In the field of industrial automation and control, self-operated control valve as an important flow, pressure or temperature control element, its performance and application scenarios directly affect the stability and efficiency of the whole system. This article will start from the perspective of professional manufacturers, in-depth discussion of self-operated control valves in the pre-valve and post-valve application of the difference between the principle of operation, structural characteristics, application scenarios and selection points.
I. Introducción
Válvula de control automática, as the name suggests, is a kind of valve that can automatically adjust itself according to the system conditions (such as pressure, flow, temperature, etc.). It works without external energy (e.g. power supply, gas source) and achieves precise control through the interaction of internal mechanisms. According to the different measurement and adjustment points, self-regulating valves can be divided into two categories: pre-valve adjustment and post-valve adjustment. There are significant differences between the two in the working principle, structural design and application scenarios.
Ⅱ. Pre-valve self-operated control valve
2.1 Working principle
Pre-valve self-regulating valve conduit pressure point is located in front of the valve, the main role is to stabilise the pre-valve pressure, control the medium in front of the valve. Its working principle is based on the principle of force balance: the working medium of the pre-valve pressure P1 through the valve spool, valve seat throttling, resulting in a certain pressure drop, and at the same time, P1 is also through the control pipeline input to the actuator of the upper membrane chamber, the role of the top disc. The force generated by the top disc is balanced with the reaction force of the spring, which determines the relative position of the valve spool and seat, and thus controls the pre-valve pressure.
When the pre-valve pressure P1 increases, the force acting on the top disc also increases, the force of the top disc is greater than the reaction force of the spring, pushing the valve spool to leave the direction of the valve seat to move, increase the flow area, reduce the flow resistance, so that P1 gradually down to the set value. Conversely, when P1 decreases, the spool moves closer to the seat, decreasing the flow area and increasing the flow resistance to maintain P1 within the set range.
2.2 Structural characteristics
Pre-valve válvula autorreguladora usually adopts single-seat or double-seat structure, with good sealing performance between spool and seat, suitable for occasions with high requirements on leakage. The actuator part mostly adopts spring-loaded design, and the set value of pre-valve pressure can be changed by adjusting the spring preload. In addition, the pre-valve self-regulating valves are equipped with sensitive pressure-taking devices and feedback mechanisms to ensure fast response and precise control.
2.3 Applicable Scenarios
Pre-valve self-regulating valves are suitable for applications where it is necessary to stabilise the pre-valve pressure or prevent backflow of the medium, such as boiler water supply systems, steam networks, oil and gas pipelines etc. In these applications, the stability of the pre-valve pressure is essential to ensure the normal operation of the system.
Ⅲ. Post-valve self-operated control valve
3.1 Working principle
With the valve before the self-regulating valve is different, after the valve self-regulating valve conduit pressure point is located after the valve, the main role is to regulate the pressure after the valve, to prevent the system from overpressure or media flow waste. Its working principle is also based on the principle of force balance, but the role of the point and the direction of regulation is different. Pre-valve pressure P1 through the valve spool, valve seat throttling into the post-valve pressure P2, P2 through the control line into the actuator of the lower membrane chamber acting on the top disc. The force generated by the top disc is balanced with the reaction force of the spring to control the relative position of the spool and seat, thus regulating the post-valve pressure .When the pressure after the valve P2 increases, the force acting on the top disc also increases, and the force of the top disc is greater than the reaction force of the spring, which pushes the valve spool to move in the direction of the valve seat, reduces the flow area, increases the flow resistance, and gradually reduces P2 to the set value. Conversely, when P2 decreases, the spool moves in the direction away from the seat, increasing the flow area and decreasing the flow resistance to maintain P2 within the set range.
3.2 Structural characteristics
The structure of the post-valve self-operated control valve is similar to that of the pre-valve type, but the design of the actuator is different. In order to better adapt to the demand of regulating the pressure after the valve, the actuator of the post-valve self-operated control valve usually adopts a more sensitive feedback mechanism and a larger adjustment range. In addition, some post-valve self-regulating valves are equipped with a safety relief device to prevent accidents caused by system overpressure.
3.3 Application Scenarios
Post-valve self-operated control valves are widely used in systems that require pressure regulation and pressure reduction, such as chemical production plants, water supply and drainage systems, air conditioning and refrigeration systems. In these applications, the stability of the pressure after the valve is directly related to the safety and operation efficiency of the system. By adjusting the pressure after the valve, it can ensure that the medium flow and pressure in the system are in the best state, avoiding the occurrence of overpressure and waste phenomenon.
IV. Comparison of pre-valve and post-valve self-regulating valves
4.1 Pressure point location
Pre-valve self-operated control valve pressure point is located in front of the valve, the main control valve before the pressure; and post-valve self-operated control valve.
After the self-operated control valve pressure point is located after the valve, the main control valve pressure. This difference directly determines the role of both in the control system and the direction of regulation.
4.2 Control objectives and effects
Pre-valve self-regulating valve: its main goal is to maintain the stability of the pressure in front of the valve to prevent the downstream system from being affected by upstream pressure fluctuations. In applications such as boiler feed water systems and steam pipe networks, stable pre-valve pressure is essential to ensure normal operation and prolong the service life of the equipment. By adjusting the spool opening, the pre-valve self-regulating valve responds quickly to changes in upstream pressure and automatically adjusts to maintain the set pressure value.
Post-valve self-regulating valve: Its main task is to regulate the pressure after the valve to prevent the system from overpressure or to maintain a constant operating pressure. In chemical reactors, water treatment equipment, HVAC systems and other occasions, the stability of the pressure after the valve is directly related to the system’s safety and operational efficiency. Post-valve self-regulating valve monitors the post-valve pressure and adjusts the spool position accordingly to achieve stable pressure control.
4.3 Structural design and application scenarios
Structural design: Although there are similarities between the two structures, such as valve body, spool, actuator and other components, but there are differences in the design of the actuator, the way to take the pressure and the sealing structure. Pre-valve self-operated control valves usually pay more attention to sealing performance and quick response ability to cope with upstream pressure changes; while post-valve self-operated control valves may pay more attention to regulation accuracy and stability to adapt to the complex needs of the downstream system.
Application Scenario: Pre-valve self-operated control valves are more suitable for upstream pressure-sensitive and fast response occasions, such as steam networks, gas pipelines, etc., while post-valve self-operated control valves are widely used in the need to accurately control the downstream pressure or to prevent overpressure of the occasions, such as chemical process control, water treatment systems.
4.4 Selection points
When selecting a self-operated control valve, whether it is a pre-valve or post-valve type, the following key points need to be considered:
media characteristics: including media temperature, pressure, corrosivity, viscosity and other parameters to ensure that the selected valve can adapt to the working conditions.
l control accuracy: according to the system requirements for pressure or flow control accuracy, select the appropriate type of control valve and accuracy level.
l Installation position: according to the location of the pressure point and the specific layout of the system, determine the installation mode and direction of the control valve.
l Operation and Maintenance: Consider the ease of operation and maintainability of the regulating valve as well as the availability of spare parts in order to reduce operating costs and maintenance difficulties.
Safety and reliability: Ensure that the selected control valve has sufficient safety performance and reliability, and can operate stably for a long time under poor working conditions.
V. Conclusión
In summary, there are significant differences between self-operated control valves in pre-valve and post-valve applications. Pre-valve self-regulating valve is mainly focused on maintaining the stability of the pressure in front of the valve, to prevent upstream pressure fluctuations on the downstream system; while the post-valve self-regulating valve focuses on regulating the pressure after the valve, to maintain the stability of the flow and pressure of the media within the system. When selecting and using self-operated control valves, they should be reasonably selected according to specific application scenarios and control needs, and focus on their media adaptability, control accuracy, installation location, operation and maintenance, as well as safety and reliability requirements. As a professional manufacturer, we will continue to commit to technological innovation and product optimisation to provide users with more efficient, reliable and easy-to-use self-operated control valve solutions.
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