The stability for a safety valve was investigated by experiments and analysis. It was found that the dynamic instability was caused by the interaction effect between the pressure wave propagation through the inlet pipe and the valve disc motion. This dynamic instability is mitigated by the extension of the inlet pipe length because the oscillating motion of the valve disc is attenuated by the mechanical friction of the safety valve before the pressure wave generated by the safety valve opening returns back to the safety valve. This dynamic instability is completely different from the static stability caused by the pressure drop of the inlet piping system. The effect of the outlet area ratio to the orifice area for the safety valve was also investigated and the possibility of the instability tends to increase for the outlet area ratio lower than 6.0. Stability analysis based on the neutral stability searching method is effective to explain the instability for the system with relatively short pipe length of the safety valve inlet. Numerical simulation is useful to predict the stability for both of the effects of the inlet pipe length and the outlet area ratio of the safety valve.
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