Page 60 - Valve Selection Handbook

Manual Valves 47

SELECTION OF VALVES

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.

Valves for Stopping and Starting Flow

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 resis-
tance can be accepted—as is frequently the case—closing-down valves
may likewise be used for this duty.

Valves for Controlling Flow Rate

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.
Valves for Diverting Flow

These valves have three or more ports, depending on the flow diver-
sion duty. Valves that adapt readily to this duty are plug valves and ball
valves. For this reason, most valves for the diversion of flow are one of
these types. However, other types of valves have also been adapted for
the diversion of flow, in some cases by combining two or more valves
that are suitably interlinked.
Valves for Fluids with Solids in Suspension

If the fluid carries solids in suspension, the valves best suited for this
duty have a closure member that slides across the seat with a wiping