Originally Posted By: SecondMonkey
SHEESH! I just pulled apart my 6gal pancake compressor to get it ready to test. I looked inside with a flashlight and couldn't believe the rust. I whacked the outside with a rubber mallet to loosen some and nearly broke through!!
I've been basically sitting on top of this tank filled to 150psi for years. (under my workbench) It looked perfect from the outside!
Back in the good old days (well they were really bad old days), one of the integrity checks that was called for on air receivers was a "ring" check. Whack it, and see if it rings or thuds.
(We use it a lot on high temperature/pressure joints that we aren't dismantling - one retighten, and the bolts lose 2 years life - and there can be $150k of bolts on one joint)
About 20 years ago, there was a sensational case where an old inspector walked up to an elevated (head height) horizontal receiver, and gave it a good whack.
It was still pressurised, and corroded to near failure. Which he prompted with his mallet.
I'm continually amazed at how little wall thickness is needed to hold pressure when the material becomes a membrane...seen a tube down to 0.004" wall thickness over an area of 1/8" square holding 2600psi. Seen a guy put a car key through another tube not far away from it.