Page 406 - Centrifugal Pumps 2E

Mechanical Seals 371

where C 3 = 532
h = face gap, in
P 2 = pressure at face ID, lb/in. 2
PI = pressure at face OD, lb/in. 2
jj, = dynamic viscosity, CP
R 2 — outer face radius, in,
RI = inner face radius, in.

Negative flow indicates flow from the face outer diameter to the inner
diameter. The effect of centrifugal force from one of the rotating sealing
planes is very small and can be neglected in normal pump applications.
The gap between the seal faces is a function of the materials of construc-
tion, flatness, and the liquid being sealed. The face gap can range from
6
6
20 x 10~ to 50 x 10~ inches. Leakage from a seal is also affected by
the parallelism of the sealing planes, angular misalignment, coning (neg-
ative face rotation), thermal distortion (positive face rotation), shaft run-
out, axial vibration, and fluctuating pressure. Further discussions of
these problem areas will be covered later in the chapter on seal installa-
tion and troubleshooting.

Seal Wear

The types of wear found in mechanical seals are adhesive, abrasive,
corrosive, pitting or fatigue, blistering, impact, and fretting. Adhesive
wear is the dominant type of wear in well designed seals. Hie other types
of wear are related to problems with the entire sealing application and
will be discussed later in seal installation and troubleshooting.
Even though the successful operation of a seal depends on the develop-
ment of a lubricating film, solid contact and wear occur during startup,
shutdown, and during periods of changing service conditions for the
equipment. For most applications, seals are designed with a carbon
graphite primary ring considered to be the wearing part in the assembly.
The primary ring runs against the mating ring that is made of a harder
material and wears to a lesser extent. In mechanical seal designs, face
loads are sufficiently low so that only a mild adhesive wear process oc-
curs. If the application is extremely abrasive, hard seal faces for both the
primary and mating rings must be considered. Abrasive applications in-
clude sand type slurries, paint, and abrasives generated at elevated tem-
peratures from carbonized products.
The PV criterion used in the design of seals is also used in the expres-
sion of the limit of mild adhesive wear. Table 17-2 gives the PV limita-
tions for frequently used seal face materials. Each limiting value has been