## F

a where Fsp = seal spring load, lb (N) and ao = seal face area, in2 (m2)

Then the actual face pressure can be expressed as

The actual face pressure is used in the estimate of the operating pressure and velocity for a given seal installation.

Pressure-Velocity As the sealing planes 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 Nf per unit area with a coefficient of friction of unity:

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

PV = PfVm = [Ap(b - k) + Psp]Vm where Vm = velocity at the mean face diameter dm, ft/min (m/s).

The PV for a given seal installation can be compared with values developed by seal manufacturers as a measure of adhesive wear.

Power Consumption The PV value also enables the seal user to estimate the power loss at the seal with the following equation:

Nf = (PV)fao, ft • lb/min (N • m/s) where f is the coefficient of friction.

As a rule of thumb, the power to start a seal is generally five times the running value. The coefficients of friction for various common seal face materials are given in Table 1. These coefficients were developed with water as a lubricant at an operating PV value of 100,000 lb/in2 • ft/min (35.03 bar • m/s). The coefficient of friction is a function of the tribo-logical properties of the mating pairs of seal face materials and the fluid being sealed. Values in oil would be slightly higher because of the viscous shear of the fluid film at the seal faces. For a double or tandem seal, the barrier/buffer oil should have a low viscosity and be a good lubricant. The values given are suitable for estimating the power loss in a seal.

For example, let's say we have a pump having a 2-in (50.8-mm) diameter sleeve at the seal chamber is fitted with a balanced seal of this size and mean diameter. The seal operates in water at 300 lb/in2 (20.68 bar), 3,600 rpm, and ambient temperatures. The materials of construction are carbon and tungsten carbide. Determine the PV value and power loss of the seal, given the following:

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