Page 290 - Intro Predictive Maintenance

Operating Dynamics Analysis 281

• Slow down the transient. Instead of instant valve closing, throttle the system
over a longer interval. This will reduce the potential for hydraulic hammer
and prolong pump life.
• Install proportioning valves. For applications where frequent radical flow
swings are necessary, the best protection is to install a pair of proportioning
valves that have inverse logic. The primary valve controls flow to the
process. The second controls flow to a full-flow bypass. Because of their
inverse logic, the second valve will open in direct proportion as the primary
valve closes, keeping the flow from the pump nearly constant.

Design Limitations. Centrifugal pumps can be divided into two basic types: end-
suction and horizontal split case. These two major classifications can be further broken
into single-stage and multistage. Each of these classifications has common monitor-
ing parameters, but each also has unique features that alter its forcing functions and
the resultant vibration profile. The common monitoring parameters for all centrifugal
pumps include axial thrusting, vane-pass, and running speed.

End-suction and multistage pumps with inline impellers are prone to excessive axial
thrusting. In the end-suction pump, the centerline axial inlet configuration is the
primary source of thrust. Restrictions in the suction piping, or low suction pressures,
create a strong imbalance that forces the rotating element toward the inlet.

Multistage pumps with inline impellers generate a strong axial force on the outboard
end of the pump. Most of these pumps have oversized thrust bearings (e.g.,
Kingsbury bearings) that restrict the amount of axial movement; however, bearing
wear caused by constant rotor thrusting is a dominant failure mode. Monitoring the
axial movement of the shaft should be done whenever possible.

Hydraulic or flow instability is common in centrifugal pumps. In addition to the
restrictions of the suction and discharge discussed previously, the piping configura-
tion 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 result 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 centri-
fugal pumps.

13.1.6 Interpreting Operating Dynamics
Operating dynamics analysis must be based on the design and dynamics of the
specific machine or system. Data must include all parameters that define the actual
operating condition of that system. In most cases, these data will include full, high-