3.1 Introduction to Manual Valves
3.1.1 Definition of Manual Valves
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. 3.1.2 Classifications of Manual Valves Manual valves are usually classified into four types, depending on their design and use. The first classification type of manual valves is
rotating valves, which includes those manual-valve designs that use a quarter-turn rotation of the closure element. Rotating manual valves have a flow path directly through the body and closure element without any right-angle turns. The most common designs in the rotatingmanual-valve family are plug, ball, and butterfly valves. They are most commonly used for on–off, full-flow services. In some applications they can be used for throttling control, as well as diversion and combination service. Overall, because rotating valves are inexpensive and versatile, they are the most common type of manual valve used in the process industry today. As a general rule, rotating valves—except butterfly valves—perform well in less-than-clean services, because the rotation of the closure element has a tendency to sever particulates when closing. The second classification is stopper valves, which are defined as those manual-valve designs that use a linear-motion, circular closure element perpendicular to the centerline of the piping. These manual valves use a globe body to direct the flow through a right-angle turn under or above the closure element. If the valve uses an angle body, the flow continues from that right angle. If the valve has a straight-through body design, another right-angle turn is necessary after the closure element for the flow to be redirected in the same direction as the inlet. The two most common designs in the classification are the globe and piston manual valves. Because of the right-angle turns in these valves, stopper valves take more of a pressure drop than other designs. Therefore, among manual valves, they are the most frequently used throttling control and diversion applications, although they are often used for simple on–off service. Because of the stopper design, particulates can trap solids between the closure element and the seat, causing leakage; therefore, stopper valves are preferred for cleaner services. The third classification is sliding valves, which are described as those manual valves that use a...