1. Preface
In highly automated chemical control systems, the regulating valve serves as the terminal execution device of the automatic control system, receiving control signals to regulate the chemical process. Its motion sensitivity is directly related to the quality of the regulating system. According to on-site statistics, about 70% of the faults are caused by regulating valves. Therefore, in daily maintenance, summarize and analyze the factors that affect the safe operation of regulating valves and their countermeasures.
2. Stuck up
The common problem with regulating valves is blockage, which often occurs during the initial operation of new systems and major repairs. Due to welding slag, rust, and other blockages in the throttling and guiding parts of the pipeline, the flow of the medium is not smooth, or the filling of the regulating valve is too tight during maintenance, causing an increase in friction and resulting in the phenomenon of small signals not acting and large signals acting excessively.
Fault handling: Quickly open or close the auxiliary line or regulating valve to allow dirt to be washed away by the medium from the auxiliary line or regulating valve. Another method is to clamp the valve stem with a pipe clamp, and under external signal pressure, rotate the valve stem forward and backward with force to make the valve core flash past the stuck position. If not possible, increase the pressure of the gas source and increase the driving power by repeatedly moving up and down a few times to solve the problem. If it still doesn't move, it needs to be disassembled for processing.
3. Leakage
3.1 Valve leakage, stem length discomfort. When opening the valve, the valve stem is too long and the distance from the valve stem upwards (or downwards) is not enough, resulting in a gap between the valve core and the valve seat, which cannot fully contact and leads to poor sealing and internal leakage. Similarly, the valve stem of the gas shut-off valve is too short, resulting in a gap between the valve core and the valve seat, which cannot fully contact and leads to poor sealing and internal leakage.
Solution: The valve stem of the regulating valve should be shortened (or extended) to make the length of the regulating valve appropriate, so that it no longer leaks internally.
3.2 Packing leakage. After the packing is loaded into the packing box, axial pressure is applied to it through the gland. Due to the plasticity of the packing, it generates radial force and tightly contacts the valve stem, but this contact is not very uniform. Some parts are in loose contact, some parts are in tight contact, and even some parts are not in contact. During the use of a regulating valve, there is relative motion between the valve stem and the packing, which is called axial motion. During use, with the influence of high temperature, high pressure, and highly permeable fluid media, the packing box of the regulating valve is also a part where leakage occurs more frequently. The main cause of packing leakage is interface leakage, and for textile packing, leakage may also occur (pressure medium leaks outward along the small gaps between packing fibers). The interface leakage between the valve stem and the packing is caused by the gradual attenuation of the packing contact pressure, the aging of the packing itself, and other reasons. At this time, the pressure medium will leak outward along the contact gap between the packing and the valve stem.
Solution: To facilitate the installation of the packing, chamfer the top of the packing box and place a metal protective ring with a small gap that is resistant to erosion at the bottom of the packing box (the contact surface with the packing cannot be a slope) to prevent the packing from being pushed out by the pressure of the medium. The metal surfaces of each part of the packing box in contact with the packing should be precision machined to improve surface smoothness and reduce packing wear. Flexible graphite is selected as the packing material due to its good airtightness, low friction, minimal changes after long-term use, low wear and tear, easy maintenance, no change in friction force after re tightening the gland bolts, good pressure resistance and heat resistance, and no corrosion from internal media. The metal in contact with the valve stem and packing box does not undergo pitting or corrosion. In this way, the sealing of the valve stem packing box is effectively protected, ensuring the reliability and long-term sealing of the packing.
3.3 Valve core and seat deformation and leakage. The main reason for leakage of the core and valve seat is due to casting or forging defects in the production process of the regulating valve, which can lead to increased corrosion. The passage of corrosive media and the flushing of fluid media can also cause leakage of regulating valves. Corrosion mainly exists in the form of erosion or cavitation. When corrosive media pass through the regulating valve, they will corrode and impact the valve core and seat materials, causing the valve core and seat to become elliptical or other shapes. Over time, this can lead to a mismatch between the valve core and seat, resulting in gaps and leakage due to poor sealing.
Solution: The key is to ensure the selection and quality of the materials for the valve core and seat. Choose corrosion-resistant materials and resolutely eliminate products with defects such as pitting and sand holes. If the deformation of the valve core and seat is not too severe, it can be ground with fine sandpaper to eliminate traces and improve the sealing smoothness, thereby enhancing the sealing performance. If the damage is severe, the valve should be replaced with a new one.
4. Oscillation
The spring stiffness of the regulating valve is insufficient, and the unstable and rapidly changing output signal of the regulating valve can easily cause oscillation of the regulating valve. It is also said that the frequency of the valve selection is the same as the system frequency or the pipeline or base vibrates violently, causing the regulating valve to vibrate accordingly. Improper selection can result in sharp changes in flow resistance, velocity, and pressure when the regulating valve operates at a small opening. When the valve stiffness is exceeded, stability deteriorates, and in severe cases, oscillation occurs.
Solution: As the causes of oscillation are multifaceted, specific problems should be analyzed on a case by case basis. For slight vibrations, stiffness can be increased to eliminate them. If using a high stiffness spring, switch to a piston actuator structure. The severe vibration of pipelines and bases can be eliminated by increasing support to eliminate vibration interference; If the frequency of valve selection is the same as the system frequency, replace the valve with a different structure; The oscillation caused by working at a small opening is due to improper selection of the flow capacity C value. It is necessary to re select a flow capacity C value with a smaller value or use split range control or mother and child valves to overcome the problem of the regulating valve working at a small opening.
5. Valve locator malfunction
5.1 Ordinary locators use mechanical force balance principle, namely nozzle baffle technology, and mainly have the following types of faults:
1) Due to the use of mechanical force balance principle, there are many movable parts, which are easily affected by temperature and vibration, causing fluctuations in the regulating valve;
2) Using nozzle baffle technology, due to the small nozzle hole, it is easy to be blocked by dust or unclean air sources, which makes the locator unable to work properly;
3) Using the principle of force balance, the elastic coefficient of the spring changes in harsh environments, causing nonlinearity in the regulating valve and resulting in a decrease in control quality.
5.2 The intelligent locator consists of a microprocessor (CPU), A/D, D/A converters, and other components, and its working principle is completely different from that of a regular locator. The comparison between the given value and the actual value is purely an electric signal, no longer a force balance. Therefore, it can overcome the disadvantage of force balance of conventional locators. But when used in emergency parking situations, such as emergency shut-off valves, emergency vent valves, etc. These valves are required to remain stationary in a certain position and only need to operate reliably in case of emergency. Staying in a certain position for a long time can easily cause the electrical converter to lose control and result in a dangerous situation where the small signal does not operate. In addition, the position sensing potentiometer used for valves is prone to changes in resistance value due to its operation on site, resulting in dangerous situations where small signals do not operate and large signals are fully open. Therefore, in order to ensure the reliability and usability of intelligent locators, they must be frequently tested.