# Valve Flow Coefficient (Kv / Cv) Sizing Calculator for Liquids

> Source: https://netray.co/tools/valve-flow-coefficient-kv-cv-sizing-calculator
> Publisher: Netray (netray.co), AI and integration engineering for ERP and manufacturing systems.
> Section: Free Tools

## Short answer

Required Kv and Cv for a liquid control valve from flow, specific gravity and pressure drop, with a simple choked flow limit and margin.

## Category

industry-templates

## Industry

Oil and Gas

## Audience

- Process engineer
- Instrument engineer
- Sales engineer

## Tool

**Kind:** calculator

**Inputs:**

#### Flow rate

**Key:** flow

**Type:** number

**Default:** 60

**Min:** 0

**Step:** 1

**Unit:** m3/h

#### Specific gravity

**Key:** sg

**Type:** number

**Default:** 0.85

**Min:** 0.1

**Step:** 0.01

**Help:** Relative to water at 15 C. Water is 1.0, light crude about 0.85.

#### Inlet pressure (absolute)

**Key:** p1

**Type:** number

**Default:** 12

**Min:** 0.1

**Step:** 0.1

**Unit:** bar a

#### Pressure drop across valve

**Key:** dp

**Type:** number

**Default:** 4

**Min:** 0.01

**Step:** 0.1

**Unit:** bar

#### Vapour pressure (absolute)

**Key:** pv

**Type:** number

**Default:** 0.2

**Min:** 0

**Step:** 0.05

**Unit:** bar a

#### Critical pressure of fluid

**Key:** pc

**Type:** number

**Default:** 220.6

**Min:** 1

**Step:** 1

**Unit:** bar a

**Help:** Water is 220.6 bar a. Hydrocarbons are typically 30 to 50 bar a.

#### Liquid pressure recovery factor FL

**Key:** fl

**Type:** number

**Default:** 0.9

**Min:** 0.3

**Max:** 1

**Step:** 0.01

**Help:** From the valve datasheet. Globe valves are often 0.85 to 0.9.

#### Sizing margin

**Key:** margin

**Type:** number

**Default:** 20

**Min:** 0

**Max:** 100

**Step:** 5

**Unit:** %

**Outputs:**

#### Critical pressure ratio factor FF

**Key:** ff

**Expr:** 0.96 - 0.28 * sqrt(pv / pc)

**Format:** decimal

#### Choked pressure drop limit

**Key:** dpchoke

**Expr:** fl * fl * (p1 - ff * pv)

**Format:** decimal

**Description:** Pressure drop above this does not increase flow.

#### Pressure drop used for sizing

**Key:** dpused

**Expr:** min(dp, dpchoke)

**Format:** decimal

#### Required Kv

**Key:** kv

**Expr:** flow * sqrt(sg / dpused)

**Format:** decimal

**Primary:** true

**Description:** Kv in m3/h at 1 bar drop for water.

#### Required Cv

**Key:** cv

**Expr:** kv * 1.156

**Format:** decimal

**Primary:** true

**Description:** Cv in US gpm at 1 psi drop for water. Cv = 1.156 x Kv.

#### Cv to specify with margin

**Key:** cvm

**Expr:** cv * (1 + margin / 100)

**Format:** decimal

#### Choked flow (1 = yes)

**Key:** choked

**Expr:** if(gt(dp, dpchoke), 1, 0)

**Format:** integer

**Chart:**

#### Required Cv vs flow rate

**Type:** line

**X:** flow

**From:** 5

**To:** 200

**Steps:** 25

**Series:**

- cv
- cvm

**Disclaimer:** Basic incompressible flow sizing without piping geometry factors. Use the valve manufacturer sizing software for final selection, including noise and cavitation checks.

## Content

**Intro:** This calculator gives the required flow coefficient for a liquid control valve and checks whether the pressure drop you entered exceeds the choked flow limit. Results are in both Kv (metric) and Cv (US).

**Sections:**

#### How to use it

**Body:** Enter the flow rate, the specific gravity, the inlet pressure and the pressure drop you can afford across the valve. Supply the fluid vapour pressure and critical pressure, plus the valve's FL factor from its datasheet. The tool calculates Kv = Q x sqrt(SG / dP) and converts to Cv. If your pressure drop is larger than the choked limit, flow stops rising and the calculator sizes at the limit instead and flags the result. A margin of 10 to 25 percent is typical so the valve works at a sensible opening.

#### Formulas

**Body:** For non-choked liquid flow, Kv = Q x sqrt(SG / dP) with Q in cubic metres per hour and dP in bar. Cv is 1.156 times Kv. Choked flow occurs when dP exceeds FL squared x (P1 - FF x Pv), where FF = 0.96 - 0.28 x sqrt(Pv / Pc). This is the standard approach in IEC 60534 and ISA 75 for incompressible fluids, simplified here by neglecting pipe reducer and viscosity corrections.

**Bullets:**

- Higher FL means better pressure recovery resistance.
- Vapour pressure close to inlet pressure lowers the choked limit.
- Flashing and cavitation need a separate, specialist check.

#### Worked example

**Body:** For 60 cubic metres per hour of light oil at SG 0.85, a 4 bar drop and a 12 bar a inlet, Kv is 60 x sqrt(0.85 / 4) = about 27.7 and Cv is about 32. With a 20 percent margin you specify roughly Cv 38. The choked limit is 0.81 x (12 - 0.95 x 0.2), so about 9.6 bar, well above 4 bar: no choking.

#### Common mistakes

**Body:** Sizing on normal flow only leaves no room for the maximum case, and oversizing for a worst case leaves the valve nearly closed at normal flow and prone to poor control. Using gauge pressure where absolute is needed distorts the choked limit. Always compare the final Cv against the valve's own characteristic curve for its travel. In the ERP, keep the valve datasheet Cv, trim and rated pressure class on the item so the right body is bought.

**Cta:** If valve data lives in spreadsheets and datasheets, we can help you structure it in the ERP item master so engineering and purchasing see the same Cv, class and trim.

**Faq:**

#### What is the relation between Kv and Cv?

**Answer:** Kv is flow in cubic metres per hour of water at 1 bar drop. Cv is flow in US gallons per minute of water at 1 psi drop. Cv equals 1.156 times Kv, or Kv equals 0.865 times Cv.

#### Why does choked flow cap my result?

**Answer:** When the pressure at the vena contracta falls to the vapour pressure, bubbles form and flow cannot increase with more pressure drop. The valve passes a maximum flow, so sizing beyond that drop gives a wrong, too small Cv.

#### Does this work for gas or steam?

**Answer:** No. Gases and steam are compressible and use a different equation with an expansion factor. This tool is for liquids only.

#### What margin should I add?

**Answer:** Many specifications ask for the normal case to sit at 60 to 80 percent open and the maximum case at 90 percent or less. A 10 to 25 percent Cv margin over the maximum flow case is a common starting point.

## Search terms

valve cv calculator, valve kv calculator, control valve sizing liquid, cv to kv conversion, choked flow valve calculator, valve sizing formula ISA 75, required cv for pressure drop

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