1 Outline
An automatic control system, regulating valve actuator of its common. Control process is stable depends on the ability to accurately regulating valve action, making the process control flow of embodied energy and materials, the exact change. Therefore, according to the different needs of different valves. Select the appropriate design of regulating valve is the main pipeline issue, but also to ensure the safety and smooth running conditioning system key.
2 Composition of regulating valve
Regulating valve adjustment by the implementing agencies and institutions, to accept the regulator or the computer control signals, used to change the flow of charged medium, to be adjust parameters of Wei Chi in the required range, so the reach of process control automation.
2.1 Executive body
Implementing agencies in accordance with the drive into the form of pneumatic, electric and hydraulic 3. Pneumatic actuator is simple in structure, movement reliable, stable performance, low cost, easy maintenance, fire explosion-proof, the advantages of control Xitong in Zaixu Duo has been widely De applications. Electric actuator, while not conducive to fire and explosion, but its drive power to facilitate desirable, and signal transmission speed, ease of long-distance transmission, small size and reliability, easy maintenance, low price. Hydraulic actuator thrust the biggest adjustment of high precision, fast action, running smoothly, but because the equipment is big, complex process, so much current.
Implementing agencies of whatever type, the output power are used to overcome the load of effectiveness (mainly refers to the unbalanced force and unbalanced moment, friction, sealing force and gravity, and the role of the force). Therefore, in order to work properly regulating valve is equipped with the implementing agencies to be able to generate sufficient output power to overcome resistance and ensure a high degree of sealing and the valve opening. The implementation of the output power is confirmed. Environmental requirements should be based on technology use, select the appropriate enforcement agency. For example, explosion-proof requirements for the scene when the pneumatic actuator should be selected, and the terminal box for the explosion-proof type. If no explosion-proof requirements, pneumatic or electric actuators can be used, but energy considerations, should try to use electric actuator. Required adjustment for high precision, movement speed and smooth running condition, should use hydraulic actuators. CN HDFM.
Comprehensive features of various types of actuators, automatic control systems commonly used by electric actuator. Such as simple structure and small size of the ZAZ straight trip class and ZAJ rotary type, 3610L (R)-based electronic and multi-turn electric actuator SKD-based institutions. Despite the wide range of implementing agencies are not identical in structure, but the basic structures, including amplifiers, reversible motor, deceleration devices, thrust mechanism, mechanical limit components, flexible coupling and the position feedback and other components (Figure 1).
Figure 1, Block diagram of actuator
Electric actuators and servo amplifier supporting the general need to receive the signal modulator, the signal through the servo amplifier, converted to three following the electrical signals, control the forward or reverse reversible motor to drive adjusting mechanism, causing the valve to open or close.
2.2 Regulatory agencies
Regulating valve is a valve regulating body, according to the requirements of the control signal to change the size of the valve opening to regulate the flow, is a local resistance to change in the spending components. Mainly by the upper and lower valve regulating valve cover, valve, flap valve, valve seat, such as packing and plate components. An automatic control system, the main regulator valve medium such as water and steam. Low pressure, the use of a single case, the common regulating valve of a through-regulating valve, three-way valves and butterfly valves.
Through valves through a single seat and double seat valves divided. Single-seat valve is simple, inexpensive, off a small amount of leakage, but there’s pressure on the valve seat around the valve flap resulting unbalanced force is high, suitable for low dropout of occasions, such as water supply or return pipes in. Valve seat valve has two valve seat, in its closed state, the two valve flaps may be partially offset by the force, unbalanced force suffered flap valve small, but due to thermal expansion and contraction effects, and closing of its poor, and higher cost, only after pressure on the valve closed higher but less demanding situations, such as water supply or return water pressure difference between the bypass valve.
Three-way valve has three entrances and the channels connected to the total amount of water was constant for fixed water systems, and requires the installation of a fixed direction, not anti-wear, not suitable for large temperature difference occasions. Confluence of three-way valves are valves and shunt valves divided. Confluence valve is the mixing of fluids from the two entrance to an exit. Shunt valve sucked a fluid entrance port sent by two.
Butterfly structure is relatively simple, by the body, such as disc shaft and seal components, the stroke is 0 ° ~ 90 °. Two butterfly-style control and proportional control of 2 ways. Butterfly valve is characterized by resistance loss, small size, light weight, easy to install and open the valve and close the valve to allow pressure more, but the regulator valve closed performance He Guan poor performance, often for Yachai But the performance of less demanding greater regulation places. Except for two-way valve, the valve can also be combined with two to complete a three-way valve function. In the automatic control system on / off electric butterfly valve used in cold water and hot water systems, as water connectivity on and off control.
3 Performance
3.1 Working principle
According to known fluid mechanics, regulating valve is a local resistance to change in the spending components. For the incompressible fluid, the flow regulating valve can be expressed as:
The formula: Q-regulating valve opening of a flow, mm3/s
P1-regulating valve inlet pressure, MPa
P2-regulating valve outlet pressure, MPa
A-cutting cross-sectional area, mm2
ξ-regulating valve resistance coefficient
ρ-fluid density, kg/mm3
From (1) We find that when A certain, ΔP=P1-P2 are constant, the flow through the valve Q changes with the drag coefficient ξ, that the greater the drag coefficient ξ, the less traffic. The drag coefficient ξ and the valve is the structure and opening the. Therefore, the output signal control regulator valve on or off, can change the valve’s resistance coefficient, thus changing the flow of media to be transferred.
3.2 Flow characteristics
Regulating valve flow characteristics is to be transferred medium flow through the valve and the relative flow regulating valves between the relative opening. Its mathematical expression is:
Type in: Qmax-regulating valves fully open when the flow, mm3/s
L – a valve opening trip, mm
Lmax-regulating valves fully open Time travel, mm
Regulating valve flow characteristics, including good flow characteristics and work flow characteristics. Ideal flow characteristics is the pressure drop in the fixed valve flow characteristics of cases, there is linear, equal percentage, quick opening parabola, and 4 properties (Table 1).
| Flow characteristics | Nature | Feature |
|---|---|---|
| Straight line | The relative flow regulating valve opening and the relative linear relationship between the unit relative to changes in the relative stroke change is a constant flow. | ① small opening, the flow changes, while the large flow changes when opening small ② small load, adjust the performance was too sensitive and oscillation, a large load regulation rather than slow adaptation time ③ Poor ability to adapt to |
| Equal percentage | Relative changes in stroke units relative flow changes caused by the relative flow to this point is proportional to. | ① flow changes caused by the change unit trip percentage is equal ② in the whole range of work travel more smoothly, especially in the large opening, the amplification factor is also large. More sensitive and effective work ③ widely used adaptive |
| Parabola | Line characteristics and other characteristics between the percentage of properties between the percentage of use of such features often replaced. | ① characteristics between the percentage of straight-line characteristics and other characteristics of the performance between the ② better adjustment process more difficult, but the valve flap |
| Quick open | In the valve stroke is small, the relatively large increase in traffic there, and soon reached the maximum. | ① In the small opening, the flow has been greatly, with the stroke increases, the flow quickly reaches the maximum ② General conditioning and process control for double |
In the real system, the valve pressure drop on both sides is not constant, it changes There are two main reasons. On the one hand, due to the characteristics of the pump, when the system flow decreases, the system generated by the pump pressure increases. On the other hand, when the flow rate decreases, the resistance of the coil is also reduced, resulting in a larger increase in pump pressure valve. Therefore, the import and export of pressure regulating valve is usually variable, in this case, the relative flow regulating valve and the relationship between the relative opening. Called the work flow characteristics. Concrete pipes can be divided into series and parallel flow characteristics of the work when the work of the pipeline flow characteristics.
(1) Series pipeline flow characteristics of the work
Regulating valve connected in series with the pipeline, due to changes in valve opening will cause traffic changes, known by the hydrodynamic theory, friction loss in pipe flow proportional to the square. Once the motion regulating valve, flow is changed, the system resistance is also changed accordingly, so a corresponding change in the pressure drop valve. Series pipeline flow characteristics and pressure drop in the work of distribution ratio. The smaller the pressure drop valve, regulating valve fully open flow rate decreases accordingly, so that a straight line features ideal for the quick distortion characteristics, the percentage of properties such as distortion of the ideal characteristics for the line. In actual use, when the regulating valve is selected too large or the production of non-full load condition, the regulating valve is working in a small opening, sometimes in order to have a certain valve opening, and will be small in order to adjust the valve opening increase the channel resistance to flow through the flow regulating valve decreases the pressure drop effect is to make the distribution ratio decreased, so that distortion of flow characteristics, deteriorating the quality of regulation.
(2) Parallel pipeline flow characteristics of the work
Regulating valve in parallel with the pipe, the ordinary and the bypass pipe from the valve slip slip composition, slip regulating valve installed in the valve on the pipeline. Regulating valve in the parallel pipeline, a certain resistance in the system, the regulating valve fully open flow and maximum flow ratio of mains pipes in parallel with the bypass valve opens gradually reduced. At this point, although the flow characteristics of regulating valve itself did not change, but narrowed the scope of the system’s adjustable, regulating valve in the course of their work beyond the control of the flow range is also greatly reduced, or even not achieve regulation. To make a better adjustment valve performance, bypass flow is generally believed that no more than 20% of the total flow.
4 Control Valve Selection
4.1 Flow characteristics of selected
Flow characteristics of the choice of two ways, one through the mathematical calculation method, the other is summed up in practical engineering experience in France. As the analysis complex and time-consuming, so the general engineering experience method are used. Specifically, should regulate the quality, working conditions, load and characteristics of several considerations.
(1) According to the regulation of the quality of automatic control system
According to the characteristics of automatic control theory in the compensation principle, in order to adjust the system to maintain good quality, and hope that the open-loop part of the total magnification factor and the amplification factor of the product remain constant. Thus, appropriate choice of the characteristics of valve to valve amplification coefficient to compensate for changes in magnification factor of the object, so the total magnification factor of the system remain unchanged.
(2) According to changes pipeline system pressure drop
Regulating valve pressure drop ratio S defined as the regulating valve to regulating valve corresponding to the largest import and export flows and system differential pressure ΔP differential pressure ΔP1m ratio:
Regulating valve flow characteristics and pressure drop ratio is closely related to S (Table 2).
| Piping system pressure drop than the S | 1~0.6 | 0.6~0.3 | 0.3~0 |
|---|---|---|---|
| Flow characteristics of the actual work | Straight line, Equal percentage | Straight line, Equal percentage | Regulation is not appropriate |
| Flow characteristics of the selected | Straight line, Equal percentage | Equal percentage, Equal percentage |
(3) Under load change
Straight line when the valve opening in a small flow changes, adjusting too sensitive, easy to oscillation. In the large opening, the regulation has become weak, resulting in adjustment is not timely, insensitive. Therefore, pressure drop less than S, the load should not change places with large linear valves. Equal percentage valve to ease the work towards the closure of a smooth and almost fully open state, the amplification coefficient, sensitive and effective work, so it applies to large variations in load situations. Quick opening valve characteristics of the stroke is small, the flow on larger, with the stroke increases, the flow quickly reaches the maximum, it is generally used for double-conditioning and process control of the situation.
(4) According to adjust the characteristics of the object
Generally self-regulating balance can be selected object properties such as the percentage of flow regulating valve, do not have the self-regulating balance the objects selected characteristics of linear flow regulating valve.
4.2 Select caliber
Regulating valve diameter is based on the flow ability to determine process requirements, and to provide the process conditions according to calculate the flow capacity of valve, and then choose according to their ability to flow regulating valve of the caliber. Flow capacity is fully open when the regulating valve, pressure valve at both ends for the 9.81×104Pa, fluid density 1g/cm3, the hourly traffic flows through the valve value to m3/h or kg/h for the unit. The flow regulating valve capacity is a reasonable choice valves and valve diameter is an important parameter, by the calculation of regulating valve flow capacity, compared to manufacturers with the technical parameters to determine the size of the valve diameter. For the automatic control system, the water is flowing through the valve is one common medium, so as to introduce water as an example the flow regulating valve capacity C:
Practical projects, the valve diameter is graded, C value usually is not a continuous value (the value of formula C is continuous). The same types of products from different manufacturers have different C values correspond to the table with the caliber. In calculating the expected value of C, you can access the corresponding product manufacturers data sheet to determine the required valve diameter. Select Valve diameter principles should be as close as possible to or greater than the results should not be less than the calculated results.
4.3 Selection Note
(1) Regulating valve selection based directly on the receiver diameter is unreasonable. Valve to take over the regulation of quality and nothing to do with velocity or diameter, the valve regulating water quality, only the resistance and volume. Once the system equipment that is determined, the theory for the system only an ideal valve diameter, and will not be multiple choice.
(2) Regulating valve diameter can not be too small. Select the valve diameter is too small, on the one hand increases the resistance of the system, there may even be 100% open valve diameter, the system still can not meet the capacity requirements set, leading to serious consequences. On the other hand the valve will need to provide greater pressure through the system to maintain adequate flow, increase the load on the pump, valve vulnerable, a great influence on the life of the valve.
(3) Regulating valve diameter can not be too large. Select the valve diameter is too large, not only increased the project cost, but also cause the valves often run in the low percentage range, lead to reduced regulation accuracy, so that the control performance becomes worse and make the system easy to shock and vibration by.
(4) To ensure the quality control system, the best way is to choose the system to the extent permitted access to a larger diameter valve pressure drop, the valve pressure drop during operation changes in value as small as possible. Valve fully open state, the pressure drop the higher the percentage of the entire pump, the valve pressure drop, the smaller the relative changes in value, the more characteristics of the valve installed close to its intrinsic properties.
(5) valve control system should work in as much as possible under the conditions of constant pressure drop, because it matches the valve coil depends on its intrinsic properties and the flow factors, and these valves parameters depend on a constant valve pressure drop .
5 Conclusion
Design of regulating valve, the valve requires the composition, classification and characteristics have a clear understanding of, and on this basis to master the correct selection. Moreover, the actual system configuration for a regulating valve, you also need to loop the whole pipeline system a detailed analysis of the factors taken into account. The only way to correctly select the regulating valve, adjusting the system to ensure quality control.