Will pressure fluctuations downstream of the valve affect the flow control of the electric control valve?

This post was last edited by HEJIYUER on 2015-12-14 22:42. The flow control loop FIC is governed by flow rate. The liquid flow rate reading (i.e., the measured value) does not change with variations in downstream pressure; it truly measures the flow rate. In a constant FIC control loop, if there is a change in the flow measurement, the FIC will necessarily command the valve to act in order to correct this deviation and maintain the set flow rate. However, the actual flow rate through the valve is related to the opening degree and the actual valve pressure drop, and they are in direct proportion to each other. If an increase in back pressure leads to a decrease in the pressure drop across the valve, then the flow rate through the valve will decrease, and the measurement values will also drop. In response, the FIC command increases the opening degree of the control valve, using the flow area to compensate for the difference in pressure, thereby maintaining a constant flow rate through the valve. Theoretically, it can be controlled; in practice, it is required that the back pressure not experience frequent large fluctuations, otherwise the valve will keep \"following\" the changes in back pressure, and moreover electric valves operate relatively slowly. If the pressure difference is 1 BAR and the flow rate is 1.2 M3, it seems that a DN50 valve is a bit too large; an inch-sized valve would be sufficient. @jiaguoyun

Can it be understood in this way: based on the actual flow demand, I set a fixed flow value through the system, and then the system sends commands to the control valve taking into account various factors related to the pipeline (including pressure differential fluctuations), instructing the control valve to continuously adjust its opening degree in order to achieve the desired flow rate? Is that correct? As various factors change frequently, does the operating frequency of the control valve increase as well?

Can it be understood in this way: based on the actual flow demand, I set a fixed flow value through the system, and then the system sends commands to the control valve taking into account various factors related to the pipeline (including pressure differential fluctuations), instructing the control valve to continuously adjust its opening degree in order to achieve the desired flow rate? Is that correct? As various factors change frequently, does the operating frequency of the control valve increase as well? Basically correct:D. I’ll give an example to make it easier to understand. (The flow meter is used for detection; it cannot control the flow rate.) If the control by the FIC and the control valve was stable previously, but now the pressure in the downstream process system has increased (which means the pressure drop across the valve has decreased), then if the valve opening remains unchanged, the flow rate through the valve will decrease. The flow meter will detect this decrease in flow rate (a deviation from the set value), and in response to this deviation, the FIC will adjust to increase the valve opening. As a result, the flow area of the valve increases until the flow meter reading reaches the set value, at which point the valve opening is \"fixed\" at the new position. Therefore, as the back pressure changes frequently, the valve opening also needs to change accordingly, in order to \"match\" the differential pressure and ensure that the actual flow rate remains as close as possible to your set value. Control is a process of keeping an eye on the target and making continuous adjustments. Just like driving, you need to turn the steering wheel left or right depending on road conditions, otherwise the vehicle will drift off course. Frequent changes in the opening degree (=steering wheel angle) are not good for controlling the flow rate (=vehicle stability). @jiaguoyun

Thanks for the advice, I’ve learned it: handshake

One more question: In this operating condition, is it better to use an electric version or a pneumatic one?

If you have instrument air, pneumatic valves are the best choice. Pneumatic valves have a safe position (referring to the position of the valve when air supply is cut off, as determined by process engineers based on safety principles). It is generally believed that pneumatic valves operate faster than electric ones, as large motors present issues related to installation and temperature rise. However, if the back pressure fluctuates frequently, the control valve will frequently change its opening degree (see the analysis above). I think we can first find a way to stabilize the back pressure of the process.

I would like to ask you again: under these operating conditions, can a \"butterfly valve\" or a \"V-ball valve\" be used to replace the \"control valve\"?

This post was last edited by HEJIYUER on 2015-12-25 00:25. Butterfly valve products are generally 4 inches or larger; yours is not suitable. V-type ball valves have the advantage of a wider adjustable range, and 2\" versions are also relatively rare. For your operating conditions, a valve with CV=4–5 is suitable (the exact value needs to be determined by the manufacturer); in other words, a 1-inch control valve will suffice. My suggestion is to use a regular linear actuator control valve. @jiaguoyun

Regarding the material of the valve body, I would like to ask: Is it appropriate to use 316L? Or would WCB + lining be more appropriate? Or rather, what are their respective advantages and disadvantages?