What is Cv in valve sizing?

Cv (flow coefficient) is the number of US gallons per minute of water at 60°F that will flow through a valve with a 1 psi pressure drop. It quantifies the flow capacity of a valve. A higher Cv means a larger flow capacity. The metric equivalent is Kv (m³/hr of water at 1 bar pressure drop). Conversion: Kv = 0.865 × Cv.

How do you calculate Cv for a liquid?

For incompressible (liquid) flow: Cv = Q × √(SG / ΔP), where Q = flow rate in US GPM, SG = specific gravity of liquid (water = 1.0), ΔP = pressure drop across valve in psi. This is the basic ISA/IEC 60534 sizing equation for non-choked liquid flow.

How do you calculate Cv for gas or steam?

For compressible (gas) flow: Cv = Q / (963 × P1 × Y × √(x × M / (T × Z))), where Q = flow in SCFH, P1 = upstream pressure (psia), Y = expansion factor, x = pressure drop ratio, M = molecular weight, T = absolute temperature (°R), Z = compressibility factor. For steam: Cv = W / (63.3 × Y × √(x × P1 × γ1)), where W = mass flow (lb/hr), γ1 = specific weight (lb/ft³).

What valve size do I need for a given Cv?

General rule: select a valve where the required Cv falls between 60-80% of the valve's rated Cv at full open. Common Cv values by size: DN25 (1") ≈ 30, DN50 (2") ≈ 120, DN80 (3") ≈ 270, DN100 (4") ≈ 480, DN150 (6") ≈ 1100, DN200 (8") ≈ 1900, DN250 (10") ≈ 3000, DN300 (12") ≈ 4500. Actual Cv varies by valve type and manufacturer.

What is the difference between Cv and Kv?

Cv is the US/imperial flow coefficient (US GPM, psi). Kv is the metric flow coefficient (m³/hr, bar). Conversion: Kv = 0.865 × Cv, or Cv = 1.156 × Kv. Both measure the same thing — the flow capacity of a valve — but in different unit systems. ISA uses Cv, IEC uses Kv.

Valve Sizing Calculator

Calculate the required flow coefficient (Cv / Kv) for your valve application. Select liquid, gas, or steam service below. Based on ISA / IEC 60534 sizing equations.

Liquid Valve Sizing (Cv)

Formula: Cv = Q × √(SG / ΔP)
Q = Flow rate (US GPM) | SG = Specific gravity | ΔP = Pressure drop (psi)

Flow Rate (Q)

US Gallons per Minute (GPM)

Specific Gravity (SG)

Water = 1.0

Upstream Pressure (P1)

psig

Downstream Pressure (P2)

psig

Sizing Result

Pressure Drop (ΔP)–

Required Cv–

Required Kv–

Recommended Valve Size–

Flow Velocity (approx.)–

Gas Valve Sizing (Cv)

Formula (subcritical): Cv = Q / (1360 × P1 × sin(3417 × √(x/Fk) / N) × √(M / (T × Z)))
Simplified: Cv = W / (63.3 × √(ΔP × (P1 + P2) / T))
W = mass flow (lb/hr) | T = absolute temp (°R = °F + 460)

Flow Rate (W)

lb/hr (mass flow)

Molecular Weight (M)

Air=29, N₂=28, CH₄=16, CO₂=44

Upstream Pressure (P1)

psia (absolute)

Downstream Pressure (P2)

psia (absolute)

Temperature

°F

Compressibility (Z)

Ideal gas = 1.0

Specific Heat Ratio (k)

Air=1.4, Steam=1.3, CH₄=1.31

Sizing Result

Pressure Drop (ΔP)–

Pressure Drop Ratio (x)–

Flow Regime–

Required Cv–

Required Kv–

Recommended Valve Size–

Steam Valve Sizing (Cv)

Saturated Steam: Cv = W / (46.2 × √(ΔP × P2))
Superheated Steam: Cv = W × (1 + 0.00065 × T_sh) / (46.2 × √(ΔP × P2))
W = steam flow (lb/hr) | T_sh = degrees of superheat (°F)

Steam Flow Rate (W)

lb/hr

Upstream Pressure (P1)

psig

Downstream Pressure (P2)

psig

Steam Type

Degrees of Superheat

°F above saturation

Sizing Result

Pressure Drop (ΔP)–

Required Cv–

Required Kv–

Recommended Valve Size–

Typical Cv Values by Valve Size

Use this table to cross-reference your calculated Cv with standard valve sizes. Select a valve where the required Cv is 60–80% of the rated Cv.

Valve Size	DN	Gate Valve	Globe Valve	Ball Valve	Butterfly Valve
½"	15	8	5	10	–
¾"	20	14	9	18	–
1"	25	25	15	30	30
1½"	40	55	30	65	70
2"	50	100	55	120	130
3"	80	225	120	270	300
4"	100	400	210	480	540
6"	150	900	470	1100	1200
8"	200	1600	830	1900	2200
10"	250	2500	1300	3000	3500
12"	300	3600	1900	4500	5000
16"	400	6400	–	8000	9000
20"	500	10000	–	12500	14000
24"	600	14400	–	18000	20000

Selection Rule: Choose a valve where the required Cv is 60–80% of the rated Cv at full open. This ensures the valve operates in the most controllable range and allows margin for process upsets.

Understanding Valve Sizing

What is Valve Sizing?

Valve sizing is the process of determining the correct valve size and flow coefficient (Cv or Kv) to achieve the desired flow rate at a given pressure drop. Proper sizing ensures the valve operates efficiently, provides adequate flow control, and avoids problems like cavitation, flashing, noise, and vibration.

Key Concepts

Cv (Flow Coefficient): US gallons per minute of water at 60°F flowing through the valve with 1 psi pressure drop
Kv (Flow Factor): Cubic meters per hour of water at 20°C flowing through the valve with 1 bar pressure drop. Kv = 0.865 × Cv
Choked Flow: When the pressure drop exceeds a critical ratio, flow becomes choked (sonic velocity at vena contracta). Further increasing ΔP does not increase flow.
Cavitation: In liquid service, when local pressure drops below vapor pressure, vapor bubbles form and collapse violently. Causes noise, vibration, and damage.
Flashing: When downstream pressure is below vapor pressure, liquid permanently converts to vapor. Requires special trim design.

Standards

IEC 60534-2-1: Industrial-process control valves – Flow capacity – Sizing equations for fluid flow under installed conditions
ISA-75.01: Flow Equations for Sizing Control Valves (ANSI/ISA standard)
ANSI/FCI 70-2: Control Valve Seat Leakage

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