DESIGN OF TORISPHERICAL AND ELLIPSOIDAL HEADS IN PRESSURE VESSEL AND REVIEW OF RESTRICTIONS FOR THE ONSET OF BUCKLING-INTERNAL PRESSURE IN ASME SEC.VIII DIV.1: A COMPARATIVE STUDY OF VARIOUS CODES OF CONSTRUCTION

Pressure Vessels are made with various types of end closures. Commonly used end closures are torispherical dished ends, ellipsoidal dished ends, and hemispherical dished ends. For the 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 Codes for pressure vessel design like PD 5500 [3], and EN 13445[4]. The difference in the design methodology is due to the design approach used in developing the equations in these Codes of Construction. ASME Sec. VIII Div.1 in Para. UG-32(c) and (d) for the design of all ellipsoidal heads and torispherical heads subject to internal pressure having ts/L ratio less than 0.002 directs the user to Para. 1-4 (f) of Mandatory Appendix 1 in code [1]. The requirements in Para. 1-4 (f) are added to prevent the possibility of buckling of the heads as the thickness is reduced as compared to the spherical radius. Buckling as a mode of failure is only envisaged when the ts/L ratio is less than 0.002 and no check on buckling is required when the ts/L ratio is greater or equal to 0.002. If we refer to other Codes like ASME Sec. VIII Div.2, PD 5500 and EN 13445, one can observe that the design of the dished head is based on plastic collapse and buckling failure approach and these Codes make it mandatory to check both the plastic collapse and buckling failure modes in all the circumstances. The work reported in this paper is an attempt to reviews the requirements of ASME Section VIII Division 1 for thin-walled dished heads with large internal diameter whose ratio of ts/L lie near the vicinity of 0.002 and comparative study of requirements imposed by ASME Section VIII Division 1 is compared with other International Codes. Also, FEA of the dished ends is carried out to study the stress pattern and to determine the stresses induced, buckling. Further comparison is made against various Codes' classical formula requirements.