The max allowable working pressure on a tubesheet of a combustion chamber

The maximum allowable working pressure on a tubesheet of a combustion chamber, where the crown sheet is not suspended from the shell of the boiler:
D = least horizontal distance between tube centers on a horizontal row
d = inside diameter of tubes
P = maximum allowable working pressure
t = thickness of tubesheet
W = distance from the tubesheet to opposite combustion
chamber sheet