Dear experts, I have a question regarding the forward and reverse action of control valves
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The mode of operation of a control valve is relevant only when selecting a pneumatic actuator; it is determined by the combination of the forward and reverse actions of the actuator along with those of the valve. There are 4 combination types: positive-positive (air-shut type), positive-negative (air-open type), negative-positive (air-open type), and negative-negative (air-shut type). The operation modes of the control valves resulting from these four combinations are air-open and air-shut. I don’t quite understand. Could that person explain it?
The control valve consists of a pneumatic actuator and a valve body； Definition of forward and reverse action for pneumatic actuators: An increase in air pressure causes the valve stem to move downward, which is considered forward action; an increase in air pressure causes the valve stem to move upward, which is considered reverse action ； Definition of positive and negative action for the valve body: A decrease in the flow area when the valve stem moves downward is considered positive, while an increase in the flow area under the same condition is considered negative ； So there are four combinations.
In that PID specification, gas-operated valves are specified as having a positive action – what is the relationship between this and the concept of positive or negative action for the actuation mechanisms/valves you mentioned?
Considering the control valve as a single unit, an increase in the signal acting on it results in an increase in the flow rate through the control valve; this is considered a positive action (with a positive amplification factor). From the perspective of the entire control system, the loops formed by various components ensure negative feedback, which in turn determines whether the control valve operates on air opening (direct action) or air closing (reverse action) principle. At this point, there is no need to consider the reaction forces on the actuator/valve body; only the overall operating mechanism of the control valve needs to be taken into account.
For example: a valve is used to control the addition of water to a water tank; when the valve is open, the water level rises – this is positive action； The water level drops when the valve is used to control the drainage from the water tank; this is the reaction effect.
Both the action and reaction are relative; it doesn’t really matter what is used in between. The key is that the controlled quantity and the system output must correspond logically. It can be said that a controlled increase in output has a positive effect on the volume growth, while a controlled decrease in output has a negative effect.
The reason you understand it this way is probably related to some reference books you have read. The previous valves had forward and reverse actions because automation was in its infancy at that time – there were no DCS systems, no PLCs, and no intelligent positioners. So, the counteraction can only be achieved on the valve body and the mechanical mechanism. But with the advancement of technology. The action and reaction are already something that can be adjusted at will based on electrical signals. But the information is still old, so it’s confusing. As a young person in the new era, you should abandon old and outdated knowledge. These days, it’s almost never necessary to consider the direction of action on the valve body anymore; it can be ignored.
Combination of actuator and controller