Page 106 - Intro Predictive Maintenance

Machine-Train Monitoring Parameters 97

Hydraulic Instability (Vane Pass). Hydraulic or flow instability is common in cen-
trifugal pumps. In addition to the restrictions of the suction and discharge discussed
previously, the piping configuration in many applications creates instability. Although
flow through the pump should be laminar, sharp turns or other restrictions in the inlet
piping can create turbulent flow conditions. Forcing functions such as these results in
hydraulic instability, which displaces the rotating element within the pump.

In a vibration analysis, hydraulic instability is displayed at the vane-pass frequency
of the pump’s impeller. Vane-pass frequency is equal to the number of vanes in the
impeller multiplied by the actual running speed of the shaft. Therefore, a narrowband
window should be established to monitor the vane-pass frequency of all centrifugal
pumps.

Running Speed. Most pumps are considered constant speed, but the true speed
changes with variations in suction pressure and back-pressure caused by restrictions
in the discharge piping. The narrowband should have lower and upper limits sufficient
to compensate for these speed variations. Generally, the limits should be set at speeds
equal to the full-load and no-load ratings of the driver.

There is a potential for unstable flow through pumps, which is created by both the
design-flow pattern and the radial deflection caused by back-pressure in the discharge
piping. Pumps tend to operate at their second-mode shape or deflection pattern. This
operation mode generates a unique vibration frequency at the second harmonic (2X)
of running speed. In extreme cases, the shaft may be deflected further and operate in
its third (3X) mode shape. Therefore, both of these frequencies should be monitored.

Positive Displacement
A variety of positive-displacement pumps is commonly used in industrial applications.
Each type has unique characteristics that must be understood and monitored; however,
most of the major types have common parameters that should be monitored.

With the exception of piston-type pumps, most of the common positive-displacement
pumps use rotating elements to provide a constant-volume, constant-pressure output.
As a result, these pumps can be monitored with the following parameters: hydraulic
instability, passing frequencies, and running speed.

Hydraulic Instability (Vane Pass). Positive-displacement pumps are subject to flow
instability, which is created either by process restrictions or by the internal pumping
process. Increases in amplitude at the passing frequencies, as well as harmonics of
both shafts’ running speed and the passing frequencies, typically result from
instability.

Passing Frequencies. With the exception of piston-type pumps, all positive-
displacement pumps have one or more passing frequencies generated by the gears,
lobes, vanes, or wobble-plates used in different designs to increase the pressure of the