Page 396 - Centrifugal Pumps 2E
P. 396
Mechanical Seals 361
Figure 17-4. Hydraulic pressure acting on the primary ring.
Seals can be balanced for pressures at the outside diameter of the seal
faces as shown in Figure 17-4B. This is typical for a seal mounted inside
the stuffing box. Seals mounted outside the stuffing box can be balanced
for pressure at the inside diameter of the seal faces. In special cases, seals
can be double balanced for pressure at both the outside and inside diame-
ters of the seal. Seal balance can range from 0.65 to 1.35, depending on
operating conditions.
Face Pressure. This is an important factor in the success or failure of a
mechanical seal. Hydraulic pressure develops within the seal faces that
tend to separate the primary and mating rings. This pressure distribution
is referred to as a pressure wedge (Figure 17-5). For most calculations it
may be considered as linear. The actual face pressure Pf in pounds per
square inch is the sum of the hydraulic pressure P h and the spring pres-
sure P sp designed into the mechanical seal. The face pressure P f is a far-
ther refinement of P which does not take into account the liquid film
pressure or the load of the mechanical seal:
