Series piping design with uncertainty analysis

Hydraulic design of piping systems usually consists of computing the pump head required to overcome the system pressure head, static head, and resistance losses at the design flow rate. This design point is compared with pump manufacturers' composite pump curves to select a candidate pump. Once a candidate pump is selected, a system head-capacity curve is plotted along with the pump head-capacity curve. The operating point is the intersection of these two curves. Because of inherent uncertainties in the process specifications and in the flow resistance calculations, designers have traditionally used safety factors to ensure that the final system is adequate. Safety factor values are based on the designer's experience (or sometimes lack of experience) and vary greatly. A 25% overrating is typical. The compounding effect of safety factors often leads to pumps that are greatly oversized. Hicks and Edwards give a good discussion of the problem of compounding safety factors. Safety factors assume minimum knowledge of the nature of the inherent uncertainties. However, the designer usually can make a reasonable estimate of the uncertainties in the process specifications. The result of the uncertainty analysis is the probably operating range of the candidate design. This information gives the designer a more rational basis for design decisions than that provided by the use of safety factors. With hand calculations, uncertainty analysis can be tedious; however, the uncertainty analysis can be easily combined with piping system analysis computer programs. This article presents the application of uncertainty analysis to series piping design computations and gives a computer program that combines them.