The fundamentals of a particular type of valve relate to its sealing characteristics, which include in-line seat sealing when closed and where applicable stem sealing which should prevent potential leaks into the atmosphere. In the case of process systems handling hazardous fluids, harmful to both the atmosphere and personnel, stem sealing is considered to be of more importance.
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.
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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