The pressure in a propane tank depends on the temperature, as long as there
is any liquid left in the tank. A tank with a little slosh in the bottom
will have the same pressure as a full tank, if both are at the same
temperature.
I have seen quite a few air tanks made from old propane tanks. They differ
from purpose built air tanks by having much thinner walls. Plenty strong
enough for use, and fatigue through pressure cycles is not the problem - it
is rust.
When used for propane, there is little chance of internal rust, but the
tanks are required to be inspected regularly to check for rust. "Portable
tanks that have a DOT number stamped on them are required to be recertified
twelve (12) years after the date of manufacture and every five (5) years
thereafter." This involves looking into the interior with an inspection
tool, and perhaps removing light rust by steel shot & a rotating fixture.
External paint touch up is done. A new date is then stamped on the
cylinder.
When used for air service, they will be much more likely to have condensate
collect in the tank. And there is no drain valve to release the water.
The walls will begin to rust.
Portable air tanks also collect water, and begin to rust. They just have a
longer way to go before releasing the contents.
Still, I think the idea of using an old propane tank for oil delivery is a
great idea. So lets manage the risk:
* keep the pressure down, like 30 psi instead of 100
* use dry shop air to pressurize
* check for external rust
* keep that safety release valve on your conversion
If your shop air compressor runs at 180 psi (common for 2 stage units) it
will wring out a lot of moisture in its storage tank. When it expands
through the regulator, it will have less moisture than the discharge of a
single stage system. Better for painting, too.
I live in Florida, so do not see cold, thick oil. Perhaps one of you cold
weather maintainers will try the setup with 30 psi & report the results.
Jack
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