What distinguishes a linear valve from an equal-percent valve and how does it affect control loop design?

• A. Linear valve has proportional flow with position; equal-percent valve increases more as opening increases; this affects valve gain and nonlinearity compensation.
• B. Valve material determines the flow characteristic.
• C. Both valves provide identical flow characteristics, so there is no effect on the control loop.
• D. Equal-percent valve provides proportional flow with position.

Answer: (D)

The main idea here is how valve flow relates to how far the valve is opened and what that means for a control loop. A linear valve behaves so that flow changes roughly in direct proportion to the valve opening. If you open the valve 40%, you get about 40% of the maximum flow, and the valve gain (the change in flow per change in opening) stays roughly constant across its range. That makes the process easier to model and tune with a fixed-gain controller, since the relationship between input (opening) and output (flow) doesn’t change much as the loop operates.

An equal-percent valve, on the other hand, is designed so that equal increments of opening produce equal percentage changes in flow, which creates a nonlinear relationship between opening and flow. Near the closed position, small moves in opening produce only small flow changes; as the opening increases, the same stroke yields progressively larger flow changes. This means the valve gain is not constant—it varies with position. In control loop design, that nonlinearity shows up as a changing process gain and nonlinear response, so a single fixed-tuning set may not work well across the entire range. To handle it, you’d either linearize the valve around the operating point (gain scheduling or a nonlinear controller) or use multiple models/tuned regions so the controller can adapt as the valve position shifts.

So the distinction is simple: a linear valve offers proportional flow with opening (constant gain), while an equal-percent valve yields a nonlinear flow curve with position-dependent gain, which directly affects how you design and tune the control loop. The other options don’t reflect this difference—valve material doesn’t fix the flow characteristic, and the two valves do not provide identical characteristics.