Page 18 - Valve Selection Handbook

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FUNDAMENTALS

FLUID TIGHTNESS OF VALVES

Valve Seals

One of the duties of most valves is to provide a fluid seal between the
seat and the closure member. If the closure member is moved by a stem
that penetrates into the pressure system from the outside, another fluid
seal must be provided around the stem. Seals must also be provided
between the pressure-retaining valve components. If the escape of fluid
into the atmosphere cannot be tolerated, the latter seals can assume a
higher importance than the seat seal. Thus, the construction of the valve
seals can greatly influence the selection of valves.
Leakage Criterion

A seal is fluid-tight if the leakage is not noticed or if the amount of
noticed leakage is permissible. The maximum permissible leakage for the
application is known as the leakage criterion.
The fluid tightness may be expressed either as the time taken for a
given mass or volume of fluid to pass through the leakage capillaries or
as the time taken for a given pressure change in the fluid system. Fluid
tightness is usually expressed in terms of its reciprocal, that is, leakage
rate or pressure change.

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