DESIGN OF ELLIPSOIDAL AND TORISPHERICAL HEADS IN PRESSURE VESSEL AND REVIEW OF RESTRICTIONS IN MATERIAL STRENGTH IMPOSED IN ASME SEC.VIII DIV.1: A COMPARATIVE STUDY OF VARIOUS CODES OF CONSTRUCTIONS

Pressure Vessels are made with various types of end closures. Commonly used end closures are ellipsoidal dished ends, torispherical dished end and hemispherical dished ends. For design of these end closures the rules provided in ASME Sec. VIII Div. 1[1] and ASME Sec. VIII Div. 2[2] differ significantly and a similar phenomenon could be observed when a review is done against popular international Code(s) for pressure vessel design like PD 5500 [3], EN 13445 [4]. The difference in the design methodology is due to the design approach used in developing the equations in these Code(s) of Construction. ASME Sec. VIII Div.1 in Para UG-32(d) and in Appendix 1-4 (c)& (d), end note 88 for design of all torispherical heads subject to internal pressure states that the maximum allowable stress used to calculate the required thickness cannot exceed 20 ksi (138 MPa) at room temperature irrespective of the strength of the material. This restriction is further reduced in proportion at elevated temperatures. Similar kinds of a restriction in the allowable stress is imposed for design of ellipsoidal heads when the factor K (shape factor in the design of ellipsoidal head) exceeds 1.0 in ASME Sec. VIII Div. 1. If we refer to other Code(s) like ASME Sec. VIII Div.2 and PD 5500 and EN 13445, one can observe that the restriction of allowable stress in the design of ellipsoidal and torispherical heads are not imposed when high strength materials are selected for the construction of dished ends. The work reported in this paper is an attempt to review the design methodology adopted for design of ellipsoidal dished ends with K exceeding 1.0 and all torispherical heads according to ASME Sec. VIII Div. 1 with ASME Sec. VIII Div. 2, PD 5500 and EN 13445 and to evaluate whether the restriction in the allowable stress imposed by ASME Sec. VIII Div. 1 is really required. Also, FEA of the dished ends are carried out to determine the stresses induced and a comparison is made against various Code(s) classical formula requirements.