Control valve characteristics and selection method

Regulating valve is a commonly used actuator in automatic control system, which is used to adjust the flow of controlled object. Correct choice of regulating valve, is to adjust the quality assurance system control. On the composition of the control valve, flow characteristics were described in detail, and gives the control valve selection method and should pay attention to the problem.

0 Foreword

Regulating valve is a commonly used actuator in automatic control system, which is the main control element of automatic control, which directly controls the delivery volume of the measured medium. Control valve by the implementing agencies and regulatory agencies, to accept the control signal of the regulator or computer, used to change the flow of the controlled medium, to be transferred parameters to maintain the required range, so as to achieve process control automation. In the field of automatic control, whether the control process is steady or not depends directly on whether the control valve can act accurately, so that the process control can be reflected as the precise changes of the material energy and the flow rate. So, according to different needs to choose a different control valve. Select the appropriate control valve is the main problem of pipeline design, but also to ensure the regulation of system security and smooth operation of the key.

1 type selection

Regulating valve generally by the implementing agencies and valves. Regulating valve is regulating valve regulating mechanism, which according to the requirements of the control signal to change the size of the valve opening to adjust the flow rate, is a local resistance can change the throttle element. Regulating valve is mainly composed of upper and lower bonnet, valve body, valve core, seat, packing and platen components. In the automatic control system, the main regulating medium for the valve is water and steam. The pressure is relatively low, the use of a single case, the commonly used control valve through the control valve, three-way control valve and butterfly valves and other types. According to the type of energy used, the actuator can be divided into three types: pneumatic, electric and hydraulic. Pneumatic adjustment with compressed air as power source, electric adjustment with electric power source, and pressure with liquid medium (such as oil) The hydraulic adjustment. Among them, the pneumatic actuator has the advantages of simple structure, reliable operation, stable performance, low price, convenient maintenance, fire protection and explosion proof and has been widely used in many control systems. Although the electric actuator is not conducive to fire and explosion-proof, but its convenient drive power desirable, and the signal transmission speed, easy to long-distance transmission, small size, reliable action, easy maintenance, cheap. Hydraulic actuator maximum thrust, and the regulation of high precision, fast and smooth movement, but the equipment size, process complexity.

(1) adjust the valve type of choice

The valve body type selection is the most important part of the control valve selection. Before selecting a valve, carefully analyze the medium, process conditions, and parameters of the control process, understand the system requirements for the regulator, and determine the type of valve to be used based on the data collected. In the specific choice, from the following aspects:

① spool shape and structure of the main characteristics of the selected flow characteristics and unbalanced forces and other factors to consider;

② wear resistance When the fluid medium is a suspension containing high concentration of abrasive particles, the valve core, the seat surface each closed will be severely friction. Therefore, the flow of the valve should be smooth, the valve's internal material to be hard;

③ corrosion-resistant medium is corrosive, to meet the regulatory function of the situation, try to choose a simple structure of the valve;

④ medium temperature, pressure when the medium temperature, high pressure and changes in the large, the valve core and valve seat material should be selected by the temperature, pressure changes in small valves;

⑤ to prevent flash and cavitation flash and cavitation only in the liquid medium. In the actual production process, flash and cavitation not only affect the calculation of flow coefficient, but also the formation of vibration and noise, so that the valve life is shorter, so the valve should be selected to prevent flash and cavitation.

(2) control valve actuator choice

① consider the actuator output force regardless of the type of actuator, the output force is used to overcome the effective load (mainly refers to unbalanced and unbalanced torque, friction, sealing force, gravity and other related forces effect). Therefore, in order for the regulating valve to work normally, the matched actuator should be able to generate enough output force to overcome various resistances and ensure a high degree of sealing and valve opening.

② Considering the use environment requirements of implementing agencies After determining the output power of implementing agencies, select the appropriate implementing agencies according to the process environment requirements. For example, when there is an explosion-proof requirement on site, the pneumatic actuator should be used, and the junction box is explosion-proof type, and the electric actuator can not be selected. If there is no explosion-proof requirements, the pneumatic and electric actuators can be used, but from the energy-saving aspects, should try to use electric actuators. For the hydraulic actuator, although use as pneumatic, electric actuators widely, but it has a high adjustment precision, high speed and stable operation characteristics, so that in exceptional circumstances, in order to achieve good conditioning effect will be employed. Finally, we must also consider the implementing agencies of reliability, economy, choice of action is smooth, light weight, beautiful appearance, simple structure, easy maintenance and implementing agencies.

2 flow characteristics analysis

2.1 working principle

According to the fluid mechanics shows, the control valve is a local resistance can change the throttle element. On the incompressible fluid, the flow control valve p1 --- regulator valve before the pressure;

p2 --- regulator valve pressure;

A --- throttle cross-sectional area;

ξ --- regulating valve resistance coefficient;

ρ --- fluid density.

From equation (1), when A is constant and Δp = p1 -p2 is also constant, the flow rate Q through the valve changes with the resistance coefficient ξ, that is, the larger the resistance coefficient ξ, the smaller the flow rate. The resistance coefficient ξ and the valve structure and opening related. Therefore, the regulator output signal to control the valve on or off, the valve can change the resistance coefficient, thereby changing the flow of media being adjusted.

2.2 traffic characteristics analysis

Regulating valve flow characteristics refers to the medium to be transferred through the control valve relative flow and the relative opening of the control valve.

Flow characteristics of the control valve include ideal flow characteristics and flow characteristics. The ideal flow characteristics refer to the flow characteristics under the condition that the pressure difference is constant before and after the control valve. There are four kinds of characteristics such as straight line, equal percentage, parabola and quick opening. In the actual system, the pressure drop on both sides of the valve is not constant, so there are two main reasons for its change:

Due to the characteristics of the pump, the system pressure generated by the pump increases as the system flow decreases;

② When the flow decreases, the resistance on the coil also decreases, resulting in a larger pump pressure on the valve. Therefore, the pressure difference before and after the control valve is usually changed, in this case, the relative flow control valve and the relative opening between the relationship, known as the flow characteristics. Specific can be divided into series pipe when the work flow characteristics, work flow characteristics of parallel pipe.

Table 1 4 ideal flow control valve characteristics

Flow characteristics of the nature of the characteristics

Linear flow control valve relative flow and the relative opening was a linear relationship, that unit relative changes in relative flow caused by a constant change. (1) When the flow rate is small and the opening degree is small, the flow rate changes little when the flow rate changes greatly; (2) When the load is small, the regulating performance is too sensitive and oscillation occurs, and the regulation is sluggish when the large load is not timely; (3) difference.

The relative flow change caused by the change of relative stroke of equal percentage units is proportional to the relative flow at this point. (1) the percentage of flow changes caused by the change of unit stroke is equal; (2) the work is stable within the full stroke range, especially when the opening is large, the work is more sensitive and effective; (3) Wide range of applications, adaptability

Parabolic characteristics between the straight line characteristics and the percentage of the same characteristics, the use often use the same percentage characteristics instead. (1) characteristics between the straight line and equal percentage characteristics; (2) better performance adjustment but the valve core processing more difficult.

Quick opening in the valve stroke is small, the traffic will have a relatively large increase, up to the maximum. (1) The flow is very large at small opening, with the increase of the stroke, the flow reaches the maximum quickly; (2) It is generally used for two-position regulation and program control.

(1) Working flow characteristics in tandem pipeline

When the pipeline control valve in series, modulating valve opening degree changes cause changes in the flow rate, apparent from hydrodynamic theory, the resistance loss proportional to the square of the flow duct, once the operation of the control valve, the flow rate is changed, the system resistance is also changed, So the pressure regulator valve also changes accordingly. The characteristics of the working flow in tandem pipeline are related to the pressure drop distribution ratio. The smaller the pressure drop across the valve, the corresponding reduction of the full-flow control valve, the ideal straight-line characteristic is distorted to the quick-open characteristic, and the ideal equal percentage characteristic is distorted to a straight line characteristic. In actual use, when the control valve is too large or the production is not under full load condition, the control valve is working in a small opening, and sometimes in order to make the control valve has a certain degree of opening, and the valve is closed to increase small pipe resistance , So that the flow through the control valve to reduce, in fact, is to make the pressure drop distribution ratio, the flow characteristics of distortion, deteriorating the quality of regulation.

(2) Working flow characteristics in parallel pipelines

Parallel pipeline generally by the valve branch and bypass branch composition, control valve installed in the valve branch pipeline. Control valve in the parallel pipeline, the system resistance is constant, the ratio of full-flow control valve and the maximum flow of the manifold with the parallel pipeline bypass valve gradually opened and reduced. At this moment, though the flow characteristics of the regulating valve itself are unchanged, the adjustable range of the system is greatly reduced, and the range of the flow that the regulating valve can control during the working process is also greatly reduced or even not regulated. To make the control valve have better regulation performance, the bypass flow is generally considered not to exceed 20% of the total flow.

3 traffic characteristics of choice

Flow characteristics of the selection method:

① through mathematical analysis of the analytical method;

② in the actual project to sum up the experience of law. Due to the complexity and time-consuming analysis, the general engineering method is based on experience. Specifically, we should consider the following aspects:

(1) Consideration of the adjustment quality of the automatic adjustment system

According to the principle of compensation in the principle of automatic control, in order to maintain good regulation quality of the system, it is desired that the product of the total open-loop amplification factor and the amplification factor of each link be kept constant. In this way, the appropriate choice of valve characteristics to the valve's amplification factor changes to compensate for changes in the object amplification factor, the system's total amplification factor remains unchanged.

(2) Choose according to the pressure drop of pipeline system

Control valve pressure drop ratio is defined as the maximum flow control valve can be controlled by the corresponding valve differential pressure Δp1m and the system pressure than the ratio of Δp

Regulating valve flow characteristics and pressure drop than the S has a close relationship, as shown in Table 2.

Table 2 according to the pressure drop pipeline system selection valve characteristics table

Pipeline pressure drop ratio S 1-0.6 0.6-0.3 0.3-0

The actual work flow characteristics such as straight line percentage percentage adjustment is not appropriate

Selected flow characteristics Straight Percentage Straight Percentage

(3) from the load changes in the situation to analyze

Linear valve in the small opening of large flow changes, the regulation is too sensitive, easy to oscillation; in the large opening, the regulatory role and became weak, resulting in regulation is not timely, insensitive. Therefore, the pressure drop ratio S is small, large changes in load occasions should not be used linear valve. Equal percentage valve work close and close to ease and smooth, and close to fully open state, the amplification factor, work sensitive and effective, so it is suitable for large load changes in the occasion. Fast opening characteristics Valve in the smaller stroke, the flow is larger, with the increase of stroke, flow to reach the maximum, it is generally used for two-bit regulation and program control of the occasion. (4) adjusting the control valve to select all objects having a generally self-balancing capability and the like can be selected according to characteristics of percentage flow characteristics of the adjustment object, the object does not have a self-adjusting balance of the flow control valve is selected characteristic line.

4 control valve diameter selection

Select the purpose of regulating the valve diameter is to make the valve and pipe combination, resulting in a reasonable combination of linear characteristics, the system can effectively regulate the control. Control valve diameter is determined according to the technical requirements of the flow capacity, provided by the process conditions to calculate the flow capacity of the control valve, and then according to their ability to select the flow control valve caliber.

To properly adjust the caliber of the valve, we must understand the flow control valve. Flow capacity refers to when the control valve is fully open, the pressure drop across the valve 9.81 × 104 Pa, the fluid density of 1 g cm3 hourly flow through the control valve flow value. Regulating valve flow capacity is a reasonable choice of the valve and valve caliber an important parameter, through the control valve flow capacity of the calculation, compared to the manufacturer to provide the technical parameters to determine the size of the valve diameter. For automatic control system, the water is the most common and most common medium through the control valve, so here to water as an example to introduce the flow control capacity of the control valve.

In practical projects, the valve diameter is graded, C values ​​are usually not continuous (C values ​​calculated by the formula is continuous). Different manufacturers of the same type of product have different C value and caliber corresponding table. After calculating the desired C value, the manufacturer's corresponding product data sheet can be consulted to determine the desired valve size. Select the principle of the valve diameter should be as close as possible or greater than the calculated results should not be less than the calculation results.

5 Precautions

(1) It is unreasonable to adjust the valve directly according to the take-over pipe diameter, because the quality of the valve is not related to the take-up flow speed or pipe diameter. The regulating quality of the valve is only related to the resistance and flow of water. That is, once the system equipment is identified, the valve ideally suited to the system has only one ideal caliber, without multiple options.

(2) In terms of valve selection, the valve of choice is too small, on the one hand will increase the system resistance, or even appear 100% open the valve diameter, the system can not meet the set capacity requirements, leading to serious consequences; the other Aspects of the valve will need to provide a larger pressure differential through the system to maintain adequate flow, aggravating the pump load, the valve vulnerable, and has a significant impact on valve life.

(3) selection valves, the selection valve diameter is too large, not only increase the cost of construction, but also often causes the valve to run in a low percentage range, resulting in modulation accuracy is lowered, so that the control performance is deteriorated, but also easy to make the system Shock and vibration.

(4) In order to ensure the quality of the system control, the best way is to choose the valve diameter that can get a larger pressure drop within the allowable range of the system, so as to make the change value of the pressure drop as small as possible during the operation of the valve. When the valve is fully open, the higher the percentage of total pressure drop, the smaller the relative change of the valve pressure drop. The closer the valve is installed, the closer it is to its inherent characteristics.

(5) The regulating valve in the control system should work as much as possible under a constant pressure drop, because whether the valve matches the coil depends on its inherent characteristics and flow rate factors, and these valve parameters depend on a constant valve pressure drop.

In summary, a regulating valve head design is required to ensure maximum quality control system, which requires the composition of the control valve, classification, characteristics have a clear understanding of, and to master the correct selection method based on . Moreover, when configuring a control valve for an actual system, detailed analysis of the entire piping loop is also required, taking into account various factors. Only in this way can the correct choice of regulating valve to ensure that the control system to adjust the quality.