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

564 Centrifugal Pumps: Design and Application
dard arrangement had about Vie-inch radial clearance between the seal
and the stuffing box bore. When this clearance was increased to l li inch,
the seal face temperature decreased by 38 °E In response to this sort of
information and experience, many pump manufacturers are now supplying
"oversized" stuffing boxes for their pumps.
Shaft Axial Movement. Whenever the shaft moves axially, one of the
sealing elements must move in the opposite direction to maintain the existing
gap between the faces. If this movement occurs repeatedly, fretting
may occur between the seal and shaft/sleeve. Table 21-3 shows that fretting
is a significant cause of seal failures.
For reasons that are not completely clear, many manufacturers assemble
their pumps so that the shaft can "float" axially for 3 mils to 8 mils.
This is a built-in opportunity for the seal to fret. To reduce this potential,
many pump users go against the manufacturer's recommendation and
modify the pumps to restrict "float" to less than 1 mil.
Excessive Piping toads. A centrifugal pump is not a good pipe anchor,
API 610 specifies maximum loads that must be used by both pump manufacturer
and piping designer. Normally this is not a problem except for
end suction pumps used in high temperature services.
Beginning with the sixth edition, API 610 specifies piping loads that
are considerably higher than previous editions. While this makes the job
of the piping designer easier, it has imposed a severe constraint on the
pump manufacturer. Most pumps that were designed to meet the early
editions of API 610 will not meet the more stringent requirements of the
sixth and seventh editions. Because they are not as rigid, older pumps
may require even more attention to installation and hot alignment than
the newer models.
Seal Failures Caused by Pump Repair and Installation
The mechanical seal is sensitive to the environment in which it operates.
In particular, the mechanical condition of the pump has a significant
effect on seal reliability. Some of the major considerations include;
• Rotating imbalance.
• Alignment to driver.
« Flushing the seal.
Rotating Imbalance. Vibration reduces seal life and any rotating imbalance
will create vibration. The frequency of this vibration will be the