Abstract: Pneumatic Diaphragm Control Valve detailed working principle of rule flow characteristics, C value, the determination of diameter, and the combination of aluminum slurry Section of Journal of Shanxi Branch of slurry transport in control, as to the correct selection and application process The right to amend the actual situation, and the installation process for attention.
With the rapid development of China’s nonferrous metals industry, pneumatic membrane valves as simple structure, convenient operation, reliable, fire proof, etc., are widely used in the production of alumina. The automatic control system as an important part of its working condition is good or bad will directly affect the control process, Based on its own based on the practical work described in detail Pneumatic Diaphragm Regulating Valve’s working principle, selection process, installation and maintenance.
A main structure and working principle
Pneumatic actuator, the executing agency and the regulator agencies. Pneumatic actuator, including : pneumatic membrane pneumatic piston, pneumatic long journey of three implementing agencies, regulatory agencies are: valve, gate, regulating valve, a straight, angular stroke 2.
How it works: When the 0.2 ~ 1kg/cm2 Input signal when the pressure chamber in the film, putting parts of thrust to move the spring is compressed the pressure generated by reaction with the signal to generate thrust in the film Equilibrium. Putting air film movement that is the executing agency of the trip. Positive -action : when the film chamber pressure signal is zero, putting parts in the bottom, when the film indoor air pressure input signal, so putting downward moving parts; reaction formula: When the film chamber pressure signal 0:00, putting parts in the top of the indoor air when the film pressure of the input signal, so putting upward components.
2 pneumatic flow control valve film characteristics and the principle of selection t
Flow characteristics is the opening of valve position and flow rate relationship, direct impact on regulating the quality and stability of the system, and was transferred parameters and the device object, process the.
2.1 Ideal Flow Characteristics
Pressure regulating valve at both ends of a relatively constant flow and the relative opening ( stroke ) of the relationship : Q / Qmax= F × l / L
The formula, Q is an opening, the flow control valve and valve stem stroke; f factor for the spool; Qmax, L for the control valve fully open when the flow rate and stem the whole trip. Ideal flow characteristics depend on the size of the spool, the spool surface of different ideals, different flow characteristics.
2.2 Work Flow Analysis
Front and rear pressure control valve changes the flow characteristics obtained. Divided into linear features, on the number of features, parabolic characteristics. Between linear and parabolic characteristics between the logarithm.
The calculation, analysis, linear characteristic valve opening hours of work regulation in the small and strong, the relative change rate of flow is too intense and difficult to control, small interfering large to overcome easily overdone, causing the system oscillation, and in the large opening, the relative change rate Next, adjust the performance weak, too slow, a large disturbance can not be quickly overcome; on the number of features is the unit opening change, caused by changes in the relative flow value and this point is proportional to the relative flow. The calculation, analysis, number of valves in the small opening when the magnification is small, relaxed balance, beneficial to operation control, and in the large opening when the large magnification, the work can be sensitive and effective, is the most common valve.
2.3 Flow characteristics of selection rules
Commonly used in industrial production control valve, such as linear , logarithmic, quick open feature, the general selection line, on the number of properties can be adjusted to meet the process requirements, features quick adjustment to adapt to two, more difficult to control and demands for more stringent targets From the following aspects:
- Nonlinear control valve used to compensate the nonlinear process, the system changes the overall gain of less stable;
- Process pipe, considering the situation of process piping resistance;
- Adapt to the system load fluctuations;
- Consider adjusting the working conditions and service life of valve;
- Valve job performance improvement.
3 flow capacity C value is calculated and the valve diameter to determine
C value definition: China before and after the pressure control valve provides for the 1kPa, liquid heavy as 1kPa3 Cases, by adjusting the valve to the fluid per hour m3 Value that the value of the size of the flow capacity C (in case of alumina slurry. )
Pressure regulating valve : S = ΔP / (ΣΔPF+ ΔP)
The formula, ΔP the differential pressure regulating valve; ΣΔPF Maximum flow pipe pressure drop.
C = Q (r / ΔP)1/2= G / (ΔP * r)1/2
The formula, Q, G for the process provided by the volume or weight of traffic; ΔP the pressure difference before and after valve; r severe.
C value is selected and the nominal diameter Dg And the valve seat diameter dg Determination process provided by the maximum flow and minimum pressure drop corresponding. Calculated CmaxCan select the appropriate valve.
4 Applications
Settlement Section of Shanxi Aluminum Company is responsible for dissolving pulp by high pressure washing, sedimentation tank under the influence of the red mud settling, separation. In the slurry transport process requires a lot of pneumatic control valve to adjust the flow. Process according to site conditions or terms, pneumatic valve FISHER-ROSEMOUNT by the U.S. company’s pneumatic control valve, the valve positioner and valve component. Valve Positioner uses displacement pneumatic valve positioner, the negative feedback loop. Figure 1.
Figure 1 settlement slurry loop system
A diagram for the effective area of bellows; C for measuring the stiffness of components; K for the three power amplifiers magnification ; KL Lever for the transmission ratio of the input signal; KF Leverage ratio for the feedback signal transmission; KV/ (TVs +1) is a pneumatic control valve of the transfer function, is a first order cycle links, KV To adjust the amplification factor, and the implementing agencies of the film and the spring stiffness and the effective area of the structural adjustment and other factors; TV Time constant for the control valve, but also with the chamber size and other factors.
The negative feedback system, the valve stem displacement Y and the output of the regulator input pressure signal P led the transfer function between :
W (s) = Y / P ≈ AEKL/ CKF
AE, KL, C, KF Certain time, Y and P into one correspondence between the proportional relationship. In other words, through the electric valve positioner spaces in the electrical conversion, locator accepted regulator or control system from the current signal (4 ~ 20mA), the signal change in the pressure chamber executive body P, the location of the valve Reach a given value Y, so as to achieve the purpose of regulation.
4.1 The reverse movement and control valve flow characteristics
In the application process, because of production need to be a gas off -type valve into open air regulator, in the past on the need to install anti- spool, or a reaction -type actuator. Modification in the field is too much trouble, and the need for some spare Caixing. With valve positioner, the positioner is the role of the input signal changes from 20 ~ 100kPa, its output signal from the 20 ~ 100kPa into a 100 ~ 20kPa can. Concrete structure, the use of a cam and two nozzles. Nozzle to achieve positive role of left and right nozzle to achieve reaction. Left and right nozzle and the amplifier back-pressure pneumatic switch board to communicate with. Control valve flow characteristics can change the feedback to change the geometry of cams. Feedback to change the geometry of cams can change the amount of feedback control valve, so that positioning the output characteristics of change, which amended the flow characteristics.
4.2 Manual configuration of institutions
When the gas source signal or signal failure, or when the main executive body components ( diaphragm, spring, etc.) is damaged, you need the manual to automatic operation required to turn the hand wheel valve to maintain the regulatory function; On the other hand, such institutions could also serve as valve stroke of the stopper, when the signal pressure is zero, not the fully open valve is fully closed, if the process required a small amount of flow control valve, hand wheel can be used to Achieve their goals. Hand wheel bodies top -loading and side -mounted, top -loading only restriction for the one-way trip, if selection or installation, use side mounting can process requirements of safety limit in the left or right side of Shi Xian. In short, the manual valve bodies can improve the reliability, particularly additional bypass control valve station, using the larger diameter valves, the use of hand wheel bodies are from the area of investment costs or cost-effective. Therefore, the normal automatic operation is complete and the implementing agencies without fault, because they do not use the manual organization, this often fuel anti-rust.
5 Install control valve should be noted that several issues
(1) pneumatic control valve should be installed to facilitate maintenance, repair place.
(2) When the selected control valve nominal diameter and the process should be installed in different diameter connectors to connect different.
(3) installed in a vibration source of the occasion, should increase anti-vibration measures.
(4) The installation must be made on the body or the flange on the media to point the direction of the arrow direction.
(5) Before installation, require careful cleaning pipe welding slag and other debris, after installation, should be the biggest opening in the spool, and once again cleaning pipes and valves to prevent debris and damage stuck throttle Pieces.
6 Conclusion
Pneumatic control valve of the correct film selection, installation, use, maintenance, not only can improve the reliability of process control, and can quickly resolve valve failure, increased valve life energy consumption of enterprises with considerable economic benefits.