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
2. linear
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
2. Linear
- 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.
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