I am trying to calculate the minimum required thickness for an ASME F&D head under internal pressure.  Actually, I'm trying to enter the calculation into a spreadsheet.

There are two separate equations given depending on the ratio of L/r.  One where L/r = 16.6667 and one where L/r < 16.667.  Both depend on L.

I have Eugene F. Megyesy's Pressure vessel handbook, and the F&D head dimension table shows the knuckle radius (r) is approx. equal to the greater of 1) 6% of the head diameter or 2) 3 times the head thickness.

I cannot, however, determine how the inside radius of the dish of the head (L) is calculated.  

I could enter all the data into my spreadsheet and reference it, but it would save a ton of time and space if someone could tell me how it (L) is determined, or at least point me to a reference that could.

Thanks in advance!

jproj