Technical documents list
Supplementary documents list
What is "Cv factor"?
About "Cv factor"?
The Cv value is a specific coefficient indicating the ease at which a fluid can flow. This value increases in inverse proportion to pressure loss, such that a higher value indicates less pressure loss and easier flow of the fluid.
At the time of valve open, volume or weight of fluid passed into the valve is called capacity of valve. it's fixed depends on kind of valve and port size generally.
But even if the same size and same kind of valve, the capacity is different if maker is different. And it is changed by various factors such as the kind of fluid / differential pressure / temperature / viscosity / specific gravity / pressure / port design, dimensions and etc.. And furthermore, even the same size valves has both big pressure loss and small pressure loss also depends on kind of valve. Therefore slection of valve is difficult because it is neccessary to check and consider about many kinds things.
The evaluated these fluid conditions are called coefficient capacity, it can be selected kind of valve and size easily.
There are Cv factor, Kv factor and Av factor for coefficient capacity. Cv factor is using generally, it is expressed factor for fluid 60°F that flow into valve by US gal/min. in a situation of special travel and differential pressure 1 lbf/in2.
If differential pressure is same, flow rate is big so that Cv factor is big. And if Cv factor is same, flow rate is big so that differential pressure is big.
Cv factor can be calculated as following formula.
- Cv factor
- Flow rate [gal/min]
- Specific gravity (For water =1)
- Differential pressure [lbf/in²]
Kv factor is expressed flow rate for 5～40°C water pass the valve by m3/h. in condition of differential pressure is 1bar,
And Cv factor and Kv factor are different unit, but addapted the following.
Cv value of 2-way full bore-type ball valve (Reference value when fully open)
The CV value is a capacity coefficient indicating the ease at which fluid flows for a specific valve and it is mainly used for determining the bore of control valves for purposes of flow regulation.
Although ball valves are mainly used in on/off applications because pressure loss is small enough to be negligible due to no part of the valve disc remaining in the fluid passage when fully open, a feature of ball valves, there have also been many cases of ball valves being equipped with instruments such as positioners to perform opening control.
The figure below indicates reference CV values for 2-way full bore-type ball valves. In comparison with other types of valves, cavitation and flashing readily occurs with ball valves because of the low pressure difference due to the valve structure. For this reason, you must keep in mind that it might not always be possible to maintain the flow rate according to the calculation formula.
* Using a soft seat ball valve at an intermediate open angle can result in seat deformation. Please contact HISAKA if you want to use for flow regulation applications.
|2-way full bore
Specific characteristics of ball valves
A specific characteristic required of ball valves is that the pressure difference is maintained uniform when using an incompressible fluid such as room temperature water whose density is thought not to change.
The graph below shows an example of characteristics specific to ball valves indicating how flow rate depends on the opening angle.
The horizontal axis indicates valve opening angle (%) and the vertical axis indicates the flow rate (%).