general rule of thumb is if you use a flat end on a pressure vessel it will have about 30 percent more force on the flat end than on the pipe itself...yes the presusre is the same (50lb psi is 50lb psi) but it accumulates on the flat surface. think of a hydraulic cylinder with a bore of say 3 inches..add 1000 psi and it may lift 3 or 4 tons now take a cylinder with a 10 inch bore and the same 1000psi and it may lift 100 tons..same pressure but more square inches to push on...when you test your OWB ith a flat end on it you are essentually pushing on the flat end the same as a hydraulic cylinder...the bigger the bore the more pressure you exert on the flat surface. that is why your oil barrel bulged (failed) and an OWB with a flat end will burst or warp long before the same rounded side walls will give way
3"=3.53 tons
4"=6.28 tons
10"=39.27 tons
Pressure will not accumulate or build up more on the flat ends of a cylinder. Because of its inherent design, the metal on the ends will move more, and because of the stress associated with that movement it will fail first.
With all the respect that's due to you Willie, I don't buy into your rule of thumb(30%?) for flat ended pressure vessels. The force will be equal over every square inch of the inside of the vessel.
Speak to a metallurgist as I have. 36" ductile iron well pipe @ .375 wall thickness with virgin .375 ends welded properly(two passes inside and out) with 80000 tensile strength wire will barely move(ends) at 45 psi. You will be shocked at the pressure it will take before the ends eventually fail.
I agree with you all about low pressures being used for open systems to check for leaks. Every winter I do many fabricating projects, including building power units,manure pumps, booster pumps,manure applicators,knife-in applicators, etc. I understand what metal is capable of, and more importantly what it is not capable of.
A portable air tank(.125 wall thickness) with a flat end(.125) properly welded on will not fail at 100 psi. That is fact.
The flat end of the 4" square pipe measured 3.5" square(I.D. of 4" is 3.5).