Valve duty
Water 30 °C ≈ 0.042
Awaiting Valid Duty Parameters
Adjust the parameters on the left to compute live hydraulic values, ranges, and sizing verdicts.
Valve hunting or starving your line?
Wrong Cv means unstable control loops and worn trim. Our engineers size the valve and the pump together for dosing and transfer lines.
Control valve sizing explained
What Cv means
Cv is the valve’s flow capacity: the US gallons per minute of water at 60 °F that pass with 1 psi pressure drop. In metric terms, Cv = 1.156·Q(m³/h)·√(SG/ΔP bar). The valve is chosen so its Cv at the required opening sits in the controllable range — typically 20–80 % open at design flow.
The calculator returns the required Cv; compare it with the valve manufacturer’s table and pick the trim whose Cv at 60–70 % opening matches.
Choked flow and cavitation
A liquid valve chokes when the pressure drop approaches half the difference between inlet pressure and vapour pressure: ΔP_max ≈ 0.5·(P1 − Pv). Beyond that, flow no longer follows the opening, and flashing or cavitation destroys the trim. The calculator flags choked and near-choked conditions so the system is fixed, not just the valve.
Fixes: increase downstream pressure, split the drop across two valves, or choose a low-recovery (anti-cavitation) trim.
Trim selection
Equal-percentage trim suits level and pressure loops, where the same percentage change matters at any flow; linear trim suits flow loops with stable pressure drop. For dosing valves, matching the pump and valve together matters more than either alone — Teflow sizes both for dosing and transfer duties. Call +91 98251 62709 for a joint check.
Technical & Operational FAQs
Q1.What is Cv in valve sizing?
Cv is the flow capacity coefficient: US gallons per minute of water passing with 1 psi drop. Metric: Cv = 1.156 × Q(m³/h) × √(SG/ΔP bar). Select the valve so design flow occurs at 20–80 % opening.
Q2.How do I size a control valve?
Compute Cv at design flow, check the choked-flow limit ΔP_max ≈ 0.5(P1 − Pv), then pick the valve trim whose Cv at 60–70 % opening matches. The calculator does the math and the checks.
Q3.What causes choked flow in a valve?
When the pressure drop approaches half of (inlet pressure − vapour pressure), the liquid flashes and flow stops following the opening. Symptoms: noise, erosion, and flow that does not rise with opening. Re-distribute the drop or use anti-cavitation trim.
Q4.Equal percentage or linear trim?
Equal-percentage trim for level and pressure loops — it gives equal percentage change across the range. Linear trim for flow loops with steady pressure drop. For dosing, equal-percentage with a correctly matched pump is the usual choice.
Q5.Why does my control valve hunt?
Hunting usually means oversizing: the valve operates below 10 % opening where small movements swing the flow. Recompute Cv at the real minimum and design flow, and consider a smaller valve or reduced trim.
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Need a custom hydraulic audit or specific duty curve verification? Call +91 98251 62709 or Request a Detailed Engineering Quote.