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

Mechanical Seals 415
Figure 17-53. Condition of external seal parts at stuffing box. (A) Unstable operation
with surface disturbance, (B) Stable operation without any surface disturbance.
poor lubricants. In this case, it is also a good idea to check for mechanical
distortion as well.
Mechanical seals are applied to an infinite number of environmental
conditions. Some of the liquids to be sealed are extremely poor lubricants
for the seal faces and can result in severe wear problems. Figure 17-54
illustrates the faces of an inboard double seal from an agitator. The material
of construction for both the primary and mating rings is tungsten carbide.
This is the result of running in a liquid with a very low viscosity.
The seal lubricant in this case was a silicone oil. The seal life was 11
months. During that time the primary ring wore into the mating ring by
as much as 0.125 inch. This type of wear condition was overcome by
replacing the lubricant in the seal chamber. In this case, water was selected
as the replacement liquid. The result was a substantial increase in
seal life.
Normally, in poor lubricating medias, the liquid to be sealed cannot
develop a sufficient film to separate both the primary and mating rings.
Figure 17-55 illustrates the results of running a silicon carbide primary
ring against a tungsten carbide mating ring in a liquid with poor lubricating
qualities. The tungsten carbide heat checked and the silicon carbide
wore approximately 0.030 inch into the tungsten carbide. The liquid