Toolbox / Valve Cv / Kv
Valve Flow Coefficient (Cv / Kv) & Sizing Calculator
Enter the flow, pressures and fluid and get the required Cv and Kv, a check for choked flow, cavitation and flashing, a suggested size for each valve type and a valve line you can paste into an enquiry. Everything is calculated in your browser.
The simple equations from a manufacturer bulletin: liquids and gases, no choked-flow check for liquids.
Service
Conditions
Result
Kv = 0.865 × Cv, so Cv = 1.1561 × Kv. verified Fisher Catalog 12 and Valmet print 0.865.
| P1 (absolute) | 100 | psi |
|---|---|---|
| P2 (absolute) | 50 | psi |
| Pressure drop ΔP | 50 | psi |
| ΔP / P1 | 0.5 | |
| P2 / P1 (critical at ≤ 0.5) | 0.5 |
- Gas flow is at standard conditions (60 °F, 14.7 psia), the basis of the published constants.
- P2 is half of P1 or lower: critical flow, so the flow no longer depends on P2.
Valve line for my enquiry
VALVE LINE FOR ENQUIRY Valve type: Globe (control) Size: NPS ____ (see the sizing table; no sized row for this type) Required Cv: 0.2034 (Kv 0.1759) at design flow; rated Cv of the offered valve to be stated by the maker Fluid: Gas (custom), SG 0.907 Flow: 10 SCFM P1 (inlet): 100 psia P2 (outlet): 50 psia dP: 50 psi Temperature: 60 °F Flow regime from the calculation: Critical gas flow (choked) End connection: ____________ (flanged ASME B16.5 class ___ / butt-weld / socket-weld / threaded) Body / trim material: ____________ Seat: ____________ Actuation: ____________ Shut-off class: ____________ Note: estimate for specification only. The valve maker confirms size and rated Cv for the selected model.
Fill the end connection, material and actuation before you send. Rated Cv differs by maker and model: the maker confirms the size.
These are estimates for specification. The final valve selection, size and rated Cv are confirmed by the valve maker for the model they offer.
Suggested valve size
Required Cv 0.2034 (Kv 0.1759). Each type shows the share of rated Cv that the service would use. Rated Cv is for the fully open valve of the one maker and model named in each option and differs by maker and model: pick the model closest to the one you want to buy.
Valmet / Neles GU-STD-LIN: Series GU single-seat globe, unbalanced standard trim, linear (Nelprof code GU-STD-LIN), table p. 26. single source Rule: required Cv between 20 % and 80 % of rated Cv.
| Size | Rated Cv | Rated Kv | Share of rated | Approx. opening | Verdict |
|---|---|---|---|---|---|
| NPS 0.5 (DN 15) | 7 | 6.055 | 2.91 % | below 10 % | too large |
| NPS 0.75 (DN 20) | 9 | 7.785 | 2.26 % | below 10 % | too large |
| NPS 1 (DN 25) | 13.5 | 11.68 | 1.51 % | below 10 % | too large |
| NPS 1.5 (DN 40) | 28 | 24.22 | 0.726 % | below 10 % | too large |
| NPS 2 (DN 50) | 49 | 42.38 | 0.415 % | below 10 % | too large |
| NPS 3 (DN 80) | 100 | 86.5 | 0.203 % | below 10 % | too large |
| NPS 4 (DN 100) | 190 | 164.3 | 0.107 % | below 10 % | too large |
| NPS 6 (DN 150) | 295 | 255.2 | 0.0689 % | below 10 % | too large |
No listed size fits. Sizes up to DN 600 are stored; for a very small Cv ask the maker about reduced-trim valves.
Click a row to use that size in the enquiry line and the velocity check.
Line velocity
Pick a line size to see the velocity.
Other makers’ typical factors, for context
- Warren Controls table 3: trims 1840, 1843, 2820, 2920, 2922, 2923, 5840, 5843 print FL 0.81, xT 0.72; trim 3800 flow-to-open averages FL 0.74, xT 0.63, flow-to-close FL 0.48, xT 0.42. warren
- Fisher Catalog 12 sample problems: an ED valve with linear cage is called representative at FL 0.84 (NPS 3 at maximum travel FL 0.82; NPS 4 xT 0.688); a V250 NPS 8 valve has xT 0.137 at rated travel and 0.525 at 83 degrees. emerson
- Warren Controls: a rated FL "may vary from .99 for a special multiple stage trim to .30 for a ball valve". warren
Where the numbers come from
- Equations verified Swagelok bulletin MS-06-84 (N1 = 1.0, N2 = 22.67, critical factor 0.471); reproduces its worked check. The Swagelok online calculator was not used.
- Kv = 0.865 × Cv verified Fisher Catalog 12 and Valmet.
- Rated Cv by size single source Each maker and model is its own list (Valmet / Neles 2022; Fisher Catalog 12, April 2022), fully open. Every list comes from one publisher, so none is verified. Rated Cv differs by maker, model and edition. Gate valves: not held.
- Line velocity bands verified Engineering ToolBox and Powder & Bulk Solids Processing; typical pipe values, not erosion limits.
- Sizing rule your value 20 to 80 % of rated Cv for control valves and a 1.25 margin for on/off valves are design defaults of this tool, not published constants. Edit them above.
Not held, and not guessed: Fd and incipient-cavitation factors, vapour pressure for liquids other than water, superheated steam density, gate valve coefficients, other makers’ rated Cv. Full source log with deep links: src/lib/toolbox/valve-cv/SOURCES.md.
Frequently asked questions
How do you convert Cv to Kv?
Kv = 0.865 × Cv, and Cv = 1.156 × Kv. Cv is the flow of water in US gallons per minute at a 1 psi pressure drop; Kv is the flow of water in cubic metres per hour at a 1 bar pressure drop.
How is valve Cv calculated for a liquid?
In the simple form, Cv = Q × √(SG / ΔP), with Q in US gpm, ΔP in psi and SG the specific gravity. For a control valve the IEC 60534 method also checks whether the flow chokes. Water at 100 gpm and 25 psi drop gives Cv 20.
What is choked flow, cavitation and flashing?
Choked flow is when more pressure drop no longer increases the flow. In a liquid, cavitation is vapour bubbles forming and collapsing inside the valve; flashing is when the outlet pressure stays below the vapour pressure so the liquid leaves partly as vapour. Both damage valves and need a different valve or trim.
How do I pick the valve size from Cv?
Choose a valve whose rated Cv puts the required Cv in its working range: the tool uses 20 to 80 % of rated Cv for control valves and at least 1.25 times the required Cv for on-off valves, both editable. Rated Cv differs by maker and model.
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