ArticleModal Testing. Part 3: Case Histories & Operating Deflection Shapes
by Robert Sayer, P.E.
This last of three papers on the topic of modal analysis presents two case histories where both experimental modal analysis (EMA) and operating deflection shape analysis (ODS) were performed in an effort to solve machinery vibration problems. The first case history involved high vibration on skid mounted cooling water pumps at a nuclear plant. The second case history involved high vibration on a vertical pump at a nuclear plant. Both problems and their solutions are described in detail.
“Modal Case History
Two stator water cooling pumps for a generator at a nuclear power facility were supported on a common skid. The pumps were single-stage centrifugal units directly coupled to 3,600 RPM motors. Figure 1 contains a sketch of the equipment arrangement. The pump skid consisted of a deck plate welded to a framework of structural steel beams. The locations of the structural beams are indicated as hidden lines in Figure I. Note that the motors were attached to steel blocks (6″ x 6″ x length of motor), and the pumps were attached to trapezoidal pedestals. The steel blocks and trapezoidal pedestals were directly attached to the deck plate, not to the steel beams.
The pumps were referred to as pump A and pump B. The motors driving the pumps were referred to as motor A and motor B. Only one pump was required to maintain sufficient cooling water to the stator of the generator. The other pump was placed on standby in case the primary pump failed.
Motor B vibrated excessively even when it was not operating. The vibration of motor B exceeded 0.60 IPS in the axial direction when pump B was in the standby mode and pump A was operating. In fact, the vibration of the idle motor B exceeded that of the operating motor A. Vibration data can indicate possible resonant conditions including, but not limited to, directional sensitivity (high ratio of vibration in one direction versus the other directions), speed sensitivity, and unstable phase. Another indicator of resonance is unusual occurrences. The fact that the vibration of motor B was excessive when it was not even operating certainly falls in the category of unusual occurrences.
Because resonance was suspected as the root cause of the vibration problem, impact tests were performed to determine the natural frequencies of the pumps and motors. Figure 2 is the result of an impact test in which the excitation force was applied to motor A using an instrumented force hammer. The response was measured at motor B. The most significant natural frequency was identified at 60.5 Hz which is nearly identical to the rotational speed of the pumps and motors. Another less sensitive natural frequency was identified at 49.0 Hz.
A complete experimental modal test was then performed to obtain the mode shapes associated with each natural frequency. A grid of data points was established over the entire pump skid.”
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