What is a control valve and how does it regulate flow?

A control valve adjusts flow by varying the restriction to flow based on a signal from a controller. Components include: body (contains flow passage and trim), trim (plug/seat that modulates flow), actuator (pneumatic, electric, or hydraulic - moves the stem), and positioner (ensures accurate stem position). The valve creates a variable pressure drop - more closure means more dP for a given flow. Valve characteristics (linear, equal percentage, quick-opening) describe how flow changes with stem position.