How to Choose a Diaphragm Pump?
Diaphragm pump is a new type of mechanical transportation equipment, which can be used for all kinds of corrosive liquids, liquids with particles, high viscosity, volatile, flammable and highly toxic liquids. And the diaphragm pump is available in plastics, aluminum alloys, stainless steel, cast iron, etc. for everyone to choose from. So, how to choose a right diaphragm pump?
Action mode of the diaphragm pump selection
The action mode of the diaphragm pump is only available when the pneumatic actuator is selected, and its mode of action is formed by the combination of the positive and negative action of the actuator and the positive and negative action of the valve. There are 4 types of combination forms, namely positive and negative (air-off type), positive and negative (air-on type), reverse (air-on type), and reverse-reverse (air-off type). Both on and off. For the selection of the action mode of the diaphragm pump, three aspects are mainly considered:
- Process production safety.
- Properties of the medium.
- Ensure product quality and minimize economic losses.
Flow characteristics of diaphragm pump selection
The flow characteristics of the diaphragm pump refer to the relationship between the relative flow of the medium flowing through the valve and the displacement (relative opening of the valve). The ideal flow characteristics mainly include linear, equal percentage (logarithmic), parabolic and quick opening. The commonly used ideal flow characteristics are only linear, equal percentage (logarithmic), and quick opening. The parabolic flow characteristic is between linear and equal percentage, which can generally be replaced by equal percentage characteristic, while quick opening characteristic is mainly used in two-position regulation and program control, so the choice of diaphragm pump characteristic is actually linear and equal percentage flow characteristic. s Choice.
The selection of the flow characteristics of the diaphragm pump can be calculated theoretically, but the methods and equations used are very complicated. At present, empirical guidelines are mostly used, which are considered from the following aspects:
- Analyze and select from the adjustment quality of the adjustment system;
- Considering the process piping situation;
- Analysis from the load change situation.
By selecting the flow characteristics of the diaphragm pump, the shape and structure of the valve core can be determined according to its flow characteristics. However, for diaphragm valves, butterfly valves, etc., due to their structural characteristics, it is impossible to change the curved shape of the valve core to achieve the desired effect. The required flow characteristics can be achieved by changing the shape of the feedback cam of the equipped valve positioner.
Diaphragm pump diameter selection
The selection and determination of the diameter of the diaphragm pump is mainly based on the flow capacity of the valve, that is, cv. In the instrument design and selection of various projects, the cv calculation of the diaphragm pump should be carried out, and the design manual of the diaphragm pump should be provided. From the calculation of the cv of the diaphragm pump to the determination of the diameter of the valve, the following steps are generally required:
- Determination of calculation flow. The existing production capacity, equipment load and the condition of the medium determine the qmax and qmin of the calculated flow.
- Determination of the pressure difference before and after the valve. Select s (resistance coefficient) according to the selected valve flow characteristics and system characteristics, and then determine the calculated pressure difference.
- Calculate cv. Select the appropriate calculation formula and chart according to the adjusted medium to obtain cmax and cmin.
- Select cv. According to cmax, in the selected product standard series, select the level c.
- Check the opening of the diaphragm pump. Generally, the opening degree when calculating the maximum flow rate is ＞ 90%, and the opening degree when calculating the minimum flow rate is ＜ 10%.
- Check calculation of the actual adjustable ratio of the diaphragm pump. Generally, the actual adjustable ratio is required to be ＜10.
- Determination of valve seat diameter and nominal diameter. After verification is appropriate, determine according to c.
Selection of diaphragm pump type
Selection of valve body type of diaphragm pump
The selection of valve body is the most important part in the selection of diaphragm pump. There are many types of diaphragm pump valve bodies, commonly used are straight-through single-seat, straight-through double-seat, angle, diaphragm, small flow, tee, eccentric rotation, butterfly, sleeve, spherical, etc. 10 kinds. Before selecting a valve, it is necessary to carefully analyze the medium, process conditions and parameters of the control process, collect sufficient data, understand the requirements of the system for the diaphragm pump, and determine the type of valve to be used according to the collected data. In the specific selection, the following aspects can be considered:
- Wear resistance. When the fluid medium is a suspension containing high concentration of abrasive particles, the valve core and valve seat joint surface will be severely rubbed every time they are closed. Therefore, the flow path of the valve should be smooth, and the inner material of the valve should be hard.
- Corrosion resistance. Since the medium is corrosive, in the case of satisfying the adjustment function, try to choose a valve with a simple structure.
- The shape and structure of the valve core. It is mainly considered according to the selected flow characteristics and unbalanced force.
- The temperature and pressure of the medium. When the temperature and pressure of the medium are high and the change is large, the valve core and the valve seat material should be selected with small changes in temperature and pressure.
- Prevent flash evaporation and cavitation Flash evaporation and cavitation only occur in liquid media. In the actual production process, flash evaporation and cavitation not only affect the calculation of the flow coefficient, but also form vibration and noise, which shorten the service life of the valve. Therefore, the valve should be prevented from flashing and cavitation when selecting the valve.
Selection of diaphragm pump actuator
- Consideration of output force
Regardless of the type of the actuator, its output force is the effective force used to overcome the load (mainly refers to the unbalanced force and unbalanced moment plus friction, sealing force, gravity and other related forces). Therefore, in order to make the diaphragm pump work normally, the equipped actuator must be able to generate enough output force to overcome various resistances and ensure a high degree of sealing and opening of the valve.
For double-acting pneumatic, hydraulic, and electric actuators, there is generally no return spring. The magnitude of the force has nothing to do with its running direction. Therefore, the key to choosing an actuator is to find out 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, and the force on the diaphragm pump will also affect the motion characteristics, so it is required to establish a force balance over the entire opening range of the diaphragm pump.
- Determination of the type of implementing agency
After determining the output force of the actuator, select the corresponding actuator according to the requirements of the process use environment. For explosion-proof requirements on site, pneumatic actuators should be selected, and the junction box is explosion-proof, and electric actuators cannot be selected. If there is no explosion-proof requirement, both pneumatic and electric actuators can be used, but from the perspective of energy saving, electric actuators should be selected as far as possible. Hydraulic actuators are not as widely used as pneumatic and electric actuators, but have the characteristics of high adjustment accuracy, fast and stable action. Therefore, in some cases, in order to achieve better adjustment effects, hydraulic actuators must be selected. Mechanisms, such as speed regulation of transparent machines in power plants, temperature regulation and control of catalytic reactors in oil refineries, etc.