Positive and negative action of valves
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Should the forward and reverse effects of the valve be taken into consideration during operation? What are the differences when sending signals to valves with such forward/reverse effects? ? 、
Regarding the forward and reverse action of the valve, there is no need to consider it during operation; in DCS control, it is simply turned on when open and turned off when closed, with corresponding settings in place.
There is one called a reverse valve, which is used to increase the flow rate in the control room while reducing it at the site. The site you mentioned is the same as the control room
It wasn’t explained clearly enough. If it’s a control valve, of course there is a difference; especially for globe valves, reverse installation is strictly prohibited. As for shut-off valves, globe valves aren’t suitable for this purpose, while gate valves can be used
For pneumatic control valves, there are air-actuated open valves (FC) and air-actuated closed valves (FO). The opening degree can be defined manually in DCS; air-actuated open valves generally operate in a proportional manner, while air-actuated closed valves can be set to operate proportionally or inversely in DCS
The air shut-off valve closes when there is air flow and opens when there is no air flow； The wind-opening valve opens when there is wind and closes when there is no wind. For the air shut-off valve, the greater the signal you send, the more it closes; when the internal control signal is 0, it opens, while a value of 100 causes it to close! The wind-operated valve does the exact opposite
Positive action and negative action are concepts considered from the perspective of process production. To put it simply, take an example: in a level tank, there is a valve used for adding material, and the liquid level regulates this valve to enable automatic control. If the liquid level exceeds a set value, the valve closes – this is negative action. In another type of tank, there is a valve used for removing material, and again the liquid level regulates this valve for automatic control; if the liquid level exceeds the set value, the valve opens – this is positive action
During operation, the forward and reverse actions of the valve need not be considered, as the valve is automatically adjusted based on the given process signal. However, when configuring the system, it is essential to determine the direction of action accurately; otherwise, during operation the valve will respond to signals in the opposite direction, leading to increasing chaos.
Pneumatic actuators have forward and reverse operating modes, and valves also come in forward and reverse installation styles, thereby enabling four different combinations for pneumatic control valves: open on air pressure, close on air pressure, etc. Actuator Valve body Control valve Forward Forward Air-shut Off Forward Reverse Air-open Forward Reverse Air-open Reverse Reverse Air-shut Off
For pneumatic control valves, there are air-actuated open valves (FC) and air-actuated closed valves (FO). The opening degree can be defined manually in DCS; air-actuated open valves generally operate in a proportional manner, while air-actuated closed valves can be set to operate proportionally or inversely in DCS
In short: it turns everything off at 0% and turns everything on at 100%, and the reverse effect can be modified.