Energy Dissipation Valves (EDVs) at a water filtration plant were exhibiting high vibration amplitude at the piping/ elbow down-stream from the EDV. The newly installed water filtration plant had been designed to dissipate a differential static head of treated water at approximately 155 ft elevation, from high service water at approximately 290 ft of elevation (~50 psig). This operation was performed when the filtration plant is shut-down and “back fed” flow of treated water passes through the pipe (by gravity) using Energy Dissipating Valves down to the wet well.
MSI performed specialized vibration testing that included experimental modal analysis (EMA), continuous monitoring, and operating deflection shape (ODS) characterization. EMA involved impacting each pipe/ elbow and measuring the vibratory ring-downs to identify system natural frequencies and characterize their corresponding vibration mode shapes. The continuous monitoring test involved instrumenting three EDV's to study their dynamic behavior under transient and steady operating conditions. Finally, the ODS characterization involved applying steady-state vibration data to a computer model of the system, then animating it at certain vibration frequencies of interest to study the relative motion between components.
The high vibration of the piping/ elbow (~100 mils pk-pk overall) downstream the EDV#2 valve was due to a structural resonance of its 1st below-ground dominated cantilever mode excited by typical flow-induced vibration or vortex shedding frequency (VSF) at 4.06 Hz, generated from the EDV valves. This mode was measured to be at 4.75 Hz, while the valve was 65% open. However, at lower flow rate, the vibration was reduced due to increased separation margin between the structural natural frequency and the VSF. In order to mitigate the vibration issue at any flow rate, shifting the offending mode should be considered. This can be accomplished by supporting the end of the vertical section of the pipe in a horizontal plane in two orthogonal directions.
