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Back pressure
The pressure exerted on the downstream side of a valve seat.
Ball valve
A quarter-turn valve with a spherical closing element held between two seats. Characteristics include quick opening and good shut-off. Ball valves are widely used as on/off valves in the chemical process and other industries. Special designs (with V notches or fingers) are available for throttling applications. Larger valves with heavier balls (eg, on pipelines) may use trunnions to help support the ball and prevent damage to soft internals. Designs are typically, one, two or three piece.
Bellows
Sealing device which prevents line media leaking between the stem and the body.
Butterfly valve
A quarter-turn valve which has a circular disk as its closing element. The standard design has the valve stem running through the disk, giving a symmetrical appearance. Later designs off-set the stem, so that the disk ‘cams’ into the valve seat. Advantages include less wear and tear on the disk and seats, and tighter shut-off capabilities. Many design types are available including inexpensive Teflon or resilient seats for use in water (treatment) plants, etc. More expensive metal seats can be used where high temperatures or aggressive chemicals are encountered. So-called “High Performance” butterfly valves offer zero leakage designs and have been applied in both the chemicals and hydrocarbon processing sectors.
Bypass valve
A small bore valve fitted in parallel to a larger main valve. Bypass valves are used to reduce the differential pressure across the main valve before this latter valve is opened (as otherwise this larger, more expensive valve, may suffer damage to internal components).