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

Double-Case Pumps 213
ally by split rings on the suction side of each impeller hub. This is the
preferred mounting.
An alternative mounting employs a stack with sliding fits along the
shaft. A spacer sleeve is fit snugly between each pair of impellers and an
outside locknut is used to secure the impellers and sleeves. Great care is
required to assure that all spacer sleeve and impeller faces are parallel to
each other, perpendicular to the shaft, and smooth. Small errors in parallelism
or perpendicularity will misalign the stack,
Impeller Wear Rings
Impeller wear rings are generally specified for refinery pumps [1], but
must be secured with great care on high-speed, high-pressure pumps. Because
boiler feedwater is a relatively clean liquid and rapid wear or seizure
is unusual, impeller wear rings are seldom used in large, high-speed
double-case boiler feed pumps. These boiler feed pump impellers are designed
with extra stock on the wearing surfaces. When worn, impellers
are skim cut true, and the pump then fitted with case wear rings that are
undersized to match the impeller wearing surfaces,
Shaft Seals
Shaft seal failure is the most common cause of unscheduled outage for
double-case pumps. Selection of a shaft seal system designed for the application
is therefore critical to reliable pump operation.
Face-Type Mechanical Seals. Oil refinery pumps almost universally use
mechanical seals. Reliable seals for high temperature oil and light hydrocarbons
now exist. New federal air quality laws limiting hydrocarbon emissions
encourage the use of tandem-type seal systems. Figure 12-7 shows a tandem
seal assembly with bellows-type seals for hot oil service.
In the United States, only small to medium-size (up to 7,000 hp and
5,000 rpm) boiler feed pumps use mechanical seals. In Europe, they are
also used in medium to large boiler feed pumps. A typical boiler feed
pump seal is shown in Figure 12-8.
Most waterflood pumps have mechanical seals, operating at ambient
temperatures, but in corrosive liquid.
Mechanical seals are described in detail in Chapter 17.
Throttle Bushings. The most reliable shaft sealing system for large
boiler-feed pumps consists of throttle bushings with a custom designed,
cold (90°-120°F) condensate injection system. Pumps have operated for
40 years or more with their original throttle bushings.