Butterfly Valve Cv and Kv Values: Complete Guide to Flow Coefficients

Introduction

When selecting a butterfly valve for any application, understanding Cv and Kv values is essential. These flow coefficients determine how much fluid a valve can pass at a given pressure drop, directly impacting system performance, pump sizing, and energy efficiency.

1. What are Cv and Kv?

1.1 Cv — Flow Coefficient (US Standard)

Cv (Flow Coefficient) is the number of US gallons per minute (GPM) of 60°F (15.6°C) water that will flow through a valve with a 1 psi pressure drop. A valve with Cv = 100 will pass 100 GPM of water with a 1 psi pressure drop. The higher the Cv, the greater the flow capacity.

1.2 Kv — Flow Coefficient (Metric/European Standard)

Kv is the number of cubic meters per hour (m³/h) of 15°C water that will flow through a valve with a 1 bar (100 kPa) pressure drop. Cv and Kv are directly convertible.

1.3 Cv to Kv Conversion

Example: A valve with Cv = 1000 has Kv = 1000 × 0.865 = 865 m³/h/bar.

2. Why Cv and Kv Matter for Butterfly Valves

2.1 Low Pressure Drop (High Cv)

Compared to gate valves, globe valves, and ball valves of the same size, butterfly valves have higher Cv values (lower pressure drop) when fully open. A butterfly valve's Cv at full opening approaches that of a straight pipe section of equivalent length (2-5 pipe diameters equivalent length).

2.2 Cv Varies with Disc Angle

Unlike gate or ball valves, butterfly valves are frequently used for throttling service. The Cv value changes significantly with disc position:

• 90° (fully open): Maximum Cv
• 45° (half open): Approximately 40-50% of full Cv
• 30°: Approximately 15-25% of full Cv
• 0° (closed): Cv = 0

2.3 Flow Characteristic

Concentric butterfly valves exhibit an approximately equal-percentage flow characteristic. Equal increments of disc rotation produce proportional percentage changes in flow rate, providing stable control across the full operating range.

3. Typical Cv Values for Butterfly Valves

The following table shows typical Cv values for concentric resilient-seated wafer butterfly valves (Class 150, fully open at 90°). These are reference values — actual Cv varies by manufacturer and specific design.

* Flow range at approximately 2-5 psi pressure drop, water service.

4. Cv at Different Disc Opening Angles

For throttling service, it is essential to understand how Cv changes with disc position:

5. How to Calculate Flow Rate Using Cv

For liquids (water and similar fluids), the basic flow equation is:

Where: Q = Flow rate (GPM), Cv = Flow coefficient, ΔP = Pressure drop (psi), SG = Specific gravity of fluid (water = 1.0)

Example: An 8" butterfly valve with Cv = 2,400 (fully open). The valve is in a water line (SG = 1.0) with 3 psi available pressure drop. What flow rate can it pass?

Q = 2,400 × √(3/1.0) = 2,400 × 1.732 = 4,157 GPM

For gases and steam, the calculation requires compressibility considerations. Consult manufacturer data for gas sizing coefficients.

6. Factors That Affect Butterfly Valve Cv Values

6.1 Valve Size

Cv increases approximately with the square of the valve diameter. Doubling diameter increases flow area by 4x, resulting in Cv increase of 3.5-4x.

6.2 Body Configuration

Wafer and lug valves have nearly identical Cv. Double flange valves have Cv approximately 5-10% higher due to slightly larger bore.

6.3 Disc Design

• Standard concentric disc: baseline Cv
• V-port / fish-tail disc: Cv 10-20% lower, better throttling
• Eccentric disc: Cv similar to concentric

6.4 Seat Type

• Vulcanized seat: standard Cv
• Cartridge seat: 2-5% lower Cv
• PTFE seat (double eccentric): 5-10% lower Cv
• PTFE lined (split body): 8-15% lower Cv

7. Sizing a Butterfly Valve Using Cv

Rule of thumb: On-off: select valve where design flow passes at 80-90°. Throttling: select valve where normal flow passes at 40-70°. If valve operates below 20°, it is oversized. If above 85° to meet flow, it is undersized.

8. Cv for Different Butterfly Valve Types

9. Frequently Asked Questions About Butterfly Valve Cv

What is the typical Cv of a 6" butterfly valve?

A standard 6" (DN150) concentric resilient-seated wafer butterfly valve has a typical Cv of approximately 1,300 (Kv = 1,125) when fully open at 90°. Always use the manufacturer's published Cv data for final sizing.

How does butterfly valve Cv compare to a gate valve?

A butterfly valve typically has Cv 20-30% higher than a gate valve of the same size when fully open. For example, a 10" butterfly valve (Cv ≈ 3,900) has approximately 25% more flow capacity than a 10" gate valve (Cv ≈ 3,100).

Can I use butterfly valve Cv for gas sizing?

Yes, but additional factors must be considered. The liquid Cv equation does not apply to gases because the fluid compresses across the valve. Use the Cg or Kg method, or apply compressibility corrections. Most manufacturers provide gas-sizing coefficients for this purpose.

Does Cv change with pressure?

For liquids, Cv is essentially constant for a given valve opening. It does not change with pressure. The published Cv value is measured with water at 1 psi pressure drop and can be used for liquid sizing at any pressure within the valve's rated range.

What is the Cv of a butterfly valve at 45° open?

A concentric butterfly valve at 45° open delivers approximately 40-45% of its full-open Cv. For example, an 8" valve with Cv = 2,400 (fully open) has Cv ≈ 960-1,080 at 45° disc angle.

How do I obtain Cv data for Laux Valve butterfly valves?

Contact our engineering team with the specific valve size, type, and pressure class. We provide detailed Cv tables for every valve configuration, including Cv at 10° increments of disc opening for throttling applications.

10. Summary

• Cv (US) and Kv (European) measure valve flow capacity
• Conversion: Kv = Cv × 0.865; Cv = Kv × 1.156
• Butterfly valves have high Cv vs gate and globe valves
• Cv varies with disc angle — equal-percentage characteristic
• Avoid operation below 20° — cavitation risk
• On-off: select for 80-90° at design flow
• Throttling: select for 40-70° at design flow
• Different valve types have slightly different Cv (DF +5%, DE -5%, PTFE lined -8-15%)

Contact Laux Valve for Cv Data

For detailed Cv and Kv data for our complete range of butterfly valves, contact our engineering team.

Email: james@lauxvalve.com | Tel: +86 18920833829 | Website: lauxvalve.com