Diaphragm pump selection and classification
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The function of the diaphragm pump in the process control is to receive the control signal of the regulator or the computer and to change the flow of the medium to be adjusted so as to maintain the adjusted parameter within the required range so as to automate the production process. If the automatic adjustment system compared with the manual adjustment process, the detection unit is the human eye, the control unit is to adjust the human brain, then the execution unit - diaphragm pump is the human hand and foot. To achieve a certain parameters of the process such as temperature, pressure, flow, level and other regulatory control, are inseparable from the diaphragm pump. Therefore, the correct choice of diaphragm pump in process automation is of great significance. 1 diaphragm pump composition and classification diaphragm pump, also known as the control pump, is the main type of actuator, by receiving the control signal conditioning control unit, with the power to change the fluid flow. Diaphragm pump generally by the implementing agencies and valves. If you press the actuator used by its power, the diaphragm pump can be divided into pneumatic, electric, hydraulic three, namely compressed air as the power source of the pneumatic diaphragm pump to electric power source electric diaphragm pump to liquid medium (Such as oil, etc.) pressure-driven electro-hydraulic diaphragm pumps, in addition, according to their functions and characteristics of points, as well as solenoid valves, electronic, intelligent, field bus diaphragm pumps. Diaphragm pumps are many types of products, the structure is also diverse, but also constantly updated and changing. Generally speaking, the valve is universal, it can be matched with the pneumatic actuator, but also with the electric actuator or other actuator. 2 diaphragm pump type of choice 2.1 diaphragm pump body type selection The choice of diaphragm is the most important part of the diaphragm pump selection. Diaphragm valve body types, commonly used through a single seat, through two-seater, angular, diaphragm, small flow, tee, eccentric rotation, butterfly, sleeve, spherical and other 10 species. Before choosing a valve, carefully analyze the medium, process conditions, and parameters of the control process, gather enough data to understand the system requirements for the diaphragm pump, and determine the type of valve to be used based on the data collected. In the specific choice, the following aspects can be considered: (1) spool shape and structure based on 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 valve seat joint surface will be severely rubbed each time it is closed. Therefore, the flow of the valve should be smooth, the valve's internal material to be hard. (3) Corrosion resistance Due to the corrosive medium, try to choose a simple structure of the valve in the case of regulatory function. (4) Medium temperature, pressure When the medium temperature, high pressure and changes, should use the valve core and valve seat material by temperature, pressure changes in small valves. (5) to prevent flash and cavitation flash and cavitation only produce 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. 2.2 The choice of diaphragm pump actuator 2.2.1 Output Force Consideration Regardless of the type of actuator, its output force is used to overcome the effective load (mainly refers to unbalanced and unbalanced torque plus friction, Sealing force, gravity and other related forces). Therefore, in order for the diaphragm pump to work properly, the associated actuator must be capable of producing enough output force to overcome various resistances and ensure a high degree of sealing and valve opening. For double-acting pneumatic, hydraulic, electric actuator, there is usually no return spring. The size of the force has nothing to do with its direction of operation, so the key to choosing an actuator lies in knowing the maximum output force and the torque of the motor. For single-acting pneumatic actuators, the output force is related to the opening of the valve. The force appearing on the diaphragm pump will also affect the kinematics and therefore a force balance across the opening of the diaphragm pump is required. 2.2.2 Determination of the type of implementing agencies After determining the output of implementing agencies, according to the requirements of the process environment, select the appropriate implementing agencies. For on-site explosion-proof requirements, pneumatic actuators should be used, and the junction box for the explosion-proof, you can not choose the electric actuator. 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, its use is not as good as the pneumatic, electric actuators a wide range, but with high precision, fast speed and smooth characteristics, therefore, in some cases, in order to achieve better regulation effect, you must use hydraulic actuator Institutions, such as power plant transparent speed control, refinery catalytic reactor temperature control and so on. 3 diaphragm pump mode of action The function of the diaphragm pump is selected only when the pneumatic actuator only, the role of the positive and negative effect through the implementing agencies and the positive and negative valve action combination. There are four types of combination that is positive (air-off type), positive and negative (air-type), anyway (air-type), anti-anti (anti-off type) Open and Qi off two. For the diaphragm pump mode of action, mainly from three aspects: a) process safety; b) the characteristics of the media; c) to ensure product quality, economic loss is minimal. 4 diaphragm pump flow, the choice of characteristics Flow characteristics of the diaphragm pump is the relationship between the relative flow through the valve medium and displacement (relative opening of the valve) relationship between the ideal flow characteristics are straight, equal percentage (logarithm), parabolic And fast open, etc. 4, the characteristic curve and spool shape shown in Figure 1 and Figure 2. The only commonly used ideal flow characteristics, such as straight line, such as the percentage (logarithm), quickly open three. Parabolic flow characteristics between the straight line and equal percentage, the general percentage of the characteristics can be used instead of the other, while the fast opening characteristics are mainly used for two-bit adjustment and program control, so the selection of diaphragm pump is actually a straight line and equal percentage flow characteristics s Choice. The choice of diaphragm pump flow characteristics can be calculated theoretically, but the methods and equations used are complicated. At present, the use of more experience guidelines, specifically from the following aspects: ①from the regulatory system to adjust the quality of analysis and selection; ② from process piping considerations; ③ from the load changes. Select the flow characteristics of the diaphragm pump, you can determine the valve spool according to its flow characteristics of the shape and structure, but for such as diaphragm valves, butterfly valves, because of their structural characteristics, it is impossible to change the shape of the valve spool to achieve The required flow characteristics can be achieved by changing the feedback cam profile of the valve positioner fitted. 5 Diaphragm Pump Diaphragm Diaphragm selection and determination of the main valve based on the flow of capacity that Cv. In a variety of engineering design and selection of instruments, should be on the diaphragm Cv calculations, diaphragm pumps and provide design specifications. Calculated from the diaphragm pump Cv to determine the diameter of the valve, generally required by the following steps: 1) to determine the flow rate. The existing production capacity, equipment load and the condition of the medium determine the Qmax and Qmin of the calculated flow rate 2) Determination of the differential pressure before and after the valve. According to the selected valve flow characteristics and system characteristics selected S (resistance coefficient), and then determine the calculation of pressure. 3) Calculate Cv. According to the adjusted medium select the appropriate calculation formula and chart, find Cmax and Cmin. 4) choose Cv. According to Cmax, in the selected product standard series> Cmax and closest to the level of C. 5) diaphragm pump opening checking. The general requirements of the maximum flow when the calculation of the opening ≯ 90%, the minimum calculation of flow when the opening ≮ 10%. 6) Diaphragm pump actual adjustable ratio checking. Generally require the actual adjustable ratio ≮10. 7) Determination of seat diameter and nominal diameter. After verification, according to C to determine.