ASME: Vertical Fire Tube Boiler- Longitudinal Pitch between Staybolts

A furnace for a vertical firetube boiler 38 in. (970 mm) or less in outside diameter that requires staying shall have the furnace sheet supported by one or more rows of staybolts, the circumferential pitch not to exceed 1.05 times that given by the equation in PG-46. The longitudinal pitch between the staybolts shall not exceed that given by the following equation:
L = longitudinal pitch of staybolts
P = maximum allowable working pressure
R = outside radius of furnace
t = thickness of furnace sheet
When values by this formula are less than the circumferential pitch, the longitudinal pitch may be as large as the allowable circumferential pitch.