the ends.

Pete

[Dean Los Angeles]

McMaster-Carr has 1 gallon tanks 6" dia.
If I was an inspector I would want an ASME certified tank.
Custom/home made? Too risky.

[olepaw]

go to heavy truck parts dealer get a air tank for a air brake truck.  that should work