In-depth discussion on pneumatic control valves and pneumatic on/off valves

I will answer each one of you one by one. 1. Positioners are used to enable precise control of the opening degree, whereas on-off valves require rapid switching; therefore, five-way spool valves or solenoid valves are typically used in such cases. 2. A control valve equipped with a positioner can be used for regulation. 3. The price difference isn’t only evident in the positioners; control valves typically use cylinders, while common control valves employ diaphragm heads, and there is already a price difference between the two!

Valves used for control typically have several types of characteristic curves: linear, parabolic, equal percentage, etc. In terms of its regulation performance, equal percentage characteristics are the optimal choice, offering stable regulation and good performance. Moreover, the parabolic characteristic offers better adjustment performance than the linear one, allowing any of these flow characteristics to be chosen depending on the requirements of the application. Since the function of on-off valves is to cut off flow quickly, they are usually designed as butterfly valves, ball valves, plug valves, etc. As mentioned on the second floor, valve positioners provide a regulating function, but the regulating performance of such valves is not necessarily ideal. The price difference between pneumatic control valves and pneumatic on/off valves lies not only in the cost of the positioners related to the air supply, but primarily in the valve body itself. Generally, the price of ball valves and butterfly valves is much lower than that of control valves.

OP, 1. It’s certain that pneumatic control valves are equipped with valve positioners, while pneumatic on/off valves are equipped with solenoid valves. However, they all require air filtration and pressure reduction devices to ensure that the air supply for the pneumatic actuators is clean, dry, and has a stable operating pressure. 2. A pneumatic on/off valve equipped with a valve positioner can be modified into a pneumatic control valve. But it depends on what kind of valve structure it is Generally, it involves modifying angle-action pneumatic on-off valves: 1. If the butterfly valve is completely universal. 2. If it is a ball valve, a V-type ball valve is also acceptable. Because the above can meet the regulation characteristics. If it is an O-type ball valve, I do not recommend making any changes, as its adjustment characteristics are too poor. 3. For angle-controlled regulating valves and on/off valves that have the same valve body structure, the same valve core material, and the same pneumatic actuation mechanism, there are no special requirements regarding the choice of accessories. The difference in price indeed lies in the cost of the positioner, but this is a relative difference! If the positioning device is the Chunxin CCCX4000 series and the solenoid valve is an original imported ASCO model, then the total cost of the on/off valve will be much higher than that of the control valve.

The first answer is fine; as a supplement: two-way three-port valves are generally used more often. I disagree with the second point; how can a on/off valve be replaced with a control valve? I agree with the third point as well.

Reply to 5# backhamm1982: There are no strict definitions for control valves and on/off valves; they simply serve different functions. A positioner is installed on the on/off valve to regulate the air pressure in the cylinder; it can also be used for adjustment purposes

Reply to 6# zzyslhj: Control valves are not suitable for use in regulation. You can definitely use it, but performance and lifespan won’t be guaranteed.

We have them in our factory; we use positioners to adjust the cylinders. I’ve been using it for many years; it works fine! !

A technical upgrade project ran into this issue, so I’ll just share a few thoughts on it. We are the owners; there is a pressure control circuit in which the medium is liquid chlorine. It was designed by a small design institute; the valve used is a shut-off valve that closes when the pressure increases and opens when the pressure decreases. We (including the engineering team) were completely puzzled and raised our concerns on multiple occasions: why not choose a control valve? The response was “Vaporization occurs after the pressure at the valve outlet drops, resulting in cavitation.” . . Or something like that; in any case, it’s not safe. The problem is that our original unit uses the same process, designed by Tianchen Design Institute, and it has worked very well to this day, remaining safe throughout. There was no other choice; to comply with the design requirements and to ensure successful approval, we decided to use a control valve, temporarily functioning as a on/off valve. The DCS program was developed by me, and the user interface allows the operator to choose between \"on/off control mode\" or \"adjustment control mode\". In switch mode, if the pressure is high, the circuit outputs 0%; if the pressure is low, the circuit outputs 100%. If it is in regulation mode, it is the same as a regular PID control loop. However, with the above methods, whether it is switching the valve or using a shut-off valve to regulate pressure, it is not appropriate; frequent opening and closing of valves, no matter how high the quality of the valves, can easily lead to leaks, so how can safety be ensured? Let’s talk about why on-off valves cannot be used as control valves. A control valve is essentially a throttling element with specific flow characteristics, whereas a on/off valve is a quick-opening valve whose flow characteristics do not meet the requirements for regulation. The Cv of on/off valves of the same diameter is generally much larger than that of control valves. Can that control valve be used as a shut-off valve? It should not be done unless it is absolutely necessary, and it is not recommended. Firstly, the cut-off valve has better sealing performance (lower leakage) ； Secondly, control valves are not suitable for applications involving frequent opening and closing (this can be understood by examining the structure of the valve body and the valve core). Therefore, the appropriate valve must be selected based on the process requirements.

A pneumatic on-off valve, when equipped with a valve positioner, can be converted into a control valve. But it should be a measure taken in emergency situations. A few years ago, I installed a DURCO rotary plug valve with an SMC electrical valve positioner to use as a level control valve, and it worked fine. There were no spare control valves available at the time, so a temporary solution was used; there were concerns that it might not work, but the actual result was fairly good.

Reply to 9# denghl: Good technical skills; the explanation was quite comprehensive.