From Valve Selection Handbook: Engineering Fundamentals for Selecting the Right Valve Design for Every Industrial Flow Application, Fifth Edition

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.

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.

Four broad...

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Products & Services
Butterfly Valves
Butterfly valves control flow through a circular disc or vane by turning the valve's pivot axis at right angles to the direction of flow in the pipe. They are normally used as throttling valves to control flow.
Industrial Valves
Industrial valves are classified in many different ways. They can be distinguished by material of construction, media handled, and/or application.
Air Valves
Air valves allow metered fluid flow in one or both directions. They are used in pneumatic circuits to regulate the rate of activation or exhaust of cylinders and other pneumatic devices.
Ball Valves
Ball valves provide tight shut-off and characterizable control. They have high rangeability due to the design of the regulating element and few complications from side loads typical of butterfly or globe valves.
Globe Valves
Globe valves are linear motion valves with rounded bodies, from which their name is derived.  They are widely used in industry to regulate fluid flow in both on/off and throttling service. 

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