Stress calculations are essential to piping system design, but stress calculations are only as accurate as their input driving forces. Force calculations are complex for static systems, and fluid systems are rarely static. Further, a transient acting on the fluid (valve closure, pump trip, check valve slam, etc.) creates pressure and velocity waves which directly impact the load on the pipe. These waves similarly affect momentum and frictional losses as flow through the pipe changes, impacting forces further.
Problem:
The complexity of calculating forces due to fluid transients leads to simplified methods that can severely miscalculate the force. Often, pressure is considered the dominant driver of force considered in these analyses, an overly simplified approach which neglects real physical effects internal to the pipe. As with most fluid transient analyses, engineers should not speculate on whether such simplifications are acceptable for their system.
Calculation Method:
Incomplete methods may generate misleading or incorrect force values. Remove all doubt by computing the true force with the Acceleration Reaction Method for any liquid or gas piping system. Learn about the Acceleration Reaction Method and see how it is not difficult to utilize if the results of a transient fluid simulation are available.
Authors:
Scott A. Lang, PE and Trey W. Walters, PE, Applied Flow Technology, USA; Presented at the ASME PVP Pressure Vessels & Piping Conference, July 2022
Part 1: Fundamentals
ABSTRACT: Changes in the operation of piping systems – like valve closures or pump starts – propagate pressure waves that travel at acoustic velocity throughout the fluid. These pressure waves have considerable effect on forces, potentially generating dynamic loads upwards of 10,000 lbf (50 kN) in common configurations. Some estimation methods used in industry for estimating transient forces neglect terms that may be important in some cases. Calculating forces due to these transients without simplification for transient liquid or gas flow is presented here in detail.
CONCLUSION: The authors have seen numerous erroneous reaction calculations. Often, the incomplete Endpoint Pressure Method is used due to its apparent simplicity. To be emphatically clear: the Endpoint Pressure Method is very often incorrect in realistic situations, and the authors do not recommend its use under any circumstances. The complete Acceleration Reaction Method presented here is not excessively difficult to utilize if the results of a transient fluid simulation are available. Instead of speculating on the importance of the correct force, the fundamental approach is recommended for any liquid or gas piping system.
Part 2: Applications
ABSTRACT: Part 1 of this series discussed in detail how to accurately calculate the reactions induced by pressure transients that travel at acoustic velocity in either liquid or gas piping systems. Part 2 applies these methods to a variety of realistic examples to further illustrate their use and to demonstrate areas in which traditional or simplified methods may impart significant error.
CONCLUSION: Calculating transient forces on a piping assembly may at first glance appear to be a straightforward task. However, the complexities of control volume definition, accurate bookkeeping of signs, and challenging determination of some transient terms creates complication. This complication leads some engineers to use methods such as the Endpoint Pressure Method as an attempt to streamline the problem. Unfortunately, this method is incomplete, and in some cases dramatically misestimates the force values. Determining the true forces may be accomplished by careful application of Newton’s Second Law. The authors strongly recommend the use of the Acceleration Reaction Method described herein, even for simple systems. Without taking the time to make this calculation, the engineer can never be sure if the force values from an incomplete method are valid.
