H/3 Distance in Pressure Vessel Head

One-third the depth of the head....

Possibly the skirt width has been specified to have this dimension?

The difference in volume between a hemispherical or semielliptical head is less by πr²h/3 than the volume of the circumscribing cylinder. Is this where the number came up?

i am referring to the ASME Section VIII Div 1 code fig UG 28.1 .............. is it refer to the tangent length (T.L) in head........................??

I'm sorry I don't have easy access to a copy of the code, so I can't look at the figure and see what their dimensions are.

"Tangent length" and "skirt width" are just different terms for the same thing. It is possible that standards have changed over the years so that skirt width/tangent length is now h/3, but if so it is new to me. I think this used to be unspecified, but commonly 2 inches (50 mm).

My pressure vessel shop days were 1986-1994, so I may well be out of date.