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320    Centrifugal Pumps: Design and Application

         reinforcing with either a metallic back up or extra strength reinforcing
        cloth. When designing the stuffing box area, the heat transfer of the in-
        jection fluid around the seal should be considered. The larger this area,
        the better the life of the mechanical seal.
          The shaft sleeve can be a separate piece that is usually made by injec-
        tion molding or it can be made integral with the impeller. There are ad-
         vantages and disadvantages to both. When integral with the impeller, the
        entire impeller sleeve mechanism needs to be replaced if something goes
        wrong with the sleeve. When a separate shaft sleeve is used, there is an
        additional sealing surface between the impeller and the sleeve to prevent
        fluid from coming in contact with the shaft. When using nonmetallk
        sleeves, mechanical seals with teflon wedges should not be employed be-
        cause of the excess fretting. Also, the designer has to be concerned with
        the extrusion from holding force of set screws on soft nonmetallic
        sleeves. Split clamping rings using a radial type of set screw are some-
        times used to prevent damage to the shaft sleeve.

        Mechanical Seals

          Because of the corrosive properties of the fluids being used within
        nonmetallic pumps, many pumps use outside mechanical seals. As a re-
        sult, the only wetted pieces are the stationary seat and the compatible ro-
        tating surface. The remaining springs and secondary seals are external to
        the stuffing box. However, care should be taken that if an outside seal
        fails it could be catastrophic. It is recommended that a seal guard be em-
        ployed when outside mechanical seals are used. This subject is discussed
        in detail in Chapter 17.

        Impellers


          Many of the materials that have to be used for the liquids being pumped
        cannot be readily adhered or mechanically attached to themselves.
        Therefore, it is difficult to obtain closed impellers and consequently,
        most impellers are open vane design. Another basic problem with the
        nonmetallic pumps is the attachment of the impeller to the shaft. Depend-
        ing on the speed and horsepower, most nonmetallic pumps use more than
        one key for attachment due to the stress levels of the material. Many im-
        pellers are attached by using threaded inserts that are molded within the
        impeller. The problem here is that care has to be taken that excess stress
        doesn't occur around the surface of the molded insert that would result in
        a weak surface between the two materials. Another method of attachment
        is a multi-keyed or polygon shape that does not require an internal insert
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