How to choose the function and use mode of diaphra

  • Detail

How to choose the function and use mode of diaphragm pump

the function of diaphragm pump in process control is to receive the control signal of regulator or computer, change the flow of regulated medium, and maintain the regulated parameters within the required range, so as to achieve the automation of production process. If the automatic adjustment system is compared with the manual adjustment process, the detection unit is human eyes, and the adjustment control unit is human brain, then the execution unit diaphragm pump is human hands and feet. In order to realize the regulation and control of a certain parameter in the process, such as temperature, pressure, flow, liquid level, etc., it is inseparable from the diaphragm pump. Therefore, the correct selection of diaphragm pump is of great significance in process automation. The universal experimental machine synchronously displays the experimental force, peak value, displacement, speed, experimental state, experimental curve, etc; Universal testing machine (auto parts testing machine) has the functions of limit, automatic overload protection, automatic shutdown due to sample fracture, etc. 1 Composition and classification of diaphragm pump

diaphragm pump, also known as control pump, is the main type of actuator. It changes the fluid flow by receiving the control signal output by the regulating control unit and using power operation. Diaphragm pump is generally composed of actuator and valve. According to the power used by the actuator, the diaphragm pump can be divided into pneumatic, electric and hydraulic, that is, the pneumatic diaphragm pump with compressed air as the power source, the electric diaphragm pump with electricity as the power source, and the electro-hydraulic diaphragm pump with the pressure of liquid medium (such as oil) as the power. In addition, according to its functions and characteristics, there are solenoid valves, electronic, intelligent and Fieldbus diaphragm pumps. There are many types of products and structures of diaphragm pumps, which are constantly updated and changed. Generally speaking, the valve is universal, which can be matched with pneumatic actuator, electric actuator or other actuators

2 selection of diaphragm pump type

2.1 selection of valve body type of diaphragm pump the selection of valve body is the most important link in the selection of diaphragm pump. There are many kinds of diaphragm pump valve bodies, including 10 kinds, such as straight through single seat, straight through double seat, angle, diaphragm, small flow, tee, eccentric rotation, butterfly, sleeve, spherical, etc. Before selecting the valve, carefully analyze the medium, process conditions and parameters of the control process, collect enough data, understand the requirements of the system for the diaphragm pump, and determine the type of valve to be used according to the collected data. In the specific selection, the following aspects can be considered:

(1) the shape and structure of the valve core is mainly considered according to the selected flow characteristics, unbalanced force and other factors

(2) wear resistance when the fluid medium is a suspension containing high concentration of abrasive particles, the valve core and valve seat interface will be seriously rubbed every time they are closed. Therefore, the flow path of the valve should be smooth and the internal material of the valve should be hard

(3) corrosion resistance because the medium is corrosive, try to choose a valve with simple structure when it can meet the regulation function

(4) temperature and pressure of the medium when the temperature and pressure of the medium are high and change greatly, the valve whose material of the valve core and valve seat is less affected by the change of temperature and pressure should be selected

(5) prevent flash and cavitation flash and cavitation are only produced in liquid medium. In the actual production process, flashing and cavitation not only affect the calculation of flow coefficient, but also form vibration and noise, which shortens the service life of the valve. Therefore, when selecting the valve, it is necessary to prevent flashing and cavitation of the valve

2.2 selection of diaphragm pump actuator

2.2.1 consideration of output force

regardless of the type of actuator, its output force is effective for overcoming the load (mainly refers to the effect of unbalanced force and unbalanced torque plus friction, sealing force, gravity and other related forces). Therefore, in order to make the diaphragm pump work normally, the actuator should be able to produce enough output force to overcome various resistances and ensure high sealing and valve opening

generally, there is no return spring for double acting pneumatic, hydraulic and electric actuators. The magnitude of the force has nothing to do with its running direction. Therefore, the key to selecting the actuator is to find out the maximum output force and the rotating torque of the motor. For single acting pneumatic actuators, the output force is related to the opening of the valve, and the force on the diaphragm pump will also affect the motion characteristics. Therefore, it is required to establish a force balance in the entire opening range of the diaphragm pump

2.2.2 determination of actuator type

after determining the output force of the actuator, select the corresponding actuator according to the requirements of the process use environment. When there are explosion-proof requirements on site, pneumatic actuators should be selected, and the junction box is explosion-proof, so electric actuators cannot be selected. If there is no explosion-proof requirement, pneumatic and electric actuators can be selected, but in terms of energy conservation, electric actuators should be selected as far as possible. For hydraulic actuators, they are not as widely used as pneumatic and electric actuators, but they have the characteristics of high adjustment accuracy, fast action speed and stability. Therefore, in some cases, in order to achieve better adjustment effect, hydraulic actuators must be selected, such as the speed of transparent machines in power plants, including dynamic fatigue load test degree adjustment from road load data, temperature adjustment and control of catalytic device reactors in refineries, etc

3 selection of action mode of diaphragm pump

the action mode of diaphragm pump is only available when pneumatic actuator is selected, and its action mode is formed through the combination of positive and negative action of actuator and positive and negative action of valve. There are four types of combination, namely positive (air off type), positive and negative (air on type), negative (air on type) and negative (air off type). The diaphragm pump action mode formed by these four combinations includes air on and air off. For the selection of diaphragm pump action mode, three aspects are mainly considered: a) process production safety; b) Characteristics of media; c) Ensure product quality and minimize economic losses

4 diaphragm pump flow, selection of characteristics

the flow characteristics of diaphragm pump refers to the relationship between the relative flow of medium through the valve and displacement (relative opening of the valve). The ideal flow characteristics mainly include straight line, equal percentage (logarithm), parabola and quick opening. The characteristic curve and valve core shape are shown in Figure 1 and Figure 2. There are only three commonly used ideal flow characteristics: straight line, equal percentage (logarithm) and fast opening. The parabolic flow characteristic is between straight line and equal percentage, which can generally be replaced by equal percentage characteristic, while the quick opening characteristic is mainly used in two position regulation and program control, so the selection of diaphragm pump characteristics is actually the selection of straight line and equal percentage flow characteristics

the selection of flow characteristics of diaphragm pump can be calculated theoretically, but the methods and equations used are very complex. At present, empirical criteria are mostly adopted, specifically considering the following aspects: ① analyze and select the regulation quality of the regulation system; ② Consider from the process piping situation; ③ From the analysis of load changes

after selecting the flow characteristics of the diaphragm pump, the shape and structure of the valve core can be determined according to its flow characteristics. However, for diaphragm valves, butterfly valves, etc., due to their structural characteristics, it is impossible to change the surface shape of the valve core to achieve the required flow characteristics. At this time, it can be achieved by changing the digital display, which can only simply display the experimental data, and the feedback cam shape with the valve positioner

5 selection of diaphragm pump caliber

the selection and determination of diaphragm pump caliber is mainly based on the flow capacity of the valve, namely cv. In the instrument design and model selection of various projects, CV calculation shall be carried out for the diaphragm pump, and the design specification of the diaphragm pump shall be provided. From the CV calculation of the diaphragm pump to the determination of the valve diameter, the following steps are generally required:

1) the determination of the calculated flow. The existing production capacity, equipment load and medium conditions determine the Qmax and qmin of the calculated flow

2) determination of the pressure difference before and after the valve. Select s (resistance coefficient) according to the selected valve flow characteristics and system characteristics, and then determine the calculated differential pressure

3) calculate cv. According to the adjusted medium, select the appropriate calculation formula and chart to obtain Cmax and cmin

4) choose cv. According to Cmax, select the peak value key of Cmax point in the selected product standard series, and type the closest level c.

5) diaphragm pump opening checking calculation on page 3. Generally, the opening at the maximum calculated flow is ≯ 90%, and the opening at the minimum calculated flow is ≮ 10%

6) checking calculation of actual adjustable ratio of diaphragm pump. Generally, the actual adjustable ratio is ≮ 10

7) determination of valve seat diameter and nominal diameter. After verification, determine according to C

6 Conclusion

the selection of diaphragm pump is a very meticulous work, which requires not only solid professional theoretical knowledge, but also rich practical experience. A good choice is not only conducive to the adjustment of PID parameters of the control loop, so that the adjusted parameters can get a better control effect, but also greatly increase the service life of the diaphragm pump. The selection of diaphragm pump should be adjusted to local conditions, not unchanging. It should be summarized and innovated in the process of practice. Especially with the application of electromechanical integration technology, computer and digital information technology, the structure and function of diaphragm pump become better and more comprehensive, which provides great convenience for the selection of diaphragm pump

Copyright © 2011 JIN SHI