For a clean, single-phase liquid in turbulent, non-choked flow, a preliminary metric capacity estimate is Kv = Q × √(SG / ΔP), with Q in m³/h and ΔP in bar. This simplified equation estimates a required coefficient; it does not select a valve diameter.
Keep the units and assumptions visible
Specific gravity is the liquid density relative to water at the stated reference basis. ΔP is the pressure difference across the valve at the operating point, not the entire pump discharge pressure. The simple expression assumes that corrections for attached fittings, viscosity and choking are not material. Gas, steam and many slurry duties require a different treatment.
Worked example
Suppose a qualifying water-like liquid requires 25 m³/h at a valve pressure drop of 1 bar and SG = 1. The required Kv is 25 × √(1/1) = 25. Using Kv ≈ 0.865 Cv gives Cv ≈ 28.90. If the same flow has only 0.25 bar available across the valve, the required Kv becomes 50. The lower pressure drop requires more capacity, even though the flow and pipe size have not changed.
Why the example does not give a DN
Rated capacity corresponds to a particular valve and trim at a stated opening. Choosing a valve whose full-open coefficient merely equals the calculated normal requirement leaves no information about its minimum-flow resolution or maximum-load reserve. Compare all operating coefficients to the manufacturer’s curve and establish useful travel at each case. There is no universal opening percentage that proves the selection is correct.
When the simple result must be replaced
Check liquid vapor pressure, upstream absolute pressure and the selected valve’s pressure-recovery factors. If choking or damaging cavitation is possible, the simple pressure-drop equation can overstate useful capacity. Significant viscosity, non-Newtonian behavior or entrained gas also changes the analysis. Reducers and other attached fittings may need correction. Obtain a sizing calculation for the exact fluid and configuration before ordering.
Turn a calculation into an RFQ
Provide minimum, normal and maximum Q, P1, P2 and temperature as linked operating cases rather than independent maxima. Include density, viscosity, vapor pressure, solids and the required shut-off differential pressure. The supplier can then evaluate capacity, severity, materials and actuator torque together. Keep the final selected coefficient and the underlying assumptions on the approved datasheet.
Practical checklist
- Use the pressure drop across the valve.
- Keep Q, P1 and P2 together for each operating case.
- Convert Cv and Kv before comparing offers.
- Confirm the selected trim curve and pressure-recovery review.
Discuss your application
Send the process conditions and required valve function for a configuration review. Request a technical quotation ↗
Worked example: three operating cases
For an illustrative clean, non-viscous, non-flashing liquid with SG = 1, use Kv = Q / √Δp, where Q is in m³/h and Δp is in bar. These are invented teaching cases, not test results for a VPORTA valve.
| Case | Q, m³/h | Valve Δp, bar | Required Kv | Approx. Cv |
|---|---|---|---|---|
| Minimum | 10 | 4 | 5.00 | 5.78 |
| Normal | 35 | 2 | 24.75 | 28.61 |
| Maximum | 50 | 1 | 50.00 | 57.80 |
The required coefficient ratio is 10:1 although the flow ratio is only 5:1. The available differential pressure changes across the operating cases. This is why process flow turndown cannot simply be copied into a valve rangeability requirement. Here Cv = Kv / 0.865.
What the example does not tell you
The table does not select a valve diameter or predict travel. For each candidate size and trim, obtain the actual coefficient-versus-travel curve. Locate the operating cases on that curve and assess useful resolution, proximity to seat contact and available margin. Avoid adding arbitrary capacity allowances: oversizing can move normal operation into a small, sensitive opening region.
Nor does the simplified equation establish cavitation safety, noise, choked capacity or service life. Evaluate absolute inlet pressure, liquid vapor pressure, pressure recovery and the selected valve’s sizing factors using the applicable engineering method. Gas and steam require their own equations; viscous, flashing or multiphase fluids require additional analysis.
Information that prevents a misleading calculation
- Label pressure as absolute or gauge, and make inlet and outlet values consistent.
- State whether flow is actual volume, standard volume or mass flow.
- Use the fluid density and viscosity at operating temperature, not automatically those of water.
- Separate maximum closed-valve differential pressure from normal flowing pressure drop.
- Identify reducers, nearby fittings, pump behavior and abnormal operating sequences.
Use the website sizing tools for the stated clean-liquid screening calculation, then request a reviewed sizing sheet for the selected assembly. The RFQ worksheet provides a structured starting point.