Control Valve Authority (Beta)

Authority (beta) = the valve's pressure drop fully OPEN / the total drop across the VARIABLE branch it modulates (valve + coil + variable piping). A 5 psi valve in series with a 3 psi coil is beta = 5/8 = 0.625. A valve only realizes its inherent (equal-percentage/linear) characteristic if it keeps most of the branch drop; a LOW-authority (oversized) valve dropping little while a fixed coil drops most distorts the installed characteristic into an on/off curve that hunts. Target beta >= 0.5 (near-linear), 0.25-0.5 tolerable, < 0.25 poor -- the fix is a SMALLER (lower-Cv) valve, which is why control valves are undersized vs line size. Separate from Cv flow-capacity sizing. A design/commissioning check; the design pressures, valve trim, and engineer/balancer govern.

valve_authority (beta) = valve_pressure_drop_psi / (valve_pressure_drop_psi + controlled_circuit_drop_psi). Target beta >= 0.5; 0.25-0.5 tolerable; < 0.25 poor.

Control valve authority (beta) for the installed valve characteristic (ASHRAE Systems; control-valve manufacturer guidance, Belimo / Honeywell), by name; the design pressures, valve trim, and the engineer / balancer govern.

The authority ratio is public control-engineering practice; the valve open pressure drop and the controlled-circuit drop are the design pressure values.

Estimate. AHJ and licensed professional govern.

Field names used by the API: valve_pressure_drop_psi, controlled_circuit_drop_psi, valve_authority

• Authority = beta beta = valve open drop / (valve open drop + controlled-circuit drop); target >= 0.5 for a near-linear installed characteristiccontrol-valve practice (Belimo / ASHRAE)
• Undersize for authority low authority (oversized valve) distorts the installed characteristic and hunts; fix with a smaller (lower-Cv) valve; separate from Cv flow sizing (valve-flow-coefficient)control-valve selection practice