Allowable stress for nozzle load 2

[Bangrod]

When a nozzle load on a ASME Section VIII, Div.1 vessel is based on pure thermal stresses, what allowable stress value may be used ?

Our customer believes that for a pure thermal case, that the allowable material stress may be multiplied by 1.5X, but can not point to specific code section that says this.

Can someone please advise on any knowledge of this ?

Thanks
Bangrod

 

[jte]

Bangrod-

See VIII Div. 1 UG-23(e). For a better understanding, take a look at Div. 2 App 4, in particular Fig. 4-130.1. Going to 1.5*S would be considered by most to be conservative with a thermal loading: You could go as high as 3*S. However, going to 3*S requires a lot of confidence that you have evaluated everything. Try to keep local primary + secondary stresses below 2*S and you'll save yourself some trouble.

Seipp, George, and Morrison have presented two papers at the ASME PVP conferences which provide at least one example in which thermal loadings induced stresses which behaved as primary, not secondary stresses. If you're looking for the paper at the ASME web site, it's number PVP2005-71535. I'd ask your piping stress guy what the free thermal growtn of the piping would be if the vessel wasn't there. If its 3 feet, I'd be wary. If its 1/2 inch it'll behave as a secondary stress.

jt

 

[bvi]

External loads on vessel nozzles are considered the same, irrespective of whether they are due to restrained thermal expansion of the piping, or due to weight, pressure, or other loads. Membrane stress in the nozzle neck is primary (note that this includes the stress due to axial loads on the nozzle and the M/Z stress due to gross bending; that is how the M/Z type stress is treated in the vessel code, the bending stress in that code is through wall bending). Membrane stress in the shell is primary or local membrane, depending upon the extent of the region of the stress. Bending stress in the shell, in combination with the membrane stress) is subject to a 3S, primary plus secondary stress limit because the bending stress in the shell due to external loads on the nozzle is considered to be secondary.