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

406 Centrifugal Pumps: Design and Application
pJished by using a tandem seal arrangement as previously discussed. Upgrading
the seal materials may be another solution to consider. An
example of an extrusion failure is shown in Figure 17-43, This is a corrosion
resistant elastomer that had been in chemical service at-700 psig and
a temperature of 420°E The seal had also been subject to excessive axial
motion. The seal leakage problem was corrected by reducing the temperature
in the seal cavity and eliminating the excessive axial motion from
the equipment.
Secondary elastomeric seals may also be subject to excessive volumetric
change, either swell or shrinkage. Volumetric swell will result in the
loss of seal face flexibility. Volumetric swell of a secondary seal may
also result in seal face distortion that can also contribute to leakage.
Shrinkage of the secondary seal will result in the toss of interference between
the shaft and primary ring, resulting in seal leakage. An 0-ring
subjected to shrinkage and corrosion is shown in Figure 17-44. This type
of condition was corrected by upgrading the material of construction,
Elastomeric O-rings are also subject to a condition referred to as explosive
decompression. This can occur when sealing some liquefied gases
such as carbon dioxide. Small amounts of gas can penetrate the O-ring at
high pressure. As long as pressure is kept on the seal, nothing will happen
to the O-ring, Once the pressure in the seal cavity is reduced, the O--
ring will expand and burst at the point where the gas has been absorbed
into the O-ring. This blistered condition can be avoided by upgrading the
material used for the O-ring.
The advantages of using an O-ring as a secondary seal outweigh the
disadvantages. This type of secondary seal offers an excellent solution to
high pressure applications.
Metal bellows seals offer an excellent solution for temperatures greater
than 400°F and pressures to 500 psig. A seal fitted with a metal bellows
can be applied to those applications that are essentially non-corrosive and
where angular misalignment is not a problem. A bellows used in an environment
with a large amount of angular misalignment will fail from fatigue.
Cracking of a metal bellows will occur above the weld near the
inside diameter of the bellows, as shown in Figure 17-45. This type of
seal problem can be eliminated by correcting the motion from angular
misalignment.
Motion from angular misalignment, parallel misalignment, shaft runout
or whirl, and shaft end play have an effect on seal operation. Motion
from parallel misalignment is believed to aid the lubrication process.
Motion from other types of conditions will result in larger than normal
forces on a primary seal ring. This will lead to unstable operation for the
mechanical seal. The increase in load from angular misalignment can be
many times larger than the design load of the seal. This increase in load