During the commissioning of a 400 MW coal fired power plant with supercritical steam parameters, severe vibration and intense noise at approximately 4 kHz were produced by the high-pressure piping used to by-pass the steam turbine. This vibration caused fatigue damage to several components attached to the piping as well as to the stem of the bypass valve. Since the vibration frequency was within the frequency range of valve noise, attention was first focused on modifications of the bypas valve to alleviate its coupling with any other possible sources in the piping system. However, all design modifications carried out on the valve alone influenced neither the level nor the frequency of vibration, and therefore modifications in the layout of the high-pressure piping became unavoidable. Tests on a one-third scale model of the piping system without the valve indicated that the strongest excitation source in the system is a sphere-shaped elbow located upstream of the valve. After eliminating this spherical elbow and re-installing the original version of the bypass valve, the system operated without any vibration problems and the noise level was reduced to the desing value.
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