． Problems in control valve applications
① There are many varieties, specifications and parameters of control valves. In order to adapt to the control requirements of different industrial production processes, such as temperature, pressure, medium characteristics, etc., control valves have nearly a thousand products of different specifications and types, making the selection, installation and application of control valves inconvenient, and maintenance inconvenient. , Management is inconvenient.
②The control valve has poor reliability. The characteristics of control valves when they leave the factory are very different from those after a period of operation. For example, leakage increases, noise increases, valve reproducibility deteriorates, etc., which brings difficulties to long-term stable operation.
③The control valve is bulky and requires transportation and installation of the control valve. Maintenance brings inconvenience. Usually, the weight of a control valve is several to hundreds of times heavier than that of a general instrument. For example, a DN200 control valve weighs 700kg. Transportation, installation and maintenance require some mechanical equipment, which brings great challenges to the application of control valves. Inconvenient.
④The flow characteristics of the control valve do not match the characteristics of the controlled object in the industrial process, causing the quality of the control system to deteriorate. The ideal flow characteristics of the control valve have been determined when the product leaves the factory. However, the characteristics of the controlled objects in the industrial process are different. The pressure drop ratio on the pressure port changes, so that the operating flow characteristics of the control valve cannot match the characteristics of the controlled objects, and the control system cannot control the control valve. Quality deteriorates.
⑤The control valve makes too much noise. In industrial applications, control valve noise has become the main noise source of industrial equipment. Therefore, reducing control valve noise has become an important current research topic and has attracted the attention of governments around the world.
⑥Control valves are energy-consuming equipment. As energy becomes increasingly scarce, energy-saving technologies should be adopted to reduce the energy consumption of control valves and improve energy utilization.
2. The development direction of control valves The development direction of control valves is mainly intelligence, standardization, miniaturization, rotation and safety.
() Intelligentization and standardization. The intelligence and standardization of control valves have been put on the agenda. Intelligence mainly uses intelligent valve positioners. Intelligence is manifested in the following aspects.
① Intelligent functions such as self-diagnosis of control valves and remote communication of operating status make the management of control valves convenient and fault diagnosis easy, and also reduce the skill requirements for maintenance personnel.
② Reduce product types and simplify the production process. The use of intelligent valve positioners can not only easily change the flow characteristics of the control valve, but also improve the control quality of the control system. Therefore, the requirements for the flow characteristics of control valves can be simplified and standardized (for example, only linear characteristic control valves are produced). Intelligent functional modules are used to match the characteristics of the controlled objects, so that the types and varieties of control valve products are greatly reduced, making The manufacturing process of control valves is simplified and is proven and recognized in production and market.
③Digital communication. Digital communication will be widely used in control valves. Based on the HART communication protocol, some valve positioners of control valves implement input signals and valve position signals on the same transmission line; based on fieldbus technology, control valves and valve positioners , PID control function module is combined to realize the control function at the field level, disperse the danger, and make the control more timely and faster.
④Intelligent valve positioner. The intelligent valve positioner has all the functions of the valve positioner. It can also improve the dynamic and static characteristics of the control valve and improve the control accuracy of the control valve. Therefore, the intelligent valve positioner will become an important auxiliary equipment for control valves in the future. widely used. The standardization of control valves is reflected in the following aspects.
①In order to achieve interchangeability, control valves produced by different manufacturers of the same size and specification can be interchanged, so that users do not have to spend a lot of time choosing a manufacturer.
②In order to achieve interoperability, control valves produced by different manufacturers should be able to work together with products from other manufacturers without signal mismatch or impedance mismatch.
③Standardized diagnostic software and other auxiliary software enable control valves from different manufacturers to diagnose operating status and analyze operating data.
④Standardized selection procedure. Control valve selection is still a matter of great concern to automatic control designers. Using standardized calculation procedures and based on the data provided by the process, we can correctly calculate the flow coefficient of the required control valve, determine the piping, and select the appropriate valve body, valve core and valve Internal parts materials, etc., standardize the design process and improve design quality. (2) Miniaturization. In order to reduce the weight of the control valve and facilitate transportation, installation and maintenance, the following measures have been adopted to minimize the control valve.
①Adopt fine and small actuator. Use lightweight materials and use multiple sets of springs instead of one set of springs to reduce the height of the actuator. Generally, the control valve composed of fine and small pneumatic diaphragm actuators is about 30% lower in height and weight than the control valve composed of the same type of pneumatic diaphragm actuator. It is reduced by about 30%, while the circulation capacity can be increased by about 30%. ②Change the flow path structure. For example, changing the movement of the valve core to the movement of the valve seat, changing the linear displacement to angular displacement, etc., can reduce the size and weight of the control valve.
③ Use electric actuator. Not only can the air source device and auxiliary equipment required for pneumatic actuators be reduced, but the weight of the actuators can also be reduced. For example, the height of Fisher’s 9000 series electric actuator of type 20 is less than 330mm, which reduces the mass of the entire control valve (with digital controller and actuator) to 20-32kg.
(3) Rotation: Rotary control valves, such as ball valves, etc., have the advantages of relatively small volume, small flow path resistance, large adjustable ratio, good sealing, good anti-blocking performance, and large flow capacity. Therefore, among new varieties of control valves, the proportion of rotary valves increases. Especially in large-diameter pipelines, ball valves, butterfly valves and other types of control valves are commonly used. Judging from foreign products in recent years, the proportion of rotary valve applications is increasing year by year.
(4) Safety The safety of instrument control systems has received attention from all aspects, and safety instrumented systems (SIS) have increasingly higher requirements for control valves, which are reflected in the following aspects.
① The requirements for diagnosis and processing of control valve fault information have increased. It is not only necessary to perform passive maintenance on control valves after a fault occurs, but also perform preventive maintenance and predictive maintenance before a fault occurs. Therefore, it becomes more important to conduct statistics and analysis on the relevant components that make up the control valve, and to make timely maintenance recommendations.
② For control valves used in emergency stop systems or safety interlock systems, requirements for timely, reliable and safe action are required. Make sure these control valves are responsive and accurate.
③ For control valves used in hazardous locations, the certification procedures should be simplified. For example, fieldbus instruments for intrinsically safe applications can be simplified to adopt the FISCO fieldbus intrinsically safe concept, which simplifies the certification process for intrinsically safe products.
④Similar to the safety of other field instruments, for the safety of control valves, explosion-proof technology, fire protection technology, increased safety technology, intrinsic safety technology, non-sparking technology, etc. can be used; for fieldbus instruments, entity concepts, Intrinsically safe concept, FISCO concept and non-incendive (FINCO) concept, etc.
(5) Energy saving: Reducing energy consumption and improving energy utilization is a development direction of control valves. There are mainly the following development directions.
① Use a control valve with a low pressure drop ratio. Reduce the proportion of the control valve in the pressure drop of the entire system, thereby reducing energy consumption. Therefore, designing control valves with low pressure drop ratios is one of the development directions; another development direction is to use low-impedance control valves, such as butterfly valves and eccentric valves. Rotary valve, etc.
② Use self-operated control valve. For example, the pressure of the medium behind the valve is directly used to form a self-operated control system, and the energy of the controlled medium is used to control the pressure behind the valve.
③ Control valve using electric actuator. Pneumatic actuators require a certain amount of air pressure during the entire operation of the control valve. Although amplifiers with small consumption can be used, the air consumption is still huge over time. Using an electric actuator requires power supply when changing the opening of the control valve, and no longer supplies power when the required opening is reached. Therefore, from the perspective of energy saving, electric actuators have obvious energy-saving advantages over pneumatic actuators.
④ Use piezoelectric control valve. The piezoelectric control valve is used in the intelligent electric valve positioner, and the air source is only consumed when the output signal increases.
⑤ Use a valve core with a balanced structure to reduce the thrust or thrust torque of the actuator, reduce the diaphragm head air chamber, and reduce energy requirements.
⑥ Use variable frequency speed regulation technology to replace the control valve. For applications with high voltage drop ratio, if the energy consumption is very large, frequency conversion speed regulation technology can be used, and the frequency converter can be used to change the rotation speed of the relevant operating equipment to reduce energy consumption.
(6) Protect the environment Environmental pollution has become a public hazard. The main environmental pollution caused by control valves includes control valve noise and control valve leakage. Among them, the environmental pollution caused by control valve noise is even more serious.
① Reduce control valve noise. Develop various methods to reduce control valve noise, including from control valve flow path design to control valve trim design, from noise source analysis to noise reduction measures, etc. It mainly includes the design of noise reduction control valves and noise reduction control valve trims; reasonable distribution of pressure drop and use of external noise reduction measures, such as increasing isolation and using mufflers.
② Reduce air pollution from control valves. Air pollution from control valves refers to the “running”, “popping”, “dropping” and “leakage” of control valves. These leakages not only cause a waste of materials or products, but also cause pollution to the atmospheric environment. Sometimes, they can also cause the death of personnel. Casualties or accidents such as equipment explosions. Therefore, developing control valve packing structures and packing types, developing control valve seals, etc. will be an important research topic for control valves in the future.
The development of high-tech science and technology such as computer science, control theory and automated instrumentation has promoted the development of control valves, such as the development of fieldbus control valves and intelligent valve positioners, and the implementation of digital communication in control valves. The development of control valves has also promoted the development of other science and technology, such as research on anti-corrosion materials, research on methods to weaken and reduce noise, research on fluid dynamics, etc. With the development of fieldbus technology, control valves will also be open, intelligent and more reliable. They will be more closely related to each other, have more obvious control effects, and play a more important role in my country’s modernization construction with other industrial automation instruments and computer control devices. At the same time, the functions of industrial production process control are more perfect and the control is precise.
Link to this article：Shortcomings and development trends in the application of control valves
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