Instrumentation: [Weekly Question] Week 48 of 2011: Stability issues of control valves

The last edit to this post was made by denghl on 2012-12-12 at 21:29. The stability of a control valve refers to its ability to resist various disturbances when the signal pressure remains constant; if it can resist such disturbances, then it is stable, otherwise it is not. It is related to the following factors: ① The direction of the unbalanced force. When the direction of this unbalanced force pushes the valve core open, stability is good ； The direction of the unbalanced force is such that it presses the valve core shut; when Ft

The last edit to this post was made by denghl on 2012-12-12 at 21:28. The stability of a control valve refers to its ability to resist various disturbances when the signal pressure remains constant; if it can resist such disturbances, then it is stable, otherwise it is not. It is related to the following factors: ① The direction of the unbalanced force. When the direction of this unbalanced force pushes the valve core open, stability is good ； The direction of the unbalanced force is such that it presses the valve core shut; when Ft

The last edit to this post was made by denghl on 2012-12-12 at 21:28. The stability of a control valve refers to its ability to resist various disturbances when the signal pressure remains constant; if it can resist such disturbances, then it is stable, otherwise it is not. It is related to the following factors: ① The direction of the unbalanced force. When the direction of this unbalanced force pushes the valve core open, stability is good ； The direction of the unbalanced force is such that it presses the valve core shut; when Ft

The last edit to this post was made by denghl on 2012-12-12 at 21:28. The stability of a control valve refers to its ability to resist various disturbances when the signal pressure remains constant; if it can resist such disturbances, then it is stable, otherwise it is not. It is related to the following factors: ① The direction of the unbalanced force. When the direction of this unbalanced force pushes the valve core open, stability is good ； The direction of the unbalanced force is such that it presses the valve core shut; when Ft

What is the stability of a control valve? What factors influence it? How can it be determined? My personal understanding is that stability refers to the valve’s ability to meet the requirements for use, to achieve the desired control accuracy, to reach the specified opening degree within a given time frame, without experiencing oscillations around that opening degree. Factors: the control precision of the actuator, the frictional force of the packing at the sealing points, air supply pressure, etc. How to determine it: whether the valve adjusts back and forth around its opening position during operation

The last edit to this post was made by huangbhu1987 on 2011-12-12 at 17:42. The stability of a control valve refers to its ability to resist various disturbances when the signal pressure remains constant; if it can resist such disturbances, it is stable, otherwise it is not. The common solutions for poor stability of control valves and oscillations are as follows: 1) Change the direction of the unbalanced force Ft. Typically, the direction of FI is changed by altering the flow direction. For example, by changing a straight-through single-seat valve with dg≥20mm from a flow-closing type to a flow-opening type, the stability issue of the valve can be easily resolved. (2) By avoiding the alternating points where the direction of the unbalanced force FI changes in the valve’s own unstable region, oscillations in the valve can be prevented. For example, in the case of butterfly valves, the rotation angle alternates between 5°–10° and 75°; therefore, the minimum opening angle should be greater than 20%, with the full opening angle being 70° ； Another example is the two-seat valve, which generally exhibits variation within 10% and at opening degrees of 80% to 90%; this should be avoided during use. (3) Replace it with a valve that has good stability – such valves exhibit minimal changes in unbalanced forces and provide good guidance, and sleeve valves possess this characteristic. When single- and double-seat valves have poor stability, sleeve valves can be used as a substitute. (4) Increase spring stiffness. This is a common and simple method for improving stability, such as replacing springs with a stiffness of 20–100 kPa with ones having a higher stiffness of 60–180 kPa. This method is mainly used for valves equipped with positioners; otherwise, a separate positioner must be installed. (5) Reduced response speed: When the system requires that the valve’s response or adjustment speed not be too fast (for example, when fine tuning of flow rate is needed) but the valve operates at a high speed, or when the system itself is already a fast-response system and the valve is equipped with a positioner to further accelerate its action, overshoot and oscillation will occur. To address this, the response speed should be reduced. The solutions are: ① Change the linear characteristic to a logarithmic characteristic ； ②Those with positioners can be replaced by converters or actuators. What are the reasons for the unstable operation and oscillations of the 876 pneumatic diaphragm control valve? ① The output signal of the regulator is unstable. ②The pipeline or base vibrates violently. ③The valve positioner has excessive sensitivity. ④The circulation capacity C value has been set too high. The control valve operates at a low opening degree. ⑤The fit of the throttle element and the guiding clearance are too large. ⑥The valve stem has high friction, which can easily lead to hysteretic oscillations. ⑦Insufficient stiffness of the actuator can cause oscillations throughout its full stroke ； Insufficient spring pre-tensioning can occur at low strokes

1: The valve itself is brand new. 2: Working environment. 3: Quality of the purified air

The last edit to this post was made by denghl on 2012-12-12 at 21:29. The stability of a control valve refers to its ability to resist various disturbances when the signal pressure remains constant; if it can resist such disturbances, it is stable, otherwise it is not. It is related to the following factors: 4 k/ c |3 j1 _- L4 Q, W6 m+ i ① The direction of the unbalanced force. When the direction of this unbalanced force pushes the valve core apart, stability is good ； The direction of the unbalanced force is such that it presses the valve core shut; when Ft

Reply to 11# huangbhu1987: Upvote floor 11! ! ! Praise: Stability of valves: Personal summary 1. Manufacturing technology level of valves and their accessories ； 2. Correct model selection ； (It is necessary to understand the characteristics and operating conditions of various types of valves, and to carry out thorough design calculations.) 3. The quality of piping installation cannot be ignored ； 4. The locator is properly calibrated ； 5. Ensure regular maintenance is carried out.

Reply to 10# shell regarding stability: 1. Whether the flow characteristics and capacity of the valve meet the control requirements. 2. The magnitude of torque is also very important in practical applications. 3. High and low temperatures in the environment have a significant impact on whether normal regulation functions properly. 4. Manufacturing process, material, and brand are other considerations. 5. Installation and system debugging are the foundation for stability.