Diaphragm pump type distinction and selection techniques

The function of the diaphragm pump in the process control is to accept the control signal of the regulator or the computer, change the flow of the regulated medium, and maintain the adjusted parameter within the required range, so as to achieve the automation of the production process. If the automatic adjustment system is compared with the manual adjustment process, the detection unit is the human eye and the adjustment control unit is the human brain, then the execution unit - diaphragm pump is the human hand and foot. To achieve a certain process parameters such as temperature, pressure, flow, liquid level adjustment control, are inseparable from the diaphragm pump. Therefore, the correct choice of diaphragm pump is of great significance in process automation.

1 The composition and classification of diaphragm pumps Diaphragm pumps, also known as control pumps, are the main types of actuators. They accept the control signals output by the control unit to change the fluid flow by means of power operation. Diaphragm pumps generally consist of actuators and valves. According to the power used by its actuators, diaphragm pumps can be divided into three types: pneumatic, electric, and hydraulic, that is, pneumatic diaphragm pumps with compressed air as the power source, and electric diaphragm pumps with electric power as the source, with liquid media. (such as oil, etc.) pressure-driven electro-hydraulic diaphragm pumps, in addition, according to their functions and characteristics, as well as solenoid valves, electronic, intelligent, fieldbus diaphragm pumps. Diaphragm pumps have many types of products and a variety of structures, and they are constantly updated and changed. In general, valves are universal and can be matched with pneumatic actuators or electric actuators or other actuators.

2 Selection of diaphragm pump type Valve body type selection of diaphragm pump The choice of valve body is the most important part in the selection of the diaphragm pump. Diaphragm pump body type is many, commonly used straight through single seat, straight through two seats, angle, diaphragm, small flow, three links, eccentric rotation, butterfly, sleeve, spherical and other 10 kinds. Before choosing the valve, carefully analyze the media, process conditions and parameters of the control process, collect enough data to understand the requirements of the system on the diaphragm pump, and determine the type of valve to be used based on the collected data. When choosing specific options, consider the following aspects:

(1) The valve core shape structure is mainly based on selected flow characteristics and unbalanced forces.

(2) Wear resistance When the fluid medium is a suspension containing a high concentration of abrasive particles, the valve core and the valve seat interface are severely rubbed each time they close. Therefore, the flow path of the valve should be smooth, and the internal material of the valve should be hard.

(3) Corrosion resistance Due to the corrosive nature of the medium, a valve with a simple structure can be selected as far as possible while satisfying the adjustment function.

(4) Temperature and pressure of the medium When the temperature and pressure of the medium are high and the change is large, the valve whose valve core and valve seat materials are affected by the temperature and pressure change should be selected.

(5) Preventing flashing and cavitation Flashing and cavitation are only produced in liquid media. In the actual production process, flashing and cavitation not only affect the calculation of the flow coefficient, but also create vibration and noise, so that the service life of the valve is shortened. Therefore, the valve should be prevented from flashing and cavitation when selecting the valve.

3 The mode of action of the diaphragm pump The mode of action of the diaphragm pump is selected only when the pneumatic actuator is selected. The mode of action is formed by combining the positive and negative action of the actuator and the positive and negative action of the valve. There are four types of combination: positive (air-closed), forward-reverse (air-opened), positively-inverted (air-opened), and reverse-reacted (air-closed). The diaphragm pumps formed by these four types of combinations are gas-free. Open and air off two. For the choice of the diaphragm pump mode of action, it is mainly considered from three aspects: a) process production safety; b) the characteristics of the medium; c) ensure product quality, and minimize economic losses.

4 diaphragm pump flow, the choice of characteristics Diaphragm pump flow characteristics refers to the medium flow through the valve relative flow and displacement (relative valve opening) relationship between the ideal flow characteristics are mainly linear, equal percentage (logarithm), parabola There are four kinds such as quick opening, and the characteristic curve and spool shape are shown in Figs. 1 and 2. The commonly used ideal flow characteristics are only linear, equal percentage (logarithmic), and quick opening. The parabola flow characteristics are between linear and equal percentages and can generally be replaced by equal percentage characteristics. The quick opening characteristics are mainly used for two-position adjustment and program control. Therefore, the diaphragm pump characteristics are actually selected as linear 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 complex. At present, more experience criteria are used, specifically considering the following aspects: (1) Analysis and selection from the regulation quality of the regulation system; (2) Considerations from the process piping; (3) Analysis from the load change situation.

Selecting the flow characteristics of the diaphragm pump can determine the shape and structure of the valve plug according to its flow characteristics, but for diaphragm valves, butterfly valves, etc., due to their structural characteristics, it is impossible to change the shape of the valve core to achieve The required flow characteristics, in this case, can be achieved by changing the shape of the feedback cam of the assigned valve positioner.