Regarding step control and the forward/reverse action of PID
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Please take a look at the flowchart in the attachment as well as the relevant diagrams. In simple terms, when the pressure is low, valve A is opened to introduce nitrogen; when the pressure is high, valve B is opened to release gas. Valve A is of the open-type, while valve B is of the closed-type. Question: 1. Is the control graph showing the relationship between the PID output in the image and the valve position drawn correctly? 2. Is the action mode of the PID in this case positive or negative? Thank you!
This post was last edited by oceans on 2019-6-21 20:59 to control A with 0-50. 50-100: Control B; gas A is shut off, gas B is turned on
The diagram clearly shows that the controller is a feedback controller. Valve A operates in fail-closed mode (open on air), with a range of 50–100% from fully closed to fully open; Valve B operates in fail-open mode (closed on air), with a range of 0–50% from fully open to fully closed. Assuming that the pressure inside tank 133-D108 remains stable at a certain level, the controller output stays at 50%, and both valves are in the closed state. When the pressure decreases, the controller’s measured signal drops, and the controller’s output increases (due to the feedback effect); valve A begins to open to fill the tank with nitrogen (while valve B remains closed), and the pressure inside the tank is restored ； As the pressure increases, the controller detects an increase in the signal, and its output decreases (due to feedback action); valve B begins to open to release nitrogen (while valve A remains closed), causing the pressure inside the tank to drop.
1 The relationship shown in the diagram is correct. 2 Reaction force
1. If valve B is changed to an FC (air-operated) type, will the diagram change? Is it still independent of whether the valve operates on air open or air close principle? 2. In this case, will setting the PID value at 50% cause valves A and B to operate frequently, thereby affecting their lifespan? Is it necessary to establish a buffer range of around 49%–51% so that both valves remain closed? Or is it normal for the control valve to operate frequently for adjustment, and it doesn’t matter? Thank you!
1. Valve B cannot be changed to the FC (air-operated) type, because the circuit from the controller to each control valve must be of negative feedback; if Valve B is changed to the FC type, this circuit would become positive feedback； 2. Since tanks have a certain pressure-bearing capacity and fluctuations within a certain range do not cause problems, a dead zone can be set around 50%, for example, 49%–51%.
Valve B definitely cannot be used for accident shutdown; otherwise, in the event of an accident, if the pressure exceeds the tank’s tolerance limit, there is a risk of the tank exploding
This step-by-step control and the VOC nitrogen inflation pressure maintenance control are based on the same principle
It is recommended not to use 50%, as it tends to fluctuate; use values between 0 and 47, and between 53 and 100, leaving a small gap in between.