How do control valve characteristics affect control loop performance?

Valve characteristics describe the relationship between valve position and flow. Linear valves provide proportional relationship, equal percentage valves give same percentage flow change per unit position change (small flow at low opening, large at high), and quick opening provides large flow change at low opening. Process gain varies with valve position for non-linear valves. Equal percentage valves compensate for process non-linearities, providing more consistent loop gain across operating range. Proper matching of valve to process characteristics ensures stable control throughout the operating range.