The design external pressure, Pe, for an independent tank type C must be calculated by the following formula: Pe = P1 + P2 + P3 + P4 where: P1 = the vacuum relief valve setting for tanks with a vacuum
relief valve, or 24.5 kPa gauge (3.55 psig) for tanks without
a vacuum relief valve.P2 = 0, or the pressure relief valve setting for an enclosed
space containing any portion of a pressure vessel.P3 = total compressive load in the tank shell from the weight
of the tank, including corrosion allowance, weight of
insulation, weight of dome, weight of pipe tower and piping,
the effect of the partially filled tank, the effect of
acceleration and hull deflection, and the local effect of
external and internal pressure.P4 = 0, or the external pressure from the head of water from
any portion of the pressure vessel on exposed decks.