failing of hemispherical head in combined loading

While designing vessel as per ASME Sec VIII Div 2 Ed 2007 Add 2008 in PVElite 2009 version, the top and bottom hemispherical head fails in combined loading even if there is no wind and earthquake load only self weight is there on vessel. The minimum thickness required to withstand design pressure is not sufficient. So, thickness of heads needs to be increased. can anyone tell me why the heads fail in combined loading? Has anyone faced this sort of issue?

Dear Anil,

This is difficult to tell without looking at your file, please send your PV ELite file and brief discription to "support at coade dot com" and we will review it.

I have sent PVElite file to techsupport@coade.com. Kindly review it.

Hi Mandeep,
Can we please have an update on the resolve of the issue above? I'm sure other people would be interested to find out the outcome of the above topic.
Cheers, gr2vessels

Hi All,

We have reviewed the new Division 2 and have made some intersting observations in this area. If you have a hemispherical head and the required thickness is equal to the actual thickness, then there will be a problem when you plug into the combined stress intensity equations on pages 4-54 and 4-55 of Division 2 2007 edition 08 addenda.

I created a simple model and the stress in the sphere due to internal pressure was 23200 psi and the design pressure was 500 psig, the operating allowable was 23200 psi and E = 1.

This means sigma1 = sigma2 = 23200 psi. Sigma3 = -0.5*500 = -250 psi

From equation 4.3.44 the stress intensity is:

1/sqrt(2)[(sigma1-sigma2)^2 + (sigma2-sigma3)^2 + (sigma3-sigma1)^2)]^0.5

= 0.7071[0^2 + (23450)^2 + (-23450)^2]^0.5
= 23450 psi

Since 23450 > 23200, it would appear the for combined loads the sphere is overstressed even when the only load is pressure.