Vessel Design and Construction

A pressure vessel is a container designed to hold a fluid at a pressure different from ambient. In oil and gas that covers separators, knockout drums, scrubbers, surge vessels, columns, filters and air receivers.

Design basis

Every vessel has a design pressure and design temperature, a minimum design metal temperature, a corrosion allowance, a nominated fluid service and a defined set of nozzle loads. These are recorded on the nameplate and in the vessel data book, and they are the boundary of what the vessel may be used for. Changing the service, the pressure, the temperature or the fluid is a re-rating exercise under management of change, not an operational decision.

Construction

• Shell — rolled plate, longitudinally and circumferentially welded, thickness set by pressure, diameter, material allowable stress and joint efficiency.
• Heads — ellipsoidal, torispherical or hemispherical. A hemispherical head is the most efficient shape and the most expensive to make.
• Nozzles — each opening weakens the shell and requires reinforcement, usually a compensating pad or a thicker forged nozzle.
• Supports — saddles for horizontal vessels, skirts or legs for vertical ones, designed to allow thermal movement at one end.

Post-weld heat treatment and testing

Thick sections and certain services require post-weld heat treatment to relieve residual stress and reduce hardness, which is particularly important in sour service. Vessels are examined by non-destructive methods to the extent the code requires and hydrostatically tested before certification.

Protection

Every vessel is protected against overpressure by a relief device sized for the governing relieving case — blocked outlet, fire exposure, control valve failure, thermal expansion or tube rupture. It is also protected against vacuum where the design does not allow it: a vessel that can be steamed out and then cooled, or drained with the vent closed, can collapse inward long before anyone notices.