Is it true that the adjustment accuracy of valve handwheels is higher than that of process valves? Let’s discuss this
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This post was last edited by zhaohh3211 on 2017-5-2 at 16:45. I encountered a quite interesting question related to valves, and I’d like to discuss it with everyone: When communicating with users, someone suggested adding a handwheel to control valves, arguing that using a handwheel allows for greater precision in adjustment compared to process valves. Is that true? In my opinion, both can be adjusted in the same way; that is to say, the adjustment precision of a process gate valve is not lower than that of a control valve’s handwheel. What do everyone think? It would be best if there were evidence or illustrations to support it; I hope everyone will share their opinions. Thank you very much!
The handwheel is intended to be used to operate the control valve by hand in case of a failure in the control system (electrical or pneumatic), and it is not meant to replace the process valve for regulation purposes.
What was said on the second floor makes sense – the adjustment accuracy is closely related to the quality of the valves; in our case, even a change of 0.05% in the opening degree of a high-precision valve results in a corresponding change.
Based on your description, adjusting the control valve knob indeed provides higher precision than adjusting a process globe valve! Can’t you figure this out? Think about the respective functions of control valves and globe valves.
Under normal circumstances, the adjustment accuracy of electrical positioners is higher than that of manual controls; however, compared to handwheels, positioner control is dynamically variable, even when the input signal remains constant. But comparing the two isn’t very meaningful, as they are roughly on par. What determines the regulation accuracy of a control valve is the valve body itself, that is, its structure or qualitative design, rather than the positioner or handwheel. We have modified many control loops with poor accuracy; when PID and positioners fail to solve the problem, modifying the valves can work.
Under normal circumstances, the adjustment accuracy of electrical positioners is higher than that of manual controls; however, compared to handwheels, positioner control is dynamically variable, even when the input signal remains constant. But comparing the two isn’t very meaningful, as they are roughly on par. What determines the regulation accuracy of a control valve is the valve body itself, that is, its structure or qualitative design, rather than the positioner or handwheel. We have modified many control loops with poor accuracy; when PID and positioners fail to solve the problem, modifying the valves can work.
The handwheel is intended to be used for operating the control valve by hand in case of a failure in the control system (electrical or pneumatic), and it is not meant to replace the process valve for regulation; however, the precision of the control valve is certainly higher than that of the process stop valve.
In other words, since there are process valves, there is actually no need to install a control valve handwheel anymore.
Under normal circumstances, the adjustment accuracy of manual valves is higher than that of control valves. For example, if the travel distance of a control valve is 50 mm, then a 1% adjustment corresponds to 0.5 mm, which is very difficult for a positioner to achieve. In contrast, a manual valve can easily make an adjustment of 0.5 mm by rotating its threads; therefore, I believe that manual valves have higher adjustment precision than automatic control valves.
Firstly: I agree with the view expressed on the second floor – the addition of a handwheel is mainly intended to allow manual operation of the control valve in case of failures in the control system (whether electrical or pneumatic). Secondly, the adjustment accuracy of the valve should refer to the change in flow rate that occurs when the valve stem extends (or retracts) by a fixed length. And this change amount is determined by the internal structures such as the valve body and the valve core. Of course, positioners can optimize valve control. Furthermore, control valves and globe valves have completely different structures and functions; the primary purpose of a globe valve is not to regulate flow but to shut it off, so its flow control capabilities are certainly inferior to those of control valves. Finally: The regulating performance of control valves with handwheels is higher than that of globe valves. But this is not due to the handwheel; it is determined by the internal structure of the control valve. Therefore, in addition to installing a handwheel on the control valve, it is also possible to purchase a process handwheel mechanism for the related parts of the control valve; it works in the same way. Note that this will add another valve to the pipeline, which increases the cost, as well as creating an additional point of leakage and failure. Finally: It is recommended to meet the owner’s requirements. After all, the shoes are on his feet, and he knows best whether they fit or not. This is just my personal opinion for your reference; I wish you success in winning the bid!