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Control Valve Cheat Sheet

Control valve essentials on one page: Cv and Kv sizing for liquids, choked flow, valve characteristics, sizing rules, leakage classes and fail action terms.

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Liquid sizing

Cv (US units)	Cv = Q (US gpm) × √(SG ÷ ΔP (psi))
Kv (metric)	Kv = Q (m³/h) × √(SG ÷ ΔP (bar))
Conversion	Cv = 1.156 × Kv, Kv = 0.865 × Cv
Flow from Cv	Q = Cv × √(ΔP ÷ SG)

Choked flow (IEC 60534)

Maximum useful ΔP	ΔP_max = F_L² × (P₁ − F_F × P_v)
F_F	0.96 − 0.28 × √(P_v ÷ P_c)
Cavitation index	σ = (P₁ − P_v) ÷ (P₁ − P₂)

Flow does not increase once ΔP exceeds ΔP_max; expect cavitation or flashing.

Inherent characteristics

Type	Behaviour	Typical use
Linear	Flow ∝ travel	Level, constant ΔP
Equal %	Each step of travel changes flow by the same %	Pressure, flow, varying ΔP
Quick opening	Most flow in first travel	On/off, relief

Sizing rules of thumb

Maximum flow at about 70 to 85 % open; minimum flow above about 10 %
Valve authority: valve ΔP at least 25 to 50 % of system ΔP
Body often one size smaller than the line; include reducer losses in sizing
Check noise (85 dBA at 1 m is a common limit), cavitation and outlet velocity

Seat leakage (ANSI/FCI 70-2)

Class	Maximum leakage
II	0.5 % of rated capacity
III	0.1 % of rated capacity
IV	0.01 % of rated capacity
V	5 × 10⁻¹² m³/s water per mm port diameter per bar ΔP
VI	Bubbles per minute by size, for example 1 at 1", 3 at 2", 11 at 4"

Fail action terms

Term	Meaning
ATO	Air to open = fail close (FC)
ATC	Air to close = fail open (FO)
FL / FAI	Fail last position / as is
Direct acting positioner	Increasing signal increases output pressure

Rangeability

Definition	R = Cv_{max, controllable} ÷ Cv_{min, controllable}
Typical	Globe 30:1 to 50:1, ball (V port) 100:1 or more

Common problems

Symptom	Check
Hunting	Oversized valve, tight packing, tuning
Cannot reach set point	Undersized valve, low supply air
Noise, vibration	Cavitation or high gas velocity
Seat leakage	Damaged trim, wrong bench set

Related Calculators and Tools

Liquid Cv CalculatorFree on instrumentationblog.in Gas Valve Sizing CalculatorFree on instrumentationblog.in Valve Leakage Class CalculatorFree on instrumentationblog.in Flow Coefficient ConverterFree on instrumentationblog.in

Frequently Asked Questions

What is the difference between Cv and Kv?

Cv is US gallons per minute of water at 1 psi drop; Kv is m³/h at 1 bar drop. Cv = 1.156 Kv.

What leakage class is tight shut off?

Class VI is often called bubble tight; class V is also used for tight metal seated valves.

Is ATO the same as fail close?

Yes. An air to open valve closes on loss of air.

Learn more on the blog

Control valve flow coefficient (Cv and Kv)

A practical guide on instrumentationblog.in that explains the theory behind this cheat sheet.

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