Some Considerations in the Design of Class 1 Pressure Vessels

Abstract

The fusion-welded mild steel pressure vessels generally known in this country as “Class 1” welded pressure vessels are those of a sufficiently high standard of manufacture to comply with the requirements of certain recognized specifications, such as those referenced in the paper. Such vessels are constructed of good-quality mild steel plate joined together by a metallic arc fusion-welding process. A radiographical examination is made of the full length of all the main seams, which are machined or dressed so as to be level with the surfaces of the plate on either side of the joint, and specified mechanical tests are carried out to ensure that the quality of the welded joint is of the required standard. The vessels receive a stress-relieving heat treatment on completion of the welding operations. The magnitude of the stress in the material is a dominant factor in the design of a pressure vessel, but difficulties arise in connexion with the design of Class 1 fusion-welded pressure vessels on account of the different viewpoints of various authorities on the stress to be applied to the welded joint. The extent of these differences is shown in the paper by a comparison of vessels designed in accordance with the specification requirements of four leading authorities (two British and two American). An analysis is made of the factors involved in the formulae employed by these authorities for determining the wall thickness, and it is shown that the formulae have been based on the riveted form of construction despite the very different conditions of stress at Class 1 welded joints. The characteristic physical properties of such joints are discussed in conjunction with the results of recent investigations on their behaviour under fluctuating stress conditions. A recommendation is made for a much-needed simplification in design formulae to take into consideration the actual characteristics of the welded form of joint. Illustrations are given of typical welding groove preparations for the attachment of flanges, branches, etc., designed to provide adequate accessibility for manipulation of the welding rods.