Are there any people who work in design? This is the information provided by the designer; please check if there are any issues

This post was last edited by jcicwht on 2017-5-4 15:50; the positive effect should be selected

Thank you, expert. The information provided by the designer is as follows; can this be considered a design error? Do they have other rules, is that how it is for FO valves?

This post was last edited by tiandiyi*an on 2017-5-4 20:37. 1. The direct and reverse action of valves have nothing to do with FO and FC; FO can be either direct or reverse in action. Air-operated valve: It requires compressed air to open, and it closes when the air supply is cut off. (FC) Air-operated valve: It requires compressed air to close; the valve opens when the air supply is cut off. (FO) Direct action: When the actuator moves the valve stem downward as the pressure of the actuating gas increases, this is called direct action. Reaction: The upward movement of the valve stem by the actuator as the pressure of the actuating gas increases is called reaction. 2. The direct and reverse action of the controller in the system loop are related to FO and FC. Direct Control Action: When the input (error PV-SV) increases, the output (MV) increases, which is a positive action ； In simple terms, as PV increases, MV increases as well; when PV decreases, MV also decreases. Reverse Control Action: When the input (error PV-SV) increases, the output (MV) decreases, which is a positive-action system ； In simple terms, when PV increases, MV decreases; when PV decreases, MV increases. A single-loop system consists of four components: the regulator, the transmitter, the valve, and the process being controlled. For the loop to be controllable, it is necessary to maintain negative feedback, which means that the product of these four components must be negative. Both the transmitter and the controlled object are positive, FO is negative, and FC is positive. Therefore, it is concluded that the control effect should be positive.

This post was last edited by einsamdog on 2017-5-4 20:19. In a simple control loop, it can be understood as follows: Direct Control Action: When PV increases, MV increases; when PV decreases, MV also decreases. Reverse Control Action: When PV increases, MV decreases; when PV decreases, MV increases. Personally, I think Direct control should be used in this case, rather than Reverse control. Or a valve of an FC. It’s recommended to talk to the technician and ask what he needs.

What my brother said is absolutely correct – from a safety perspective, it’s indeed right to choose FO valves. I think a direct-acting type should be selected in this case. However, the screenshots shown above were provided by the designers, who recommended a reverse-acting type. Could this be considered a design mistake? Is there another reason behind the design? If configuration is required, decide whether to use direct or reverse action

When it comes to modifying the action and reaction forces, it’s a matter of seconds. But try to be as precise as possible.

We can’t rush to say whose fault it is, because the screenshot is only part of this circuit. On the FIC, in addition to the FT input, there is another input signal, which has not been clearly explained. What everyone has said above is based only on this screenshot. If this FIC is only part of a complex control loop, the outcome might be different. So, it’s best for you to talk to the process engineers before making a decision.

A VOC project? Don’t listen to those upstairs; they don’t understand. This should be a cascade loop; is the main loop for temperature control? If so, select “-” for the main controller, “-” for the flow rate control unit, FO for the valve, and invert the valve output.

Agree with what was said on floor 8 – the screenshot shows only part of this circuit; if the other one is a temperature signal. As the temperature rises, reduce the valve opening; it has a reverse effect.

This is the secondary loop of cascade control. Just from the perspective of the secondary circuit, it is indeed wrong