Example data table
| Case | Service | Inputs | Output |
|---|---|---|---|
| 1 | Liquid (US) | Q=120 gpm, P1=80 psi, P2=60 psi, SG=1.0 | Required Cv ≈ 26.83, rated Cv ≈ 33.54 |
| 2 | Liquid (Metric) | Q=50 m³/h, P1=3.5 bar, P2=2.0 bar, SG=0.85 | Kv ≈ 37.64, Cv ≈ 43.51 |
| 3 | Gas / Vapor | Q=500 SCFM, P1=50 psia, P2=40 psia, T=70°F, Gg=0.6, Z=1.0 | Required Cv ≈ 0.324 (preliminary) |
Formula used
Liquid (US): Cv = Q × √(Gf / ΔP) ÷ (Fp × FR)
Q in gpm, ΔP in psi, Gf is specific gravity relative to water.
Liquid (Metric): Kv = Q × √(Gf / ΔP) ÷ (Fp × FR), then Cv ≈ 1.156 × Kv
Q in m³/h, ΔP in bar.
Vapor / Gas (SCFM): Cv = Q × √(Gg × T × Z) ÷ (1360 × Fp × P1 × Y × √X)
T in °R (°F + 460), P1 in psia, X = (P1 − P2)/P1, FK = k/1.4, Y = 1 − X/(3 × FK × Xt).
If X exceeds Xt, choked flow is indicated and X is limited to Xt for sizing.
How to use this calculator
- Select the service type that matches your application.
- Enter flow rate and inlet/outlet pressures for your line.
- Provide specific gravity (liquid) or gas properties (vapor).
- Keep Fp and FR at 1.0 unless piping effects are known.
- Click Calculate to view required Cv and a rated Cv target.
- Download CSV or PDF to attach to design submittals.
Sizing inputs that drive Cv
Start with a design case, not an average. Use maximum required flow, minimum upstream pressure, and the highest credible downstream backpressure. Typical preliminary data includes line size, fluid temperature, density or specific gravity, viscosity range, and vapor pressure. For gas, capture k, compressibility, and allowable noise. Document units and base conditions to prevent later rework.
Managing pressure drop and recovery
Compute ΔP as P1 minus P2 at the valve location, then confirm the system can supply that drop without starving equipment. A rule of thumb is keeping the control valve at 20–40% of total system drop to maintain authority. High recovery trims can increase local velocity and reduce effective margin, so compare inlet and outlet piping losses before locking the case.
Selecting rated Cv and travel window
After the required Cv is calculated, choose a rated Cv that places normal operation between 30% and 70% travel for good controllability. Oversizing often causes hunting at low openings, while undersizing can limit throughput during upset conditions. If multiple operating points exist, check a minimum, normal, and maximum case and choose a trim that covers all three.
Avoiding cavitation, flashing, and choking
For liquids, compare downstream pressure to vapor pressure and watch for cavitation risk when recovery is high. If P2 can fall near vapor pressure, select anti‑cavitation trim or stage the drop. For gases, monitor the pressure ratio factor and choked flow indication; once choked, increasing ΔP will not increase flow, so either increase valve capacity or reduce required drop.
Turning results into submittal-ready notes
Use the CSV and PDF exports to capture the design basis: service type, flow, pressures, temperature, correction factors, and the selected rated Cv. Add a brief narrative stating assumptions, such as Fp and FR set to 1.0, and flag any red conditions like choked flow. This makes vendor review faster and keeps procurement aligned with process intent. Include actuator fail position and leakage class when known for completeness.
What does the calculator output represent?
It estimates the required flow coefficient and suggests a rated Cv with a sizing margin, based on your selected service and correction factors.
Should I size for normal flow or maximum flow?
Use the worst credible design case: maximum required flow with minimum upstream pressure. Then check normal and minimum cases to avoid oversizing and poor control.
Why are Fp and FR included?
Fp accounts for piping geometry effects, while FR adjusts for liquid Reynolds number at low viscosity or small openings. If you do not have test data, keeping both at 1.0 is a common preliminary approach.
What does “choked flow” mean for gases?
It indicates the flow has reached a sonic or limiting condition inside the valve. Beyond that point, increasing pressure drop does not increase flow, so capacity must come from a larger valve or different trim.
How do I choose a rated Cv from the result?
Select a rated Cv that places typical operation around mid-travel and still covers the maximum case. Many designs target 30–70% travel to maintain stable control and provide reserve.
Can I use the result for procurement?
Use it as a preliminary sizing note. Final selection should consider noise, cavitation, leakage class, actuator sizing, and vendor-specific trim data before ordering.