Valve Flow Coefficient Cv, Kv, Av

Flow Coefficient Cv

The flow coefficient Cv states the flow capacity of a valve in gal (U.S.)/min of water at a temperature of 60℉ that will flow through a valve with a pressure loss of one pound per square inch at a specific opening position, as defined by the equation:

Where

Q = U.S. gal/min

Δp0 = reference differential pressure = 1 lb/in.2

Δp = operating differential pressure in lb/in.2

ρ0 = density of reference fluid, water = 62.4 lb/ft3

ρ = density of operating fluid in lb/ft3

Because ρ/ρ0 = specific gravity and the numerical value of Δp0 is unity,

Equation 2-12 is normally presented in the form:

Where

G = specific gravity

This relationship is incorporated in the International Standards Associ- ation (ISA) standards S.39.1 to S.39.4 and Publication 534- 119 of the International Electrotechnical Commission (IEC), and applies to single- phase and fully turbulent flow of Newtonian liquids. For other flow conditions, refer to the above ISA standards, IEC Publication 534-2 Parts 1 and 2, or Reference 20.

Flow Coefficient Kv

The flow coefficient Kv is a version of coefficient Cv in mixed SI units. It states the number of cubic meters per hour of water at a tem- perature between 5◦ and 40◦ C that will flow through the valve with a pressure loss of one bar at a specific opening position, as defined by the equation:

here

Q = m3/hour

Δp0 = reference differential pressure = 1 bar

Δp = operating differential pressure, bar

ρ0 = density of reference fluid (water = 1, 000 kg/m3 )

ρ = density of operating fluid, kg/m3

Because ρ/ρ0 = specific gravity and the numerical value of Δp0 is unity, Equation 2-14 is normally presented in the form:

where

G = specific gravity

This relationship is contained in the IEC Publication 534- 119 and applies to single-phase and fully turbulent flow of Newtonian liquids. For other flow conditions, refer to IEC Publication 534-2, Parts 1 and 2, or to manufacturer’s catalogs.

Flow Coefficient Av

The flow coefficient Av is a version of the flow coefficient Kv incoherent SI units. Av states the number of cubic meters per second of water at a temperature between 5。and 40 。C that will flow through the valve with a pressure loss of one Pascal at a specific opening position, as defined by the equation:

where

Q = flow rate, m3/s

p = operational differential pressure, Pa

ρ = density of Newtonian liquid, kg/m3

Av is derived from Equation 2-8, which maybe presented in the following forms:

v = flow velocity of fluid, m/s

A = cross-sectional area, m²

The expression   is replaced in Equation 2-16 by a single expression Av .

This relationship is contained in IEC Publication ^{534- 119} and applies to single-phase and fully turbulent flow of Newtonian liquids. For other flow condition, see IEC Publication ^534-2, Parts 1 and 2.