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Practical Application of Automatic Regulating System for Regulating Valves
In the automatic control system of the production process, the control valve is an important and indispensable link, which is called the "hand and foot" of the production process automation, and is one of the terminal control elements of the automatic control system.
The regulating valve has a simple flow path and low resistance, and is generally suitable for forward use (installation). However, in high-pressure drop situations, the reverse use of the regulating valve can improve the imbalance force and reduce damage to the valve core, while also facilitating the flow of media and avoiding coking and clogging of the regulating valve. When using the regulating valve in reverse direction, it is particularly important to avoid opening the valve with a small opening for a long period of time to prevent strong oscillation and damage to the valve core. Especially during the trial production stage of chemical plants, due to the low load during the trial production and the inability of the design process conditions to quickly meet the requirements, the control valve used in reverse direction should be avoided from being opened with a small opening for a long time as much as possible to prevent damage to the control valve.
The regulating valve is composed of an actuator and a valve. From a hydraulic point of view, a regulating valve is a throttling element that can change local resistance. The regulating valve changes the resistance coefficient by changing the stroke according to the input signal, thereby achieving the purpose of regulating flow.
Structure and Use
Structure of regulating valve
Except for the angle type valve body, the other structures of the regulating valve are similar to those of the single seat valve. Its characteristics determine that its flow path is simple and its resistance is small, which is particularly conducive to the regulation of high pressure drop, high viscosity, fluid containing suspended solids and granular substances. It can avoid phenomena such as coking, binding, and clogging, and is also convenient for cleaning and self-cleaning.
Comparison of forward and reverse use of control valves
Generally, regulating valves are installed in a forward direction, that is, bottom in and side out. Reverse installation, i.e., material side entry and bottom exit, is recommended only in situations of high pressure differential and media with high viscosity, easy coking, and suspended particulate matter. The purpose of reverse use of the regulating valve is to improve the unbalanced force and reduce wear on the valve core, while also facilitating the flow of media with high viscosity, easy coking, and suspended particles, and avoiding coking and clogging.
Reverse Usage Analysis
It is recommended to use the regulating valve in reverse direction under high pressure drop process conditions. During the test run, the regulating valve vibrated strongly and made a harsh noise. After 4 hours of test run, the valve core broke. However, this is not a quality issue, but due to unreasonable use. The fracture reason is analyzed below.
Except for butterfly valves and diaphragm valves that are completely symmetrical in structure, all control valves of other structures are asymmetric. When the regulating valve changes the flow direction, changes in the flow path will cause changes in the value. The normal flow direction of various regulating valves is the direction in which the valve core is opened (for forward use), and the manufacturer only provides the flow capacity and flow characteristics in normal flow direction. When the regulating valve is used in reverse direction, the flow capacity of the regulating valve will increase when the fluid flows in the direction that closes the valve core. During the water linkage test run, the simulated process conditions cannot quickly reach the normal state. If the regulating valve is used with a small opening for a long time, serious instability may occur due to the effect of unbalanced forces. Therefore, the regulating valve will produce strong vibrations and emit harsh noise, which will cause the valve core to break quickly. Under normal process conditions, the opening of the regulating valve is moderate, and even a small opening is temporary, so the regulating valve can be used normally and safely.
conclusion
Generally, reverse use of control valves is not recommended, and only recommended for high pressure differential, high viscosity, coking prone, and suspended particle containing media. When using in reverse direction, it is necessary to avoid long-term operation with small opening, especially during commissioning.
Factors Affecting the Normal Operation of Regulating Valves and Countermeasures
1. Preface
In chemical control systems with a high degree of automation, the regulating valve serves as the terminal executing device of the automatic regulating system, receiving control signals to regulate the chemical process. Its action sensitivity is directly related to the quality of the regulating system. According to on-site actual statistics, about 70% of the faults are caused by the regulating valve. Therefore, in daily maintenance, summarize and analyze the factors that affect the safe operation of the control valve and their countermeasures.
2. Jamming
The common problem with regulating valves is blockage, which occurs during the initial stages of commissioning of new systems and major overhauls. Due to the blockage of welding slag and rust in the pipe at the throttle and guide parts, the flow of media is impeded, or the filler is too tight during the maintenance of regulating valves, resulting in increased friction, leading to small signal inaction and large signal excessive action.
Troubleshooting: Quickly open and 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 rod with a pipe wrench, and under the applied signal pressure, forcefully rotate the valve rod in both directions to allow the valve core to flash past the clamp. If not, increase the air supply pressure, increase the drive power, and repeatedly move up and down several times to solve the problem. If it still does not work, it needs to be disassembled.
3. Leakage
3.1 The valve has internal leakage and the length of the valve rod is not suitable. When the valve is opened by air, the valve stem is too long and the upward (or downward) distance of the valve stem is insufficient, resulting in a gap between the valve core and the valve seat that cannot be fully contacted, resulting in lax closing and internal leakage. Similarly, the valve rod of the air shutoff valve is too short, resulting in a gap between the valve core and valve seat that cannot be fully contacted, resulting in lax closing and internal leakage.
Solution: The valve rod of the regulating valve should be shortened (or extended) to make the length of the regulating valve suitable and prevent internal leakage.
3.2 Packing leakage. After the packing is loaded into the stuffing box, axial pressure is applied to it through the gland. Due to the plasticity of the packing, it produces a radial force and is in close contact with the valve stem, but this contact is not very uniform. Some parts are loosely contacted, some parts are tightly contacted, and even some parts are not in contact. During the use of a regulating valve, there is a relative movement between the valve stem and the packing, which is called axial movement. During use, due to the influence of high temperature, high pressure, and highly permeable fluid media, the control valve stuffing box is also a location where leakage occurs frequently. The main reason for filler leakage is interface leakage, and leakage can also occur for textile fillers (pressure media leaking outward along the tiny gaps between the filler fibers). The interface leakage between the valve rod and the packing is caused by the gradual attenuation of the packing contact pressure, 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 rod.
Solution: To facilitate the loading of fillers, chamfer the top of the stuffing box and place a metal protective ring with a small erosion resistance gap at the bottom of the stuffing box (the contact surface with the filler cannot be an inclined surface) to prevent the filler from being pushed out by medium pressure. The metal surface of each part of the stuffing box in contact with the filler should be finely machined to improve surface smoothness and reduce filler wear. Flexible graphite is selected as the filler because of its good air tightness, low friction, small change after long-term use, small wear and burn, easy maintenance, no change in friction after the gland bolts are retightened, good pressure resistance and heat resistance, and no erosion by internal media. The metal in contact with the valve rod and stuffing box does not undergo pitting or corrosion. In this way, the sealing of the valve stem stuffing box is effectively protected, ensuring the reliability and long-term performance of the sealing of the packing.
3.3 Deformation and leakage of valve core and valve seat. The main reason for core and valve seat leakage is that casting or forging defects in the production process of control valves can lead to enhanced corrosion. The passage of corrosive media and the flushing of fluid media can also cause leakage of the control valve. Corrosion occurs mainly in the form of erosion or cavitation. When corrosive media pass through the regulating valve, they will cause erosion and impact on the valve core and valve seat materials, causing the valve core and valve seat to form an oval shape or other shapes. Over time, they will result in mismatched valve core and valve seat, resulting in gaps, and leakage if not tightly closed.
Solution: The key is to control the selection and quality of valve core and valve seat materials. Select corrosion resistant materials and resolutely eliminate products with defects such as pitting and trachoma. If the deformation of the valve core and valve seat is not too serious, they can be ground with fine abrasive paper to eliminate traces and improve the sealing smoothness to improve sealing performance. If the damage is serious, replace the valve with a new one.
4. Oscillation
The spring stiffness of the regulating valve is insufficient, and the output signal of the regulating valve is unstable and changes rapidly, which can easily cause the regulating valve to oscillate. It is also said that the frequency of the selector valve is the same as the system frequency or the pipe or base vibrates violently, causing the control valve to vibrate accordingly. Due to improper selection, the control valve operates at a small opening with sharp changes in flow resistance, flow rate, and pressure. When the valve stiffness is exceeded, the stability becomes poor, and in severe cases, oscillation occurs.
Solution: Due to the multiple reasons for oscillation, specific problems will be analyzed. For slight vibrations, stiffness can be increased to eliminate them. If a large stiffness spring is selected, a piston actuator structure is used instead. Severe vibration of pipes and bases eliminates vibration interference by adding support; If the frequency of the selected valve is the same as the system frequency, replace the valve with a different structure; The oscillation caused by operating at a small opening is due to improper selection of a larger flow capacity C value. It is necessary to select a new type with a smaller flow capacity C value or use split range control or sub mother valve to overcome the problem of operating at a small opening of the regulating valve.
5. Valve positioner failure
5.1 Ordinary positioners operate using the mechanical force balance principle, namely nozzle baffle technology, and there are mainly the following types of failures:
1) Due to the use of mechanical force balancing principle, there are many movable components, which are easily affected by temperature and vibration, resulting in fluctuations in the control valve;
2) Using nozzle baffle technology, due to the small nozzle hole, it is easy to be blocked by dust or unclean air sources, causing the positioner to not work properly;
3) Using the principle of force balance, the elastic coefficient of the spring changes under harsh conditions, resulting in nonlinear control of the control valve and reduced control quality.
5.2 The intelligent positioner consists of a microprocessor (CPU), A/D, D/A converters, and other components, and its working principle is completely different from that of an ordinary positioner. The comparison between the given value and the actual value is purely an electrical signal and is no longer a force balance. Therefore, the disadvantage of force balance of conventional positioners can be overcome. However, when used in emergency parking situations, such as emergency shut-off valves, emergency vent valves, etc. These valves are required to be stationary in a certain position, and only when an emergency occurs, reliable action is required. Staying at a certain position for a long time can easily cause the electrical converter to lose control and cause a small signal to not act. In addition, the position sensing potentiometer used for valves is prone to change in resistance due to its on-site operation, resulting in a dangerous situation where small signals do not operate and large signals are fully opened. Therefore, in order to ensure the reliability and availability of intelligent locators, they must be frequently tested.
