Page 354 - Centrifugal Pumps 2E
P. 354

Chemical Pumps Metallic and Nonmetaliic   319

         Bolting

           As threaded studs will impose tension in the composite case during as-
         sembly, it is preferable to use through bolting. This leaves the casing in
         compression rather than in tension. When through bolting cannot be
         used, the bolts or studs are fastened into stainless steel or alloy inserts
         and are molded into the piece. The inserts are gnarled and grooved on the
         outside diameter to prevent twisting or pulling within the piece when
         torque is applied to the fastener. The inserts are usually a class 3 fit on the
         inside diameter for the fasteners. A blind end insert is used to give a posi-
         tive stop for tiie studs. When inserts are used, it is best to mold them
         within the piece rather than post insert them. When they are molded in
         the piece they should be located at least Vs in. below the finished surface
         so that when machining is being done, the cutting tool does not have an
         interrupt cut against the insert resulting in weakening of the mounting of
         the insert. When inserts are used, care has to be taken by the designer
         that there is proper flow of the composite material to avoid a path for
         leakage during hydrostatic testing.


         Gaskets
           With nonmetallic pumps, most main gaskets are O-rings, These can be
        either round or square cross sectional O-rings. O-rings result in less bolt
        loading on the main bolts. If gasket surface requires final machining,
        then it is recommended that the surfaces be coated with the base resin to
        prevent wicking of the pump fluid through the exposed ends of the glass
        reinforcement resulting in leakage of the gasket.

        Sack-up Support for Bolting

          To reduce the bolt head or nut loading, it is recommended that when
        washers are used their diameter should be at least three times the diame-
        ter of the bolt. Casing covers usually have inserts for the gland studs as
        well as inserts for jacking bolts to aid in the disassembly of casing cov-
        ers. Inserts require optimum strength to absorb radial and axial forces
        and must be compatible with the atmospheric conditions and in many
        cases with the liquid being pumped.

        Stuffing Box Area
          If the glands are made of a composite material, they must be capable of
        withstanding the torque that is applied without creeping. Depending
        whether an inside or outside seal is used, the gland may need additional
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