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

Mechanical Seal Reliability 565
running speed of the pump. The best way to prevent this is to dynamically
balance the rotating assembly. Although many technicians will
work with an assembly until no further improvement can be detected, a
good commercial standard is 0.0045 ounce-inch per pound of assembly.
Alignment to Driver. Misalignment between the pump and driver is another
cause of vibration. This vibration occurs at two times the running
speed of the pump. Both radial and axial components may be present. In
order to reduce this vibration, many pump users take great pains to align
pumps to drivers.
Flushing the Seal. Because a mechanical seal is a rubbing device that
generates heat, it must be lubricated and cooled. Cooling and lubrication
are normally accomplished by circulating a suitable liquid around the
seal. This is called flushing the seal. The most common arrangement is
the discharge bypass flush, sometimes called API Plan 11 (see Chapter
17, Figure 17-9), but there are many others (see Chapter 17, Figure 17-
26).
Perhaps the most important feature of a good flushing system is that
the liquid circulates around the rubbing interface of the seal. This is accomplished
by injecting or withdrawing the liquid in close proximity to
the faces.
In general, seal flush systems should be as simple as possible to avoid
installation and operating errors. This means designing the system to
minimize valves, orifices, filters, etc. A common problem with flushing
systems is that liquid does not actually flow because a valve is closed, a
filter is blocked, etc,
Seal Failures Caused by Pump Operation
Although mechanical seals should be designed to survive operational
upsets, seal life can certainly be enhanced by observing a few simple precautions.
Some of the more common operating problems include:
• Operation at low flow.
• Operation at no flow.
• Cavitation.
Low Flow. In spite of the performance curve, which shows operation
from no flow to maximum, centrifugal pumps may not operate smoothly