by Paul Boyadjis & Chris Hurrell

Potential vertical turbine pump vibration problems were avoided through design-stage finite element analysis.

The Lower Cape Fear Water and Sewer Authority had begun to expand its main raw-water pump station at Kings Bluff, near the Lower Cape Fear River in southeastern North Carolina. The core of this municipal project revolved around three, identical, new vertical turbine pumps. One of these pumps was a replacement for an existing pump, and two were part of an addition that included a newly constructed extension of the station building. Charles R. Underwood, Inc., the original equipment manufacturer (OEM) of the pumps, requested that Mechanical Solutions, Inc. (MSI) perform a detailed finite element analysis (FEA) of the vertical turbine pumps prior to their manufacture, installation and acceptance testing at the pump facility.

The structural natural frequencies of vibration and their mode shapes would be predicted through the FEA, and the proximity of these natural frequencies to the pump operating speed range would be revealed.

Armed with this information, the influence of the vibration on the performance and the reliability of the pumps could be assessed appropriately. Practical modifications to the design of the pumps could be implemented, if necessary, to eliminate any anticipated vibration problems well before the installation of the pumps.

Since modifying machinery during design is much easier than implementing modifications after installation, this proactive approach potentially could conserve a significant amount of the project’s budget

The Pumps Were Modeled With the Finite Element Technique

Detailed technical data, including drawings of the pumps and of the pumping facility were provided to the company that performed the FEA, by both Charles R. Underwood, Inc., the pump OEM, and the Lower Cape Fear Water and Sewer Authority. Because this municipal water project involved the extension of the pump station’s building to contain two of the three new raw-water pumps, the information from the latter source proved to be especially relevant during the analysis.

Based on the extensive set of data, two detailed finite element models that represented portions of the pump system were created. These models accounted for the differences in the construction details of the foundations to which the pumps were attached.

Each model incorporated sufficient portions of the adjacent building foundation structures and piping, Figure 1. The pump system finite element models also simulated both the minimum and the maximum submergence levels of each foundation, which accounted for the added internal and external water mass effects on the column pipes, bowls and suction bells of the pumps.

An FEA that used each of the models was performed to predict the structural natural frequencies of the pump system. The main criterion of the FEA, taken from the specification for the vertical turbine pumps, was that no structural natural frequencies should fall within 15 percent of the 9.75 Hz through 14.8 Hz running speed range of the pumps.

In parallel with this FEA, pump shaft lateral and torsional analyses were performed. Both predicted that the criteria for the shaft lateral and torsional natural frequencies would be satisfied by the existing design.

Figure 1. A solid model of one of the new vertical turbine pumps with a portion of the new structural foundation on the left and with a portion of the existing structural foundation on the right.