 conclude that Δq should be 16 gpm. when the valve opening changes from 60% to 70%.
The flow rate through the valve is therefore approximately 32 gpm. at 70% valve opening.
Installed flow characteristic
When valves are installed with pumps, piping and fittings, and other process equipment, the pressure drop across the valve will vary as the valve travel changes.
When the actual flow in a system is plotted against valve opening, the curve is called the installed flow characteristic and it will differ from the inherent valve characteristic which assumed constant pressure drop across the valve.
When in service, a linear valve will in general resemble a quick opening valve while an equal percentage valve will in general resemble a linear valve.
Typical application of quick opening, linear and equal percentage valves
i) Quick opening valve
- Frequent on-off service.
- Used for systems where ‘instant’ large flow is needed (safety or cooling water systems).
ii) Linear valve
- Liquid level and flow control loops.
- Used in systems where the pressure drop across the valve is expected to remain fairly constant.
iii) Equal percentage valve (most commonly used valve)
- Temperature and pressure control loops.
- Used in systems where large changes in pressure drop across the valve are expected.