Control valve characteristic guidelines
| Liquid Level Control System | |
| Control valve pressure drop | Best inherent characteristic |
| Constant delta pressure | Linear |
| Decreasing delta pressure with increasing load, delta pressure at maximum load > 20% of minimum load delta pressure. | Linear |
| Decreasing delta pressure with increasing load, delta pressure at maximum load & 20% of minimum load delta pressure. | Equal percentage |
| Increasing delta pressure with increasing load, delta pressure at maximum load & 200% of minimum load delta pressure. | Linear |
| Increasing delta pressure with increasing load, delta pressure at maximum load > 200% of minimum load delta pressure. | Quick opening |
| Source: Control Engineering with data from Fisher Controls | |
| Flow Control Processes | |||
| Best inherent characteristic | |||
| Flow measurement signal to controller is: | Location of control valve in relation to measuring element is: | Wide range of flow setpoint | Small range of flow but large delta pressure change at valve with increasing load |
| Proportional to flow | In series | Linear | Equal percentage |
| In bypass * | Linear | Equal percentage | |
| Proportional to flow squared | In series | Linear | Equal percentage |
| In bypass * | Equal percentage | Equal percentage | |
| * When control valve closes, flow rate increases in measuring element. Source: Control Engineering with data from Fisher Controls | |||
| Pressure Control System | |
| Application | Best inherent characteristic |
| Liquid process | Equal percentage |
| Gas process, small volume, less than 10 ft (3 m) of pipe between control valve and load. | Equal percentage |
| Gas process, large volume [process has a receiver, distribution system, or transmission line exceeding 100 ft (30.5 m) of nominal pipe volume], decreasing delta pressure with increasing load, delta pressure at maximum load >20% of minimum load delta pressure. | Linear |
| Gas process, large volume, decreasing delta pressure with increasing load, delta pressure at maximum load &20% of minimum load delta pressure. | Equal percentage |
| Source: Control Engineering with data from Fisher Controls | |
| Comments? E-mail dharrold@cahners.com |
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