Page 356 - Centrifugal Pumps 2E
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Chemical Pumps Metallic and Nonmetallic   321

         because the stresses of the material are distributed throughout its circum-
         ference relative to the shaft surface. The disadvantage of the polygon at-
         tachment is that there are more surfaces that have to be sealed to prevent
         external fluid from attacking the shaft.
           The sealing of the impellers with either the insert or a polygon fit is
         similar to that used in the metallic pumps. Most manufacturers will em-
         ploy the same sealing mechanisms for the two types.

                       Nonmetallic Immersion Sump Pumps

           Typical applications include wet pit chemical waste handling, effluent
         handling, and liquid transfer operations where broad corrosion resistance
         is required. These pumps are made of the same basic materials as hori-
         zontal nonmetallic pumps, either vinyl ester or epoxy. The hydraulics
         cover the same basic range as the horizontal pumps and in many cases,
         the casing impeller, and casing cover are the same parts as used in the
         horizontal pumps.
          The shaft material is 316, alloy 20, Hastelloy B or C, or titanium, de-
        pending on the liquid being pumped. Optional shafts of 316 coated with
        various materials such as kynar are also available. The use of pultruded
        nonmetallic shafts is being investigated to eliminate all metallic parts for
        this type of application.
          The column supporting the wet end to the mounting plate is a one-piece
         construction with inserted bearings or a multi-construction of short col-
         umns with flanges and the bearing support sandwiched between the
         flanges of the column. The column material is usually the same base ma-
        terial as the pump and impeller. The bolting of the casing and the col-
        umns can be of a nonmetallic material compatible with the fluid.
          Bearings are made out of teflon or carbon with spiral flutes. The lubri-
        cation is either external or clean product lube. Clean liquid for lubrica-
        tion is one that has less than 5 micron particle size. The lubrication to
        each bearing should be at least one half GPM at 160°F temperature or
        less and at a pressure of approximately 25 psig. Carbon bearings are fur-
        nished when external lubrication or injection pressure is not adequate,
          Figure 15-20 shows when to supply carbon or teflon bearings based on
        particle size in the fluid and the flush pressure available to these bear-
        ings. It also shows when cyclone separators are required and what flow
        for a given flush pressure is obtainable from the separators. The lower
        bearings are usually twice as long as the line bearings to absorb the radial
        thrust developed by the impeller. A gap or relief hole is placed between
        the throat bushing of the casing cover and the bearing itself so that dirty
        liquid under pressure will be relieved of pressure and not be forced into
        the bearing clearances resulting in short life.
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