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What is Flow Coefficient?
Datetime: 2016/11/22 15:23:39  Hits: 642

In this article, we'll give some explanation about the flow coefficient, and how to calculate it.


1. Flow coefficient

The flow coefficient is a constant, related to the geometry of a test specimen used to calculate the flow rate of a of the test specimen under given conditions. Test specimen can be any valve or combination of valve, reducers, expanders or other fitting for which test data are required.



Flow coefficient


2. Imperial units definition

For imperial units flow coefficient is expressed as coefficient Cv, determined  by tests under the following conditions:


- Static pressure drop p across the valve of 1 psi


- Flowing fluid is water at a temperature from 40 to 100° F


- The volumetric flow rate qv is expressed in gpm.

 

3. Metric units definition

In  metric units flow coefficient is expressed as coefficient Kv, determined  by tests under the following conditions:


- Static pressure drop p across the valve of 1 bar (105 Pa)


- flowing fluid is water at a temperature from 5 to 40° C


- the volumetric flow rate qv is expressed in m3/h.

  

Engineering conversion between Cv and Kv coefficient can be expressed as:   


Cv 1.16 Kv


General test setup

Figure 1. General test setup

 

General test setup for determination of flow coefficient is shown at Figure. 1. Test is performed by measuring differential pressure on pressure taps upstream and downstream of the valve.


 Although the flow coefficients were defined as liquid flow rates, they also can be used for valve sizing both for incompressible and compressible fluids.


The test section consist of two straight lengths of pipe as shown in Figure 2. Static pressure drop measured between upstream and downstream pressure taps located as in Figure 1 and Figure 2.


When the flow pattern across the pipe is not uniform, multiple taps may be necessary to achieve the desired accuracy of measurement.


The pressure tap diameter upstream and downstream are equal and are required to  3 - 12 mm in diameter, or one tenth nominal pipe diameter, whichever is less.


The upstream and downstream piping actual inside diameter  is equal to diameter of the test specimen  (valve) actual inside diameter  with tolerance of  ±2 %. 


This condition is for diameters up to 10" (DN250), for larger diameters with pressure rating higher than #1500 (PN100) the inside diameter at the inlet and outlet of the test specimen needs to be matched with the inside diameter of the adjacent piping.


Some of the other requirements that are necessary to be meat in order to perform test are:


- The inside surface of piping and valve needs to be free of  rust, scale, or other obstructions which may cause excessive flow disturbance.


- For standard test conditions flow in turbulent condition is considered


- No cavitation and vaporization phenomena


- Newtonian fluid.

 

Length of pipes and pressure tap positions


Figure 2. Length of pipes and pressure tap positions

 

Additionally, upstream or downstream of the test section, flow measuring instrument may be used to determine the true time-average flow rate as shown in Figure 1.

 

For standard test conditions flow coefficient is determined with following relations:



In many industrial applications, reducers or other fittings are attached to the valves. The effect of these types of fittings on the nominal flow coefficient of the valve can be significant. 


A correction factor should be introduced to account for this effect. Additional factors are introduced to take account of the fluid property characteristics that influence the flow capacity of a valve. Figure 3 and Figure 4 shows relevant equations for expressing flow coefficient in relation to flow type and fluid type.


For more detail insight into variations and correction factor one should refer to standard series IEC 60534.

 

Sizing equations for turbulent flow


Figure 3. Sizing equations for turbulent flow


Sizing equations for laminar and transitional flow


Figure 4. Sizing equations for laminar and transitional flow




 


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