Page 398 - Centrifugal Pumps 2E

Mechanical Seals 363

Pressure-Velocity. The value for a seal installation may be compared
with values developed by seal manufacturers as a measure of adhesive
wear. As the primary and mating rings move relative to each other, they
are affected by the actual face pressure and rotational speed. The product
of the two, pressure times velocity, is referred to as PV and is defined as
the power N per unit area with a coefficient of friction of unity;

For seals, the equation for PV can be written as follows:

where V m = velocity at the mean face diameter d m, ft/min

Power Consumption. The power consumption of a seal system can be
estimated using the PV value and the following equation:

where f is the coefficient of friction.
As a general rule, the power to start a seal is five times the running
value.
The coefficients of friction for various common seal face materials are
given in Table 17-1. These coefficients were developed with water as a
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lubricant at an operating PV value of 100,000 lb/in. ft/min. Wues in oil
would be slightly higher as a result of viscous shear of the fluid film at
the seal faces.

Example:

A pump having a 3-inch diameter sleeve at the stuffing box is fitted
with a balanced seal of the same size and mean diameter. The seal oper-
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ates in water at 400 lb/in. , 3,600 rpm and ambient temperatures. The
material of construction for the primary and mating rings are carbon and
tungsten carbide. Determine the PV value and power loss of the seal.