Pressure Vessels Design Rule of Thumb:

1. Design  temperature   between   ―20˚F   and   650˚F   is   50˚F   above   operating temperature; higher safety margins are used outside the given temperature range.
2. The design pressure is 10% or 10―25 psi over the maximum operating pressure, whichever is greater. The maximum operating pressure, in turn, is taken as 25 psi about the normal operation.
3. Design pressures of vessels operating at 0―10 psig and 600-1000˚F are 40 psig.