Determine A Vertical Pump’s Natural Frequency

What’s the best way to determine the natural frequency of your vertical pumps?

According to Dave Leach, CRL, CMRT, CMRP, there are two commonly used testing methods. A vibration specialist with Ludeca Inc. (Miami, ludeca.com), Leach offered the following explanations of these procedures in a 2017 post on the Ludeca blog.

STARTUP (OR COAST-DOWN) METHOD

To perform this method, a tachometer (tach) signal is required to track the pump’s speed. The unit is started and the amplitude and phase are recorded. Note, however, that this method is difficult to use when a pump is started across the line, i.e., connected directly to a power source, without a drive or soft-start circuit. The problem is that the pump goes from zero rpm to full speed so quickly there isn’t enough time to obtain valid data. When the stop is initiated, the unit quickly comes to a complete stop, with the liquid inside the pump column falling back to the wet well acting as a brake. In such cases, the startup (or coast-down) method typically isn’t successful. This type of testing, though, can be performed successfully if a pump is being operated using a VFD (variable-frequency drive), as the rate of speed can be controlled.

IMPACT METHOD

The second method for determining structural natural frequencies on a vertical pump is the impact test, commonly referred to as a “bump test.” The procedure requires that a pump be stopped and subjected to an impact using a block of wood or a large, soft-tipped hammer, i.e., modal hammer. This test provides a response curve that will identify the natural frequency and/or frequencies of the pump.

The impact/bump test should be performed in two directions. One direction is in-line with the pump’s discharge pipe; the other is 90 deg. from the discharge pipe. The different directions will usually result in two different natural frequencies, as the discharge pipe tends to stiffen a pump’s structure. Vibration data from the first direction (in-line with the discharge pipe) could show a higher natural frequency than that from the second direction (90 deg. from the discharge). This is due to the fact that pump manufacturers typically cut out part of the structure, which allows access to the coupling or seal and, in turn, dampens the structure in that direction.

Both of these methods can help determine the natural frequencies of a pump. Once those frequencies are identified, proper corrections can be made to ensure the unit is not operating at a resonance frequency. EP

Find more tips on keeping fluid-handling and other equipment running at ludeca.com/blog.