Factors Affecting the Safe Operation of Control Valves and Countermeasures

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【Keywords】Control valve, factors and countermeasures 【Abstract】In chemical control systems with a high degree of automation, control valves serve as the terminal actuating devices of automatic control systems; they receive control signals to regulate the chemical processes. Its operational sensitivity is directly related to the quality of the control system. According to on-site statistics, about 70% of the failures are caused by control valves. Therefore, in daily maintenance, it is necessary to summarize and analyze the factors affecting the safe operation of control valves and the corresponding countermeasures. 　 　1. Introduction In chemical control systems with a high degree of automation, control valves serve as the terminal actuating devices of automatic control systems; they receive control signals to regulate the chemical processes. Its operational sensitivity is directly related to the quality of the control system. According to on-site statistics, about 70% of the failures are caused by control valves. Therefore, in daily maintenance, it is necessary to summarize and analyze the factors affecting the safe operation of control valves and the corresponding countermeasures. 　　2. Jamming A common problem with control valves is jamming, which often occurs in newly commissioned systems or at the beginning of operation after major repairs. Weld slag, rust, and other debris in the pipes can cause blockages at the throttling areas and guide sections, preventing smooth flow of the medium. Additionally, if the packing is tightened too much during valve maintenance, it increases friction, resulting in the valve not responding to small signal inputs while overreacting to larger signal inputs. 　 　　Fault handling: The bypass line or control valve can be quickly opened and closed, allowing debris to be carried away by the fluid through the bypass line or control valve. Another method is to use pipe wrenches to clamp the valve stem, and under applied signal pressure, rotate the stem back and forth to move the valve core past the jammed area. If this is not possible, increase the air supply pressure and boost the driving power; moving up and down several times will resolve the issue. If no action is taken, it will need to be disassembled. 　　 　　3. Leakage 3.1 Internal valve leakage, improper length of the valve stem. The gas control valve has a too-long valve stem; the distance that the valve stem can move upward (or downward) is insufficient, which creates a gap between the valve core and the valve seat. As a result, they cannot make full contact, leading to poor sealing and internal leakage. Similarly, the valve stem of the air shut-off valve is too short, resulting in a gap between the valve core and the valve seat; this prevents proper contact and leads to poor sealing and internal leakage. 　　Solution: The valve stem of the control valve should be shortened (or lengthened) to adjust its length appropriately, so as to prevent internal leakage. 　　3.2 Packing leakage After the packing is installed in the stuffing box, axial pressure is applied to it via the gland. Due to the plasticity of the filler, a radial force is generated, causing it to make close contact with the valve stem; however, this contact is not very uniform. In some areas, the contact is loose; in other areas, it is tight, and there are even some areas where there is no contact at all. During operation of a control valve, there is relative motion between the valve stem and the packing, and this motion is known as axial motion. During operation, due to the effects of high temperature, high pressure, and fluid media with strong permeability, the packing gland of control valves is also a part where leaks occur frequently. The main cause of filler leakage is interfacial leakage; in the case of textile fillers, seepage also occurs (the pressure medium leaks out along the tiny gaps between the filler fibers). Interfacial leakage between the valve stem and the packing is caused by the gradual decrease in the contact pressure of the packing, as well as aging of the packing itself; under such conditions, the pressurized medium leaks outward through the gap between the packing and the valve stem. 　 　　Solution: To facilitate the insertion of the packing, a chamfer is provided at the top of the packing box, and an erosion-resistant metal protective ring with a small clearance is placed at the bottom of the packing box (the contact surface with the packing must not be inclined) to prevent the packing from being pushed out by the medium pressure. The metal surfaces of all parts of the stuffing box that come into contact with the packing must be finely processed to improve surface smoothness and reduce packing wear. Flexible graphite is chosen as the packing because it offers good airtightness, low friction, minimal changes over time, low wear and burnout, ease of maintenance; the friction remains unchanged after the gland bolts are tightened again. It also possesses good pressure and heat resistance, is not affected by internal media, and does not cause pitting or corrosion in the metals that come into contact with it within the valve stem and stuffing box. In this way, the sealing of the valve stem stuffing box is effectively protected, ensuring the reliability and durability of the stuffing’s seal. 　 　　3.3 Leakage due to deformation of the valve core and seat The main cause of leakage in the valve core and seat is the enhancement of corrosion resulting from casting or forging defects during the production of control valves. The passage of corrosive media, as well as the scouring by fluid media, can also cause leakage in control valves. Corrosion mainly occurs in the form of erosion or cavitation. When corrosive media pass through the control valve, it causes erosion of the valve stem and seat materials as well as impact that results in these components taking on an oval or other shape. Over time, this leads to a mismatch between the valve stem and seat, creating gaps that prevent proper sealing and result in leaks. 　 　　Solution: The key is to ensure proper selection and high quality of the materials used for the valve core and valve seat. Choose corrosion-resistant materials, and firmly reject products with defects such as pitting and sand eyes. If the valve core and seat are not severely deformed, they can be polished with fine sandpaper to remove any marks and improve the smoothness of the seal, thereby enhancing its sealing performance. If the damage is severe, a new valve should be replaced. 　 　　4. Oscillation Insufficient spring stiffness in the control valve, as well as unstable and rapid changes in the output signal of the control valve, can lead to oscillation of the control valve. It is also said that the selection frequency of the valve is the same as the system frequency, or that severe vibrations in the pipes and base cause the control valve to vibrate as well. Due to improper selection, the control valve operates at a low opening degree, resulting in sharp changes in flow resistance, flow rate, and pressure. When these changes exceed the valve’s stiffness, its stability deteriorates, and oscillations may occur in severe cases. 　　Solution: Since there are various reasons for the oscillations, each specific issue must be analyzed individually. For slight vibrations, increasing stiffness can be used to eliminate them. If a high-stiffness spring is used, switch to a piston actuation structure. Severe vibrations in pipes and bases are eliminated by adding supports to reduce vibration interference ； If the frequency of the selector valve is the same as the system frequency, replace it with a valve of a different structure ； The oscillations that occur when the control valve operates at a low opening degree are caused by an improper selection of the flow capacity C value; it is necessary to choose a value for C that is lower, or to use split-control systems or mother-and-child valves in order to overcome the problems associated with operating the control valve at low openings. 　 　　5. Valve positioner failures 　　　5.1 Conventional positioners operate on the principle of mechanical force balance, namely the nozzle damper technique. The main types of failures associated with them are as follows: 　　1) Due to the use of this mechanical force balance principle, they have numerous moving parts, which make them susceptible to the effects of temperature and vibration, leading to fluctuations in the performance of the control valve ； 　　2) The nozzle baffle technology is used; since the nozzle holes are very small, they can easily get clogged by dust or dirty air, which prevents the locator from functioning properly ； 　　3) Based on the principle of force balance, the elastic coefficient of the spring changes in harsh operating conditions, resulting in non-linearity of the control valve and a decline in control quality. 　　5.2 The intelligent locator is composed of components such as a microprocessor (CPU), A/D and D/A converters, etc., and its working principle is completely different from that of ordinary locators. The comparison between the given value and the actual value is purely an electrical signal; it is no longer a force balance. Therefore, it can overcome the disadvantage of force balance in conventional positioners. But when used in emergency shutdown applications, such as emergency shut-off valves and emergency vent valves. These valves are required to remain in a certain position, and they only need to operate reliably in emergency situations. Staying in one position for an extended period can cause the electrical converter to lose control, posing a risk of failure to respond to small signals. Furthermore, the position sensing potentiometer used for valves, operating in the field, has its resistance value prone to changes, which can lead to the situation where small signals do not trigger any action, while large signals cause the valve to open fully. Therefore, to ensure the reliability and availability of intelligent locators, they must be tested frequently. 　　 　　6. Conclusion By analyzing the causes of control valve failures and implementing appropriate corrective measures, it is possible to **improve the utilization rate of control valves and reduce the incidence of instrument failures. This has a significant impact on enhancing the production efficiency and economic benefits of process operations, as well as on reducing energy consumption. It also helps to improve the quality of the control system, thereby ensuring the long-term operational reliability of production facilities.