Centrifugal pump
A centrifugal pump is a flow machine ( pump ) that uses a rotary movement and dynamic forces to convey mainly liquids. The radial centrifugal pump, which is usually understood by it, uses the tangential acceleration of the medium as well as the centrifugal force occurring in the radial flow to deliver. For this reason it is also called a centrifugal pump . Axial centrifugal pumps , on the other hand, deliver parallel to the drive shaft.
The medium to be conveyed enters the centrifugal pump via the suction pipe, is picked up by the rotating pump wheel and, in the case of the radial centrifugal pump, is carried outwards on a spiral path. The outwardly decreasing radial velocity of the liquid (the circulating velocity increases), which is imposed by this , leads to an outwardly increasing pressure inside the pump. The pressure and the speed of rotation convey the liquid into the tangentially attached pressure pipe.
See also axial compressors and centrifugal compressors (radial compressors) for gaseous media.
Types
The following pump types are considered centrifugal pumps:
- Radial pumps ,
- Diagonal pumps (also called semi-axial pumps),
- Side channel pumps ,
- Peripheral impeller pumps ,
- Axial pumps (also called propeller pumps ).
The specialist group pumps and compressors in the Association of German Mechanical and Plant Engineering (VDMA) has over 400 different pump designs.
Working principle
An experiment explains the function of the radial centrifugal pump : If you stir a glass filled with water with a spoon, the pressure in the center drops , while the liquid at the edge of the glass rises due to the higher pressure there (see centrifugal force ). If the glass had a lateral hole or an overflow above the original level , water would flow out there. In this example, the bucket can be compared to the impeller.
In centrifugal pumps, the liquid to be pumped is mainly moved across the shaft of the drive (radial flow), in the direction of the axis mostly only at the pump inlet.
With an impeller that is open on one side and a wide housing, liquids mixed with solids (e.g. waste water) can also be pumped to the detriment of efficiency. The so-called ball passage , specified as the maximum diameter of the ball that the pump can pass, is a measure of the permissible solid size .
properties
The following parameters characterize a pump:
- Flow rate Q
- Head H
- Clutch performance P
- Efficiency
- Holding pressure head NPSH at the inlet
- Speed n
The characteristic curve of a centrifugal pump describes the relationship between pressure increase and flow rate (see graphic on the right, blue curve). The highest pressure is generated in a centrifugal pump when the flow rate is zero. This is the case when the pump delivers against a closed slide. Combined with the characteristic of the connected pipe network, the operating point results as the intersection of the pump and pipe network characteristic. When several centrifugal pumps are connected in series, the delivery pressure is added; when connected in parallel, the achievable flow rate is added. Changes in the speed of the pumps change both the delivery rate and the pressure and thus the power consumption. Affinity laws: Q ~ n ; H ~ n ²; P ~ n ³.
Suction behavior
Standard centrifugal pumps must be filled with medium before starting. If more air than the critical amount (radial centrifugal pump 8–10% by volume) enters the suction line during operation, the delivery usually collapses. There are special designs such as cell rinsing pumps or jet pumps that, when filled, can vent the suction line independently.
A side channel pump is self-priming. It can deliver up to 100% by volume of gas, as long as a residual fluid remains in the machine, which forms a liquid ring.
Cavitation
" Under cavitation (engl.cavitation) refers to the formation of vapor-filled cavities (steam bubbles) in a flowing liquid and the sudden collapse (implosion) of the vapor bubbles. Cavitation in a pump leads to a reduction in the delivery rate and, in extreme cases, to a complete interruption of the delivery flow. With longer cavitation operation, the bearings and the mechanical seals of the pump are heavily loaded and wear out quickly. In particular, the impeller and the inner pump housing are destroyed by material removal. "
Types
The centrifugal pumps are classified according to the shape of the impellers, number of stages, housing structure, drive or the pumped medium. Since there are different designs for each pump component, there are a large number of possible combinations and therefore a large variety of centrifugal pumps.
Wheel
The central component of a pump is the impeller, which transfers the mechanical energy to the fluid as an impulse. The shape of the impeller determines how the flow exits the pump. A distinction is made between radial, semi-axial and axial impellers. Pumps with an axial impeller are called propeller pumps. When it comes to the blade shape, a distinction is made between impellers with simple and spatially curved blades.
Special forms are radial gears with axially protruding blades and semi-axial gears with adjustable blades.
With regard to the design of the impeller, a distinction is made between closed, half-open and open forms or forms with and without a cover and support disc. With a closed impeller, the impeller blade is connected to a disk on both sides. This increases the hydraulic efficiency and stabilizes the wheel. Because of these advantages, it is the most frequently used impeller. However, air bubbles can only be conveyed poorly. These collect in the center due to the centrifugal forces and clog the impeller.
If an impeller cannot build up enough pressure, several impellers can be mounted one behind the other with radial and semi-axial pumps. These pumps are called multi-stage. It is also possible to build impellers with double flow. One or more impellers are mirrored, which largely cancels out the axial forces.
Classification of centrifugal pumps according to type
The following table shows the range of applications for the individual designs.
| (Specific speed) | Impeller type | Impeller shape | Max. Delivery head | Max. Efficiency |
|---|---|---|---|---|
| 7-30 | Radial pump |
|
800 m (up to 1200 m) | 40-88% |
| 50 | Radial pump |
|
400 m | 70-92% |
| 100 | Radial pump |
|
60 m | 60-88% |
| 35 | Semi-axial pump |
|
100 m | 70-90% |
| 160 | Semi-axial pump |
|
20 m | 75-90% |
| 160-400 | Axial pump |
|
2 - 15 m | 70-88% |
The (dimensionless) specific speed refers to the delivery head of one meter, the delivery volume of one cubic meter per second and the speed of one revolution per minute. Impellers with high values are called high-speed. From considerations of similarity it follows:
The designations radial, semi-axial or axial correspond to the direction of flow at the outlet of the impeller. Axial impellers are also referred to as propellers.
The radial or semi-axial impellers with a cover plate are referred to as closed impellers (the top five examples in the table). Without a cover plate, we speak of half-open impellers.
The type and size of possible solid particles are decisive for the selection of the impeller (examples: sewage pumps, thick matter pumps, solids transport).
The most primitive possible “impeller” is a simple bar. This corresponds roughly to the spoon mentioned in the section on the functional principle . The efficiency is low here.
casing
The main features of the housing are the shape of the flow inlet and outlet to the impeller. The discharge of the flow from the impeller is important for the function of the pump. There are two main forms for this: a stator with diffuser and a volute casing. These flow channels are used to convert the kinetic energy induced in the flow by the impeller into static pressure.
The circumferential gap between impeller and housing, which cannot be avoided by design, mainly determines the efficiency. Furthermore, the efficiency is significantly influenced by the following factors:
- Accuracy of the practical implementation of the calculated geometry for the volute and the impeller blades. This is partly determined by the structural design (closed or radially divided spiral), the choice of the primary forming process and the associated model and mold construction.
- Roughness of the surfaces in the pump exposed to the flow medium. These are influenced by the selected primary forming process because they can hardly be processed mechanically. In the case of the radially divided spiral and the open impeller, on the other hand, machining is possible so that the permissible roughness cannot be exceeded with the choice of the manufacturing process selected for this . This also applies accordingly to the diffusers in multi-stage pumps.
- Choice of material for housing and impeller. With rolled materials, denser material structures are achieved compared to cast materials. This usually goes hand in hand with finer surface roughnesses that can be achieved without complex surface refinement processes.
Shaft seal
The shaft must be sealed against the housing. The permissible or tolerable leakage depends on the medium. The technical effort involved with toxic or corrosive media is very high.
Historically, the stuffing box is the oldest form of shaft seal. An asbestos cord (now PTFE ) is wrapped around the shaft and pressed into a chamber. If it becomes apparent that the leakage rate is increasing due to wear, the press ram of the stuffing box is tightened. If this is no longer possible, a new sealing cord is inserted.
The radial shaft seal was added from around 1930 . A resilient lip contacts the rotating shaft. So that this happens with sufficient pressure, a circumferential tension spring is arranged inside the sealing ring. The sealing ring is replaced when worn. Today's sealing rings are often made of PE or PTFE.
Then the mechanical seal was developed . Two plates (often made of ceramic) are pressed together with springs. One plate is firmly connected to the housing, the other rotates with the shaft. Ceramic plates are extremely hard and therefore hardly wear out. A low-maintenance solution results with a comparatively high construction effort.
A non-contact seal is the labyrinth seal , which is only used in extreme operating conditions, such as high speeds and pressures and / or temperatures.
In special cases (e.g. with poisonous or corrosive media), barrier liquids or gases are used. Instead of a leakage flow from the inside to the outside, the barrier medium flows from the outside to the inside. The barrier medium must be continuously supplemented.
A magnetic coupling can be used as a further sealing system for a centrifugal pump. These pumps are called magnetic drive pumps . Compared to the dynamic seals, the drive torque is transmitted without contact by permanent magnets through the pump housing to the impeller.
drive
Smaller centrifugal pumps are offered as compact units of pump and electric motor with a common shaft (three-phase or alternating current). Medium-sized and larger centrifugal pumps are offered by the pump supplier on a standardized mounting frame. A (standardized) three-phase asynchronous motor is then usually added to this frame on site. The motor and pump shafts must be well aligned and connected with a fixed coupling.
Steam turbines are also used to drive pumps in power plants. This is especially true for some of the cooling pumps in a nuclear power plant. It is assumed here that electrical energy is not available in an emergency.
In special cases there are also other drives. Turbopumps are driven by gas or water turbines.
Areas of application
Because of their simple and robust construction, centrifugal pumps are widely used. Mostly there are designs as single or multi-stage pumps for dry installation or as submersible motor pumps, either for mobile or stationary use.
Range of services
Centrifugal pumps have the following range of services:
- Flow rates about 0.00001 to 60 m³ / s
- Heads of around 1 to 5000 m
- Speeds from <1000 to 30000 1 / min
Areas of application
Centrifugal pumps are used in plant and mechanical engineering , for water supply in waterworks and irrigation systems , for water treatment in swimming pools, for draining mines and pits or as heating circulating pumps and in cooling systems . Typical applications are emptying basements and garages in the event of flooding, irrigating fields in agriculture , filling and emptying tanks or storage containers, and pumping out dirty water. In addition, they are used very numerous and in standardized designs by the fire brigade (" fire extinguishing centrifugal pump "), in particular as a portable version with its own drive motor in the portable pumps , and as a permanently installed version, driven by the vehicle engine, in the fire fighting vehicles (see fire fighting vehicle , tank fire engine ).
Large centrifugal pumps are used in oil production (water injection), in oil refineries , in oil transport ( pipelines ) and in hydraulic engineering for flushing. In power plants, centrifugal pumps take care of the water cycle.
history
A water pumping machine, which according to the Italian historian of science Ladislao Reti “must be called the prototype of the centrifugal pump”, is included in the Trattato di Architectura by the Italian artist-engineer Francesco di Giorgio Martini (around 1475). In 1689, the physicist Denis Papin invented the centrifugal pump. Today this is the most widely used pump design worldwide. The AMAG company in Nuremberg built the first self-priming centrifugal pump in 1916.
See also
literature
- Walter Wagner: centrifugal pumps and centrifugal pump systems. 2nd revised and expanded edition. Vogel Verlag, Würzburg 2004, ISBN 3-8023-1972-9 .
- Johann Friedrich Gülich: Centrifugal Pumps. Manual for development, system planning and operation. 3rd corrected and supplemented edition. Springer Verlag, Berlin et al. 2010, ISBN 978-3-642-05478-5 .
- Igor J. Karassik (Ed.): Pump Handbook. 3rd edition. McGraw-Hill Professional, New York NY et al. 2001, ISBN 0-07-034032-3 .
- Hans Schönherr: Die Roten Hefte, Heft 44a - Pumps in the fire brigade: Part I: Introduction to hydromechanics, mode of operation of centrifugal pumps . 4th edition. Kohlhammer, Stuttgart 1998, ISBN 978-3-17-015172-7 .
- Christian Schwarze: Die Rote Hefte, Heft 44b - Pumps in the fire brigade: Part II: Fire-fighting centrifugal pumps, additional equipment, pressure proportioning and compressed air foam systems . 5th edition. Kohlhammer, Stuttgart 2005, ISBN 978-3-17-018605-7 .
Web links
Individual evidence
- ↑ Excerpt from the chemical technology book by Eckhard Ignatowitz with the assistance of Gerhard Fastert, 8th edition, EUROPA-FACHBUCHREIHE für Chemieberufe. Publishing house EUROPA LEHRMITTEL
- ^ Johann Friedrich Gülich, Centrifugal Pumps: Handbook for Development, Plant Planning and Operation , ISBN 978-3-642-40031-5
- ^ Ladislao Reti: Francesco di Giorgio Martini's Treatise on Engineering and Its Plagiarists. In: Technology and Culture. Vol. 4, No. 3, 1963, ISSN 0040-165X , pp. 287-298 (290).
- ^ Franz-Josef Sehr : Development of fire protection . In: Freiwillige Feuerwehr Obertiefenbach e. V. (Ed.): 125 years of the Obertiefenbach volunteer fire brigade . Reference 2005, ISBN 978-3-926262-03-5 , pp. 117-118 .