Resolving Structural Vibration Issues On a Multi Stage Pump

Abstract

There are three multistage pumps supplied as part of a new project for transporting diesel from one location to another. Earlier, the transportation of diesel was carried out using trucks for this facility. The availability of these pumps are critical to ensure that the diesel transportation is smooth without the needing the truck services. The machine train is equipped with online vibration monitoring & protection system with online diagnostic software. During the commissioning, it was noticed that the pumps were not able to reach the maximum continuous speed of 50.7 Hz (Max rated speed is 53.8 Hz) and were tripping on high vibration at the pump casing. The operation of these pumps were limited with the speed restrictions using the Variable Frequency Drive (VFD) system. The data review from the online diagnostics system revealed that these pumps were tripping on high vibration due to the excitation of pump vane pass frequency (195 Hz), when the pump was reaching the speed of 39Hz. Further tests concluded that the pump casing natural frequency was matching with the vane pass frequency of 195 Hz. This case study is designed to outline how the high vibration issue was successfully diagnosed, the root cause for the high vibration defined and finally how the problem was mitigated using some of the structural analysis techniques (modal analysis, Operating Deflection Shape etc). Lessons learned are also discussed in view of the discovery methodology using expert system available on site, from the project point of view as well as from design point of view.