In process control, a diaphragm pump functions by receiving control signals from a controller or computer to adjust the flow rate of a process medium, thereby maintaining specific parameters within required ranges and enabling the automation of production processes. If an automatic control system is compared to manual control, the sensing unit acts as the "eyes," the control unit as the "brain," and the final control element—the diaphragm pump—as the "hands and feet." Achieving the regulation and control of process parameters such as temperature, pressure, flow rate, or liquid level is impossible without a diaphragm pump. Consequently, the correct selection of a diaphragm pump is of great significance in process automation.
1. Composition and Classification of Diaphragm Pumps
Also known as control pumps, diaphragm pumps are a primary type of final control element (actuator). They receive control signals from a control unit and utilize an external power source to modulate fluid flow. A diaphragm pump generally consists of an actuator and a valve. Based on the power source used by the actuator, diaphragm pumps are categorized into three types: pneumatic (powered by compressed air), electric (powered by electricity), and electro-hydraulic (powered by the pressure of a liquid medium, such as oil). Furthermore, based on their functions and characteristics, there are also solenoid-operated, electronic, intelligent, and fieldbus-compatible diaphragm pumps. There is a wide variety of diaphragm pump types and structural designs, which continue to evolve and change. In general, the valve component is versatile and can be paired with pneumatic, electric, or other types of actuators.
2. Selecting the Type of Diaphragm Pump
2.1 Selection of the Valve Body Type: Choosing the valve body is the most critical step in selecting a diaphragm pump. There are many types of valve bodies available; the ten most commonly used include: straight-through single-seat, straight-through double-seat, angle-style, diaphragm, low-flow, three-way, eccentric rotary, butterfly, sleeve-style, and ball-type valves. Before selecting a valve, it is essential to carefully analyze the process medium, operating conditions, and parameters. Sufficient data must be collected to understand the system's requirements for the diaphragm pump, and the appropriate valve type should be determined based on this information. The following aspects should be considered during the selection process:
(1) Valve plug shape and structure: These are primarily determined by the required flow characteristics and factors such as unbalanced forces.
(2) Wear resistance: When the fluid medium is a suspension containing a high concentration of abrasive particles, the mating surfaces of the valve plug and valve seat are subjected to significant friction each time the valve closes. Therefore, the valve's flow path should be smooth, and the internal components should be made of hard, wear-resistant materials.
(3) Corrosion resistance: Given that the medium may be corrosive, it is advisable to select a valve with a simple structure, provided it can still fulfill the required control functions.
(4) Medium temperature and pressure: In cases where the temperature and pressure of the medium are high and subject to significant fluctuations, the valve should be constructed from materials whose properties remain stable under such varying conditions.
(5) Prevention of flashing and cavitation: Flashing and cavitation occur only in liquid media. In practical industrial processes, these phenomena not only complicate the calculation of flow coefficients but also generate vibration and noise, which shorten the service life of the valve. Consequently, valve selection should aim to prevent the occurrence of flashing and cavitation.
2.2 Selection of Actuators for Diaphragm Pumps
2.2.1 Consideration of Output Force
Regardless of the type, an actuator's output force serves as the effective force required to overcome the total load (primarily consisting of unbalanced forces and torques, as well as friction, sealing forces, and gravitational forces).
Therefore, to ensure the proper operation of a diaphragm pump, the selected actuator must be capable of generating sufficient output force to overcome these various resistances, thereby guaranteeing a tight seal and the reliable opening of the valve.
For double-acting pneumatic, hydraulic, or electric actuators—which typically lack a return spring—the magnitude of the operating force is independent of the direction of motion. Consequently, the key to selecting such an actuator lies in accurately determining the maximum output force or the motor's rotational torque. For single-acting pneumatic actuators, the output force is dependent on the valve opening (travel). The forces acting on the diaphragm pump also influence its motion characteristics; therefore, a force balance must be maintained across the entire operating range of the diaphragm pump.
2.2.2 Selection of Actuator Type
Once the required output force for the actuator has been determined, the appropriate actuator type should be selected based on the specific process environment. If the site requires explosion-proof equipment, a pneumatic actuator with an explosion-proof junction box must be used; electric actuators are not suitable in such cases. If there are no explosion-proof requirements, either pneumatic or electric actuators may be chosen; however, from the perspective of energy efficiency, electric actuators are generally preferred. Although hydraulic actuators are less common than pneumatic or electric types, they offer high control precision, rapid response, and smooth operation. Consequently, they are sometimes necessary to achieve optimal control—for instance, in speed regulation for turbines in power plants or temperature control for reactors in oil refinery catalytic units.
3. Selection of Diaphragm Pump Action Mode
The action mode of a diaphragm pump is a consideration specific to pneumatic actuators. It is determined by the combination of the actuator’s action (direct or reverse) and the valve’s action (direct or reverse). There are four possible combinations: direct-direct (air-to-close), direct-reverse (air-to-open), reverse-direct (air-to-open), and reverse-reverse (air-to-close). These combinations result in two primary action modes for the diaphragm pump: air-to-open and air-to-close. The selection of the action mode is based on three main factors: a) process safety; b) the characteristics of the process medium; and c) the need to ensure product quality while minimizing economic loss.
4. Selection of Diaphragm Pump Flow Characteristics
The flow characteristic of a diaphragm pump refers to the relationship between the relative flow rate of the medium passing through the valve and the valve’s displacement (relative opening). There are four primary ideal flow characteristics: linear, equal-percentage (logarithmic), parabolic, and quick-opening. The corresponding characteristic curves and valve plug shapes are illustrated in Figures 1 and 2. In practice, the most commonly used ideal flow characteristics are linear, equal-percentage (logarithmic), and quick-opening. Parabolic flow characteristics fall between linear and equal-percentage characteristics; in practice, equal-percentage characteristics are often used as a substitute. "Quick-opening" characteristics, meanwhile, are primarily employed for on-off control or programmed control. Consequently, selecting the flow characteristic for a diaphragm pump effectively comes down to a choice between linear and equal-percentage characteristics.
While the flow characteristic of a diaphragm pump can be determined through theoretical calculation, the methods and equations involved are highly complex. In practice, empirical criteria are generally used, taking the following factors into account: ① the required control quality of the regulation system; ② the configuration of the process piping; and ③ the nature of load variations.
Once the appropriate flow characteristic for a diaphragm pump has been selected, the shape and structure of the valve plug (or internal element) can be determined accordingly. However, for certain types of valves—such as diaphragm valves or butterfly valves—it is not possible to achieve a specific flow characteristic by altering the geometry of the internal element. In such cases, the desired characteristic can be attained by modifying the profile of the feedback cam within the valve positioner.
5. Selection of Diaphragm Pump Size (Orifice Size)
The selection and determination of a diaphragm pump’s size are primarily based on its flow capacity, expressed as the $C_v$ value. During the instrumentation design and selection phase for any engineering project, it is necessary to calculate the $C_v$ value for the diaphragm pump and provide a corresponding design specification. The process—from calculating the $C_v$ value to determining the final valve size—generally involves the following steps:
1) Determination of the design flow rate: The maximum and minimum flow rates ($Q_{max}$ and $Q_{min}$) are established based on existing production capacity, equipment load, and the properties of the process medium.
2) Determination of the pressure differential across the valve: Based on the selected flow characteristic and the specific requirements of the system, the resistance coefficient ($S$) is determined, which is then used to calculate the pressure differential across the valve.