In line/ portable air tank out of a Propane Tank

This is a old air tank that I use for starting fires and for filling my air cannon.

This tank was made with parts that where on hand a many many years ago. I had to take it apart because of a small leak and wanted to show how it was made.

Precautions
Use an EMPTY propane tank. You don't want gas leaking out and causing an explosion. Also compressed air can be dangerous. I'm not sure what a propane tank is rated for but keeping the PSI at a reasonable amount should be OK.

Step 1 Whats needed

Parts (All in sizes inches)

• standard EMPTY BBQ propane tank with vale removed. Preferably newer then the one I used
• Air gauge 0-100 PSI (could go higher then 100)
• 1/4 female T
• 1/4 T male/female though female take off
• 1/4 male nipple
• 3/4 to 1/2 reducer
• 1/2 to 1/4 reducer
• 2 female air couplings
• pipe joint compound

Tools

• 5/8 wrench
• 11/16 wrecnh
• 3/4 wrench
• 3/8 to 1-1/4 adjustable wrench (1" wrench)

If needed, a regular pipe wrench .

Comments

[KENUT0]

there should have been mercaptan in the tank. How did you dispose of it? Did it present a problem? Thanks

[batman1298]

household bleach will remove the mercaptan from the tanks. as far as disposal i used household haz-mat disposal in my community

[Rob K]

Thought that was just added to the propane gas. As for this tank it has been used for an air tank for the last 15 years or so.

[TheCaffeinatedOne]

I've been thinking about extending the capacity of my air compressor by adding three 20# propane tanks I have kicking around from a time prior to the valve design change. Given that the compressor cannot produce any more than 125 # of pressure, the tanks should be fine.

My idea is to add them inline using 3/4" Schedule M copper pipe. I'm not interested in quick removal; these are intended to stay in place.  I plan to  mount them upside down off the main copper line so they become self draining. If I set up the line in my garage / workshop horizontally along one of the roof trusses, the pipe can support the tanks in line. I'm planning to use 3/4" threaded adapters to tees inline and add a dropped water trap similar to a raised air hammer chamber in home plumbing, but with a ball valve at the end to pull off the accumulated water. Tees along the line can provide access for tools and so forth. Any thoughts?

[jkarle1106]

Propane tanks are DOT certified, and must be re-tested every 5 years. The hydro is to not only verify the pressure rating, but a "real" hydrostatic test measures the tanks ability to return to an established % or it's original size after a 1.25 to 1.5 x over the maximum opperating presure rating. The "pop off" valve needs to be sized for not only a specific pressure rating, but also a discharge capacity rating. The discharge capacity is calculated by the maximum floww rate of the filling device, or the maximum pressure temperature (gas expansion) due to exposue to fire. I have no idea what corrosion allowances are designed into the tanks, but I'll bet wet compressed air is not as health for them as dry propane. Would I do this? Probably, but it would make me a little nervous!

[Baytonian]

Very creative, simplistic and functional. Nice job and nice instructable, thanks. I have some constructive safety comments: 1) You can hydro-test the tank by filling with water and then pressurizing with the highest pressure compressor you will potentially fill the tank up with. If it fails the it will only leak (spray) water and not potentially fail catastrphically as it could if it failed with only air in it. 2) Once you have established that the tank will work with this pressure you can substitute the first "T" connector with a "X" or cross and then screw in a pressure relieving valve which is calibrated to blow (pop off) at a pressure 10 PSI (or more) below the successful hydro-tested pressure. This will ensure that you do not exceed your tested pressure and will allow for some degradation to occur and still remain safe. 3) Make it a point to regularly drain the tank to reduce corrosion. Corrosion will happen and will reduce the pressure holding capability for any steel tank. 4) Mind Exercise: When thinking of pressure and how a tank can fail , think of of PSI over a square foot. 1 PSI over 1 square foot = 144 lbs force. the same square foot with 100 PSI acting on it = 14,400 lbs force. 125 PSI =18,000 lbs force. That's a lot of force! Now where are those old propane tanks at...

[Rob K]

Thanks for the info on hydro testing. I was not sure on how that was done. The tank really needs to be replaced. Over 15 years of use or more of not using a moisture filter the tank was full of a rusty dust.

[StrangeRanger]

How the heck do you fill it with water? j

[Rob K]

You would maybe need to remove the valve first. Or find away to force water into the open valve.

[ironsmiter]

With a propane tank, 250 psi SHOULD be a safe pressure(asme standard pressure reliefe setting). I THINK standard 50# grill tanks have their blowbys set to 375#. Either way, I would highly recommend staying under 200#. Also, use a new(but recently emptied) tank. An older, banged up starting to rust tank like shown, will tend to fail much more frequently than a new tank, and at those pressures failure may be "exciting".

[Rob K]

This tank has about seen its days 15+ years. I will probably get another year or two out of it before it gets replaced. Thanks for the info on pressure. This was the standard grill 20 pound tank. I think that is what its called.

[baudeagle]

A couple precautions for all of those that are fool hardy. Open valve and vent off for a couple of days. Avoid sparks, use non-sparking tools to remove valve. touch a grounded piece of metal before attempting to remove valve avoiding a static spark. Ground propane tank before de-valving and fill with water like MacMan45 recommends.

[NathanWilliams]

To be sure the tank is empty of propane, you can fill it with water, which will push out any residual gas. (then just dump out the water and let it dry) Especially useful if you plan to cut the tank or weld anything to it!!