# Summary

C_{v} and K_{v} are singles values in units of flowrate that may be used to characterise the relationship between flowrate and pressure loss for fittings and equipment. This article demonstrates how to calculate the C_{v} or K_{v} values, and how to use these values to determine the pressure loss for a given flowrate.

# Definitions

: | Flow coefficient determined on a US customary basis | |

: | Flow coefficient determined on a metric basis | |

: | Flowrate of fluid | |

: | Specific gravity of the fluid | |

: | Pressure change through the equipment | |

: | Density of the fluid |

# Introduction

There are many ways of describing the pressure loss through pipes, fittings and equipment. The C_{v} and K_{v} methods use a single number to characterise the flowrate through the equipment at a known pressure loss, this number may then be used to estimate the pressure loss characteristics of the equipment at other flowrates.

While the C_{v} and K_{v} methods may be used to describe many types of fitting and equipment, the most common use is for describing the flow characteristics of control valves. The C_{v} value is in units of US gallons per minute, while the K_{v} value is in units of m^{3}/hr. The C_{v} values, using US customary units are far more widely reported than K_{v} values, using Metric units.

# Calculation of Pressure Loss using the C_{v} Method

A C_{v} value is defined as the rate of flow of water in US gallons per Minute at 60°F at a pressure drop of 1 psi across the equipment. C_{v} relates to pressure drop and flowrate via the following expression (imperial units):

Where Q is in USGPM and ΔP is in psi.

Thus for a given C_{v}, SG and Q the pressure drop across an item of equipment is calculated as:

C_{v} relates to pressure drop and flow rate via the following expression (Metric Units):

Where Q is in L/min, ρ is in kg/m^{3} and ΔP is in bar.

Thus for a given C_{v}, ρ and Q the pressure drop across an item of equipment is calculated as:

# Calculation of Pressure Loss using the K_{v} Method

A K_{v} value is defined as the rate of flow of water in m^{3}/hr and 4°C at a pressure drop of 1 bar across the equipment. K_{v} relates to pressure drop and flow rate via the following expression:

Where Q is in m^{3}/hr and ΔP is in bar.

Thus for a given K_{v}, ρ and Q the pressure drop across an item of equipment is calculated as:

# Conversion Between C_{v} and K_{v} Values

Values of C_{v} and K_{v} can be converted using the formulae below: