The method by which the closure member regulates the flow and the configuration of the flow path through the valve impart a certain flow characteristic to the valve, which is taken into account when selecting a valve for a given flow-regulating duty.
These valves are normally selected for low flow resistance and a straight-through flow passage. Such valves are slide valves, rotary valves, and flex-body valves. Closing-down valves offer by their tortuous flow passage a higher flow resistance than other valves and are therefore less frequently used for this purpose. However, if the higher flow resistance can be accepted as is frequently the case closing-down valves may likewise be used for this duty.
These are selected for easy adjustment of the flow rate. Closing-down valves lend themselves for this duty because of the directly proportional relationship between the size of the seat opening and the travel of the closure member. Rotary valves and flex-body valves also offer good throttling control, but normally only over a restricted valve-opening range.
Gate valves, in which a circular disc travels across a circular seat opening, achieve good flow control only near the closed valve position and are therefore not normally used for this duty.
These valves have three or more ports, depending on the flow diversion duty. Valves that adapt readily to this duty are plug valves and ball valves. For this reason, most valves for the diversion...
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