Calculation sequence
Step-by-step method
- Add valve pressure drop to the remaining controlled-circuit pressure drop.
- Divide valve pressure drop by the total.
- Compare calculated authority with the user-entered minimum criterion.
MechanicalIntermediateHydronics
Valve Authority Calculator
Calculate control-valve authority from valve pressure drop and the pressure drop of the remaining controlled circuit.
Why it matters: Low valve authority can reduce controllability even when the valve has sufficient nominal flow capacity.
Calculated result
Engineering evaluation
Calculated valve authority (a)0.333 ratio
- Valve share of controlled-circuit drop (100a)
- 33.33 %
- Total controlled-circuit drop (ΔPtotal)
- 15 psi
- Valve drop required for entered minimum (ΔPv,req)
- 3.333 psi
Engineering interpretation
Calculated valve authority is 0.3333 (33.33%). The entered minimum is 0.25; at the entered remaining-circuit drop, that criterion corresponds to 3.333 psi across the valve.
Important limitations
- Authority alone does not confirm valve sizing, rangeability, inherent characteristic, actuator authority, minimum differential pressure, or control stability.
Engineering knowledge center
Understand the calculation—not just the answer
Governing relationships
ΔPtotal = ΔPv+ΔPc
a = ΔPv/ΔPtotal
100a = 100×a
ΔPv,req = amin·ΔPc/(1−amin)
Current substitution
a = 5/(5+10) = 0.3333; ΔPv,req = 3.333 psi.
Equation legend
Inputs and calculated quantities
- ΔPv
- Design valve pressure drop — psi or kPa
- ΔPc
- Remaining controlled-circuit drop — psi or kPa
- amin
- Entered minimum authority — ratio
- a
- Calculated valve authority — ratio
- 100a
- Valve share of controlled-circuit drop — %
- ΔPtotal
- Total controlled-circuit drop — psi
- ΔPv,req
- Valve drop required for entered minimum — psi
Applications
How this calculation is used
Building design
Screen control-valve pressure-drop allocation.
Construction
Check installed valve selection and circuit pressure losses.
Commissioning
Evaluate whether measured circuit conditions support stable valve control.
Quality controls
Assumptions and limitations
Assumptions
- Entered values describe the same stable operating condition.
- Units, pressure definitions, fluid properties, and equipment boundaries are applied consistently.
Limitations
- This tool supports preliminary evaluation and field verification; it does not replace manufacturer curves or project-specific engineering analysis.
- Project criteria, applicable standards, measurement uncertainty, and authorized professional judgment remain controlling.
Field use
Verification procedure
- Confirm the approved reference condition, instrument range, and calculation boundary.
- Stabilize the system and record calibrated field measurements.
- Compare the result with current manufacturer data and project requirements before disposition.
Common engineering mistakes
- Mixing measurements from different operating conditions or equipment boundaries.
- Treating a calculated screening status as automatic acceptance.
Technical basis
References and source standards
- ASHRAE Handbook—FundamentalsCurrent handbook chapters covering fluid flow, pressure relationships, system curves, and mechanical-system fundamentals; the licensed edition must be verified during professional review.
Engineering calculation disclaimer
These engineering tools are provided for educational, preliminary evaluation, field verification, and commissioning support. Results depend on the accuracy of user-entered information and the assumptions stated for each calculation. Every colored status and comparison is a screening indicator only; project criteria, contract requirements, applicable codes, manufacturer instructions, measurement uncertainty, and authorized engineering judgment govern. The tools do not replace project-specific engineering analysis or the judgment of a licensed professional engineer.
Read the full use and limitation statement