Control valve authority is the share of the pressure drop in a defined circuit that falls across the fully open control valve at a stated flow condition. It belongs to the valve and system together, not to the valve catalogue alone. Changing the selected Cv or Kv, design flow, circuit resistance or calculation boundary changes the result.
A low value means the valve accounts for only a small part of the total circuit resistance, so its installed flow response can differ substantially from its inherent characteristic. Raising the authority gives the valve more influence over that response, but requires more available pressure drop and may increase energy loss or service risk. The calculation therefore needs a stated numerator, denominator, flow condition and circuit boundary before any target value can be judged. This scope covers conventional pressure-dependent modulating valves; three-way arrangements and pressure-independent control valves require separate treatment.
What control valve authority means
Under the common hydronic convention, control valve authority compares the pressure drop across a fully open control valve with the total pressure drop of the defined circuit at the same design flow.
The common authority formula
N = ΔPv / (ΔPv + ΔPc)
In this formula, N is the dimensionless valve authority. ΔPv is the pressure drop across the fully open control valve at the stated design flow. ΔPc is the combined pressure loss through the rest of the defined circuit, excluding the valve, at that same flow.
The denominator represents the total pressure drop across the valve and the remaining circuit. Both pressure terms must use the same unit and operating point. Combining the valve pressure drop at design flow with circuit losses measured at another flow produces an invalid ratio because component pressure losses change with flow.
Why the calculation boundary matters
The circuit boundary determines which pipework, fittings, heat exchangers, coils or other resistance elements belong in ΔPc. The boundary should enclose the part of the system whose flow the valve is expected to regulate. Authority values calculated with different boundaries cannot be compared directly.
Valve authority terminology is not internationally standardised. Process-control references may use DPm for a related pressure-drop share and define it at maximum process flow rather than by the hydronic convention above. Valmet’s explanation of control valve authority discusses this difference in terminology. A reported authority value should therefore identify the formula, flow condition and circuit boundary used.
Three-way circuits require boundaries that reflect their mixing or diverting paths. Pressure-independent control valves also change the hydraulic relationship through their pressure-regulating element. The two-way pressure-dependent formula should not be transferred to either arrangement without redefining the calculation.
How to calculate control valve authority
Begin with the pressure loss through the remaining circuit and the authority selected for the design point. The following hypothetical example uses incompressible water and assumes a relative density of approximately 1.0.
Worked water example from circuit loss to valve Kv
The design flow is 10 m3/h, the remaining circuit pressure loss is 60 kPa, and the example target authority is 0.40. This target is an input to demonstrate the calculation, not a universal recommendation.
- Rearrange the authority formula to find the required pressure drop across the fully open valve:
ΔPv = [N / (1 – N)] × ΔPc
ΔPv = [0.40 / (1 – 0.40)] × 60 kPa = 40 kPa
- Convert 40 kPa to 0.40 bar and calculate the preliminary Kv for water:
Kv = Q / √ΔP
Kv = 10 / √0.40 = 15.8 m3/h
The calculated Kv of 15.8 is the required flow coefficient at the stated water flow and valve pressure drop. An actual valve must be selected from the manufacturer’s available Kvs values, where Kvs is the rated Kv with the valve fully open.
Recalculate authority after choosing an available Kvs
Assume the nearest suitable catalogue value is Kvs 16. The actual valve pressure drop at 10 m3/h is then recalculated rather than left at the original 40 kPa assumption.
ΔPv = (Q / Kvs)2
ΔPv = (10 / 16)2 = 0.3906 bar = 39.1 kPa
N = 39.1 / (39.1 + 60) = 0.394
The available Kvs changes the authority from the assumed 0.40 to approximately 0.394. A larger Kvs would reduce the valve pressure drop and authority at the same flow, while a smaller Kvs would increase both.
This water-only calculation is preliminary. It does not check viscosity corrections, cavitation, choking, piping reducers or valve-specific pressure-recovery factors. Gas and steam require compressible-flow equations rather than this Kv shortcut. The MacoTango Cv calculator separates liquid, gas and steam cases for the next sizing check.
What low authority changes in the installed valve response
As a pressure-dependent control valve closes, its flow resistance rises and a larger share of the circuit pressure drop falls across the valve. The pressure-drop share therefore changes throughout the stroke, while the reported authority remains the design-point value calculated with the valve fully open.
An inherent flow characteristic relates valve travel to relative Cv or Kv while the pressure drop across the valve remains constant. An installed valve does not operate under that test condition. Its flow depends on both the available opening and the pressure drop that the system places across the valve.
With low authority, the rest of the circuit accounts for most of the pressure loss when the valve is fully open. The pressure-drop share shifts towards the valve as it throttles, causing the installed control valve characteristic to depart from the inherent characteristic. The amount and direction of the distortion depend on the selected characteristic and the system resistance curve.
This redistribution also changes installed gain, which is the flow change produced by a given change in valve travel. One part of the stroke may produce only a small flow response, while another part may respond much more sharply. The controller must then work with a gain that varies across the operating range.
Valve oversizing can intensify the problem. A valve with excessive Kvs produces little pressure drop at the required flow and may operate close to the seat during normal duty. In that region, trim characteristic, actuator resolution, positioner performance, friction and backlash can have more influence on each small movement. Low authority and oversizing often appear together, but one does not prove the other.
Low authority alone does not prove that a control loop will hunt. Poor tuning, process delay, stem friction, an unsuitable inherent characteristic, actuator or positioner behaviour and changing system pressure can produce similar symptoms. Treat the authority result as a reason to examine the installed curve and operating travel, not as a complete instability diagnosis.
Why 0.5 is not a universal target
At an authority of 0.5, the fully open valve and the remaining circuit each account for half of the defined total pressure drop at the design flow. This relationship can be a useful sizing reference, but it does not make 0.5 a universal acceptance limit.
Hydronic water-system guidance
In a pressure-dependent water circuit, increasing authority gives the valve a larger share of the available pressure drop. This generally reduces installed-characteristic distortion and gives the valve more influence over flow. The same decision also requires more pump head to overcome the valve pressure loss.
A lower authority reduces the design pressure drop assigned to the valve, but the system resistance then has greater influence on the installed response. The selected value is therefore a compromise between valve control influence, pump energy, available differential pressure and usable valve travel.
The required compromise changes with the pump curve, circuit resistance, design flow and expected part-load conditions. A value that works in one branch may be unsuitable after a pump change, circuit extension or balancing adjustment. The authority should be recalculated when these hydraulic conditions change.
Process liquid, gas and steam systems
Process-control sizing may use DPm as the valve’s share of the total dynamic pressure loss at maximum process flow. A preliminary process-liquid model may begin with a value such as 0.30, then revise it after the selected valve’s Cv or Kv curve, travel and installed gain have been checked. That starting value is an engineering assumption, not a pass or fail requirement.
Gas and steam services add density change, pressure-ratio effects and possible choked flow. A pressure-drop share can still help describe the system, but the water-system Kv calculation and a target of 0.5 cannot be transferred directly. Compressible-flow equations and valve-specific data such as xT are required.
Higher authority is not automatically better. Assigning more pressure drop to the valve can increase pumping or compression demand. In severe liquid service it may also increase cavitation risk, while gas or steam service may face choking, noise or high outlet velocity. The suitable value is the one that produces acceptable installed response without exceeding the energy and service limits of the actual operating cases.
Use authority before, not instead of, full control valve sizing
Control valve authority cannot confirm capacity, usable travel or service margin. It describes how pressure drop is allocated at one declared operating point. The selected valve must still pass the sizing and installed-performance checks for the complete operating range.
For a process system, establish the available valve pressure drop from at least two operating conditions. An alternative is to model maximum flow with a clearly stated DPm assumption and system boundary. Each flow rate must remain paired with its corresponding inlet and outlet pressures; combining flow from one case with pressures from another creates a false sizing basis.
- Define minimum, normal and maximum flow with the matching P1, P2, temperature and fluid state for each case.
- Allocate the valve pressure drop at the declared design point, then calculate the required Cv or Kv using the equation for the actual fluid phase.
- Select an available valve and trim, then use the manufacturer’s Cv or Kv curve to determine expected travel at every operating case. Recalculate the authority or DPm with the selected rated coefficient rather than the preliminary value.
- Review the installed characteristic and gain, then check valve-specific factors such as FL for liquid service, xT for gas or steam, and piping geometry where reducers or nearby fittings affect the calculation.
The maximum-flow case checks whether the valve can deliver the required capacity without approaching an unacceptable flow limit. Normal flow shows where the valve will spend most of its operating time, while minimum flow tests whether the selected trim retains useful travel and controllable gain near the low end.
A high authority value cannot compensate for an unsuitable inherent characteristic, excessive rated Cv, poor travel distribution or a severe-service limit. The control valve sizing guide provides the broader calculation path for checking capacity, travel and service risks across the operating cases.
A project-specific authority result is credible only when calculated at a declared flow point and circuit boundary, used to select a preliminary Cv or Kv, and followed by verification of the chosen valve’s travel, installed response and service limits across the actual operating cases.