Page 182 - Tribology in Machine Design

168 Tribology in machine design

the secondary seal and specific tests show that a fretted installation may
leak more rapidly. Fretting is initiated by adhesion and those conditions
that reduce adhesion usually mitigate fretting.

4.15.7. Parameters affecting wear
Three separate tests are usually performed to establish the performance and
acceptability of seal face materials. Of these the most popular is the PV test,
which gives a measure for adhesive wear, considered to be the dominant
type of wear in mechanical seals. Abrasive wear testing establishes a relative
ranking of materials by ordering the results to a reference standard material
after operation in a fixed abrasive environment. A typical abrasive
environment is a mixture of water and earth. The operating temperature
has a significant influence upon wear. The hot water test evaluates the
behaviour of the face materials at temperatures above the atmospheric
boiling point of the liquid. The materials are tested in hot water at 149 °C
and the rate of wear measured. None of the above mentioned tests are
standardized throughout the industry. Each seal supplier has established its
own criteria. The PV test is, at the present time, the only one having a
reasonable mathematical foundation that lends itself to quantitative
analysis.
The foundation for the test can be expressed mathematically as follows:

where PV is the pressure x velocity, A/? is the differential pressure to be
sealed, b is the seal balance, £ is the pressure gradient factor, F s is the
mechanical spring pressure and V is the mean face velocity.
All implicit values of eqn (4.194), with the exception of the pressure
gradient factor, <!;, can be established with reasonable accuracy. Seal
balance, b, is further defined as the mathematical ratio of the hydraulic
closing area to the hydraulic opening area. The pressure gradient factor, £,
requires some guessing since an independent equation to assess it has not
yet been developed. For water it is usually assumed to be 0.5 and for liquids
such as light hydrocarbons, less than 0.5 and for lubricating oils, greater
than 0.5. The product of the actual face pressure, P, and the mean velocity,
V, at the seal faces enters the frictional power equation as follows:

where N { is the frictional power, PV is the pressure x velocity, / is the
coefficient of friction and A is the seal face apparent area of contact.
Therefore, PV can be defined as the frictional power per unit area.
1
6
Coefficients of friction, at PV = 3.5 x 10 Parns" , for frequently used seal
materials are given in Table 4.3. They were obtained with water as the
lubricant. The values could be from 25 to 50 per cent higher with oil due to
the additional viscous drag. At lower PV levels they are somewhat less, but
not significantly so; around 10 to 20 per cent on the average. The coefficient
of friction can be further reduced by about one-third of the values given in