1、流體設備的流量系數（Flow Coefficient）流體設備的流量系數（Flow Coefficient）11何謂流量系數CV值21.1表述121.2表述221.3表述331.4表述431.4.1relationship for liquids in turbulent flow through an orifice31.4.2relationship for gases and vapors with subsonic flow through an orifice41.5表述551.5.1Flow of Liquid51.5.2Flow of Gases62流量系數計算72.1Flowr

2、ate Calculation for an Orifice Flowmeter72.2調節閥口徑和Cv值計算92.3Znud121 何謂流量系數CV值1.1 表述1為明確表達控制閥的特性，特以CV值來表示之。CV值為英制，當閥體出入口的差壓為 1 psi時，用清水于60在閥體全開的情況下流過，而此時的流量可用GPM來表示。 CV=Q/sqrt(P) Q 流過閥體流量GPMP 閥體全開時壓力降psi 采用公制為KV值，即閥體出入口差壓在1kgc，使20 oC清水流過時，其流量用m³hr 來表示Cv = 1.167Kv Kv = 0.857 Cv1.2 表述2閥門Cv值和Kv值的定義與

3、計算方法 (2009-11-20)Cv值 Cv值的定義：Cv值表示的是元件對液體的流通能力;即：流量系數。對于閥門來講，國外一般稱為Cv值，國內一般稱為Kv值。Cv值的測定：被測元件全開，元件兩端壓差p.=1bf/in(1lbf/in=6.89kPa），溫度為60（15.5）的水，通過元件的流量為qv，單位為USgas/min（USgas/min=3.785L/min），則流通能力Cv值為 Cv值的計算公式：Cv=qv*p0/(0*p)0.5 式中： Cv：流通能力，USgas/min qv：實測水的流量，USgas/min ：實測水的密度，g/cm； 0：60下水的密度，0=1g/cm； p

4、.=p1-p2。p1和p2是被測元件上下游的壓力差，lbf/in。Kv值的定義：Kv值是表示氣體流量特性的一個參數和表示方法。 Kv值的測定：被測元件全開，元件兩端壓差p.=0.1MPa，流體密度=1g/cm時；通過元件的流量為qv（m/h），則流通能力Kv值為 Kv值的計算：Kv=qv*p0/(0*p)0.5 式中： Kv：流通能力，m3/h； ：實測流體密度，g/cm； p.=p1-p2。p1和p2是被測元件上下游的壓力差，MPa。 Kv值與Cv值之間的關系：Cv=1.167KvKv=LPM/SQRT(kPa/0.36)Cv=LPM/SQRT(kPa/0.49)1.3 表述3Flow Co

5、efficient Cv and Flow Factor KvFluid Mechanics - The study of fluids - liquids and gases. 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 charact

6、eristics 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

7、a pressure drop of 1 kg/cm2 (1 bar). The connection between Cv and Kv can be expressed as:Cv = 1.16 Kv Kv = 0.853 Cv1.4 表述41.4.1 relationship for liquids in turbulent flow through an orificeCv = flow coefficient (gallons/min/psi1/2) Fliq = flow rate (gallons/min) FP = dimensionless factor accounting

8、 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 (see Driskell 1983 for details) FR = dimensionless factor accounting for viscosity effects for liquids; the value is 1.0 for Reynolds numbers greater than 4x1

9、04 (see Hutchison 1976 for the calculation of the valve Reynolds number and FR) Gliq = specific gravity of process fluid at 60 °F (15 °C) P = pressure drop across the valve (psi)1.4.2 relationship for gases and vapors with subsonic flow through an orificeQ=gas Volume flow rate (std. ft3/h)

10、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 factorP1=upstream pressure (psia)T1=upstream temperature (°R)Y=dimensionless expansion factor which depends on P1/P2 and the specific heat r

11、atio(Ft); ranges from 0.67 to 1.0 (see Driskell 1983),Y=1-X/3FtXt, Y=1: as long as X<<XtZ=compressibility factorX=Pressure drop ratio,X= DP/P11.5 表述51.5.1 Flow of LiquidQ=Flow in US GPMPD=Pressure Drop (PSI)SG=Specific Gravity at Flow ConditionsCV=Valve Flow Coefficient1.5.2 Flow of GasesQ=Flo

12、w in US GPMPD=Pressure Drop (PSI)P2=Outlet Pressure PSIAT=Temp. (R°) (F°+460)SG=Specific Gravity at Flow ConditionsCV=Valve Flow Coefficient2 流量系數計算2.1 Flowrate Calculation for an Orifice FlowmeterInputs Pipe (inlet) diameter upstream of orifice, Di: m in ft cm Orifice diameter (less than

13、the inlet diameter), Do: m in ft cm Pressure difference across the orifice, Dp: Pa mmHg inH2O inHg ftH2O psi Fluid density, r: kg/m3 lb/in3 kg/l Flow Coefficient, Cf: Answers Velocity at the inlet, Vi: 1.76 m/s cm/s in/s ft/s m/s mph Volumetric Flowrate, Q: 13.9 l/s m3/s l/s ft3/s in3/s cm3/s gal/s

14、gpm pt/s ft3/min Mass Flowrate: 0.0179 kg/s kg/s slug/s lbm/s Select desired output units for next calculation.As long as the fluid speed is sufficiently subsonic (V < mach 0.3), the incompressible Bernoulli's equation describes the flow reasonably well. Applying this equation to a streamline

15、 traveling down the axis of the horizontal tube gives, where location 1 is upstream of the orifice, and location 2 is slightly behind the orifice. It is recommended that location 1 be positioned one pipe diameter upstream of the orifice, and location 2 be positioned one-half pipe diameter downstream

16、 of the orifice. Since the pressure at 1 will be higher than the pressure at 2 (for flow moving from 1 to 2), the pressure difference as defined will be a positive quantity. From continuity, the velocities can be replaced by cross-sectional areas of the flow and the volumetric flowrate Q, Solving fo

17、r the volumetric flowrate Q gives, The above equation applies only to perfectly laminar, inviscid flows. For real flows (such as water or air), viscosity and turbulence are present and act to convert kinetic flow energy into heat. To account for this effect, a discharge coefficient Cd is introduced

18、into the above equation to marginally reduce the flowrate Q, Since the actual flow profile at location 2 downstream of the orifice is quite complex, thereby making the effective value of A2 uncertain, the following substitution introducing a flow coefficient Cf is made, where Ao is the area of the o

19、rifice. As a result, the volumetric flowrate Q for real flows is given by the equation, The flow coefficient Cf is found from experiments and is tabulated in reference books; it ranges from 0.6 to 0.9 for most orifices. Since it depends on the orifice and pipe diameters (as well as the Reynolds Numb

20、er), one will often find Cf tabulated versus the ratio of orifice diameter to inlet diameter, sometimes defined as b, The mass flowrate can be found by multiplying Q with the fluid density, 2.2 調節閥口徑和Cv值計算 文件類型：DOC/Microsoft Word 文件大小：字節更多搜索：調節口徑Cv計算 調節閥口徑和Cv值計算決定調節閥口徑應根據已知的流體條件,計算出必要的Cv值,然后再根據調節閥的額

21、定Cv值,選取合適的調節閥口徑.Cv值計算公式Cv值是用來表示調節閥的英制單位流量系數.其定義是:閥處于全開狀態,兩端壓差為1磅/英寸2(7KPa)的條件下,60F(+15.6)的清水,每分鐘通過閥的美加侖數.液體 (英制) (公制)Cv=Q Cv=1.17Q=Q(1) =1.17Q(1')式中Q=最大流量 gpm(美加侖/分) Q=最大流量 m3/hG=比重(水=1) G=比重(水=1)P1=進口壓力 Psia(最大流量時) P1=進口壓力 kgf/cm2(最大流量時)P2=出口壓力 Psia(最大流量時) P2=出口壓力 kgf/cm2(最大流量時)P=P1-P2注:上述公式只適用

22、于流體流動呈紊流狀態,或雷諾數大的場合,流體接近層流或雷諾數較小的場合,上述公式必須進行粘度修正.粘度修正要按粘度修正曲線(雷諾數R的實測系數值)進行修正.表示調節閥流量系數的其它符號及定義C工程單位制(MKS制)的流量系數,在我國長期使用.其定義為溫度5-40的水,在1kgf/cm2(0.1MPa)壓降下,一小時流過調節閥的立方米數.Kv國際單位制(SI制)的流量系數,其定義為:溫度5-40的水,在105Pa壓降下,每小時流過調節閥的立方米數.注:1.C,Cv,Kv之間的關系為:Cv=1.17CKv=1.01C2,我國調節閥流量系數將由C系數變為Kv系數.3,IEC推薦公式中的符號C是作為各

23、種運算單位的流量系數的通用符號,不同運算單位計算出的流量系數,用公式中的數字常數Ni來區別.因此,勿與我國長期使用的C值混淆.粘度修正液體粘度大于100SSU(賽波特秒),或者大于20CST(厘斯),計算所要求的Cv值應按下列次序進行粘度修正.1,不考慮粘度影響,用公式(1)或(1')求出Cv.2,用公式(A)和(B)或者用公式(A')和(B'),求出系數R.3,從粘度修正曲線上,求出系數R相對應的Cv的修正系數.4,用這個修正系數乘以第一步求出的Cv.5,然后,從Cv值一覽表上,選取合適的調節閥口徑.系數R的計算公式(英制) (公制)R=(A) R=(A')R

24、=(B) R=(B')式中Q=最大流量 gpm Q=最大流量 m3/hMcs=進口溫度下液體運動粘度系數cstCv=無修正過的CvMssu=進口溫度下液體粘度SSU(賽波特秒)備注:液體粘度200SSU,使用公式(B)或(B')計算,粘度小于200SSU,請把SSU粘度單位換算成cst粘度單位,再用公式(A)或(A')計算.氣體計算公式氣體計算公式是把液體計算公式的比重,經過換算后得出的.這個比重是取進口一側狀態下的比重呢,還是取出口一側狀態下的比重呢,還是取其兩者平均值呢 實驗證明.取平均值的計算結果比較接近實驗數值.所以最近普通采用比重平均值來計算Cv值.另外,氣體

25、在PP1/2狀態時,氣體的流速達到音速,流量會達到飽和狀態.壓差在增大,流量也不會增加了.因此應分PP1/2和PP2/2兩種情況加以討論.1.P< 2.P時Cv=Cv=式中:Q:最大流量(ft3/h)(在14.7psi abs, 60F條件下)G: 比重(空氣=1)T:流體溫度(°F )P1:進口絕對壓力(1bs/in2abs)P2:出口絕對壓力(1bs/in2abs)P=P1-P2(注:P1和P2為最大流量時的壓力)水蒸氣計算公式它與氣體一樣,也應分兩種情況加以討論.1.P< Cv=2.P Cv式中:W:最大流量(1bs/hr)P1*: 進口絕對壓力(1bs/in2ab

26、s)P2*: 進口絕對壓力(1bs/in2abs)K: 1+(0.0007×過熱溫度°F)P=P1-P2(1bs/in2)(注:P1和P2為最大流量時的壓力)其它蒸氣計算公式其它蒸氣的計算公式,同水蒸氣的計算,可得到Cv= Cv=注:當P2<時應用代替P,V2應用相對應的值.式中:W:最大流量(1bs/hr)V1:在進口壓力P1下蒸氣比容(ft3/1bs at P1)V2:在出口壓力P2下蒸氣比容(ft3/1bs at P2)P1*:進口絕對壓力(1bs/in2abs)P2*:進口絕對壓力(1bs/in2abs)P=P1-P2(1bs/in2)(注:P1和P2為最大流

27、量時的壓力)公稱通徑的選擇:調節閥公稱通徑選擇,是由最大Cv值,可調范圍,以及調節閥有足夠的調節余量,這幾個因素來決定的.最大Cv值和最小Cv值是分別在最大流量和最小流量條件計算出的二個數值.1,最大Cv值鑒于額定Cv值有±20%,-10%的調節誤差,建議等百分比閥在90-95%開度內的值作為最大Cv值,線性調節閥在8090%開度內的值作為最大Cv值.2,常用Cv值常在低開度下工作,閥芯易于磨損,再從控制性能上考慮,希望閥在5080%開度范圍工作.3,最小Cv值閥的最小Cv值應在固有的可調范圍之內,實際上大多數調節閥控制流體時,開度變化,閥上壓差也相應變化.開度與流量之間的固定流量特

28、性,變成了實際的流量特性,可調范圍也變小了.閥達到最小Cv值時,希望閥在1020%開度上工作,如果要使閥在更小的開度范圍內工作,應選擇可調范圍較小的調節閥,或者改用一臺大,一臺小的切換閥,用這二臺閥分程控制流量.縮小閥孔單座閥,雙座閥及角閥等調節閥,同一個公稱通徑,設計了幾組不同的縮小閥孔,它的流量系數比原來的額定值小一檔或小二檔,籠式閥只要更換套筒就可以做到這一點,蝶閥,三通閥,隔膜閥等沒有縮小閥孔的規格.縮小閥孔的調節閥用于下列場合:(1)從機械強度上考慮,(管道應用引起)必須選用大尺寸的閥體.(2)閥的流量系數今后有可能加大.(3)降低大壓差產生的噪音,或者增加閥體各部分的強度.(4)漸

29、縮管成本太高(5)閥體受閃蒸流體和高壓差流體,氣蝕破壞時,增大下流側流出截面積,可減輕氣蝕破壞3002錯誤!未定義樣式.產品選型樣本Product Manual 調節閥的選型和計算參考資料產品選型樣本Product Manual 調節閥的選型和計算參考資料- -除水以外的其它液體對于水以外的其它液體,雖然也可像水一樣采用,"臨界壓差法"或"液體氣體混合比重法"等,但這兩種方法必須已知液體的飽和壓力或臨界壓力數據.目前,僅局限于已知飽和壓力或臨界壓力的幾種液體才采用這兩種方法計算,其它液體一般不用這兩種方法.一般的計算方法是求出閃蒸的比率.然后分別計算出液體和氣體的Cv值,它