TABLE OF CONTENTS By definition, manual valves are those valves that operate through a manual operator (such as a handwheel or handlever), which are primarily used to stop and start flow (block or on off valves), although some designs can be used for basic throttling.
The best manual valves for on off service are those that allow flow to move straight through the body, with a full-area closure element that presents little or no pressure drop. Usually if a manual valve is used to start and stop flow, as an on off valve, and the manual operator is placed in a midstroke position, partial flow is possible as a throttling valve. However, some on off designs in a midstroke position are not conducive to smooth flow conditions and may even cause turbulence and cavitation. Even though a manual on off valve is being used in a throttling situation, it is not considered a control valve because it is not part of a process loop, which requires some type of self-actuation as well as input from a controlling device to a valve and position feedback. Throttling manual valves used to control flow are those that offer a definite flow characteristic inherent or otherwise between the area of the seat opening and the stroke of the closure element.
Besides on off and throttling functions, manual valves are also used to divert or combine flow through a three- or four-way design configuration.
Manual valves are usually classified into...
