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

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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.

214 Centrifugal Pumps: Design and Application Figure 12-7. Tandem seal assembly for hot oil service (courtesy BW/IP international, Inc. Seal Division, manufacturer of BW Seals). Figure 12-8. Shaft seal for boiler feed pump service (courtesy BW/IP International, Inc. Seal Division, manufacturer of BW Seals).

214 <strong>Centrifugal</strong> <strong>Pumps</strong>: <strong>Design</strong> <strong>and</strong> <strong>Application</strong><br />

Figure 12-7. T<strong>and</strong>em seal assembly for hot oil service (courtesy BW/IP international,<br />

Inc. Seal Division, manufacturer of BW Seals).<br />

Figure 12-8. Shaft seal for boiler fe<strong>ed</strong> pump service (courtesy BW/IP International,<br />

Inc. Seal Division, manufacturer of BW Seals).

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