Efficient Plant March 2018 - 35

fluid handling

You have two options for finding the natural frequencies of your vertical pumps.

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.
- Jane Alexander, Managing Editor

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

MARCH 2018

+

STARTUP (OR COAST-DOWN) METHOD

+

IMPACT 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.

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
EFFICIENTPLANTMAG.COM |

35


http://www.ludeca.com http://www.ludeca.com/blog http://www.EFFICIENTPLANTMAG.COM

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