Mechanical Solutions, Inc. (MSI) was contracted to solve excessive vibration problems on four newly installed 3-stage vertical turbine pumps at a fire pump station. All of the units are driven by diesel engines with a right-angle gear operating at constant speed (28.75 Hz). A 10-blade fan at the top of the gearbox provides gearbox cooling. The customer reported that all the pumps had been operating with high overall vibration levels exceeding the specification limit of 10 mm/s RMS (0.40 in/s RMS).
Figure 1. ODS showing the separation/looseness between main components of the pump assembly.
MSI collected detailed natural excitation vibration data on all components of each pump, including the nearby floor, baseplates, engines and discharge piping. MSI uses a specialized operating force response vibration technique that allows the data to be used to produce very detailed animated operating deflection shapes (ODS) of the pump system. The ODS animation shows the relative motion of various system components in an exaggerated fashion to help efficiently identify the problem root cause(s) (Figure 1). MSI also performed impact modal testing while the units were operating to determine the natural frequencies and mode shapes.
The pump had an overall vibration amplitude, 34 mm/s RMS, in the direction perpendicular to the pump discharge pipe. MSI installed and adjusted a vibration absorber on this pump in order to reduce the vibration at 1x running speed. The filtered vibration at 28.75 Hz was reduced from 32.4 mm/s RMS to 0.76 mm/s RMS in the perpendicular direction and from 12.8 mm/s RMS to 7.1 mm/s RMS in the parallel direction.
The source of the high frequency vibration of the pumps was found to be looseness in the gear housing assemblies, where gear housing parts were acting as impact hammers exciting all the natural frequencies in the system in all directions over a broad frequency range.
Figure 2. MSI Engineer stands next to the equipment in the field
In order to minimize the separation/ looseness between the gear/ pump assemblies, MSI recommended the following improvements:
Tighten down all bolted joints between the gear and the discharge head, and between the discharge head and the caisson flange, to the maximum allowable torque level.
Excessive motion by the discharge piping of a petrochemical ammonia reciprocating compressor startled the operators.
Troubleshooting a new oil tanker pumping station in Colombia consisting of 4 screw pumps. Specialized testing revealed that piping natural frequencies were to blame.