What are the 3 most common control valve flow characteristics?
To satisfy the needs of many applications, control valves are made with a range of flow characteristics. Fast opening, linear, and equal percentage are the three most prevalent flow characteristics. An outline of each is provided below, with straightforward explanations:
1. Quick Opening Characteristic
- A quick opening valve is made to permit significant amounts of flow with only a slight movement of the valve from the closed state. For instance, up to 90% of the maximum flow rate may be possible if the valve is opened halfway (50% lift).
- Usually, these valves are employed in “on/off” applications where prompt action is required but exact control is not. They are frequently operated by pneumatic or electrical means.
- Because the flow curve’s shape isn’t uniform, two fast-opening valves may operate marginally differently.
- Instead of being true fast-opening valves, some valves that resemble fast-opening valves but are capable of making tiny, accurate adjustments are better categorized as high-rangeability valves.
2. Linear Characteristic
- The relationship between the flow rate and the valve lift, or how much the valve is open, is constant with a linear valve. A valve that is 40% open, for instance, permits 40% of the maximum flow to go through.
- These are perfect for procedures that require a predictable flow rate change that is directly proportional to the movement of the valve.
- The orifice size, or the flow opening, is guaranteed to increase at the same rate as the valve lift by the design.
3. Equal Percentage Characteristic (Logarithmic)
- Instead of increasing the flow by a set amount, each step in valve lift with an equal percentage valve raises the flow by a percentage of the present flow rate. It is a logarithmic connection. The flow may double, for instance, if you increase the open from 10% to 20%, and the same doubling impact occurs between 20% and 40%.
- These valves are ideal for situations where controlling flow at lower levels requires minor adjustments to the valve position, whereas controlling high flows successfully requires more significant adjustments.
- This feature guarantees smooth operation across a broad range of flow rates and improves control at low flows.
How to Choose the Right Flow Characteristic ?
- Fast Opening: For straightforward on/off control or situations requiring quick reaction, use this feature.
- Linear: Select when consistent and predictable flow changes require proportionate control.
- Equal Percentage: Ideal for systems that need precise management, particularly at lower flow levels, and have large flow rate variations.