Like other points, I have covered in a selection of the control valve, here is one more consideration required which is a control valve characteristics.
All control valve have inherent flow characteristics define the relationship between valve opening and flow rate.
Here valve opening means a position of the plug to its close position against the valve seat.
To define valve flow characteristics we must know the process requirement, rate of flow that passes through a valve, etc.
There are three types of valve characteristic used based on process application.
1. Equal percentage
3. quick opening.
Let's learn and understand each one in detail.
1. Equal percentage
- This type of valve has plug shape such that with an increment in valve lift flow rate increases by a certain percentage of previous flow.
- Check below image of an equal percentage type plug. The small gap between the valve seat and plug called orifice pass area.
- For example when a valve is open 50% and the previous flow rate is of 0.9 m3/hr then it will add 48% of this flow in the next opening of 50%.
- Above example is of globe type valve with a rangeability of 50 have the same 48% increment with an opening by 10% with constant pressure drop. Valve with different rangeability has different opening and increment percentage.
- You can calculate it by the following equation,
where V = volumetric flow rate
X = ln t
T = Valve rangeability
Vmax = maximum volumetric flow rate through a valve
- A linear valve plug is designed such that with the opening of the plug, the proportional flow rate will pass through it.
- Here, in the following image, you can see the linear type plug. It achieves its linearity by keeping a linear relationship between valve lift and an orifice pass area.
3. Quick opening
- This types of valve plug give large change in flow rate by a small opening of the valve plug.
- For example, a valve have an opening of 30% allow 50% flow to pass through its opening.
- Have a look at quick opening type plug in the below image. With an increment in the opening, a gap between orifice area and valve plug change which allow more flow to pass through a small opening.
Now have a look at Control valve characteristic in the below image.