Page 395 - Centrifugal Pumps 2E

360 Centrifugal Pumps: Design and Application

Design Fundamentals

Seal Balance. This is the ratio of hydraulic closing area to seal face area:

where b = seal balance
&c = hydraulic closing area in. 2
a 0 = hydraulic opening area (seal face area), in. 2

Seal balance is used to reduce power loss at seal faces from sliding
contact. If the load at the seal faces is too high, the liquid film between
the seal rings could be squeezed out or vaporized. An unstable condition
from thermoelastic instability could result in a high wear rate of the seal-
ing surfaces. Seal face materials also have a bearing limit that should not
be exceeded. Seal balancing can avoid these conditions and lead to a
more efficient installation.
The pressure in any stuffing box acts equally in all directions and
forces the primary ring against the mating ring. Pressure acts only on the
annular area ac (Figure 17-4A) so that the closing force in pounds on the
seal face is:

where p = stuffing box pressure, lb/in. 2
F c = hydraulic closing force, pounds

The pressure in pounds per square inch between the primary and mating
rings is:

To relieve the pressure at the seal faces, the relationship between the
opening and closing force can be controlled. If a,, is held constant and a,,
decreased by a shoulder on a sleeve or on the seal hardware, the seal face
pressure can be lowered (Figure 17-4B). This is called seal balancing. A
seal without a shoulder in the design is an unbalanced seal. A balanced
seal is designed to operate with a shoulder. Only a metal bellows seal is a
balanced seal that does not require a shoulder. The balance diameter is
the mean area diameter of the bellows.