Noncircular vessel vacuum chamber ligament efficiency

[LearningENG-]

Hi all,

I'm now at the stage of calculating ligament efficiencies in designing my chamber here thread794-463774.

I have three types of plates with holes to which CF nipple tubes will be welded on. Type one is a plate with one row of 4 uniformly distributed 6" holes on the centre line of the plate, which I can find the efficiency em and eb as per UG-53; type two is a plate with two rows of 2 uniformly distributed 2.5" holes not on the centre line of the plate, which I can also find the efficiency as per UG-53 and 13-18(b);

What's really tricky is type 3, where one row with 2 equally distributed 6" holes and one row with 4 equally distributed 2.5" holes are along the longitudinal direction, not on the centre line of the plate. In 13-18(b) the third last paragraph the Code says "If the diameter of all the holes are not the same, the stresses must be calculated for each set of em and eb values."

So my question is, how the stress can be calculated for holes with different diameters? particularly when calculating the diagonal ligament efficiency? Should I just do it conservatively by using the diameter of the largest hole?

Thanks,
XS