Regulating valve opening and precision

When selecting a control valve, it is necessary to provide the primary side pressure and secondary side pressure, as well as the maximum flow rate, normal flow rate, and minimum flow rate. As long as it is within the range of maximum and minimum flow rates, the adjustment is generally quite accurate. When the flow rate exceeds the maximum value or is below the minimum value, the deviation may be quite large. It can even happen that when the opening is adjusted, the flow rate remains practically unchanged. During normal operation, the manual valves before and after the control valve are usually kept fully open; they are closed only when the control valve needs to be inspected or the system is shut down.

1. Assuming the selection of the control valve is correct, the control accuracy is determined by the control system (system margin)； 2. Each component of the control system has an amplification factor (see the transfer function block diagram); an amplification factor that is too high or too low in any single component is not conducive to the stability of the system ； 3. Generally speaking, increasing the differential pressure across the control valve improves the control quality, but it also increases pipe losses. 4. For equal percentage control valves operating in a throttled flow regime, the amplification factor remains relatively constant across the entire range of motion; however, the optimal design opening degree is between 1/4 and 3/4 ； For other characteristics and operating conditions, an opening degree of the control valve between 1/3 and 1/2 is considered ideal. 5. As the opening degree of the control valve increases, the amplification factor decreases; in the case of a PID controller without self-tuning functionality, the control quality deteriorates at high opening degrees (equivalent to a reduced proportional effect) ；

For most control valves, the best regulation accuracy is achieved when the opening degree is between 30% and 75%. If the opening degree is too high or too low, it indicates that there may be an error in the valve selection, and it may be necessary to reduce or increase the diameter of the valve

If the opening degree of the control valve is too large or too small, its control accuracy will be relatively poor. It is not possible to adjust the opening degree of the control valve by closing the upstream and downstream manual valves; rather, the control valve must be selected appropriately during the initial design phase, based on the designed normal flow rate, maximum flow rate, and minimum flow rate. If the opening degree of the control valve remains too large or too small during operation, it can be adjusted by reducing or increasing the diameter of the control valve pipe. However, closing the upstream and downstream manual valves will cause prolonged wear of the valve disc, making it difficult to remove the control valve later on.

I do not recommend using hand valves for control; let the throttle valve ensure precise regulation, as erosion and corrosion are inevitable； This makes subsequent operations more troublesome.