An alternative way to support horizontal pressure vessels subject to thermal loading Alwyn S. Tooth a , *, John S.T. Cheung b , Heong W. Ng b , Lin S. Ong b , Chithranjan Nadarajah c a Department of Mechanical Engineering, University of Strathclyde, Glasgow G1 1XJ, UK b School of Mechanical and Production Engineering, Nanyang Technological University, Singapore 639798, Singapore c Exxon Engineering Asia Pacific, Singapore 048693, Singapore Received 27 May 1998; accepted 9 June 1998 Abstract When storing liquids at high temperature in horizontal vessels, the current design methods aim to minimise the thermal stresses by introducing a sliding surface at the base of one of the twin saddle supports. However, regular site maintenance is required to ensure that adequate sliding is achieved. This may be difficult and costly to carry out. The aim of the present work, therefore, is to dispense with the sliding support and to provide saddle designs which, although fixed to the platform or foundation, do not result in the storage/pressure vessel being overstressed when thermal loading occurs. This paper provides general recommendations for the most appropriate saddle geometries, and details the way in which design-by-analysis and fatigue-life- assessments may be carried out using the stresses that arise from these designs.  1998 Elsevier Science Ltd. All rights reserved Keywords: Pressure vessels; Storage vessels; Supports; Thermal loading 1. Nomenclature A length of vessel beyond saddle overhang b p breadth of the saddle top plate in the axial direction d p basic saddle width E elastic modulus of the vessel and saddle material h e overall heat transfer coefficient h p height of the saddle, measured from nadir of the vessel to saddle base plate k s thermal conductivity of steel L s length of the vessel between supports r m mean radius of the vessel t c wall thickness of the vessel t s thickness of the saddle web and stiffeners S I stress intensity (i.e. maximum principal stress difference) DT temperature differential w extended width of the saddle top plate a linear coefficient of thermal expansion of the vessel and saddle material 2. Introduction Horizontal cylindrical storage/pressure vessels as used in the power, petroleum and other process industries are designed according to recognised codes and standards (for example; the ASME, BS, CODAP, etc. pressure vessel stan- dards) to withstand both the test and operating conditions. The common practice, in terms of support, is to provide two saddle-like supports symmetrically located along the length of the vessel. To avoid induced axial restraint stresses, as in the case of thermal loading, the codes recommend that one of the saddles be free to slide in the axial direction. This can be achieved in a number of different ways: by the use of foundation bolts positioned in slotted holes, by the use of low-friction material (such as polytetrafluoroethylene (PTFE)) bearing pads bonded to the backing plate of the saddle base and the foundation plate, or by the introduction of a roller at the base of the support. A further recommendation is that the hot liquid storage vessel and the supports be fully insulated. Such a require- ment is obviously necessary to prevent heat loss which could be detrimental to the process, to prevent fire damage thereby inducing structural weakness in the vessel and the support, and to protect personnel from inadvertent contact with the vessel or the support. Such insulation also avoids a high rate of temperature loss down the saddle itself which induces correspondingly high values of vessel stress. In spite of the long-standing practice of providing a slid- ing surface (or roller), mistakes can and do occur in practice. 0308-0161/98/$ - see front matter  1998 Elsevier Science Ltd. All rights reserved PII: S0308-0161(98)00065-9 * Corresponding author. Tel.: +44-141-552-4400; Fax: +44-141-552-5105 International Journal of Pressure Vessels and Piping 75 (1998) 617–623 IPVP 1855