Control Valve

# Relationship Between Cv and Kv in Control Valves

• The relationship between control valve Cv and Kv is essential for understanding and assessing flow capabilities among various measurement standards.
• The flow coefficient, or Cv, is more widely used in the United States, whereas the valve flow coefficient, or Kv, is more frequently used in Europe and other metric-using nations.
• The conversion coefficient of 1.156 between Cv and Kv makes it easier to translate flow capacity between these two standards.

We must first understand the basic definitions and units connected to Cv and Kv in order to fully appreciate the relevance of this conversion factor and how it was derived.

The flow coefficient, denoted as K, is a fundamental parameter in fluid dynamics, particularly pertinent to valves and other flow-restricting apparatus. It quantifies the relationship between the pressure drop ( ΔP) or head drop h) across the valve and the flow rate (Q) of the fluid passing through it.

The flow coefficient K is typically expressed as:

Q= Flow rate

ΔP=Pressure Drop

Sg= Specific gravity (1 for water)

K= Flow coefficient Kv or Cv

• Each valve possesses its own unique flow coefficient, determined by its design characteristics and the position of its opening.
• Consequently, different types of valves and variations within the same type exhibit differing flow coefficients.
• The significance of the flow coefficient lies in its ability to guide valve selection.
• For instance, valves intended for continuous operation should ideally feature a low head loss to conserve energy.
• Conversely, precise control over flow requires selecting valves with coefficients that align with the specific application requirements.
• The expression of the flow coefficient can vary based on factors such as manufacturer, valve type, and application.
• It may be dimensionless or include units, incorporating parameters like diameter or density into the equation.
• The valve industry often standardizes the flow coefficient (K), referencing it for water at a specific temperature, along with standardized units for flow rate and pressure drop.
• Additionally, for a given valve model, the coefficient may differ for each diameter, reflecting the influence of size on flow characteristics.
• Cv (flow coefficient) and Kv (valve flow coefficient) are both used to quantify the flow capacity of a control valve.
• Cv is primarily used in the United States, while Kv is more common in Europe and other metric-using regions.

Represents the flow rate of water in US gallons per minute (GPM) that will pass through a valve with a specific opening, given a pressure drop of 1 pound per square inch (psi) across the valve, at a standard temperature of 60°F.

Denotes the flow rate of water in cubic meters per hour (m3/hr) with a temperature range of 5 to 30°C and a pressure drop of 1 bar across the valve.

The conversion factor between Cv and Kv, often noted as 1.156, facilitates the translation of flow capacities between these two standards.

The conversion factor of 1.156 arises from the necessity to equate flow capacities measured in different units and under different standards.

Let’s delve into the unit conversions that lead to this factor:

• 1 US GPM = 0.227125 m3/hr
• This conversion factor accounts for the difference in volume measurements between gallons and cubic meters.
• 1 psi = 0.06894757 bar
• This conversion factor adjusts the pressure measurements from pounds per square inch to bars, which are commonly used in the metric system.
• 60°F ≈ 15.5°C
• This conversion aligns temperature measurements between the Fahrenheit scale, typical in the US, and the Celsius scale, widely used in the metric system.

Given these conversions, we can derive the relationship between Cv and Kv:

From this calculation, we find that 1 Cv is equivalent to 1.156 US GPM per psi. Therefore, to convert from Cv to Kv, we multiply by 1.156, and to convert from Kv to Cv, we divide by 1.156.

Similarly, to convert from Kv to Cv, we perform the following conversions:

Thus, 1 Kv is equivalent to 0.864 Cv. This establishes the relationship between Cv and Kv and demonstrates the significance of the conversion factor in reconciling flow capacities measured under different standards.

Understanding the relationship between Cv and Kv is crucial for engineers and technicians working with control valves. It allows for seamless comparison and interchange of flow capacity data between systems that utilize Cv and Kv, enabling efficient design, selection, and operation of control valves across diverse applications and geographical regions.

### Sundareswaran Iyalunaidu

With over 24 years of dedicated experience, I am a seasoned professional specializing in the commissioning, maintenance, and installation of Electrical, Instrumentation and Control systems. My expertise extends across a spectrum of industries, including Power stations, Oil and Gas, Aluminium, Utilities, Steel and Continuous process industries. Tweet me @sundareshinfohe

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