Author: Lin Qiangxing, Shanghai Suofu Industrial Co., Ltd

Original link: precautions for selection of micro magnetic gear pump

Micropumps usually refer to pumps with small volume and low flow. According to different technical principles, it can be divided into micro centrifugal pump, micro gear pump, micro diaphragm pump, micro plunger pump, micro peristaltic pump, micro vane pump, vortex pump, etc. according to the driving mode, it can be divided into direct connection type, magnetic drive type, reciprocating type and rotary type according to the movement mode, metal pump, plastic pump, composite pump, etc. according to the material, it can be divided into cryogenic pump High temperature pump and normal temperature pump can be divided into pneumatic pump, DC pump (12V, 24V, 48V, etc.), servo pump, AC pump, variable frequency pump, etc. according to different power sources. Users are often used to naming pumps according to their actual use needs, such as circulation pump, pressurization pump, sampling pump, cooling pump, urea pump, chromatography pump, flushing pump, atomization pump, etc. With so many subdivisions, it is inevitable to bring a lot of confusion to users in selecting pumps. Here, our company makes some analysis and suggestions based on the structure and performance characteristics of micro pumps and common applications for reference in selecting pumps.

No matter what type of pump is, it can never be separated from its origin. Its function is to make the conveyed liquid produce a flow that meets the requirements by working on the conveyed liquid. Here comes the most basic parameters of pump selection: flow and pressure. Generally speaking, it is difficult for centrifugal pumps to achieve high lift (high pressure) under small flow. Therefore, when small flow and high pressure are required, the frequency of positive displacement pumps is very high. One of them is a micro gear pump.

Micro (magnetic) gear pump features:

(1) During operation, the driving wheel and the driven wheel need to mesh with each other

(2) The clearance in the pump is small, and the clearance between the gear and the pump body and between the shaft and the bearing is in the range of tens of microns

(3) Bearings and gears are generally made of polymer materials, and the torque of the motor is transmitted to the shaft through the magnetic coupling

Therefore, when selecting this type of pump, it is necessary to consider the flow rate of liquid, inlet and outlet pressure, viscosity, corrosiveness of liquid, particle condition, temperature, sensitivity of liquid to shear force, inlet and outlet pipeline condition, type of motor, control mode of flow and pressure (precision, batch, check), etc.

1. Flow

       The well-made micro magnetic gear pump can generally work within the speed range of 50 ~ 4000rpm. By looking up the flow pressure speed curve of each model, you can easily select the type (LPM is L / min, and the oblique line represents the pressure difference)

       Due to the gap in the pump, a part of the high-pressure liquid at the outlet end of the pump will return from the pump to the inlet end of the pump along the gap in the pump, resulting in the actual output flow of the pump less than the theoretical value. And with the increase of the pressure difference between the inlet and outlet, the internal reflux flow also increases correspondingly. Actual flow of the pump = theoretical flow of the pump - internal leakage of the pump. It can be seen from the above figure that the higher the pressure is, the lower the actual flow is at the same speed. In addition, when the outlet pressure is stable, the flow rate and speed of the pump are linear, which is also the theoretical basis for the gear pump to be used as a metering pump. However, if the pressure fluctuates, the linear relationship will be destroyed, which is more obvious in the system transporting low viscosity liquid.

2. Pressure

      Generally, we only consider the inlet and outlet pressure difference, but it is important to understand the inlet pressure and outlet pressure respectively for correct selection. Because sometimes the inlet of the pump may be negative pressure, and sometimes it may be positive pressure. In case of negative pressure, it is necessary to know the specific negative pressure value. When the inlet pressure of the pump is less than atmospheric pressure, the flow rate of the pump under the same speed and pressure difference will be lower than the normal value in the performance curve. When the inlet vacuum reaches an extreme value, the pump will not work normally (npsha < NPSHr). In addition, each pump has a maximum allowable working pressure, that is, the maximum pressure of the pump. When the inlet pressure of the pump is greater than atmospheric pressure, the outlet pressure must be guaranteed to be less than the maximum pressure of the pump, that is, "inlet pressure + pressure difference < the maximum pressure of the pump", so as to ensure the long-term safe operation of the pump.

In addition, we also need to correctly understand the outlet pressure of the pump. Gear pump is a positive displacement pump. We usually say that the NP series micro magnetic pump can reach a pressure difference of 20bar. Here 20bar refers to the pump capacity. However, after the pump is installed in a system, the actual working pressure is determined by the resistance of the system. Resistance of the system = flow resistance of the liquid in the pipeline + possible vessel pressure.

       The actual working pressure of the pump in the figure = the pressure drop of the liquid flowing through the pipeline + the pressure drop of the liquid flowing through the valve + the pressure in the tank. If there is still pressure drop caused by height difference and pressure drop caused by elbow, it shall be calculated.

3. Viscosity

       The viscosity of the liquid has an influence on the performance and power of the pump, and the viscosity is also affected by the temperature and even the shear force. Because of the internal leakage of the gear pump, the smaller the internal leakage is when the pump is delivering the liquid with higher viscosity, the larger the actual output flow will be. Therefore, compared with low viscosity water, when delivering high viscosity liquid, the actual output flow of the pump of the same model will be larger under the same rotation speed and pressure difference. (add contrast curve). However, as the viscosity of the liquid increases, the fluidity of the liquid itself decreases, and the power consumed by the rotation of the gear in the high viscosity liquid also increases sharply. Therefore, with the increase of the viscosity of the liquid, the working speed of the pump should also be appropriately reduced. This can not only reduce unnecessary energy consumption, but also prevent cavitation. When the gear pump is working, the temperature rise will occur due to the meshing between gears, the friction between bearings and shafts, and the loss of magnetic coupling, which will affect the liquid viscosity. If the properties of the liquid or the process requirements are very sensitive to the temperature change, the pump speed, internal clearance, tooth shape design and other factors need to be considered in the selection to limit the influence of the pump work on the liquid temperature. If the liquid is very sensitive to shear force, it needs to be dealt with by reducing the rotation speed.

4. Corrosivity of liquids

       The corrosiveness of the liquid will seriously affect the service life of the pump. We must ensure that all parts in the pump that are in contact with the liquid can withstand the corrosion of the delivered liquid. Generally speaking, the 316L stainless steel material used in the pump body of NP series gear pump is easy to find out the corrosion resistance of various liquids. Our ceramic shaft is also very good in corrosion resistance, mainly because the applicability of PEEK / PPS gears and bearings to different chemical media needs further consideration. Peek and PPS have good corrosion resistance and a wide range of applications. However, some liquids will swell them. In particular, they only have very slight swelling in some liquids, which may be judged as "usable" in other applications. However, due to the very small internal clearance of the micropump, these very slight swelling may cause the pump to jam. Shanghai Suofu Industrial Co., Ltd. has a large amount of data to judge the compatibility of liquids and materials, but when some materials are not in our existing database, we can also provide you with sample immersion test. In particular, it should be noted that the corrosion resistance of materials is often related to temperature.

5. Particle condition

       Due to the small clearance inside the micro gear pump, any particles close to the clearance range may block the pump. In addition, even very small hard particles may permanently damage the gear or bearing when the gear is running at high speed. Some particles exist in the liquid. In many cases, the liquid may be clean. However, particles may exist in the system pipes, vessels, joints and welding joints, especially in newly installed or reconstructed systems. Therefore, it is recommended that the user install 300-400 mesh filters at the inlet of the pump to protect the safety of the pump. In addition to ensuring that the material will not be corroded by the liquid, the selection of the filter should also consider the temperature resistance, pressure resistance and flow area of the filter. If the flow area is too small, the inlet resistance will be too large, and the filter may be frequently blocked.

6. Temperature:

       When it comes to temperature, we need to consider the temperature of the liquid and the ambient temperature of the pump.

1) Liquid temperature

The parts of the pump shall be able to ensure long-term stable operation within the liquid temperature range, including the materials of the pump body, bearing, gear, shaft, magnetic drive and sealing ring. At the same time, we also consider the change of the maximum allowable working pressure of the pump under extreme temperature. Especially the pressure bearing capacity of the thinnest magnetic cover of the pump body. For the magnetic coupling, we should not only consider the temperature resistance grade of the magnetic powder used, but also consider the aging temperature after the magnet is formed. The NP series magnetic pump magnetic drive of Shanghai suofe pump industry has undergone high temperature aging at 150 ° C, and the whole pump body can work stably in the range of - 40 ~ 150 ° C for a long time. The liquid temperature will also affect the clearance in the pump. Because the materials and sizes of different parts are different, the size change of different parts is different under the same temperature rise, resulting in the gap change. Generally speaking, the flow may be too small at low temperature and the pump may be stuck at high temperature. Therefore, the temperature range of the liquid under the actual working condition should be considered in the early consultation, so as to set the proper clearance in the pump when leaving the factory. In other words, even if our pump can work at - 40 ~ 150 ℃. However, if the liquid temperature provided during the inquiry is too different from the actual range, the pump may have insufficient flow or stuck during operation. Since the pump and the motor are connected by a connecting frame, the temperature of the pump head will also be transmitted to the motor, which will also have a certain impact on the temperature resistance level of the motor.

2) Ambient temperature

The ambient temperature will have a great impact on the selection of motor. It may be necessary to configure low-temperature or high-temperature motors. If brushless DC motors are configured, the environmental temperature will have a considerable impact on the performance of the driver. In addition, some liquids will solidify under normal temperature or low temperature in winter. At this time, it is necessary to consider heating the pump head to ensure the normal startup of the pump. The more common way is to wrap the heat tracing tape on the pump head.

7. Sensitivity of liquid to shear force

       During the operation of gear pump, the meshing between driving wheel and driven wheel will produce shear force. Some liquids are very sensitive to shear force. Under high shear force, the properties of liquids will change until they can not be used. For these liquids, it is necessary to reduce the shear as much as possible by reducing the working speed of the pump and increasing the clearance in the pump. More often, it may be necessary to select diaphragm pump, peristaltic pump and other pump types with less shear force to complete the delivery. We can provide you with consulting services in this regard.

8. Inlet and outlet pipelines

       Generally speaking, the inlet and outlet pipe diameters shall not be less than the interface diameter of the pump body. In order to ensure the normal suction of the pump, the inlet pipe diameter should be as short as possible and elbows should be reduced. All factors such as the length of the inlet pipeline or the height difference between the pump and the liquid level can be attributed to the conceptual scope of npsha. The only difference is that npsha corresponds to the situation where the pump inlet is filled with liquid. If the inlet line is empty before the pump is operated, the maximum allowable operating time of the pump without liquid and the pumping capacity of the pump in this case must be considered. For the occasion requiring self-priming, the dry operation capacity of the pump and the suction capacity during dry operation are very important, otherwise the pump will be burnt directly. Shanghai sofu NP series micro magnetic gear pump can achieve a dry running time of up to 100 hours and a vertical suction capacity of about 1 meter in a fully dry state. The outlet pipe diameter will affect the system pressure at the outlet, especially when conveying high viscosity liquids. Sometimes the outlet pipe diameter is one level larger, and the outlet pipe pressure can drop by 30% or even higher, which will play a very important role in the energy saving of daily operation. The outlet pipeline system of the gear pump shall be protected against pressure build-up. The DC brushless motor provided by our company is generally provided with overcurrent protection function. We also recommend that the user install safety valves on the outlet pipeline if possible to prevent the pipeline or pump body from being damaged by the excessive outlet pressure. Our products do not provide the built-in safety valve of the pump head, because the volume of this type of products is too small. Once the safety valve in the pump is opened, all the energy will promote the high-speed circulation of the liquid in the narrow pump head space, and the temperature of the liquid will rise quickly. Including the imported micro gear pump, the pump was stuck and worn due to overheating caused by the opening of the pump head safety valve. (specific data consulting company).

       Dry operation: there are several typical cases of dry operation: 1) before the pump is started, the inlet pipe is empty, and the pump needs to suck the distant liquid into the pump by itself. 2) When the delivery is finished, the system has no self shutdown function or the self shutdown function is delayed, and the pump needs to run dry for a period of time. 3) The liquid conveyed by the pump is very easy to vaporize or it is the medium for conveying gas-liquid two phases, and there is often gas in the pump.

Dry operation is a troublesome situation for the liquid pump, mainly because the structure and material design of the pump itself are designed to transport liquid, and the transport of gas during dry operation is not considered too much. For commonly used centrifugal pumps, users all know that the inlet pipe of the pump needs to be filled with liquid before starting the pump. However, when users hear that the positive displacement pump has self-priming capacity, they often think that it is no longer necessary to consider the inlet situation, which will cause great problems.

The main problem comes from:

(1) Excessive wear caused by lack of liquid lubrication when gears mesh with each other;

(2) Excessive wear caused by lack of liquid lubrication during relative movement between shaft and bearing;

(3) Excessive wear caused by lack of liquid lubrication during relative movement between gear side and pump body. During this process, the heat generated by excessive friction will lead to the expansion and deformation of the material size, and even lead to the direct jamming of the pump and the damage of components. At the same time, the eddy current loss generated in the magnetic shield when the magnetic coupling is working will also cause the heat of the magnetic shield.

The NP series micro magnetic gear pump of Shanghai Suofu Industrial Co., Ltd. greatly alleviates the above problems by applying special gear, bearing and shaft materials and special gear profile. Therefore, the whole series of products have good dry running resistance. Np039 model has been tested for 100 hours on the test bench. The whole series of products can fully cope with a few minutes of dry running on site.

9. Motor type and power selection

Generally speaking, the motor can be divided into AC motor (ordinary AC motor, frequency conversion motor, explosion-proof motor, frequency conversion explosion-proof motor, single-phase motor, etc.), DC brushless motor, DC brushless motor (BLDC), stepping motor and servo motor. In the industrial field, various AC motors are widely used. First, it is 1x220v or 3x220v or 3x380v. After the voltage is determined, it is necessary to see whether explosion-proof (explosion-proof grade) or frequency conversion is required. Frequency conversion is usually selected because it needs to adapt to a certain flow range. However, on the one hand, the cost of frequency conversion motor is high and the frequency converter needs to be installed. Therefore, the customer can also adjust the flow through the outlet bypass. Due to the large volume and low speed regulation accuracy of AC motor, it is not easy to install in many commercial equipment. At this time, various DC motors, stepper motors or servo motors can show their magic power. Ordinary brushless DC motors can be directly regulated by the power supply voltage. However, due to the short life of carbon brushes, the small carbon powder after the wear of carbon brushes may cause harm to the system, so it is less and less used in various professional equipment. The brushless DC motor is small in size, with the traditional carbon brush removed, and its service life is extended. It can be adjusted by local manual speed regulation, remote analog signal speed regulation or remote digital signal speed regulation, and is more and more accepted by the market. When customers need small flow and high control accuracy, they can consider using stepper motor or servo motor. The cost of stepper motor is relatively low, and the speed is generally less than 800rpm. Therefore, if you need large flow span and high accuracy, it is recommended to consider using servo motor.

Motor power = (flow * pressure / efficiency) * safety factor. In many cases, the difficulty of motor power selection lies in the selection of safety margin. For AC motors, the safety factor of motors with different power is specified in the national standard. However, we should reasonably change the safety factor according to the actual use environment, such as altitude, ambient temperature, environmental ventilation, continuous working time, start and stop frequency, speed regulation frequency, overload possibility and other factors.

*The DC brushless motor customized and developed by Shanghai Suofu industry covers the power of 40 W to 1200 W. the motor below 400 W can provide the option of built-in driver to further reduce the occupation of space and facilitate on-site installation. We can provide local manual speed regulation or remote 0-5V voltage signal or PWM signal speed regulation, support speed signal output function, and support motor forward and reverse control. The ambient temperature is - 40 ~ 80 ℃, and the protection grade can reach IP66. We have also developed brushless DC motor with shielded internal drive without motor shaft for NP series products specifically for miniaturized applications.

10. Control mode of flow and pressure

The flow control of gear pump is generally achieved by controlling the rotation speed of the pump. For example, the nominal displacement of np039 is 0.39 ml / revolution, which means that the pump should theoretically output 0.39 ml per revolution. If the rotation speed of the pump is 1000 rpm, then theoretically the flow rate that the pump should output is 390 ml / min. Therefore, the required flow can be easily achieved by adjusting the rotation speed of the pump. Therefore, if the accuracy of the system is not high (+ - 2%), it is a good way to adjust the flow by controlling the speed of the gear pump with AC variable frequency motor or DC brushless motor. It is still recommended to use servo motor in the occasions with high accuracy requirements (~ 0.5%).

The flow accuracy of the pump is also affected by the pressure fluctuation of the outlet pipeline. Many people may have questions: isn't the pressure generated by the pump? How can it be said that it is affected by the pressure fluctuation of the outlet pipeline? Because the pressure after the positive displacement pump is actually determined by the resistance and load of the pipe network, if the outlet of the positive displacement pump is directly connected to the atmosphere, the actual working pressure is 0 (gauge pressure). It is a typical "strong when strong, weak when weak". Pipe network resistance is the resistance of the product when the liquid flows through the pipeline. Straight pipe, elbow, reducer, valve, etc. will bring pressure drop; The load may include height difference, vessel pressure, actuator power, etc. The fluctuation of the pump outlet pressure will not only directly cause the fluctuation of the pump output flow, but also cause the fluctuation of the torque and the motor speed due to the pressure fluctuation. Therefore, in an accurate metering system, it is necessary to install a pressure stabilizing valve behind the pump and use a motor whose rotational speed is less affected by torque change.

The structure of gear pump is different from that of plunger pump and diaphragm pump. The latter has inlet and outlet check valves in the pump, so the liquid cannot flow when the pump is stopped. The gear pump has no built-in one-way valve, so the liquid in the pipeline can flow during shutdown, which may cause "drip" phenomenon or air may be found in the pipeline after shutdown for a period of time. If you do not want this phenomenon, you can install a one-way valve with the opening pressure as small as possible at the pump outlet. In addition, the feature of gear pump without one-way valve also has many advantages. For example, the liquid in the pipeline can be discharged into the liquid inlet tank simply by reversing the motor before shutdown, and the phenomenon of "air lock" of the diaphragm pump will not occur.