Pumps convey liquids (incompressible fluids ), which also include liquid-solid mixtures ( suspensions ), pastes and liquids with a low gas content. As fluid energy machines convert pump as a working machine , a drive operating in kinetic energy of the fluid (pressure times volume).
In technical jargon, pumps are exclusively referred to as machines for the transport of incompressible fluids: The common term "air pump" is not correct in this language: In the case of compressible gases, one speaks either of a blower if volume is conveyed without significant pressure (e.g. for inflating an air mattress) or by a compressor , when gas is compressed under pressure: So " bicycle air pumps " are technically not pumps, but piston compressors .
- If compressible media are conveyed without there being a significant increase in pressure and the main objective is accordingly to increase the dynamic energy, the correct term is fan (colloquially, fan ).
- If gases and vapors as well as gas-liquid mixtures are conveyed and if their pressure increase through compression is an essential goal, the device is called a compressor .
- Machines that are used to generate a vacuum in a container are called vacuum pumps , but their function is also a compressor.
Devices that give a fluid an increase in energy by applying mechanical work, but in which the medium can flow in and out without additional guidance, are technically not pumps. Which includes:
Classification of pumps
According to their functional principle, pumps are divided into two main groups: In the following, for the sake of simplicity, both groups are referred to as pumps.
In fluid flow machines , the energy transfer is effected exclusively by fluid mechanical processes. The medium flows freely through the machine without flaps and valves. At a standstill, the medium could flow backwards through the pump. Therefore, depending on the application, slides , valves or non-return valves must be used. Flow pumps are not self-priming, so the suction lines must always be filled with liquid or there must be a sufficiently large volume of liquid in front of the actual impeller inlet. The maximum suction height is also limited here by the local air pressure and any flow resistance. Flow pumps should not be throttled during operation on the suction side, since there is a risk of the blades being destroyed by cavitation .
Positive displacement pumps
With positive displacement pumps, the medium is conveyed through self-contained volumes; backflow is prevented by valves or flaps, other media or their shape by gravity . Except for design-related leaks, the medium cannot flow through the pump in the opposite direction even when it is not running. Positive displacement pumps are usually self-priming, which means that pumps designed for liquids can also convey gases for a mostly limited period of time and thus build up a vacuum that is sufficient for suction.
The maximum suction height ( geodetic suction height ) is limited by the vacuum that can be achieved, the local air pressure , the density of the medium and the flow resistance to be overcome. Displacement machines should not be shut off on the pressure side unless suitable measures have been taken using slip clutches , pressure relief and bypass valves and the like to prevent damage to the pump, the drive or the lines up to the shut-off point.
A distinction is also made between fixed displacement pumps and variable displacement pumps. Fixed displacement pumps always displace the same volume with each revolution. With variable displacement pumps, however, the displacement can be adjusted. In addition, the pumps can be divided into rotating and oscillating displacement pumps according to their delivery principle.
- Bellows ( bellows pumps or bellows pumps)
- Diaphragm pumps
- Scroll compressor
- Rotary piston pumps
- Eccentric screw pumps
- Screw conveyors ( Archimedean screw )
- Hydraulic ram
- Impeller pump
- Chain pumps
- Piston pumps
- Rotary piston pumps
- Peristaltic pumps (including peristaltic pumps called)
- Pumping stations , in the simplest case a bucket in the well.
- Screw pumps (also screw pumps , helical piston pump or screw compressor called)
- Shaking pumps
- Sinus pumps
- Timing belt pumps
and many special constructions, as well as the heart in animals and humans .
Jet pumps occupy a special position . With them, the medium to be conveyed is accelerated by a jet of gas, steam or liquid. Although they use flow dynamic processes, they are still mostly counted as positive displacement pumps.
These are for example:
Further funding principles are:
- Mammoth pump or bubble pump (delivery by ascending gas bubble as a piston)
- Ram (hydraulic ram) (uses water to pump water)
- Horse head pump (deep pump)
In addition, there are other, sometimes exotic constructions that use electromagnetic or other physical properties of the conveying medium for energy transfer.
Examples of this:
- Ion getter pump (see also sorption pump in the article vacuum pump )
- Titanium sublimation pump
- Turbo molecular pump
- Electrochemical gas pump
NPSH (holding pressure height) is a term from the USA ( Net Positive Suction Head ) and means - loosely translated - about the minimum intake height above saturation pressure . According to DIN EN ISO 17769, the corresponding German expression is holding pressure height . NPSH is given in meters (m).
Holding pressure height and NPSH value are not equivalent due to different reference points.
With NPSH, for example, the pressure difference between the vapor pressure and the pressure at the suction port inlet of the pump, and at the holding pressure level, the vapor pressure is compared with the pressure at the impeller inlet.
It is between net positive suction head of the pump (NPSH R [required] or NPSH req. ) And holding pressure level in the system (NPSH A [available] or NPSH prev. ) Distinguished.
Required NPSH (holding head of the pump)
The net positive suction head NPSH of the pump or Inv. Corresponds to the total pressure drop from the suction nozzle (limit of supply) of the pump to the impeller inlet (pressure increase in the impeller zone) and indicates the absorbency of the pump.
Example: NPSH req. = 2 m means that the pressure losses from the pump intake to the impeller inlet (including acceleration losses) correspond to a pressure head of 2 m water column.
If the holding pressure level of the pump (NPSH req. ) Is not reached during operation, the evaporation pressure of the pumped liquid falls below the local level and thus cavitation (vapor formation with subsequent sudden condensation in the pressure increase zone of the pump). Cavitation can cause a drop in the delivery rate and the degree of efficiency, uneven running and damage to the internal parts of the pump through material removal (so-called abrasion).
The holding pressure level of the pump depends on the temperature via the saturation vapor pressure of the pumped liquid. However, it changes with each pump with the flow rate and speed. NPSH required is usually given by the pump manufacturer either as a numerical value for a specific operating point of the pump or shown in connection with a performance curve in the form of a curve.
Existing NPSH (holding pressure head of the system)
The existing NPSH value includes all the individual data of the pump system that have an influence on the pressure at the suction port of the pump, such as density, temperature and vapor pressure of the pumped medium, pressure losses in the suction line, pressure in the suction tank and geodetic suction or inlet head for a specific flow rate in one single numerical value together.
Example: NPSH existing = 4 m indicates that the liquid to be pumped at the suction port of the pump is still a positive 4 m water column away from the evaporation pressure .
The following applies to the pressure at the suction port (only the suction side is considered):
Steam is generated and thus cavitation occurs when the steam pressure equals the pressure p at the suction nozzle, i.e. that is, if NPSH exists 0 goes.
Since the holding pressure level of the pump (NPSH req .) Must not be undershot, the value for NPSH present must be at least the same, or better, slightly greater than that of the pump.
If necessary, the suction tank must be placed higher or the pump lower, the nominal size of the suction line larger or the pressure in the suction tank increased.
- Hellmuth Schulz: The pumps: working method, calculation, construction. 13., rework. Ed., Springer, Berlin 1977, ISBN 3-540-08098-8 .
- G. Vetter (Ed.): Pumps. 2nd edition, Vulkan-Verlag, Essen 1992, ISBN 3-8027-2696-0 .
- Technical comparison of different pump types (including rotary lobe pumps, eccentric screw pumps, centrifugal pumps, gear pumps) with their advantages and disadvantages