1. Introduction

Saddle support are the main (maybe the only) type of support used in horizontal vessels. It can be designed using ASME BPVC Section VIII Division 2 (which is based in a paper published by L. P. Zick((Zick, L. P., “Stress in Large Horizontal Pressure Vessels on Two Saddle Supports”, The Welding Journal Research Supplement, 1951, pp. 435-445)) and can be downloaded here) together with the Dennis R. Moss and Michael M. Basic book((Moss, D. and Basic, M. (2013). Pressure Vessel Design Manual. 4th ed. Amsterdam: Butterworth-Heinemann, p. 253)), because unfortunately ASME code doesn’t address the design of the saddle parts (base plate, web, bolt holes, etc). There are several other books((Bednar, B. (1991). Pressure Vessel Design Handbook. 2th ed. Krieger Pub Co, chapter 6))((Megyesy, E. (2008). Pressure Vessel Handbook. 14th ed. Pressure Vessel Handbook Pub, p. 86))((Lloyd, B., Young, E. (1959). Process Equipment Design. 1st ed. Wiley-Interscience, p. 203)) that approaches the design of saddle supports but they are all the based in the Zick method, you may want to read them if you want another interpretation about the subject.  According to ASME BPVC Section VIII Division 2 (which is based on the paper published by L. P. Zick – “Stress in Large Horizontal Cylindrical Pressure Vessels on Two Saddle Support” in 1951. The method of calculation is also called “Zick Method”) to determine the stresses in the vessel and to “Pressure Vessel Design Manual” from Dennis Moss and Michael Basic to design the saddle support (web, ribs, baseplate), as it isn’t addressed in the ASME standard. Needless to say, this type of support can be only used in horizontal vessels (there’s no sense in using it in a vertical vessel).According to ASME ((ASME BPVC, Section VIII, Division 1 – 1995))code:
The design method is based on an analysis of the longitudinal stresses exerted within the cylindrical shell by the overall bending of the vessel, considered as a beam on two single supports, the shear stresses generated by the transmission of the loads on the supports, and the circumferential stresses within the cylindrical shell, the head shear and additional tensile stress in the head, and the possible stiffening rings of this shell, by this transmission of the loads on the supports. The stress calculation method is based on the linear elastic mechanics and covers modes of failure by excessive deformation and elastic instability.
This method of calculation can only be used to design pressure vessel supported by two saddles symmetrically placed.
  • The stresses that can be evaluated are:
  • Maximum longitudinal stress
  • Tangential shear stress
  • Circumferential stress at the horn of the saddle
  • Additional stress in the head used as a stiffener
There are a few books that address the subject:
  • Pressure Vessel Design Manual – Dennis Moss, Michael Basic
  • Mechanical Design of Heat Exchangers and Pressure Vessel Components – Krishna P. Singh, Alan I. Soler
  • Pressure Vessel Handbook – Eugene Megyesy