control valves: "control valves
Control valves are valves used to control conditions such as flow, pressure, temperature, and liquid level by fully or partially opening or closing in response to signals received from controllers that compare a 'setpoint' to a 'process variable' whose value is provided by sensors that monitor changes in such conditions.[1]
The opening or closing of control valves is done by means of electrical, hydraulic or pneumatic systems. Positoners are used to control the opening or closing of the actuator based on Electric, or Pnuematic Signals. These control signals, traditionally based on 3-15psi (0.2-1.0bar), more common now are 4-20mA signals for industry, 0-10V for HVAC systems, & the introduction of 'Smart' systems, HART, Fieldbus Foundation, & Profibus being the more common protocols.
A valve is a device that regulates the flow of a fluid (gases, fluidized solids, slurries, or liquids) by opening, closing, or partially obstructing various passageways. Valves are technically pipe fittings, but are usually discussed as a separate category.
Valves are also found in the human body. For example, there are several which control the flow of blood in the chambers of the heart and maintain the correct pumping action (see heart valve article).
Valves are used in a variety of contexts, including industrial, military, commercial, residential, and transportation."
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