Sizing & Selection
Valve Authority
Valve authority (pressure-drop ratio) is the share of the total circuit pressure drop that the valve takes, usually written N = ΔPvalve(full open) / ΔPtotal circuit. It is the single number that tells you how faithfully the loop will see the valve's inherent characteristic.
At N ≥ 0.7 the valve dominates the circuit: installed ≈ inherent, and a linear valve controls as linear. At N ≤ 0.3 the rest of the circuit dominates: as flow rises the valve's own ΔP collapses, its installed curve bends strongly toward quick-opening, and gain varies wildly across the stroke. The engineering habit: compute N before choosing a characteristic; if N is low, specify equal percentage (or a higher-drop valve design) rather than fighting the controller.
Authority also interacts with energy economics — gaining authority by deliberately throttling away pressure costs pumping power, so in large systems the designer balances control quality against efficiency (variable-speed pumps change this calculus). The installed-vs-inherent distortion this entry describes is explained with curves in flow characteristics compared.