Is the movement direction of the actuator push rod of the control valve consistent with the opening and closing direction of the valve?
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In pneumatic diaphragm actuators, the direct-acting type means that as the signal pressure increases, the actuator’s push rod moves downward, moving from its zero position to its full-stroke position； In the reaction type, when the signal pressure increases, the push rod of the actuator moves upward, moving from its full stroke position to the zero position (this is information from Lude’s Manual on Automatic Control in the Petrochemical Industry). According to this, the zero point of the control valve’s travel is at the top, while the full travel position is at the bottom. Isn’t the travel position nothing but the opening degree of the valve? (I’m not sure if my understanding is correct?) In other words, the valve is closed when the push rod is in the upper position; the valve opens as the push rod moves downward. But as I understand it, it becomes a bit confusing when it comes to the valve part! ! Taking a globe valve as an example, in daily manual operation, it should be remembered that when the valve stem is upward, the more of the stem that is visible, the more open the valve is; whereas when the valve stem moves downward and enters inside the valve, that’s when the valve is closed! The same is true when considering the common sectional view of globe valves! Isn’t such an understanding that it opens upwards and closes downwards! ? Isn’t regulation simply achieved by adding an actuator to the valve? The push rod of this actuator moves in a direction that is different from that of the valve stem What’s going on? !
This issue was discussed before at http://bbs.hcbbs.com/viewthread.php?tid=243006
You haven’t noticed that the correct and reverse installation of the valve core, the forward and reverse operation of the valve, the forward and reverse operation of the control mechanism, as well as the forward and reverse operation of the control system are all interrelated. When the valve core of your valve is installed in the correct orientation, it is open; when installed in the reverse orientation, it is closed.
Yes, one can’t rely only on appearance; the correct or reversed orientation of the valve core, as well as the different operating modes of the actuator, mean that it’s impossible to tell just by looking at the overall appearance.
I’m very grateful for the reference you provided. I went to take a look; it’s on the 3rd floor there, and the issue of whether the solenoid valve is energized or de-energized is mentioned there. After reading that, I got confused again. What does this have to do with solenoid valves? ? Is there any connection? Also, does a regular control valve use just one supply of instrument air?» Does it refer to a signal air source of 0.2~1 kgf? Do we mean the signal air source when we talk about the air supply for opening and closing? I remember that the normal gauge air pressure isn’t 4~7 kgf, right? How is this signal air source obtained? ? Thank you! ! Hope someone with more experience can give some advice! !
One more thing I’d like to ask: is the travel scale of the control valve (0~100%) oriented from bottom to top? Mainly, it’s because I read the content in the Petroleum and Chemical Industry Automatic Control Manual compiled by Lu De – that’s why I think so! !
By looking at the principle behind this action, you’ll understand it: http://www.tj88.cn/question/answer.asp?id=403#A
Just because the actuator rod of the control valve moves downward or upward does not mean it is in the open or closed position; the valve element can be installed in either orientation. The pressure range of 4 to 5 BAR corresponds to the air supply, while the pressure range of 0.2 to 1.0 BAR relates to the pressure applied to the diaphragm head. This pressure is determined by a 4-20MA signal generated by the DCS system. The ESD system ensures that the valve closes or opens in the event of a power loss, and its response time is faster than that of the DCS!
Regarding the issue with solenoid valves: Solenoid valves are installed on the air supply line of control valves, and are generally used to cut off the air supply as part of interlock mechanisms. When selecting a valve, its FO and FC values are already determined; therefore, during emergency interlocks, the solenoid valve activates to cut off the air supply, putting the valve in a safer state. Solenoids are also divided into those that open when powered on and those that close when powered on, with the specific type determined by the process requirements.
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.