The valve is a controlled device that regulates the flow of a liquid or gas in a system. This regulation is accomplished by the varying resistance that the valve introduces into the system as the valve is stroked. As the valve modulates to the closed position the system pressure drop shifts to the valve and reduces the flow in the system.
The valve is very important to the operation of the system. Without a properly sized valve the system will never operate at an efficient level. For valves that are oversized the result is poor controllability that may cause the system to hunt or cycle. Undersizing a valve will require a larger pressure drop across the valve to maintain adequate flow and may not provide required capacity. This results in the pump working harder and leaves the valve susceptible to the effects of cavitation.

Johnson Controls (1994) Control Valves
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