Centrifugal pump

A centrifugal pump is a flow machine. It utilizes the centrifugal force to transfer liquid. For this reason it is also called centrifugal pump. The medium to be pumped enters via the draft tube into the centrifugal pump, is detected by the rotary pump wheel and carried on a spiral path to the outside. Characterized the impressed outwardly increasing radial velocity of the fluid leads to an increasing outward pressure within the pump, which conveys the fluid in the pressure tube.

• 4.1 impeller
• 4.2 Classification of Centrifugal Pumps by Type
• 4.3 housing
• 4.4 shaft seal
• 4.5 Drive

• 5.1 Range of Services
• 5.2 Areas of application

Types

The following pump types are considered as centrifugal pumps:

• Centrifugal pumps,
• Diagonal pumps ( also called Semiaxialpumpen )
• Side channel pumps,
• Peripheral,
• Axial flow pumps (also called propeller pumps ).

The professional community pumps and compressors in the German Engineering Federation (VDMA ) lists more than 400 different pump designs.

Principle of operation

An example illustrates the function: one Stir with a spoon in a glass filled with water, so the pressure drops in the center, while the liquid rises on the edge of the glass by the prevailing higher pressure (see centrifugal force ) upwards. If the glass has a lateral bore or an overflow above the original level, where water would flow out. The bucket has to be compared in this example with the wheel.

The liquid to be pumped is in the centrifugal pumps mainly transverse to the shaft of the drive moves ( radial flow ), in the direction of the axis usually only happens at the pump inlet. With an appropriate design of the impeller and casing also mixed with solids liquids can be promoted (eg waste water). A measure of the permissible solids size is the so-called ball passage, indicated as a maximum diameter of the ball, the pump could pass.

Properties

The following parameters characterize a pump:

• Flow rate Q
• Head H
• Coupling power P
• Efficiency
• NPSH NPSH at the inlet
• Speed n

The characteristic curve of a centrifugal pump describes the relationship between pressure rise and flow rate ( see chart on the right, blue curve). The highest pressure is generated in a centrifugal pump with flow zero. This is the case when the pump is pumping against a closed valve. Combined with the characteristics of the connected pipe system results in the operating point as the intersection of pump and piping characteristic curve. By connecting several centrifugal pumps be added to the delivery pressure, by paralleling the achievable flow rate. Changes in rotational speed of the pump to change both the flow and the pressure and thus the power consumption. Affinity laws: Q ~ n; H ~ n ²; P ~ n ³.

Suction

Standard centrifugal pumps must be filled prior to start with medium. Device while operating more air than the critical amount ( radial centrifugal pump 8-10 vol - %) in the suction line, breaking the promotion together usually. There are special designs such as Zellenspülpumpen or jet pumps, which, when filled, can vent the suction line independently.

A side channel pump is self-priming. It may be up to 100 vol - % further gas component, as long as a residual fluid remains in the machine, which forms a liquid ring.

Cavitation

" Cavitation ( engl.cavitation ) is defined as the formation of vapor- filled cavities (steam bubbles) in a flowing liquid and the sudden collapse ( implosion ) of the vapor bubbles. The cavitation in a pump leads to a reduction of capacity and in extreme cases, to a complete interruption of the flow. For prolonged cavitation the bearings and seals of the pump are heavily loaded and wear out quickly. In particular, the impeller and the inner pump housing are destroyed by removal of material. "

Types

The classification of centrifugal pumps is based on the shape of the wheels, number of stages, housing construction, drive or even liquid. Since there are different designs for each pump component, there is a variety of possible combinations and therefore a wide range of centrifugal pumps.

Impeller

The central component of a pump is the impeller, which transfers the mechanical energy as a pulse in the fluid. The impeller shape determines how the flow exits the pump. A distinction is radial, semi-axial and axial impellers. Pumps with an axial impeller are called propeller pumps.

Special forms are radial wheels with axially favorite blades and Halbaxialräder with adjustable blades.

Regarding the design of the impeller closed, semi-open and open forms are distinguished or forms with and without a cover and the support disc. In a closed impeller, the impeller blade is connected on both sides with a disc. This increases the hydraulic efficiency and stabilizes the wheel Because of these advantages, it is the most widely used impeller. However, air bubbles can be poorly co-sponsored. These collect by the centrifugal forces in the center and clog the impeller.

Can not build enough pressure a wheel, you can mount several wheels one behind the other in radial and mixed flow pumps. These pumps are called stages. It is also possible to build double suction impellers. In this case, one or more impellers are each mirrored what the axial forces acting largely cancels.

Classification of Centrifugal Pumps by Type

The following table shows the range of applications of the individual designs.

The ( dimensionless ) specific speed refers to the head of one meter on the delivery volume of one cubic meter per second and the speed of one revolution per minute. Impellers with high values ​​are referred to as schnellläufig. From similarity considerations follows:

The terms radial, semi-axial or axial direction of flow at the outlet of the impeller match. In axial impellers is also called a propeller.

The radial or semi-axial impellers with shroud are called closed impellers ( in the table, the top five examples). Without cover plate is spoken by semi-open impellers.

The type and size of possible solid particles are crucial for the selection of the impeller ( Examples: sewage pumps, slurry pump, sediment transport ).

The primitivstmögliche " wheel " is a simple beam. This corresponds approximately to the mentioned in the section Operating principle spoon. The efficiency here is low.

Housing

Main features of the housing, the shapes of Strömungszu and removal to the impeller. Important for the function of the pump, the discharge of flow from the impeller. There are two major forms: a stator with a diffuser and volute casing. These flow channels are used to convert the voltage induced in the fluid by the impeller kinetic energy into a static pressure.

The constructive unavoidable circumferential gap between the impeller and housing mainly determined the efficiency.

Shaft seal

The shaft must be sealed against the housing. The allowable or tolerable leakage is dependent on the medium. Thus, the technical effort in toxic or corrosive media is very high.

Historically, the gland is the oldest type of shaft seal. An asbestos cord ( day PTFE) is wound around the shaft and pressed into a chamber. Indicates that the leak rate increases due to wear, then the ram of the gland is tightened. Is not possible that, then a new sealing cord is inserted.

Since about 1930, the radial shaft sealing ring was added. An elastic lip contacts the rotating shaft. For this to occur with sufficient pressure, inside the sealing ring has a circumferential tension spring is arranged. When wear of the sealing ring is exchanged. Today's sealing rings are often made ​​of PE or PTFE.

Then it came to the development of the mechanical seal. Two plates (often ceramic ) are pressed together by springs. One plate is fixed to the housing, the other rotates with the shaft. Ceramic plates are extremely hard and therefore have hardly any wear. With a comparatively high design effort results in a low-maintenance solution.

A non-contact seal is a labyrinth seal, but is only used in extreme operating conditions such as high engine speeds and pressures and / or temperatures are used.

In the special case (such as for toxic or corrosive media) is given to a sealing liquids or gases. Instead of a leakage flow from the inside to the outside here, there will be a flow of sealing medium from the outside to the inside. The blocking medium has to be constantly updated.

As a further sealing system for a centrifugal pump of a magnetic coupling can be considered. These pumps are called magnetic drive pumps. Compared to the dynamic seals, the drive torque is transmitted without contact of the permanent magnets through the pump housing to the impeller.

Drive

Small centrifugal pumps are available as a compact unit of the pump and electric motor with a common shaft ( rotational or alternating current). Medium and larger centrifugal pumps are available from the supplier of pumps on a standardized mounting frame. On this frame is then locally inflicted mostly a ( standardized ) three phase asynchronous motor. Motor and pump shaft must be well aligned and connected with a solid clutch.

In power plants, steam turbines are used as pump drive. This applies in particular to a part of the cooling pump in a nuclear power plant. Here it is assumed that electric power is not available in an emergency.

In special cases there are other drives.

Areas of application

Because of their simple and robust design Centrifugal pumps are widely used. Most often find designs as single or multi-stage pumps for dry installation or as a submersible pump, for either mobile or stationary use.

Power spectrum

Centrifugal pumps have the following range:

• Flow rates from 0.001 to 60 m³ / s
• Lifting heights between 1 and 5000 m
• Speeds of < 1,000 to 30,000 1/min

Applications

Find use centrifugal pumps in plant and mechanical engineering, to supply water in waterworks and irrigation systems for water treatment in swimming pools, for dewatering of mines and pits or as a heat circulating pump and cooling systems. Typical applications are draining of basements and garages in case of flooding, crop irrigation in agriculture, filling and emptying of tanks or reservoirs and pumping of dirty water. In addition, they find very numerous, and in standardized versions in the fire service application ( " fire pump " ), in particular as a portable version with its own drive motor in the portable pumps, and as permanently installed and powered by the vehicle engine running in the firefighting vehicles (see Löschgruppenfahrzeug, Tanklöschfahrzeug ).

Large centrifugal pumps in oil production (water injection), in petroleum refineries, petroleum transport (pipelines ) and hydraulic used when flushing. In power plants, centrifugal pumps get the water cycle.

History

A water carrier, which "must be designated as the prototype of the centrifugal pump " According to the Italian historian of science Ladislao Reti, is included in Trattato di Architectura of the Italian artist-engineer Francesco di Giorgio Martini ( 1475 ). In 1689 the physicist Denis Papin invented the centrifugal pump. Today this is the most widely used type of pump.