Guide to Selecting Pneumatic Diaphragm Pumps

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The use of pneumatic diaphragm pumps can **improve work efficiency and quality, provided that a high-quality and suitable diaphragm pump is selected. 　　In general, the selection of a pump requires consideration of the following basic parameters: fluid viscosity, flow rate requirements, fluid properties, inlet and outlet specifications, pipeline length, pump suction pressure, and whether it will operate continuously or intermittently. Based on these parameters, and by referring to the chemical tolerance tables and performance curves, it is possible to select the appropriate pump type. 1. Fluid viscosity Viscosity refers to the resistance that arises during the flow of a fluid. For common products in daily life, the viscosity of water is 1 centipoise, that of beer is 3 centipoise, SAE30 engine oil is 200 centipoise, flavored ketchup is 50,000 centipoise, mineral oil is 64,000 centipoise, toothpaste is 190,000 centipoise, and peanut butter is 250,000 centipoise. 　　Pneumatic diaphragm pumps are primarily used for transporting low-viscosity fluids, and they can handle fluids with a viscosity of up to 1000 centipoise without the need for any additional equipment. 2. Flow rate requirements The flow rate is a key requirement for applications, and the units used to measure it are gallons per minute or liters per minute. 3. Fluid properties This is the most important factor in selecting a pump. The so-called principles of adapting to local conditions and teaching according to students’ aptitudes also apply, of course, to selecting pumps based on the fluid. Refer to the chemical tolerance table to select the appropriate pump for each fluid; otherwise, incompatibility between the fluid and the pump can lead to serious consequences. 4. Import information – that is, the inner diameter of the inlet pipe. It is important to make the connection dimensions of the pump as similar as possible to those of the container, so as to ensure a proper connection and optimal operation. Generally, the maximum dry suction height of a pneumatic diaphragm pump is 300-500 centimeters. 5. Outlet information This depends on the inner diameter of the pipeline, as well as the length of the pipe at the pump outlet. 6. Inlet Pressure The inlet pressure of the pump determines whether it can meet the application requirements. An adequate gas supply is essential for the correct selection. 　　1 GPM US gallons per minute = 1 CFM cubic feet per minute. 7. Application Information Number of cycles: Intermittent operation refers to the pump being turned on and off over a certain period of time; continuous operation, on the other hand, refers to operation without interruption for more than 8 hours. 　　And most pneumatic diaphragm pumps are used for intermittent operation. 8. Performance curve graph: We generally use curve graphs to determine whether a pump meets the requirements of its application. As shown in the example above, the X-axis represents flow rate, while the Y-axis represents back pressure. 　 Next, we will provide a detailed explanation of the performance charts:
a. How to read the flow rate
Example: Back pressure = 4 bar, inlet pressure = 6.9 bar, flow rate = 22 lpm
Reading the chart:
Step 1: Locate the intersection point where the back pressure of 4 bar meets the inlet pressure curve at 6.9 bar.
Step 2: The value indicated on the X-axis at this vertical intersection is 22 lpm.
As shown in the chart, this pump can deliver a flow rate of around 22 lpm.

b. How to read the air consumption
Example: Back pressure = 4 bar, inlet pressure = 6.9 bar, flow rate = 22 lpm
Reading the chart:
Step 1: From the flow rate of 22 lpm, draw a vertical line upward to find the intersection point with the inlet pressure curve at 6.9 bar.
Step 2: Read the value on the X-axis at the intersection point closest to this line; this value represents the air consumption.
Result: In this example, the air consumption is around 7.5 lps.

c. How to read the net positive suction head
Example: Back pressure = 4 bar, inlet pressure = 6.9 bar, flow rate = 22 lpm
Reading the chart:
Step 1: From the point at 22 lpm on the X-axis, draw a vertical line upward to find the intersection point with the net positive suction head curve.
Step 2: The value on the Y-axis at the intersection point to the right of this line represents the net positive suction head.
In this example, the net positive suction head is greater than 4 m.

d. Calculating pressure drop
Pressure drop formula:
DP = m x Q x L / (0.000245 × (i.d.)⁴)
Where:
m = viscosity of the fluid
Q = flow rate
L = length of the outlet pipe
0.000245 = coefficient
(i.d.)⁴ = fourth power of the pipe’s inner diameter

Example:
m = 500 cPs
Q = 20 gpm
L = 50 ft
i.d. = 1.5”
Calculation:
DP = 500 × 20 × 50 / (0.000245 × 5.0625)
DP = 24.19 psi
For this pump to function properly, the inlet pressure needs to be higher than 25 psi.

The above summarizes the key points for selecting a diaphragm pump based on its basic performance parameters. Next, we will further determine the most suitable diaphragm pump model by considering its specific structural features. Specific methods for selecting diaphragm pumps: 1. Selection of the operating mode of the diaphragm pump: The operating mode of a diaphragm pump is relevant only when choosing 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 choosing the operating mode for diaphragm pumps, three main factors are considered: a) process safety; b) the properties of the medium; c) improving product quality while minimizing economic losses. 　　2. Selection of the flow characteristic of 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-opening. 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 diaphragm 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, it is possible to determine the shape and structure of the valve core 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 in the valve actuator. 　　3. 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 CV calculations 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: a. Determination of the flow rate. The existing production capacity, equipment load, and condition of the medium determine the calculated flow rates qmax and qmin. b. Determination of the pressure difference before and after the valve. Based on the selected valve flow characteristic and system properties, s (the resistance coefficient) is determined, after which the calculated pressure difference is obtained. 　　c. Calculate cv. Select the appropriate calculation formulas and charts based on the medium being used, in order to determine cmax and cmin. d. Choose cv. Based on cmax, select from the chosen series of product standards the value of y*c that is greater than cmax and close to its z-value. e. Verification of diaphragm pump opening degree. Generally, it is required that the opening degree at the maximum calculated flow rate be ≤90%, and the opening degree at the minimum calculated flow rate be ≥10%. 　　f. Verification of the actual adjustable ratio of the diaphragm pump. Generally, the actual adjustable ratio is required to be ≮10. 　　g. Determination of the valve seat diameter and nominal diameter. After verification is successful, it is determined based on c. 　　4. Selection of diaphragm pump type (1) 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 choice, the following factors can be considered: a. Wear resistance – When the fluid medium is a suspension containing high concentrations of abrasive particles, the mating surfaces of the valve core and seat experience 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. 　　b. Corrosion resistance: Since the medium is corrosive, whenever the regulating function can be satisfied, valves with a simple structure should be preferred. 　　c. The shape and structure of the valve core are primarily determined by factors such as the selected flow characteristics and unbalanced forces. 　　d. Temperature and pressure of the medium: When the temperature and pressure of the medium are high and subject to significant fluctuations, it is necessary to choose valves whose valve core and seat materials are less affected by such temperature and pressure changes. 　　e. Prevent 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. 　　(2) Selection of diaphragm pump actuator a. Consideration of output force Regardless of the type, the output force of an actuator is used to overcome the load forces (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 actuator used must be capable of generating sufficient output force to overcome various resistances, as well as ensure 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 rotational 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. 　　b. Determination of actuator type Once the output force of the actuator has been determined, 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. 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 better 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. When selecting a diaphragm pump, the following factors can be taken into consideration: 1. The shape and structure of the valve core are determined primarily based on the desired flow characteristics and the forces involved. 　　2. Wear: When the fluid medium is a suspension containing high concentrations of abrasive particles, the mating surfaces of the valve core and seat experience 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, where possible, choose valves with a simple structure as long as 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, it is necessary to choose valves whose valve core and seat materials are less affected by such temperature and pressure changes. 　　5. Prevent 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. 　　Through the above explanations and clarifications, I believe you now have a certain understanding of how to select diaphragm pumps.