Q. External Pressure Design of unfired pressure vessels?

Determine requirements for external pressure based on the expected operation of the vessel, and add a suitable operating margin to establish the external design pressure and coincident temperature.

Consideration must be given to the effects of introducing air into the vessel, if vacuum relief devices are used. Vessels in steam service shall be designed for full vacuum, and consideration shall also be given for vessels in services that may be subject to steam out.

Required stiffening rings for shells under external pressure shall be placed on the outside of the vessel, have a thickness not less than 3/8 in (9.5 mm), and have a ring width-to-thickness ratio no greater than 10. Stiffening rings shall be attached by continuous fillet welds on both sides of the ring.