Control
Many valves are controlled manually with a handle attached to the stem. If the handle is turned ninety degrees between operating positions, the valve is called a quarter-turn valve. Butterfly, ball valves, and plug valves are often quarter-turn valves. If the handle is circular with the stem as the axis of rotation in the center of the circle, then the handle is called a handwheel. Valves can also be controlled by actuators attached to the stem. They can be electromechanical actuators such as an electric motor or solenoid, pneumatic actuators which are controlled by air pressure, or hydraulic actuators which are controlled by the pressure of a liquid such as oil or water. Actuators can be used for the purposes of automatic control such as in washing machine cycles, remote control such as the use of a centralised control room, or because manual control is too difficult such as when the valve is very large. Pneumatic actuators and hydraulic actuators need pressurised air or liquid lines to supply the actuator: an inlet line and an outlet line. Pilot valves are valves which are used to control other valves. Pilot valves in the actuator lines control the supply of air or liquid going to the actuators.
The fill valve in a toilet water tank is a liquid level-actuated valve. When a high water level is reached, a mechanism shuts the valve which fills the tank.
In some valve designs, the pressure of the flow fluid itself or pressure difference of the flow fluid between the ports automatically controls flow through the valve.
Read more about this topic: Valve
Famous quotes containing the word control:
“For the mother who has opted to stay home, the question remains: Having perfected her role as a caretaker, can she abdicate control to less practiced individuals? Having put all her identity eggs in one basket, can she hand over the basket freely? Having put aside her own ambitions, can she resist imposing them on her children? And having set one example, can she teach another?”
—Melinda M. Marshall (20th century)
“There are many things children accept as “grown-up things” over when they have no control and for which they have no responsibility—for instance, weddings, having babies, buying houses, and driving cars. Parents who are separating really need to help their children put divorce on that grown-up list, so that children do not see themselves as the cause of their parents’ decision to live apart.”
—Fred Rogers (20th century)
“... the black girls didn’t get these pills because their black ministers were up on the pulpit saying that birth control pills were black genocide. What I’m saying is that black men have exploited black women.... They didn’t want them to have any choice about their reproductive health. And if you can’t control your reproduction, you can’t control your life.”
—Joycelyn Elders (b. 1933)