Analysis of PRV Stability In Relief Systems Part III

How to avoid the singing PRV problem. Excitation of acoustic standing waves in a main process flow line closed side branch, such as the inlet line of a pressure relief valve (PRV), can occur due to vortex shedding generated by increased flow in the main process line. The flow velocity for process lines where pressure relief devices are mounted via a side branch should be limited to where ce is the effective isentropic speed of sound of the main process flow pipe fluid system, u is the maximum allowable fluid flow velocity in the main process line, and d/L is the pressure relief device inlet line diameter to length ratio. This limit can be very restrictive for flashing two phase flow.

As a result of increasing steam flow rates, several boiling water reactor (BWR) nuclear power plants have recently experienced the excitation of acoustic standing waves in closed side branches, e.g., safety relief valves (SRVs), due to vortex shedding generated by steam flow in the main steam lines (see Figure 1). Flow past a valve entrance cavity excites a standing wave, resulting in noise and vibration [2]. A similar tone is produced when air is blown across the mouth of a glass bottle. There are many similar installations of pressure relief valves in the process industries where the pressure relief valves are mounted on large process lines such as overhead lines for distillation columns.

The amplitude of the acoustic pressure waves can be several times higher than the dynamic pressure present in the system (see Figure 2). The acoustic waves propagate in the steam lines, eventually reaching sensitive components such as steam dryers and turbine stop valves. In addition, the acoustic waves generated in the side branches may generate vibration problems locally and may lead to complications such as valve-seat wear. Therefore, the structural components are subjected to high-cycle fatigue loads, which over time may severely impact those components functionality and safety.

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