流体设备的流量系数（Flow Coefficient）
流体设备的流量系数（Flow Coefficient） 1
1 何谓流量系数CV值 2
1.1 表述1 2
1.2 表述2 2
1.3 表述3 3
1.4 表述4 3
1.4.1 relationship for liquids in turbulent flow through an orifice 3
1.4.2 relationship for gas and vapors with subsonic flow through an orifice 4
1.5 表述5 5
1.5.1 Flow of Liquid 5
1.5.2 Flow of Gas 6
2 流量系数计算 7
2.1 Flowrate Calculation for an Orifice Flowmeter 7
2.2 调节阀口径和Cv值计算 9
2.3 Znud 12
1 何谓流量系数CV值
1.1 表述1
为明确表达控制阀的特性，特以CV值来表示之。CV值为英制，当阀体出入口的差压为 1 psi时，用清水于60℉在阀体全开的情况下流过，而此时的流量可用GPM来表示。
CV=Q/sqrt(ΔP)
Q ＝ 流过阀体流量GPM
ΔP ＝ 阀体全开时压力降psi
采用公制为KV值，即阀体出入口差压在1kg／c㎡，使20 oC清水流过时，其流量用m³／hr 来表示
Cv = 1.167Kv
Kv = 0.857 Cv
1.2 表述2
阀门Cv值和Kv值的定义与计算方法 (2009-11-20)
Cv值 Cv值的定义：Cv值表示的是元件对液体的流通能力;即：流量系数。对于阀门来讲，国外一般称为Cv值，国内一般称为Kv值。
Cv值的测定：被测元件全开，元件两端压差△p.=1bf/in(1lbf/in=6.89kPa），温度为60℉（15.5℃）的水，通过元件的流量为qv，单位为USgas/min
（USgas/min=3.785L/min），则流通能力Cv值为
Cv值的计算公式：Cv=qv*[ρ*△p0/(ρ0*△p)]^0.5
式中：
Cv：流通能力，USgas/min
qv：实测水的流量，USgas/min
ρ：实测水的密度，g/cm；
ρ0：60℉下水的密度，ρ0=1g/cm；
△p.=p1-p2。p1和p2是被测元件上下游的压力差，lbf/in。
Kv值的定义：Kv值是表示气体流量特性的一个参数和表示方法。
Kv值的测定：被测元件全开，元件两端压差△p.==0.1MPa，流体密度ρ=1g/cm时；通过元件的流量为qv（m/h），则流通能力Kv值为
Kv值的计算：Kv=qv*[ρ*△p0/(ρ0*△p)]^0.5
式中：
Kv：流通能力，m^3/h；
ρ：实测流体密度，g/cm；
△p.=p1-p2。p1和p2是被测元件上下游的压力差，MPa。
Kv值与Cv值之间的关系：Cv=1.167Kv
Kv=LPM/SQRT(kPa/0.36)
Cv=LPM/SQRT(kPa/0.49)
1.3 表述3
Flow Coefficient Cv and Flow Factor Kv
Fluid Mechanics - The study of fluids - liquids and gas. Involves various properties of the fluid, such as velocity, pressure, density and temperature, as functions of space and time.
For control valves it is often convenient to express the capacities and flow characteristics in terms of a flow coefficent Cv defined as the flow of water at 60 oF in US gallon/minute at a pressure drop of 1 lb/in2 across the valve.
The metric equivalent called the flow factor Kv defined as the number of cubic meters per hour of water at 20 oC which will flow through the valve with a pressure drop of 1 kg/cm2 (1 bar).
The connection between Cv and Kv can be expresd as:
Cv = 1.16 Kv
Kv = 0.853 Cv
1.4 表述4
1.4.1 relationship for liquids in turbulent flow through an orifice
Cv = flow coefficient (gallons/min/psi1/2)
Fliq = flow rate (gallons/min)
FP = dimensionless factor accounting for difference in piping due to fittings for piping changes at inlet and outlet; values range from 0.80 to 0.98 and are typically about 0.95 (e Driskell 1983 for details)
FR = dimensionless factor accounting for viscosity effects for liquids; the value is 1.0 for Reynolds numbers greater than 4x104 (e Hutchison 1976 for the calculation of the valve Reynolds number and FR)
Gliq = specific gravity of process fluid at 60 °F (15 °C)
ΔP = pressure drop across the valve (psi)
1.4.2 relationship for gas and vapors with subsonic flow through an orifice
Q=gas Volume flow rate (std. ft3/h)
Gg=specific gravity of the process fluid relative to air at standard conditions
N=unit conversion factor (equal to 1380 for English units)
FP=Pipe geometry factor
P1=upstream pressure (psia)
T1=upstream temperature (°R)
Y=dimensionless expansion factor which depends on P1/P2 and the specific heat ratio(Ft); ranges from 0.67 to 1.0 (e Driskell 1983),Y=1-X/3FtXt, Y=1: as long as X<<Xt
Z=compressibility factor
X=Pressure drop ratio,X=P/P1
1.5 表述5
1.5.1 Flow of Liquid
Q=Flow in US GPM
PD=Pressure Drop (PSI)
SG=Specific Gravity at Flow Conditions
CV=Valve Flow Coefficient
1.5.2 Flow of Gas
Q=Flow in US GPM
PD=Pressure Drop (PSI)
P2=Outlet Pressure PSIA
T=Temp. (R°) (F°+460)
SG=Specific Gravity at Flow Conditions
CV=Valve Flow Coefficient
2 流量系数计算
2.1 Flowrate Calculation for an Orifice Flowmeter
Inputs
Pipe (inlet) diameter upstream of orifice, Di: m in ft cm
Orifice diameter (less than the inlet diameter), Do: m in ft cm
Pressure difference across the orifice, Dp: Pa mmHg inH2O inHg ftH2O psi
