Often piping configurations that are acceptable by stress criteria are limited by the loads which they place on connected equipment and vessels.
Piping loads on the nozzles of equipment such as pumps, compressors, turbines, and heat exchangers may have the tendency to deform or overstress equipment casings, overload bearings, or cause shaft binding. Normally manufacturers should provide allowable nozzle loadings to which their equipment may be subjected, or they may reference industry standards, such as NEMA SM-23 (Steam Turbines), API 610 (Centrifugal Pumps), API 617 (Centrifugal Compressors), API 661 (Air Cooled Heat Exchangers), or HEI (for Closed Feedwater Heaters). These standards provide look-up tables or simple calculations which serve as a common reference for equipment vendor and pipe stress engineer.
Piping attached to vessels induces stresses in the vessel walls, in the form of membrane and bending stresses. These stresses must be evaluated against the requirements of the ASME Boiler and Pressure Vessel Code, Section VIII, Division 2. Calculation of stresses in a vessel wall is difficult without a finite element analysis; the best means of doing a hand calculation is to use aids such as Welding Research Council (WRC) Bulletin 107 "Local Stresses in Spherical and Cylindrical Shells due to External Loadings".
Piping attached to vessels may also tend to bend, buckle, or otherwise deform the vessel wall, permitting some displacement or rotation of the connection under load. Therefore, totally rigid restraint models may not be accurate representations of piping to vessel connections. Flexibilities of the connection should be estimated when possible; this can be done with the assistance of literature such as Welding Research Council Bulletin 297 "Local Stresses in Cylindrical Shells Due to External Loadings on Nozzles — Supplement to WRC Bulletin No. 107".
The three types of possible analyses which may be done on nozzles — evaluation of equipment loads, calculation of vessel stresses, and calculation of piping/vessel connection flexibilities — are described in this section.
Modeling And Analysis Of The Piping System
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