Dynamic Amplification Factor for Rigid and Flexible Piping System due to Steam Hammer Transient Load

Abstract

Pressure surges and fluid transients, such as steam and water hammer, are events that can occur unexpectedly in operating power plants causing significant damages. The steam hammer normally occurs when one or more valves suddenly close or open. In a power plant, the steam hammer could be an inevitable phenomenon during turbine trip, since valves (e.g., main steam valves) must be closed very quickly to protect the turbine from further damage. When a valve suddenly stops at a very short time, the flow pressure builds up at the valve, starting to create pressure waves along the pipe runs which travel between elbows. Furthermore, these pressure waves may cause large dynamic response on the pipeline and large loads on the pipe restraints. The response and vibrations on the pipeline depend on, one hand, on the pressure waves amplitudes and frequencies of the applied transient loads and on the other hand, on the natural frequencies and dynamic characteristics of the pipeline itself. The piping flexibility or rigidity of the pipeline (piping configuration plus support locations and types), determines how the pipeline will respond to these waves and eventually the magnitude of the loads on the pipe restraints. Consequently, the design of the piping system (pipe configuration and pipe support arrangement) is closely related to the dynamic amplification of applied steam hammer loads. This paper investigates how the pipe support arrangement and the flexibility of the pipeline relate to the dynamic amplification of the piping system due to steam hammer transient loads. The paper examines the correlation that exists, between the applied steam hammer transient spectrum and the pipe frequencies of the pipeline with the dynamic load amplification. Furthermore, it establishes the relation between the dynamic amplification of the pipeline and the pipe support configuration by examining various piping systems from very rigid (many supports) to very flexible (fewer supports). The typical pipe configuration of hot reheat line in a power plant is analyzed for steam hammer due to sudden valve closure (turbine trip) for six cases of closure time (50, 100, 200, 300, 400, 1600 ms). The transient loads on each pipe segment are generated using the PIPENET program and used as an input to CAESAR II stress analysis program to perform a dynamic time history analysis. The dynamic analysis is performed for a range of different pipe support arrangements, from very rigid to a very flexible. The magnitude of the restraint loads from the time history analysis, for each selected pipe support arrangement, is compared with the transient load spectrum associated. The analysis is performed using the PIPENET transient module and CAESAR II pipe stress analysis program, for a range of different pipe support arrangements, from very rigid to a very flexible. The results from the dynamic analysis are compared with the applied transient spectrum, and a dynamic load factor is established for the selected pipe support configuration. The dynamic load factor calculated based on the analysis results is compared with the factor proposed in standard industry practice. The information and the methodology presented in this paper aim to assist the design engineer in the power plant industry to select an optimum support arrangement that will cause the minimum dynamic amplification on the pipeline, thus reducing the steam hammer effects, the size and the cost of pipe supports.