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The role of diaphragm pumps in process control is to receive control signals from regulators or computers, thereby changing the flow rate of the medium being controlled and keeping the relevant parameters within the desired range, thus achieving automation of the production process. If we compare the automatic control system with the manual control process, the detection unit is the human eye, the control unit is the human brain, and then the actuation unit—the diaphragm pump—is the human hands and feet. To achieve regulatory control of certain parameters in a process, such as temperature, pressure, flow rate, and liquid level, diaphragm pumps are essential. Therefore, selecting the right diaphragm pump is of great significance in process automation. Composition and classification of diaphragm pumps Diaphragm pumps, also known as control pumps, are one of the main types of actuators; they receive control signals from a control unit and use power to adjust the flow rate of fluids. Diaphragm pumps generally consist of an actuator and a valve. Based on the power source used by their actuators, diaphragm pumps can be classified into three types: pneumatic, electric, and hydraulic. Pneumatic diaphragm pumps use compressed air as a power source, electric diaphragm pumps use electricity as the power source, while electro-hydraulic diaphragm pumps utilize the pressure of a liquid medium such as oil. Additionally, depending on their functions and characteristics, there are also solenoid valve-type, electronic, intelligent, and fieldbus-type diaphragm pumps. There are many types of diaphragm pumps, with diverse structures, and these continue to evolve and change. Generally speaking, valves are versatile and can be used with pneumatic actuators as well as electric actuators or other types of actuators. Selection of diaphragm pump type. Selection of the valve body type for diaphragm pumps – the choice of valve body is the most important aspect in selecting a diaphragm pump. There are many types of diaphragm pump valve bodies; the commonly used ones include straight-through single-seat, straight-through double-seat, angular, diaphragm, low-flow, three-way, eccentric rotary, butterfly, sleeve-type, and ball-type – a total of 10 types. Before selecting a valve, it is necessary to conduct a thorough analysis of the medium involved in the control process, as well as the process conditions and parameters. Sufficient data should be gathered to understand the requirements of the system regarding diaphragm pumps, and the type of valve to be used should be determined based on this collected data. When making a specific selection, the following aspects can be taken into consideration: (1) The shape and structure of the valve core are primarily determined by factors such as the desired flow characteristics and unbalanced forces. (2) Wear resistance: When the fluid medium is a suspension containing high concentrations of abrasive particles, the mating surfaces of the valve core and seat are subjected to severe friction every time they close. Therefore, the flow path of the valve must be smooth, and the internal materials of the valve must be hard. (3) Corrosion resistance: Since the medium is corrosive, valves with a simple structure should be preferred as much as possible, provided that they can fulfill the regulation function. (4) Temperature and pressure of the medium: When the temperature and pressure of the medium are high and subject to significant fluctuations, valves should be selected in which the materials of the valve core and seat are less affected by such temperature and pressure changes. (5) Preventing flashing and cavitation: Flashing and cavitation occur only in liquid media. In actual production processes, flashing and cavitation not only affect the calculation of the flow coefficient but also cause vibrations and noise, thereby reducing the service life of the valves; therefore, when selecting valves, it is necessary to prevent flashing and cavitation from occurring. Selection of diaphragm pump actuators – Considerations regarding output force. Regardless of the type, the output force of an actuator is the force used to overcome the loads (primarily referring to unbalanced forces and unbalanced torques, in addition to forces such as friction, sealing forces, and gravity). Therefore, for a diaphragm pump to function properly, the associated actuator must be capable of generating sufficient output force to overcome various resistances, ensuring a tight seal and the proper opening of the valve. Double-acting pneumatic, hydraulic, and electric actuators generally do not have a return spring. The magnitude of the acting force is independent of its direction of movement; therefore, the key to selecting an actuator lies in determining the maximum output force and the torque of the motor. For single-acting pneumatic actuators, the output force is related to the valve opening, and the force generated on the diaphragm pump also affects its operating characteristics; therefore, it is necessary to establish a force balance across the entire range of valve openings of the diaphragm pump. Determination of actuator type: After determining the output force of the actuator, the appropriate actuator is selected based on the requirements of the operating environment. When explosion protection is required at the site, pneumatic actuators should be used, with explosion-proof junction boxes; electric actuators cannot be chosen. If there are no explosion-proof requirements, both pneumatic and electric actuators can be used; however, from an energy-saving perspective, electric actuators should be preferred as much as possible. For hydraulic actuators, their use is not as widespread as that of pneumatic and electric actuators. However, they feature high control precision, fast response speeds, and smooth operation. Therefore, in certain situations, in order to achieve optimal control results, hydraulic actuators must be used, such as for adjusting the speed of turbines in power plants or for controlling the temperature of reactors in catalytic units in oil refineries. Selection of the operating mode of diaphragm pumps: The operating mode of diaphragm pumps is a consideration only when selecting a pneumatic actuator; it is determined by the combination of the forward and reverse actions of the actuator along with those of the valve. There are 4 combination types: positive-positive (air-shut type), positive-negative (air-open type), negative-positive (air-open type), and negative-negative (air-shut type). The operation modes of the diaphragm pump resulting from these four combinations are air-open and air-shut. When selecting the operating mode for diaphragm pumps, three main factors are considered: a) process safety; b) the properties of the medium; c) ensuring product quality while minimizing economic losses. Selection of flow characteristics for diaphragm pumps: The flow characteristic of a diaphragm pump refers to the relationship between the relative flow rate of the medium passing through the valve and its displacement (the relative opening degree of the valve). The ideal flow characteristics include four types: linear, equal percentage (logarithmic), parabolic, and quick-open. The commonly used ideal flow characteristics are only linear, equal percentage (logarithmic), and quick-opening. The flow characteristic of a parabolic type lies between linear and equal percentage types; generally, the equal percentage characteristic can be used as a substitute. The quick-open characteristic is mainly used for two-position control and program control. Therefore, the choice of diaphragm pump characteristic essentially comes down to choosing between linear and equal percentage flow characteristics. The selection of the flow characteristic of a reciprocating pump can be done through theoretical calculations, but the methods and equations used are quite complex. Currently, empirical criteria are commonly used, with considerations drawn from the following aspects: ① Selection based on an analysis of the control quality of the control system; ② Consideration of the process piping layout; ③ Analysis of load variations. Once the flow characteristics of the diaphragm pump have been selected, the shape and structure of the valve core can be determined based on those characteristics. However, for valves such as diaphragm valves and butterfly valves, it is not possible to achieve the desired flow characteristics by altering the shape of the valve core’s surface; in such cases, this can be accomplished by changing the shape of the feedback cam used with the valve actuator. Selection of diaphragm pump diameter: The selection and determination of the diaphragm pump diameter are primarily based on the valve’s flow capacity, namely Cv. When designing and selecting instruments for various projects, it is necessary to perform a Cv calculation for diaphragm pumps and provide design specifications for them. From calculating the Cv of the diaphragm pump to determining the valve diameter, the following steps are generally required: 1) Determination of flow rate calculation. The existing production capacity, equipment load, and condition of the medium determine the calculated flow rates Qmax and Qmin. 2) Determination of the pressure difference before and after the valve. Based on the selected valve flow characteristic and system characteristics, S (the resistance coefficient) is determined, after which the calculated pressure difference is established. 3) Calculate Cv. Select the appropriate calculation formulas and charts based on the medium being used, and determine Cmax and Cmin. 4) Choose Cv. Based on Cmax, select the grade C with a value > Cmax and closest to it from the selected series of product standards. 5) Verification of diaphragm pump opening degree. Generally, the opening degree at the maximum calculated flow rate should be ≯90%, and at the minimum calculated flow rate it should be ≮10%. 6) Verification of the actual adjustable ratio of the diaphragm pump. Generally, the actual adjustable ratio is required to be ≮10. 7) Determination of the valve seat diameter and nominal diameter. After verification is satisfactory, it is determined based on C. Selecting a diaphragm pump is a highly detailed task that requires not only solid professional theoretical knowledge but also extensive practical experience. A good selection not only facilitates the tuning of the PID parameters in the control loop, thereby achieving better control over the parameter being regulated, but also **extends the service life of the diaphragm pump**. The selection of diaphragm pumps should be tailored to specific conditions and is not fixed; it requires continuous summarization and innovation through practical use. Especially with the application of mechatronics technology, computers, and digital information technology, the structural functions of diaphragm pumps have become better and more comprehensive, which greatly facilitates their selection. High-temperature reciprocating pumps use compressed air, steam, or industrial waste gas as power sources. They are capable of pumping out various types of corrosive liquids, liquids containing particles, as well as liquids that are highly viscous, volatile, flammable, or highly toxic. They possess many advantages over pumping mechanisms such as self-priming pumps, submersible pumps, shielded pumps, slurry pumps, and dirt pumps, and are widely used in industries such as petroleum, chemicals, electronics, ceramics, and textiles. These pumps are installed in various special applications to pump fluids that conventional pumps are unable to handle, and they have achieved satisfactory results in all such cases