BIZOMAT 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). |
What each input means
Open an input to see what it is, its units, the worked example and the common mistakes, with a small diagram.
What this means: Design flow
- What it is
- The liquid flow the control valve must pass at the design operating point.
- Units and format
- Enter a number in m³/h: cubic metres per hour. The form accepts 0.000001 m³/h to 100000 m³/h. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 10 m³/h as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The Kv needed rises in direct proportion.
What this means: Valve pressure drop at design flow
- What it is
- The pressure difference across the control valve when it passes the design flow, as allocated in the circuit design.
- Units and format
- Enter a number in bar: bar (1 bar = 100 kPa). The form accepts 0.000001 bar to 100 bar. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.5 bar as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The Kv needed falls with the square root of this drop, and a larger valve drop raises authority.
What this means: Rest of the circuit pressure drop at design flow
- What it is
- The pressure drop across everything else in the controlled circuit at design flow: pipes, fittings, coils and heat exchangers.
- Units and format
- Enter a number in bar: bar (1 bar = 100 kPa). The form accepts 0 bar to 100 bar. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.5 bar as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A larger circuit drop lowers the valve authority.
- Get it right
- Leave the valve drop out of this figure; authority is valve drop ÷ (valve drop + this drop).
What this means: Specific gravity
- What it is
- The density of the liquid relative to water at the same reference conditions.
- Units and format
- Enter a plain number with no unit. The form accepts 0.3 to 3. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The Kv needed rises with the square root of specific gravity.
- Get it right
- A ratio with no unit, not a density in kg/m³.
What this means: Chosen valve rated Kvs
- What it is
- The flow coefficient of the valve you have chosen when fully open, from the valve catalogue: the water flow in m³/h it passes at one bar pressure drop.
- Units and format
- Enter a number in m³/h: cubic metres per hour. The form accepts 0.000001 m³/h to 100000 m³/h. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 16 m³/h as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
What this means: Largest opening ratio Kv ÷ Kvs at design
- What it is
- The largest design Kv as a fraction of the valve’s Kvs that you accept, leaving reserve opening above the design point.
- Units and format
- Enter a plain number with no unit. The form accepts 0.3 to 1. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.9 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Type it as a fraction of 1, not a percentage; the results show it as a percentage.
What this means: Smallest opening ratio at design
- What it is
- The smallest design Kv as a fraction of Kvs that you accept; below it the valve is oversized and controls on only a small part of its stroke.
- Units and format
- Enter a plain number with no unit. The form accepts 0.05 to 0.9. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.3 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Type it as a fraction of 1, not a percentage, and keep it below the largest opening ratio.
What this means: Least valve authority
- What it is
- The lowest valve authority you accept: valve drop divided by the sum of valve and circuit drops at design flow. Higher authority gives steadier control.
- Units and format
- Enter a plain number with no unit. The form accepts 0.1 to 0.9. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.5 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
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