The control valve flow coefficients (Cv, Kv and C) of different unit systems are control valves under a fixed differential pressure, the volume of water circulating in a unit of time when the control valve is fully open, Cv, Kv and C there is a relationship between Cv = 1.156Kv, Cv = 1.167C. This article shares the definition, unit, conversion and exhaustive derivation process of Cv, Kv and C.
1、Definition of flow coefficient
Control valve flow capacity is a specific fluid at a specific temperature, when the valve ends for the unit differential pressure, the number of fluid volume flowing through the control valve in a unit of time, using a different system of units when there are different ways of expression.
Definition of flow coefficient C
Given the stroke, the temperature of 5-40 ℃ water, valve pressure difference between the two ends of 1kgf/cm2, the volume of flow through the valve per hour (expressed in m3).C is the flow coefficient of the common metric, our country in the past has been used for a long time, formerly known as the circulation capacity of the C. The flow coefficient C is the flow coefficient of the common metric.
② Definition of flow coefficient Kv
Given the stroke, the pressure difference between the two ends of the valve is 102kPa, the temperature of 5-40 ℃ water, the volume of water flowing through the control valve per hour (expressed in m3). kv is the international system of units flow coefficient.
③ Definition of flow coefficient Cv
The volume of water at a temperature of 60°F that flows through a regulating valve per minute (expressed in U.S. gallons U.S gal) for a given stroke with a differential pressure of 1lb/in2 at each end of the valve.Cv is the imperial flow coefficient.
2、Derivation of formulas for different unit systems
①Circulation capacity C formula and units
当γ/γ0=1，Q=1m3/h，△P=1kgf/cm2时，如C定义为1，则N=1。则流通能力C的公式及单位如下：
When γ/γ0=1, Q=1m3/h, △P=1kgf/cm2, if C is defined as 1, then N=1. The formula and unit of circulation capacity C are as follows:
In the formula C is the circulation capacity; Q unit is m3/h; γ/γ0 is the specific gravity; △P unit is kgf/cm2.
② Flow coefficient Cv calculation formula and unit
When ρ/ρ0=1, Q=1USgal/min, ∆P=1lb/in2, and if Cv=1 is defined, then N=1. The formula and units of the flow coefficient Cv are as follows:
where Cv is the flow coefficient; Q is in USgal/min; ρ/ρ0 is the specific density; and ∆P is in lb/in2.
③ Flow coefficient Kv calculation formula and unit
When ρ/ρ0=1, Q=1m3/h, ΔP=100kPa, if Kv=1, then N=0.1. The formula and unit of flow coefficient Kv are as follows:
where Kv is the flow coefficient; Q is in m3/h; ρ/ρ0 is the specific density; ΔP is in kPa.
3、Conversion of circulation capacity C, flow coefficient Kv, flow coefficient Cv
① flow coefficient Cv and circulation capacity C relationship
where it is known that Q is in USgal/min; ρ/ρ0 is the specific density; and ∆P is in lb/in2.
When C=1, Q=1m3/h, γ/γ0=1 (i.e., ρ/ρ0=1), and ∆P=1kgf/cm2, substituting the Cv formula with the condition of C=1 is:
From the calculations, we know that C=1 and Cv=1.167 are equivalent (i.e., Cv=1.167C).
② Cv and Kv conversion
When Kv = 1, Q = 1m3 / h, ρ / ρ0 = 1, △ P = 100kPa substituting Cv formula for unit conversion:
That is, Kv = 1 is equivalent to Cv = 1.156 (i.e., Cv = 1.156Kv).
Due to some information and samples of the control valve flow capacity C, flow coefficient Kv and flow system Cv three lack of derivation process, the use of easy to produce confusion. Changhui Instrumentation C, Kv, Cv from the definition, unit application and the relationship between the three to be clarified, to help engineering designers in the process of regulating valve selection and calculation of different expressions of flow coefficients (C, Kv, Cv) for conversion and comparison, to facilitate the selection of regulating valves than the selection.
The CV values of butterfly valves of Tianjin Zhongfa Valve are as follows, if necessary, please refer to.