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TEN FACTORS YOU MUST CONSIDER TO SELECT THE RIGHT MICRO MAGNETIC DRIVE GEAR PUMP
2019-08-29


“ Micro pump” often means the small-size pump with low flow rates。 If according to the technical principle, micro pumps can be divided into micro centrifugal pumps, micro gear pumps, micro diaphragm pumps, micro piston pumps, micro peristaltic pumps, micro rotary vane pumps, vortex pumps, etc。; if according to the driving method, they can be divided into direct drive pumps and magnetic drive pumps; if according to the movement mode, they can be divided into reciprocating type and rotary type; if according to the pump body material, it can be divided into metal pumps, plastic pumps, composite material pumps, etc。; if according to the temperature of the liquid transported, they can be divided into cryogenic liquid pumps, high-temperature pumps and normal-temperature pumps; if according to power sources, they can be divided into pneumatic pumps(air-driven pumps), AC pumps(at 12V, 24V, 48V, etc。),DC pumps, servo pumps, variable-frequency pumps, etc。 On the other hand, the pump users tend to classify the pumps by their applications, such as circulator pumps, booster pumps, sampling pumps, cooling pumps, urea pumps, chromatography pumps, flushing pumps, atomizing pumps, etc。Faced with such a variety of names, users inevitably feel confused when selecting the pump。

Here, based on typical applications, I would like to analyze the structures and performance features of different micro pumps and make some suggestions as references for pump selecting。

Whatever the pump type, one thing is invariable that all the pumps are used to drive a liquid flow that meets specific requirements by working on the transported liquid. Here come two basic parameters of the pump selection: flow rate and pressure. Generally speaking, centrifugal pumps can hardly achieve high pressures/heads at low flow rates.That’s why positive displacement pumps are widely used to meet such demands, and one of these pumps is the micro gear pump.

Main features of a micro (magnetic drive) gear pump:

1.The driving gear and the driven gear (two gears pump)mesh with each other during operation

2.The gap between gears and the pump body and the gap between the shaft and the bearing are strictly limited to dozens of micrometers

3.Pump shafts and bearings are often made of polymer materials

4。The torque of the motor is transmitted to the shaft through the magnetic coupling 

Therefore, to select the right micro magnetic gear pump you must consider the flow rate, inlet and outlet pressure, liquid viscosity, the corrosiveness of the liquid, the type of particles in the liquid, the liquid temperature, the sensitivity of the liquid to the shear force, the condition of the inlet and outlet lines, the type of motor, the control method of the flow rate and the pressure (accuracy, batch, backflow) and so on.

1.Flow rate

In general, a well-made micro magnetic gear pump can work at flow rates from 50 to 4000 rpm(revolutions per minute).By looking up the performance curve of each model,you can easily find the pump you need.


(in the curve below: lpm represents litre/min; different lines represent different pressure levels)

Because of the clearance inside the pump, there is always a part of pressurised liquid flowing back from the outlet to the inlet through such gaps, causing the actual flow rate lower than the theoretical one(actual flow rate = theoretical flow rate — internal leakage).

Moreover, the amount of this internal backflow is augmented as the inlet/outlet differential pressure increases. As can be found from the curve above, at the same rotation speed, the higher the pressure, the lower the actual flow rate. In addition, it is clear that the flow rate and the rotational speed of a pump have a positive linear correlation when the outlet pressure is fixed. This key property constitutes the theoretical basis for the gear pump to serve as the metering pump. However, this linear correlation can be destroyed by pressure fluctuations, which become more pronounced in systems that transport low-viscosity liquids.

2. Pressure

Often, pump users only focus on the differential pressure between a pump’ s inlet and outlet. Nevertheless, correctly comprehending the features of both inlet and outlet pressure is also indispensable to the pump selection.

A pump’s inlet pressure can be both positive and negative (relative pressure), and in the latter case, users must know exactly the value of negative pressure. This is because a pump’s flow rate, at the same differential pressure and rotational speed, will be lower than the one lied in its performance curve in conditions where the inlet pressure is lower than the ambient atmospheric pressure. Also, when a pump’s inlet vacuum reaches a threshold(NPSHa<NPSHr), it can no longer properly function.

On the other hand, each pump has its own “maximum allowable working pressure” (MAWP), which is the maximum pressure that a pump’s weakest part can handle when in normal operation. In conditions where a pump’s inlet pressure is higher than the ambient atmospheric pressure, the user must make sure that outlet pressure(inlet pressure + differential pressure) is lower than MAWP, which is necessary for the pump to work safely and continuously.

The term “outlet pressure/discharge pressure” also deserves consideration and accurate understanding. The gear pump is a type of positive displacements pumps. For such pumps, the term “differential pressure(differential head)” describes a pump’s inherent capability, like “20 bar for SuperFluid NP micro gear pumps”, and is irrelevant to the pump’s actual outlet pressure in operation. The latter is determined by the actual resistance force of the system in which a pump is installed.

The system resistance = the resistance to fluid flow in tube + possible vessel pressure

The system resistance directly affects the system pressure drop(head loss), which means the difference in pressure between two points of a fluid carrying network, and in the graph above it refers to the differential pressure between the pump’s outlet and the vessel in the end. Pressure drop occurs mainly because of the resistance from the frictional forces acting on the liquid as it flows through the pipeline(frictional pressure drop), and the resistance from the parts like valves, fittings, bends(minor pressure drop).In general, we have:

Outlet pressure (system pressure drop + vessel pressure) = pressure drop through the pipeline + pressure drop through the valves + vessel pressure

That’s why the actual outlet pressure is determined by pipeline resistance(pressure drop)and load(vessel pressure).


Besides, the changes in liquid elevation and velocity(by pipe inside diameter changes)can also cause the pressure drop. For further information, you can consult articles about Bernoulli's principle and Continuity Equation.

3. Viscosity

The viscosity of the liquid transported has effects on the performance and the power of a pump, while the viscosity is influenced by both temperature and shear force. When a pump delivers a liquid with a higher viscosity, the internal leakage turns smaller, and the actual flow rate turns larger. In the meantime, higher power is needed for the gears to rotate in high-viscosity liquids. In this case, we often advise the users to properly lower the motor’s rotational speed so as to reduce unnecessary energy consumption and to protect the pump from cavitation.

During operation, the liquid temperature will rise due to the meshing between the gears, the friction between bearings and shafts, and the eddy current loss of the magnetic coupling. This increase in temperature affects the viscosity of the liquid. If the properties of the liquid or technical requirements are very sensitive to temperature changes, then, factors such as rotational speed, internal clearance, and tooth profile design of gears should be considered to limit the influences of the pump’s operation on liquid temperature.

On the other hand, If the liquid is very sensitive to shear forces, then the rotational speed should be lowered to alleviate potential impacts.

4. Corrosion

Corrosive liquids can significantly shorten a pump’s life expectancy, so users must ensure that all the liquid-contacting parts of the pump selected are corrosion resistant to the liquids transported。 Usually, it is easy to find out the corrosion resistance properties of 316L stainless steel, the main material of our pump bodies, to different liquids, and the corrosion resistance of our ZrO2 ceramic shafts is not a thing to worry about。 The only thing that deserves further consideration is the adaptability of our PEEK/PPS gears when transporting certain corrosive liquids。 PEEK and PPS can well withstand the corrosion of a wide range of liquids, but there still exist the liquids that may cause the swellings (a transient abnormal enlargement throughout the body or on a specific area) on them。 Some slight swellings may be judged as “acceptable” in other applications, but due to the extremely small size of internal clearances inside NP micro gear pumps (dozens of micrometers), these minor enlargements can still cause a pump to become stuck。

Our company has accumulated a large amount of data for users to judge PEEK/PPS’s adaptability in different liquids, and we offer the sample soaking test in case the liquid to be transported has not yet included in our database。 In addition, it is important to note that the corrosion resistance of many materials is temperature dependent。


5. Solid particle

The internal clearances inside SuperFluid pumps (dozens of micrometers) are extremely tiny. Any solid particle with a size close to these clearances can make a pump stuck, especially when the particles are of high hardness, which may permanently damage the gears or bearings when the gears are running at high speed.

Some solid particles are inherently present in the liquids, while in many cases the pure liquids flow with solid particles coming from pipes, containers, joints, solder dots, etc.

The latter condition often occurs in newly-installed systems. To tackle this problem and protect the pumps, we recommend users to add a 300—400 mesh filter at a pump’s inlet.

When selecting the filter for your system, you should ensure not only its corrosion resistance to the liquid transported but also its ability to withstand the temperature and pressure. What’s more, a filter with an inadequate area for the liquid to pass can cause excessively higher inlet resistance and frequent jam.

6. Temperature

When it comes to temperature, we often talk about liquid temperature and ambient temperature.


- Liquid temperature

All the components of a pump, including pump body, bearings, gears, shafts, magnetic drive and seals, should be able to work continuously and stably at the temperatures of the liquids transported. For example, to determine the materials for a magnetic coupling, we have to consider the temperature resistance not only of raw magnetic powders, but also of the moulded magnet after aged. That’s why we conduct an ageing process on each SuperFluid NP magnetic pump magnetic drive at 150 ℃ before assembling to make sure that the whole pump can work stably under a temperature range of -50 ℃ ~150 ℃ over a long period.

At the same time, users must be well aware of changes in maximum allowable working pressure (MAWP) at extreme temperatures, particularly the pressure resistance of the magnetic cover, which is the thinnest part of a pump.

Besides, the liquid temperature will affect sizes of internal clearances, given that the dimensional changes of different materials caused by temperature variation are not consistent. In general, at low temperatures the pump’s flow rate decreases while at high temperatures the pump is at risk of being stuck. Therefore, for the initial consultation, we suggest the clients to make clear the liquid temperature range under actual working conditions, so that proper internal clearances can be set when design and assembling.

In the end, since the pump and the motor are connected by a connecting frame, the temperature of the pump head is also transmitted to the motor, whose resistance to liquid temperature should also be taken into consideration.

- Ambient temperature

The ambient temperature at which a pump works significantly affects motor selection.

Depending on different ambient temperatures, perhaps you need to equip your pumps with specially designed high-temperature/low-temperature motors. Moreover, for a brushless DC motor, you must consider the impact of ambient temperature on its drive.

Besides, it is possible that under low temperatures some liquids solidify。 In this case, heating the pump head is usually necessary for the pump to normally start operating。 One common way to do this is to wind heating cables on the pump head。


7. Liquid’s sensitivity to shear force

When a gear pump works, a shear force is generated due to the meshing of driving gear and driven gear.

Some liquids are quite sensitive to shear forces, and their properties change continuously under strong shear forces until they are completely unusable. For these liquids, the shear force must be minimized by reducing the pump’s rotational speed and increasing the clearance inside the pump. More often, you have to select some types of pumps with a smaller shear force, such as diaphragm pumps or peristaltic pumps, to complete the liquid transportation. We can provide you with consulting services in this regard.

8. Pipe condition of inlet and outlet

Generally speaking, the diameters of inlet and outlet pipes should not be smaller than the interface diameters of the pump bodies. In order to ensure the pump’s regular suction, the inlet pipe diameter is desirably as short as possible, and the number of bends should be limited. All the parameters about inlet pipe lengths and level differences between a pump’s inlet and liquid level can be attributed to the concept of NPSHa. The only exception is that NPSHa corresponds to the conditions where a pump’s inlet is filled with liquid.

If a pump’s inlet pipe is empty before operation(self-priming), then the longest allowable dry running time and the pump’s suction capacity during dry running become vital to prevent pump burn out. As for SuperFluid NP gear pumps, we provide a longest dry running time of 100 hours and vertical suction capacity of 1 meter in the complete dry running.

The outlet pipe diameter directly affects system pressure at the outlet. When a pump transports high-viscosity liquids, one-grade enlargement of the pipe is accompanied by a discharge pressure reduction of 30% or even higher, which will significantly save the energy for daily operations.

It is also essential to avoid pressure build-up at the outlet, and most of our brushless DC motors are equipped with overcurrent protection. Besides, we recommend the installation of a relief valve in the outlet pipe system if possible to prevent damages on the pump body or the pipe caused by excessive discharge pressure. However, we do not provide any built-in relief valve inside, because the micro pump is so small that the lifting of the relief valve will result in a high-speed liquid circulation in the narrow pump head and thus a rapid temperature increase. There have been many cases where a pump is worn-out or stuck because of the pump head overheating after the opening of the relief valve(for detailed data you can contact us).

P.S. there are three typical types of dry running:

1。Inlet pipe is dry before operation, and self-priming is needed to pump in water。

2.When the transportation is over, the system does not have a self-stop function or this function has a delay, and the pump still needs to run for a while.

3.The pump transports an easy-to-vaporize liquid or a gas-liquid two-phrase medium, resulting in inherent presence of air inside the pump.


Dry running is a troublesome situation for liquid pumps, mainly because the structure and materials of these pumps are dedicated to transporting liquids, and designers seldom take into account the possibility of gas transportation during dry running. As for commonly used centrifugal pumps, users are well aware that the inlet pipe is supposed to be filled with liquids before starting up, while users of positive displacement pumps tend not to consider inlet condition and damages of dry running once they are informed that the pump is capable of self-priming. Such negligence may incur some severe problems:

1.excessive wear due to lack of liquid lubrication when the gears mesh with each other

2。excessive wear due to lack of liquid lubrication during relative movements between shafts and bearings

3.excessive wear due to lack of liquid lubrication during relative movements between gear teeth and pump body (during this process, the heat generated by excessive friction causes the materials to expand and deform, resulting in pump’s getting stuck and damages to components)

Such problems have been greatly alleviated in our gear pumps through specially selected materials of gears, bearing and shaft, and patented gear teeth design。 Therefore, all product series have good dry running resistance, and can work normally without liquids for minutes。In particular, previous experiments have proven NP039 series pumps’ dry running capacity for up to 100 hours。

9. Type and power of electric motor

Electric motors can be roughly divided into AC motors(common AC motors, variable- frequency motors, explosion-proof motors, explosion-proof variable-frequency motors, single-phase motors, etc.), brushed DC motors, brushless DC motors (BLDC), stepper motors and servo motors.

In industrial applications, AC motor is the most popular type. To select an electric motor for your pump, you should firstly decide on a suitable voltage, including 1x220V, 3X220V, and 3X380V in IEC standards. Secondly, you need to judge if explosion proof and variable frequency are needed. Often, users require variable frequency to meet the demand of flow rate regulation within a specific range. However, a variable-frequency motor costs more than a constant-frequency motor and requires the addition of a frequency inverter, so you can otherwise regulate the flow rate through outlet bypasses.

Since AC motors are usually of large sizes, and their rotational speed regulations are not accurate enough, it is inconvenient to install AC motors in commercial devices, where DC motors, stepper motors, and servo motors are widely applied。 The rotational speed of an ordinary brushed DC motor can be controlled directly by the supply voltage, but the short life expectancy of carbon brushes and the potential damage to the system owing to tiny carbon powders from worn-out brushes have resulted in its being less and less used in delicate equipments。 On the other hand, brushless DC motors have a longer service life by removing traditional carbon brushes, and support on-site manual speed control, remote analog signal speed control, and remote digital signal speed control。 These advantages make BLDC more and more accepted in the market。


If your system demands a low flow rate and a high-accuracy control, then the stepper or servo motor is the one you need. Stepper motors are relatively cheaper and generally have a speed of less than 800 rpm, while servo motors can provide a more extensive flow rate range(by a wider speed range)and higher control accuracy.

As it is known to us, motor power = α*(flow rate * pressure/efficiency) * safety factor.

Many times the difficulty in motor power selection lies in the choice of safety margin。 For AC motors, there are specific safety factor values in the national standard for motors of different powers。 Nevertheless, users need to reasonably adjust safety factors according to the actual working conditions, including altitude, ambient temperature, environmental ventilation, continuous working time, start-stop frequency, speed regulation frequency, the possibility of motor overload, etc。

The brushless DC motors that are exclusively developed for us cover 40 watt to 1200 watt, and we can provide build-in drive for those of 400 watt or lower to reduce space occupation and facilitate on-site installation. Besides, our BLDC support on-site manual speed regulation, remote 0-5V voltage signal or PWM signal speed regulation, speed signal output, and reversible rotation control. They can work at ambient pressures from -40℃ to 80℃, and achieve an explosion proof of IP66.

For applications in restricted space, we equip our NP pumps with specially developed non-shaft canned magnet integrated-drive BLDC。

10. Flow rate and pressure control method

A gear pump’s flow rate control is achieved by rotational speed control。 Take NP 039 as an example: its nominal displacement is 0。39 mL/rev, that is to say, this pump theoretically outputs 0。39 mL of liquids by each revolution。 If the rotational speed is 1000 rpm, then we get the theoretical flow rate of 390 mL/min。 In this way, users can easily have the expected flow rate through speed regulation。

If your system only requires low flow rate accuracy(+-2%), the speed regulation of AC variable frequency motors or DC brushless motors is good to control the pump’s flow rate, while servo motors are recommended for applications where flow rate accuracy is higher (~0。5%)。

In practice, the pressure fluctuation of the pump’s outlet pipe can affect its flow rate accuracy, because the discharge pressure varies as the outlet pipe pressure fluctuates, and the discharge pressure determines the internal leakage(Part 1-Flow Rate). Some people may ask: “ Does not a pump’s discharge pressure only depend on the pump itself? Why is it said that the pressure fluctuation of the outlet pipe also has an impact?” This issue has been discussed in detail in Part 2-Pressure. In fact, the discharge pressure depends on the outlet pipe network resistance and load, so if the outlet is directly connected to the atmosphere, the discharge pressure will be zero(gauge pressure). Here, pipe network resistance, due to frictional forces and pipe structure changes, refers to the resistance acting on the liquid when it is flowing through the pipe network; load includes differential elevation of liquids, vessel pressure, force to move actuators, etc.

The discharge pressure fluctuation leads to not only the flow rate fluctuation, but also the rotational speed fluctuation because of the motor torque fluctuation caused by pressure changes. Therefore, for a high-accuracy metering system, it is necessary to install a pressure regulator at the pump’s outlet and adopt a motor whose speed is little affected by torque changes.

The structure of the gear pump is different from that of the piston pump and the diaphragm pump. The latter two are both equipped with an inlet and an outlet check valve, so liquids cannot flow when the pump stops. The gear pump does not have a built-in check valve, and the flow inside the pipe continues even after the pump stops, producing a “liquid dropping” or the air’s entering into the pump after a while. To avoid this phenomenon, a check valve with a minimum opening pressure can be installed at the outlet.

Nevertheless, removing the check valve also brings many advantages. For example, the liquid left in the pipe can be easily discharged into the liquid tank by the motor’s reverse rotation before shut-off, preventing the "airlock" problem, which often occurs in diaphragm pumps.

  • Company:ShangHai Super Fluid Industrial Co.,Ltd
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