Page 238 - Know and Understand Centrifugal Pumps

Failure Analysis of Mechanical Seals

Elastomer parts and gaskets

Springs
Chemical compatibility with the face materials
Metallurgy: sleeve, shaft, set screws, gland, metallic parts, drive
pins, clips, keyways, anti rotation pins, etc.
13. Very High Pressures Use balanced O-ring cartridge seals up to
about 500 psi. Above 500 psi use the tandem double seal with the
barrier fluid pressurized at ’/2 the seal chamber pressure. Remember
as pressure goes up the O-rings will extrude and metal parts will
distort. Use a torsion balanced seal.
14. Hard Vacuum Use a balanced O-ring seal for industrial vacuum.
For absolute pressure less than 1 kpa (1 kilopascal) use a torsion
balanced seal. (Must verify this measurement)
15. Extremely High Velocity Shaft Speed Some pump companies use
very high velocity, 30,000 rpm, to improve efficiency and generate
high head with small equipment. Use a stationary seal, with the
springs in the stationary element.
A big part of the overall problem with adequate mechanical seal life is
trying to make a precision mechanical seal run into the same space that
previously was occupied by the packing rings. Pump design evolved
over time to accommodate the packing rings.

For example, the restriction bushing in the bottom of the packing
stuffing box is designed to prevent the gland follower from pushing the
packing out of the bottom of the box. With a mechanical seal, the
restriction bushing in the bottom of the packing box is a ‘dinosaur of
design’. It performs no function with the mechanical seal, except to
shorten its life by holding heat, preventing clean cooling liquid from
arriving to the mechanical seal faces, and trapping abrasives, sediment,
crystals, and dirt. If you were to remove the restriction bushing by
placing the part on a lathe and machining it away, certain pumping
applications could immediately quadruple the service life of the
mechanical seal.
The bore of the packing box serves to hold the packing around the
shaft so that the pressure from the gland follower can axially compress
the rings to affect a shaft seal. With a mechanical seal, the reduced tight
bore of the packing box is another dinosaur of design. It could be
opened on a lathe in the same function as the removal of the restriction
bushing at the bottom of the box. This would immediately triple or
quadruple the service life of most mechanical seals.
Many packed pumps have an installed discharge bypass line running
from the discharge nozzle of the pump to the packing box. This line

221