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1. Why can’t control valves operate at low opening degrees? When operating at a low opening degree, control valves experience sharp changes in flow resistance, flow velocity, pressure, etc., which lead to the following problems: ① The throttling gap is at its smallest, the flow velocity is at its highest, and erosion is severe, significantly affecting the valve’s service life ； ②When the flow rate and pressure changes are severe enough to exceed the stiffness of the valve, its stability is compromised, and severe oscillations may occur ； ③For valves operating in a flow-blocking condition, jumping closure or jumping startup phenomena occur, and control valves are unable to perform regulation within this opening range ； ④When the opening is small, the sealing surface of the valve core is close to the throttle orifice, which undermines the sealing performance of that surface ； ⑤Some valves are not suitable for operation at low opening degrees. For example, in the case of butterfly valves, the unbalanced torque is high at low opening degrees, which can cause the valve to open or close suddenly. Similarly, with double-seat valves, one of the valve cores is in the flow-open position while the other is in the flow-close position; thus, their stability is poor at low opening degrees, and oscillations tend to occur. In summary, in order to improve the service life, stability, and proper regulation performance of valves, control valves should be avoided from operating at low opening degrees; such degrees should typically be above 10%–15%. However, for high-pressure valves, two-seat valves, butterfly valves, and control valves in a flow-blocking state, it should be greater than 20% (for linear valves) to 30% (for logarithmic valves). 2. When the valve size is too large or the process conditions change, the control valve often operates at a low opening degree; how can this issue be addressed? ①Reduce the pressure difference ΔP across the valve. From the equation Q = C√ΔP/P, it can be seen that as ΔP decreases, Q also decreases. To maintain a constant flow rate through the control valve, it is necessary to increase the valve opening, thereby preventing the valve from operating at a low opening degree. The specific method is as follows: a. Add a flow-limiting orifice plate after the valve to consume part of the pressure drop ； b. Close the manual valve connected in series on the pipeline until the control valve reaches its optimal operating position. Both methods involve increasing the pressure drop in the pipelines in order to reduce the pressure drop across the valve, since the total system pressure drop ΔP_system = pressure drop across the valve ΔP_valve + pressure drop in the pipelines ΔP_pipelines. Since the ΔP system remains constant, as the ΔP pipeline increases, the ΔP valve must decrease. ②As can be seen from Q=C√ΔP/P, a decrease in the value of C leads to a decrease in Q. To maintain a constant flow rate through the valve, it is necessary to increase the opening degree; this also prevents the valve from operating at a low opening degree. The C value is related to the valve diameter DN and the seat diameter dN. Ways to reduce the C value are: a) replace the valve with one of a smaller diameter, such as changing from DN32 to DN25; b) keep the valve body unchanged and replace it with a spool and seat of a smaller DN size, such as changing from DN10 to DN8. When the control valve operates at a low opening degree, the following problems may occur: 1. Prone to clogging: At a low opening degree, the flow velocity of the fluid is slow, which allows particles in the fluid to settle in the flow channels of the control valve, leading to clogging or jamming of the valve. 2. Unstable flow control: At low opening degrees, the flow control of the control valve may become less stable, and it is prone to being affected by external disturbances or internal factors, resulting in flow fluctuations or inaccuracies. 3. Flashing and oscillation: When fluid passes through a control valve at a low opening degree, flashing may occur, leading to sharp changes in fluid pressure and temperature, as well as valve oscillation. Countermeasures include: 1. Selecting the appropriate valve type: For applications that require operation at low opening degrees, control valves specifically designed for low flow rates, such as micro-control valves, can be chosen to reduce the risk of clogging. 2. Add filters and mesh screens: Filters or mesh screens can be installed to prevent particulates from entering the flow path of the control valve, thereby reducing the risk of blockages. 3. Optimize valve design and materials: A well-designed valve structure made of corrosion-resistant materials can reduce sensitivity to clogging by particles. 4. Increase the operating range of the valve: By combining the control valve with a control system, it is possible to expand the valve’s operating range, thereby improving control accuracy and stability at low opening degrees. 5. Improve inspection and maintenance: Regularly checking and maintaining control valves, as well as promptly cleaning pipes and filters, can reduce the risk of blockages. By properly selecting the type of valve, implementing effective anti-clogging measures, and strengthening the maintenance of control valves, the problems that may arise when these valves operate at low opening degrees can be effectively resolved.