Cylindrical shell with holes, ligament efficiency question 1

[Spoonful]

Dear All,

For a cylindrical shell made from perforated plate. To determine the MAWP or with given pressure to work out the required shell thickness, use the basic formula based on circumferential stress of

t= (PR) / (SE-0.6P)

Where E is the joint efficiency or ligament efficiency

Rules in UG-53 deal with ligament efficiency.

However as we all know above equation is based on circumferential stress, hence shouldn't we use circumferential ligament in the equations? rather than the min of longitudinal and diagonal ligament efficiency?

According to UG53, with staggered holes on perforated plate, Figure UG-53.5 should be used to determine it long. and diag. ligament efficiency. and then use the fewer of these to to determine shell thickness. Where in UG-53 it did not mention how to calculated circ. ligament efficiency, however we can use the same method as used for long. efficiency, just changing the direction.

My confusion is that should we compare all three ligament efficiencies: long. circ. and diag. and use the least efficiency?


Also in AS1228, it has a similar approach to ASME, but it saying that circ. efficient should not be used to determine shell thickness, unless it is less than half of the other two directional efficiency. And is used should use twice of the circumferential efficiency valve.

Could some one please show me some light on why is the circumferential ligament efficiency is not receiving same attention as the other two?

Thank you

Spoonful

 

[Millar]

Think of the longitudinal ligament as you would a longitudinal weld seam when applying looking at your circ stress. Circumferential stress is usually your limiting factor in cylinder design since it is twice your longitudnal stress (usually). So your longitudinal ligament will probably limit your design. If your circ ligament spacing approaches half the long spacing, then you may see a different situation.

M

 

[Spoonful]

Thanks Millar, very well explained.