Page 332 - Centrifugal Pumps 2E

Chemical Pumps Metallic and Nonmetallic 297

and the casing cover relative to the axial O-ring. Because the axial O-ring
has a greater sealing diameter, it requires additional bolting. Corner O-
rings take more tolerancing but are a good compromise between radial
and axial machining. The O-rings are made out of EP (ethylene propyl-
ene) for hot water, buna for hydrocarbons, viton for general chemicals,
and Kalrez for highly corrosive chemicals. They are usually color coded
to designate materials.

Casing Covers

Stuffing Box

The casing cover, sometimes called a stuffing box extension, encloses
the back end of the casing. The casing cover also includes the stuffing
box or seal chamber. Originally, the ANSI standards required that the
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5
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minimum stuffing box packing size be /ie in., /s in., or /i6 in. depend-
ing on pump size. The stuffing box was designed to handle both mechani-
cal seals and packing; however, through years of experience, it was
found that the mechanical seal's outside diameter had too small clearance
between it and the bore of the box. This limited the amount of cooling
that a seal could obtain, especially in double seals. So even though there
was cooling injection into the gland and out of the box for double me-
chanical seals, there were frequent failures at the outboard seal. As a re-
sult, the specification includes optional large bores that only accommo-
date mechanical seals. This should give adequate cooling of the box to
increase the life of the seal. If a customer requires a box for both seals
and packing, he will require a box to the original specification. The taps
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into the stuffing box may be k in. minimum, but /8 in. is the preferred
NPT size,
Depending on fluid temperature, the option of a cooled or heated stuff-
ing box is usually offered. The type of mechanical seals offered on these
pumps are single seals, double seals, and tandem seals. Mechanical seals
used in ANSI pump applications are discussed in detail in Chapter 17.

Frame
The frame for the horizontal pump is composed of the support head
and bearing housing. Depending on pump size, this can be one integral
component or two separate pieces. Some manufacturers refer to the bear-
ing frame housing as the bearing housing. The bearing frame or housing
consists of the housing, the shaft, bearings, bearing end cover, flinger,
and feet. On pump sizes AA and AB, the feet are usually cast integral
with the bearing housing support head combination.