Step control: Determination of the positive and negative actions of the controller?
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The last edit to this post was made by jlj913 on 2021-12-25 at 12:44. Description of the control system: A controller PICA1234 is connected to two actuators, PV1234B (for breaking the vacuum) and PV1234A (for filling with nitrogen). The goal of this control is to fill the system with nitrogen when the pressure at PT1234 is low； Vent when the PT1234 pressure is high. Are both control valves currently set to fail-safe mode? Is there any problem with this setting? If there are no problems, should the controller PICA1234 be positive-acting or reverse-acting?
Personally, I think it shouldn’t be that way; the nitrogen filling fault should cause it to shut down, while the venting fault should cause it to turn on. The control circuit should be designed to operate based on the set pressure. Open the nitrogen valve when the pressure is below the set value ； Higher than pressure-controlled venting.
In a split-range control system, the controller forms feedback loops through two control valves, and both loops should be negative feedback. If both PV1234B (air breaking) and PV1234A (nitrogen filling) are set to fault-off (air-on) mode, assuming that the pressure controller operates in positive action, negative feedback is established through PV1234B (air breaking), while positive feedback is formed through PV1234A (nitrogen filling) ； If the pressure controller is set to reactive mode, positive feedback is established through PV1234B (air breaking), while negative feedback is created through PV1234A (nitrogen filling). There is always positive feedback along the way. Therefore, PV1234B (breaking through the air barrier) should be set to fault on, while PV1234A (nitrogen filling) should be set to fault off.
If the process requires that two control valves must be in fail-safe shut position! Can the conflict between positive and negative feedback be resolved through a locator or DCS configuration?
If the main regulator is of the direct-acting type, then the range of the cascade regulator PV1234B should be set to 50-100, with it also being a direct-acting regulator; whereas the range of the cascade regulator PV1234A is 0-50, and it is of the reverse-acting type； If the main regulator operates in negative feedback mode, then the range of the cascade regulator PV1234B is 0-50, with negative feedback, whereas the range of the cascade regulator PV1234A is 50-100, with positive feedback.
This post was last edited by jlj913 on 2021-12-25 at 13:04. Thank you~~
Split control point? What does that mean?
This post was last edited by jlj913 on 2021-12-25 at 13:35. For example, how should the values at points C and D in the figure be determined?
The valve positioner plus the control valve together constitute Kv; changing the valve positioner actually changes the sign of Kv.
Thank you; with this explanation, the connection between theory and practice becomes much clearer~
This post was last edited by HaoLiangZhiLe on 2021-12-26 at 10:48