The product directive ATEX 2014/34/EU (French “Atmosphére Explosible”) deals with the explosion danger of electrical, mechanical, hydraulic and pneumatic systems. Classifications are undertaken and technical requirements defined for the system components.

Axial thrust

The axial thrust results from the axial force imposed upon the impellers. It is affected by many factors.


Best efficiency point

The best efficiency point is described as the “design point”. In this point, the pump reaches its minimum of loss and is operated in the highest efficiency.



If there is a local evaporation of the liquid because the vapour pressure of the conveyed fluid is not reached, this is referred to as “cavitation”. Through the constriction of the free flow cross-sections, this influences the flow behavior of the centrifugal pump and results in a decrease of the head and the efficiency. Imploding gas bubbles may destroy existing protective layers on the surfaces inside the pumps. As a result, the metal surface is constantly attacked and ablated by chemicals.

Centrifugal pumps

Centrifugal pumps cause an energy transfer to the conveyed fluid through flow-technical processes. Liquid fluids that may contain portions of gases or solids are transported to a higher pressure level in this manner.

Characteristic curve

A characteristic curve shows characteristic pump data dependent on the flow rate. A distinction is made between the characteristic curve of the pump and the system characteristic curve. Normally the head, NPSH and power are illustrated dependent on the flow rate.

Characteristic diagram

A pump indicator is composed of the throttling curves of individual pumps from a single pump type. An illustration of all design sizes with respectively different impeller tip diameters results in a complete image of the design series.

Close-coupled design

Different construction types describe the connection type of the pump with the drive unit (motor). The close-coupled design is a direct assembly of the pump body on the motor shaft.

Closed impeller

A closed impeller is characterized by a hub on which the impeller blades are attached and a cover disc that covers the blades.

Control procedure

The operating behaviour of a centrifugal pump in a continual process may be adjusted to the process procedure through different control procedures. A difference is normally made between standard procedures such as constant pressure, differential pressure and temperature control.

Conveyed fluid

Conveyed fluid is generally described as a mixture with corresponding physical properties that is conveyed using a pump. The conveyed fluid may consist of one or multiple phases, whereby one phase describes a portion of the fluid that has the same physical and chemical properties.

Conveying task

The specific conveying task must be considered when designing each pump. This includes in particular the desired pressure build-up as well as the properties of the conveyed fluid such as density, viscosity or temperature.


Corrosion is the reaction of a metallic material with its surroundings, whereby measurable changes to the material are caused. Precautionary measures can be taken to prevent this from happening by selecting the right materials. A distinction is made between different corrosion types and mechanisms. (see material)



Diffusers with blades allow for a low-loss energy conversion from velocity energy into pressure energy. In contrast to volute housing, diffusers with blades also create a balance of radial force on the impeller. A harmonic design of the impeller and diffuser element must be ensured.

Displacement pump

Displacement pumps convey the medium in a self-contained volume and work according to the displacement principle. A movement is achieved through the alternating suction and displacement of the medium through the displacement body.

Double seal

A double seal is used if the conveyed fluid is dangerous and a leak must be avoided under all circumstances. A difference is made between “tandem arrangement”, where two mechanical seals are arranged after each other in the same direction and the “back-to-back arrangement”, where the mechanical seals are arranged back to back.


Depending on the area of application and construction form of the centrifugal pump, the fitting drive must be selected to satisfy the respective requirements. Electrical motors are primarily used, but diesel motors or battery-operated electric motors are used as well.

Dry run

If there is a complete dampening of the lubricating film within a slide ring seal, this is considered a dry run. The sliding surfaces of the slide ring and mating ring rub on each other without lubrication. The dry run with mechanical seals must be avoided, because this causes destruction.



The efficiency describes the relationship between the usable conveying capacity and the recorded drive performance. This is dependent on the operating point of the pump. Generally speaking: the higher the efficiency, the more efficient the pump works.
The efficiency can be calculated using the following formula:

Efficiency class

Products can be classified and compared regarding their energy consumption or efficiency with the aid of energy efficiency classes.

Electric motor

Since electric motors are normally standardized and easy to acquire, they are primarily used as pump drives. During the conversion of electrical energy into mechanical energy, high power is generated with minimal losses. The motor also provides the possibility of speed control.

Energy efficiency

The directive 2012/27/EU regarding energy efficiency stipulates requirements for the environment-friendly design of products with significant energy consumption. Using energy-efficient pumps makes it possible to have enormous financial savings.


Frequency converter

Frequency converters generate a variable voltage to directly supply electrical machines. They allow for a soft start where the output frequency as well as the voltage of the frequency converter are run up continuously from a minimum value to the specified target value. This helps to avoid strong blows to the system and extends the lifetime of the pipe system.


Gas conveyance

Centrifugal pumps are able to transport certain gas proportions in liquid conveyed fluid. Gas collections may lead to a blockage of the current and to a termination of conveying if there are high gas proportions. With multiphase flows, so-called multiphase pumps are particularly suitable as they can tolerate a particularly high gas proportion.



The head (H) of a pump describes the energy added to the conveyed fluid between the suction and pressure nozzles of the pump and is specified in the unit meters (m). The head can be calculated using the following formula:



The task of impellers is to transfer energy from the machine to the conveyed fluid. They are also described as the “heart” of the centrifugal pump.


Law of similarity

With the law of similarity, it is possible to calculate the characteristic curves of a centrifugal pump for other speeds. As a rule of thumb with the manufacturer-specific exponent n, the following applies:


A leakage describes the escaping of the fluid from an individual component or a system through a hole or leak. Solids, liquids or gases may enter or escape through this. With mechanical seals, in contrast, a leakage is necessary, so that a lubricating film can be formed between the sliding surfaces and these can run on each other without friction. This leakage is created by the liquid being pressed from an area of high pressure inside the pump to areas of low pressure in the atmosphere.

Life cycle costs (LCC)

Life cycle costs describe the different cost sources of a pump system throughout the entire lifetime. All cost categories, from the acquisition to operation and disposal of the pump, play a role here. This calculation serves as a decision-making aid to consider if the investment in a new system is worth it or if the existing one should be repaired.

Liquefied gas pump

A liquefied gas pump is able to transport liquid-gas mixtures with large pressure differences. Liquefied gas pumps are suitable, for example, when transporting liquefied petroleum gas (LPG) or liquefied natural gas, which is normally made of methane (LNG).


Losses in a pump occur, when the mechanical energy provided through the shaft does not completely transfer to the conveyed fluid. A difference between external and internal losses is made here. External losses include mechanical losses in bearings, seals, etc. that do not change the properties of the conveyed fluid. Internal losses, in contrast, include hydraulic losses (flow friction, flow separation, momentum exchange), gap and leakage losses and disc friction losses. These result in the conveyed fluid being heated up.


Magnetic coupling

Hermetically sealed magnetic couplings are needed if toxic, carcinogenic or hazardous conveyed fluid is being used. The liquid inside the pump is hermetically sealed from the atmosphere through a containment shell, which is located between an external magnet on the drive shaft and an internal magnet on the pump shaft. Magnetic forces cause a synchronous rotation of the pump shaft in which the torque of the drive unit is transferred from the rotating external magnets to the internal magnets. (see magnetically coupled pump)

Magnetically coupled pump

A magnetically coupled pump is a pump without a shaft seal that has a magnetic coupling. It is used if the conveyed fluid may not reach the atmosphere under any circumstances.


Substances are selected according to the conveying task and the resulting requirements. There are normally several possibilities for determining the matching substance. However, the conveyed fluid is given particular attention to in order to find an optimal solution.

Mechanical seal

Mechanical seals are used to seal the rotating shaft from the pump housing and consist of a rotating mechanical seal and a stationary mating ring. The rotating slide ring is pressed against the mating ring through the power of a spring and the surrounding conveyed fluid. Due to the high pressure in the pump, only a small amount of the flow rate leaks through the two sliding surfaces and forms a lubricating film. (see leakage)


The minimum efficiency index (MEI) is a comparative value for the efficiency of pumps with different construction sizes and types on the market. The standard size is currently at  ≥ 0.4.  The better the efficiency of a pump, the higher the MEI value.

Multiphase conveyance

Multiphase conveyance permits the transportation of fluids that consist of several stages (for example, liquid and gaseous phases). (see conveyed fluid)

Multiphase pump

A multiphase pump is able to transport in several phases. (see multiphase conveyance)

Multistage centrifugal pump

A multistage pump permits a correspondingly high head through several consecutively arranged pressure stages. Alternatively, this would only be possible with very large impeller tip diameters.

Multistage inline pump

A multistage inline pump is installed vertically and characterized by several pressure stages.



Generally speaking, a difference is made between the NPSHA-value and the NPSHR-value. The NPSHA-value (available net positive suction head) defines the existing pressure reserves at the suction nozzles. The pressure head difference in which the NPSHA-value is measured is made up of the difference between the total pressure in the suction nozzles and the evaporation pressure of the conveyed fluid. The following applies when calculating the NPSH-value:

The NPSHR-value describes the required pressure reserves at the pump inlet, so that the vapour pressure of the medium is not undershot because of pressure losses during entry into the pump. Thus, the following condition must be fulfilled for operation free of cavitation:


Operating point

An operating point describes the intersection of the pump and system characteristic curve. If this corresponds to the best efficiency point, the pump runs at an optimal level. (see characteristic curve)


If a pump is operated in overload, it conveys a volume flow rate that is higher than the volume flow rate in the best efficiency point. (see partial load)


Parallel connection

If there are deviations regarding the flow rate or the desired flow rate cannot be achieved with a single pump, several of the same or different pumps can be connected parallel to each other. The same pressure difference or head is overcome by all pipes.

Partial load

If the pump transports a volume flow rate that is less than the volume flow rate in the best efficiency point, this is considered partial load. (see overload)

Power input

Centrifugal pumps are driven through a rotating shaft. A certain amount of drive power is needed for this, which depends on the properties of the conveyed fluid as well as the conveying task. The power input of a pump dependent on the flow rate is represented in a characteristic curve.

Pressure losses

Pressure losses are pressure differences caused by wall friction (in pipes) and current resistance (in fittings and valves). The flow velocity of the conveyed fluid as well as the size, shape and surface properties of the pipe influence the possible pressure losses. In order to keep pressure losses as low as possible, large pipe diameters and flow velocities are needed. The consideration of these losses allows for an optimal design of the centrifugal pump as well as the efficiency of the entire system.

Pump control

In order to be able to adjust the operating point of the pump in the system operation to the conveying task, pumps can be controlled in different ways. Constructional actions can be taken on the running and diffuser elements or the pump may be temporarily controlled - for example, through throttling.


Self-priming centrifugal pumps

A self-priming centrifugal pump is able to ventilate the suction line without external equipment. The general principle of a self-priming pump is based on the fact that liquid is conveyed repeatedly through the impeller and gas from the suction line is continuously transported as well, which then escapes through the pressure nozzles. (see suction capability)


Sensors serve to collect physical and chemical variables and are converted into an electric signal for further processing. They are a key component of control loops for speed control.

Series connection

If the application case is affected by a deviating head, series connection is possible. A summed characteristic curve of the pump system is determined for this, in which the heads for corresponding flow rates are added together.

Shaft seal

Shaft seals serve to seal the rotating shaft from the casing. Different sealing concepts prevent the conveyed fluid from escaping through the gap between the casing and the shaft. Normally a slide ring seal is used for this.

Sharpening of impeller blades

To change the head of a pump with the same speed, the entrance angle of the impeller diffuser is changed by sharpening the rear edge on one side. This results in an increase in the head.

Single stage close-coupled pump

A single stage close-coupled pump connects the pump body and a drive on a shared shaft. It is installed horizontally.

Single stage inline pump

A single stage inline pump is the space-saving alternative to a normal close-coupled pump through its vertical design.

Solids transport

In order to be able to transport solids in the conveyed fluid, the ball passage of the impeller is most important. If necessary, there is also the possibility to use a torque flow pump.

Specific speed

The specific speed is the summary of performance data (head, speed of drive shaft and flow rate). A radial alignment of the impeller diffusers is used for a small specific speed with a high head and small volume flows. A large specific speed results from a comparatively smaller head and a large flow rate. Axial impellers are used here.

Speed control

The speed control is suitable for changing the operating point of a centrifugal pump system. The speed of the drive unit is varied here. With the speed control, the pump characteristic curve is changed in accordance with the laws of similarity and adjusted to the desired operating point.

Submersible pump

A submersible pump has no seal and has a cover plate with a riser. The pump body itself is located in the application underneath the liquid level.

Suction capability

The height from which a pump can suck up the conveyed fluid without any cavitation describes the suction capability. Among other things, the atmospheric air pressure must be considered, because this has effects on the suction capability of a pump.

System characteristic curve

The system characteristic curve consists of a static share and a dynamic share. This indicates the required head to convey the desired flow rate through the pipe system from the system.


Throttle control

The throttle control is suitable for changing the operating point of a centrifugal pump system. A throttling device in the pressure line of the system changes the dynamic portion of the system characteristic curve. A new head is created that is significantly above the required head. The difference is destroyed by dissipation in the throttling device. This results in altogether higher operating costs, because the pump is operated further away from the best efficiency point and with a low efficiency. This type of pump control is no longer up to date due to the high pressure losses. (see speed control)

Throttling curve

The throttling curve describes the recording of the head above the flow rate. The decreasing head with an increasing flow rate is characteristic for centrifugal pumps.

Torque flow pump

A torque flow pump offers the possibility to transport conveyed fluid containing solids. A nearly free flow path between the suction and pressure nozzles is achieved through a setback open impeller.

Trimming of impeller blades

The trimming of the impeller tip diameter has the purpose of a permanent reduction of the head and creates a finer grading of the original characteristic diagram. As a rule of thumb with the manufacturer-specific exponent m, the following applies:


Velocity energy

The velocity energy represents a portion of the total energy supply through the impeller. The velocity energy is normally converted into pressure energy using diffusers with blades or volute housing.

Volute housing

Spiral housing with a readjusted conical pressure nozzle is installed as a diffuser element in pumps which is then used to convert velocity energy into pressure energy. A harmonic design of the impeller and diffuser element is important to ensure that low-loss energy conversion is possible for the desired design point.