KEVOS® MAKEOS · Hydraulics
Control Valve Sizing (Kv, Opening, Authority, Full-Open Flow)
What Kv does this control valve need, how far open will the chosen valve run, does it have enough authority over the system, and what flow does it pass wide open?
Sizing of a liquid control valve: required Kv (and Cv) for the design flow and valve pressure drop (as the valve flow coefficient tool), the opening ratio of the chosen valve’s rated Kvs, the valve authority as its share of the total circuit pressure drop, and the flow the circuit passes with the valve wide open when the rest of the system’s loss rises with flow squared, against your limits.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Design flow | m³/h | Flow the valve must pass at design conditions. |
| Valve pressure drop at design flow | bar | Pressure drop allocated to the valve at design flow. |
| Rest of the circuit pressure drop at design flow | bar | Pressure drop of everything else in the controlled circuit at design flow (pipes, coil, exchanger). |
| Specific gravity | Specific gravity of the liquid (1 for water). | |
| Chosen valve rated Kvs | m³/h | Rated Kv of the valve fully open (from the catalogue). |
| Largest opening ratio Kv ÷ Kvs at design | Largest share of Kvs you want at design, leaving margin (your value). | |
| Smallest opening ratio at design | Smallest share of Kvs at design, to keep good control (your value). | |
| Least valve authority | Least authority for good control (often 0.5). |
Method and formulas
- Kv = Q √(sg ÷ Δp_v); Cv = 1.156 Kv (as the valve flow coefficient tool).
- Opening ratio = Kv ÷ Kvs; authority = Δp_v ÷ (Δp_v + Δp_s).
- With a constant total drop Δp_t = Δp_v + Δp_s and system loss ∝ Q²: Q_open = √{Δp_t ÷ [sg ÷ Kvs² + Δp_s ÷ Q²]}.
- Example: 10 m³/h at 0.5 bar needs Kv 14.142; a Kvs 16 valve runs at 88.388% with authority 0.5, passing 10.596 m³/h wide open.
Limitations
- Non-cavitating, non-flashing liquid; check choked flow at high pressure drops.
- Assumes a constant pump differential across the circuit.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Choose a control valve size for a heating or cooling loop
- Check an existing valve’s authority
- Find the maximum flow a valve can pass
Who it is for
- Process engineers
- HVAC engineers
- Hydraulic engineers
What you get
- Kv and Cv needed
- Opening ratio and authority
- Flow with the valve fully open
Interpreting the result
- A valve with low authority hands control to the rest of the circuit: increase its pressure drop or the pipe size elsewhere.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 3024–3027. Topic reference; no source tables or recommended settings are reproduced.
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Open the Control Valve Sizing (Kv, Opening, Authority, Full-Open Flow) calculator
