Section 50.30-20 - Class III pressure vessels(a) Class III pressure vessels must be subject to shop inspection at the plant where they are being fabricated, as or when determined necessary by the Officer in Charge, Marine Inspection. The inspection described in this section is required, unless specifically exempted by other regulations in this subchapter.(b) For Class III welded pressure vessels, one inspection must be made during the welding of the longitudinal joint. If there is no longitudinal joint, the inspection must be made during the welding of a circumferential joint. At this time the marine inspector must check the material and fit-up of the work and see that only welders who have passed the required tests are employed.