Centrifugal Pumps Design and Application 2nd ed - Val S. Lobanoff, Robert R. Ross (Butterworth-Heinemann, 1992)

Mechanical Seals 401
This design is a package seal that includes the stuffing box as an integral
part of the seal assembly. This model is built for an end suction
pump and can be removed from the pump without disassembling any
bearings. This is referred to as a Symmetrical Seal and is a patented design
by John Crane Inc. This seal design is ideally suited for incorporating
the upstream pumping concept that was previously discussed.
High speed pump applications offer a unique opportunity for the use of
a non-contacting, non-leaking seal. Through the use of the concept of upstream
pumping, a stuffing box can be run dead ended. The only lubrication
and cooling is achieved through the liquid being moved from the inside
diameter of the seal face to its outside diameter. This design is
illustrated in Figure 17-39. In this tandem seal arrangement, liquid between
the tandem seals at 100 psig is being moved to a high pressure of
1200 psig in the stuffing box. Shaft speed is 15,600 rpm and seal size is
1.500 inches. Flow rate from low pressure to high pressure side of the
seal is 17 cc/min. There is no measureable increase in surface temperatures
at the inboard seal that is handling a differential pressure of 1100
psig. Statically and dynamically, there is no leakage of the liquid being
pumped. Buffer liquid between the seals is water. Only a small amount of
circulation is required to remove the seal heat generated at the outboard
seal. Upstream pumping is a patented design by John Crane Inc. that will
begin to change the way one views a seal application. A seal can also be
used to do work in the system and reduce costly support equipment that is
sometimes required. It also allows a totally different way to flush a seal
with extremely small quantities of liquid.
Split seals are sometimes required on certain applications. Shown in
Figure 17-40 is a patented split seal by John Crane. In this case all components
are split, allowing the changing of a seal without dismantling the
entire pump. Split seals have been applied to pumps with large shafts operating
at speeds of 1800 rpm and pressures to 80 psig. The unique feature
of the floating seal faces allows this type of seal to handle large
amounts of shaft runout.
Many standard and unique seal designs are available from seal manufacturers.
The most efficient design for a given application can be determined
by making all of the operating conditions available to the seal
manufacturer.
Mechanical Seal Installation and Troubleshooting
Long trouble-free operation of equipment includes proper seal design,
selection of materials of construction, and correct seal installation. Any
shortcuts taken during seal installation can result in the equipment being
taken out of service in a few hours, days, or even weeks from startup. A