Centrifugal Pumps Design and Application 2nd ed - Val S. Lobanoff, Robert R. Ross (Butterworth-Heinemann, 1992)

428 Centrifugal Pumps: Design and Application
Piping and Structure. The pump should be relatively isolated from the
piping. The weight and thermal loading on the suction and discharge connections
should be minimized. The American Petroleum Institute (API)
Standard 610 [7] specifies allowable external nozzle forces and moments.
Most pump manufacturers specify allowable weight and thermal
toads transferred from the pipe to the pump case. Static forces from the
piping may misalign the pump from its driver, or for excessive loading,
the pump case may become distorted and cause rubs or seal and bearing
damage. Thermal flexibility analyses of the piping should be performed
to evaluate piping loads and to design the necessary supports and restraints
to minimize the transfer of piping loads to the operating equipment.
Vibrations of the piping or the support structure can be mechanically
transferred to the pump. The piping and the structure should not have
their resonant frequencies coincident with any of the pump excitations
such as vane passing frequency or multiples. The vibrations transferred
from the pipe to the structure can be minimized by using a visco-elastic
material (i.e., belting material) between the pipe and the pipe clamp.
Application
The initial stage of pump system design should include the task of defining
the range of operating conditions for pressure, flow, temperatures,
and the fluid properties. The vendors can provide the correct pump geometry
for these design conditions. Expected variations in operating conditions
and fluid composition, if a significant percentage, may influence
the design.
Improper application or changing conditions can result in a variety of
problems. Operation at high-flow, low-head conditions can cause vibrations
of the rotor and case. Inadequate NPSH can result in cavitation that
will cause noise and vibration of varying degrees.
Bearings. General purpose, small horsepower pumps in process plants
generally have rolling element bearings. Noise and vibrations are commonly
a result of bearing wear. As the rolling elements or races wear, the
worn surfaces or defects initially produce a noise and as wear increases
vibrations may become noticeable. Several vibrational frequencies may
occur that depend on the geometry of the bearing components and their
relative rotational speeds [3], The frequencies are generally above operating
speed.
Many ball bearing failures [8] are due to contaminants in the lubricant
that have found their way into the bearing after the machine has been
placed in operation. Common contaminants include moisture, dirt, and