Hydrostatic testing of dish designed for external pressure

[MHTT]

I am busy with designing a distillation column (ASME VIII Div 1) where the client requested the bottom dished end to be inverted (pressure on convex side) to minimise liquid holdup. The bottom dish is therefore designed for an "external pressure" equal to the internal pressure of the rest of the column. During pressure testing the total column will be tested to at least 1.3 x Sa/St x design pressure with no component stressed to more than 90% of yield. ASME VIII Div 1 is not clear about test pressure on components designed for external pressure. Also, ASME 8 div 1 design calculations for external pressure on a shperical dish uses Elastic Modulus and not Max allowable stresses. Does someone know if I am allowed to expose the inverted dish to a test pressure exceeding the external pressure the dish was designed for? I am tempted to design the inverted dish for an external pressure equal to the actual test pressure value but this is a costly solution.

Any feedback or suggestions would be much appreciated.

Many thanks
Pieter van Staden
Mass & Heat Transfer Technology (South Africa)

 

[deanc]

A test is a test.....it does not matter.

See UG-32 and UG-33 one of these should cover design. Not exactly? Use U-2g.

 

[bvi]

There is sufficient margin of safety in the external pressure rules for a 1.3 times over pressure test (and also for the former 1.5 times over pressure test). If it fails in the hydrotest, there is a deficiency in the design or construction, and it will have been a good thing you found out during the testing.