Why is Valve Authority Important?
Authority is used when deciding on control valve size and will help to understand how well the selected control valve is going to perform in the HVAC system. Proper valve sizing will provide high authority and consequently better flow control at any load.
If the control valve is oversized, the pressure drop across the control valve is reduced. For an un-changed maximum differential pressure drop, the authority and the pressure drop across the valve will be low. The valve will need to operate near its closed position to provide any form of acceptable control. In fact, this will result in on/off similar control and room temperature fluctuations and increased wear on the actuator.
On the other hand, if the control valve is under-sized, the pressure drop across the control valve is increased. If maximum differential pressure drop remains unchanged, the authority and the pressure drop across the valve will be high, control will be good, but the energy consumption to transfer the required flow rate through the undersized valve will be high and consequently more costly.
Clearly, a trade off exists between the consideration of control and energy consumption and control valve sizing requires a balance between the two dynamics.