Introduction to Manual Valves
Manual Valves are those valves that operate through a manual operator (such as handwheel or handlever).
Manual valves are primarily used to stop and start the flow (block or on-off valves), although some designs can be used for basic manual throttling. The best manual valves for on-off services are those that allows flow to move straight through the body, with a full area closure element that presents little or no pressure drop. The manual operator is usually placed in a mid-stroke position. Even though a manual on-off valve is being used for basic throttling, 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 the valve.
Manual Valve Classifications
Manual valves are usually classified into four types, depending on their design and use.
- Rotating Valve
- Stopper Valve
- Sliding Valve
- Flexible Valve
Rotating valves are those manual valves that uses a quarter-turn rotation of the closure element. Rotating valves have a flow path directly through the body and closure element without any right angle turns. The most common rotating valves are plug, ball and butterfly valves. Rotating valves – except butterfly valves – perform well in non clean services, because the rotation of the closure element has a tendency to server particulates while closing.
Stopper valves are those manual valves that use a linear motion, circular closure element perpendicular to the center line of the piping. Stopper valves use a globe body to direct the flow through a right angle turn under or above the closure element. The most common stopper valves are globe and piston valves. Because of the right angle turn in these valves, stopper valves take more of a pressure drop than other valve designs. Because of stopper design, particulates can trap between the closure element and the seat, causing leakage. Thus stopper valves are preferred for cleaner services.
Sliding valves are those manual valves that uses a flat perpendicular closure element that intersect the flow. Like rotating valves, sliding valves also have a body with straight through flow. The closure element – which is a flat element reaching from wall to wall – slides down from its full open position into the flow stream acting as a barrier wall. The most common sliding valves are gate and piston valve. As the closure element seats at the bottom of the valve body, particulates can prevent full seating, causing leakage. Thus sliding valves are preferred for cleaner services.
Flexible valves are those manual valves that uses a elastomeric or flexible closure element and a body that allows straight through flow. The closure element pushes against a highly flexible elastomeric or rubber insert until it meets against the bottom of the body or other side of the elastomeric insert, literally pinching the flow closed. The most common flexible valves are pinch and diaphragm valves. Flexible valves are typically used in on-off services where tight shutoff (ANSI Class IV) is important.
For more on valves, check out : Introduction to Valves
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