Page 239 - Centrifugal Pumps Design and Application
P. 239

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 paral-
        lelism 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. Be-
        cause boiler feedwater is a relatively clean liquid and rapid wear or sei-
        zure is unusual, impeller wear rings are seldom used in large, high-speed
        double-case boiler feed pumps. These boiler feed pump impellers are de-
        signed 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 ap-
        plication 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 hydrocar-
        bons 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.
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