What is control valve authority and why is it important in boiler feedwater control?

• A. Authority is the valve’s capacity to influence the process variable across its range; insufficient authority reduces control accuracy and responsiveness
• B. Authority is the owner's permission for operation
• C. Authority equals the valve's physical size
• D. Authority is the duration the valve remains open

Answer: (C)

Authority is the valve’s capacity to influence the process variable across its operating range. In boiler feedwater control, this means how much the feedwater flow can be changed by the controller output to move the drum level toward the setpoint. When the valve has good authority, a commanded change moves the drum level reliably and quickly, giving accurate control and quick rejection of disturbances. If authority is insufficient, the valve can only affect a small portion of the required flow for a given controller signal, so the system responds sluggishly, shows larger offsets, and may become harder to stabilize.

This isn’t about who is allowed to operate, nor about the valve’s physical size, nor about how long the valve stays open. Those factors don’t define how much influence the valve can exert on the process variable across its range.