In radial flow centrifugal pumps the impeller has no components of axial speed; it is common practice to define radial pumps (in the senseside) those where propeller blades are only in the plane r-Θ(radial-circumferential).
Many pumps and compressors can be included into this class.
The impeller of a pump or a centrifugal compressor can be open or closed: in the first case the blades are exposed frontallysince there is no frontal disk (on suction side); as foraxial flow machines, there is the possibility of a leakage from a blade compartment to the next through the gap between the impeller and the front chest.
A diffuser is positioned at the discharge of the impeller (in both cases of centrifugal pumps or compressor), followed by (in the case of machinerysingle-stage, or at the final stage in multistage machines) a collector vane which collects the flow over 360°, and by a flanged discharge.
The diffuser has the fundamental task of converting the kinetic energy to the form of pressure.
Cavitation is a physical phenomenon consisting in the formation of zones of gas vapour in a fluid which suddenly implode, producing a characteristic noise.
Cavitation appears when local pressure reaches the vapor pressure of the liquid, so that gas diluted in liquid undergoes a phase change, forming bubbles (cavities) containing steam.
The dynamics of the process is very similar to boiling.
The main difference between cavitation and boiling is that, due to temperature, the vapor rises up to overcome the pressure of the liquid, resulting in a mechanically stable bubble, because full of steam at the same pressure as that of surrounding liquid.
The cavitation is rather fluid pressure to drop suddenly, while the temperature and vapor pressure remain constant.
For this reason, the bubble cavitation resist until bubbles remain into the area of low hydrostatic pressure: just back in the fluid at rest, the pressure of steam is not sufficient to counteract the hydrostatic pressure and cavitation bubbles implode instantly.
Cavitation occurs when the suction pump transfer liquid at low pressure, allowing formation of steam at the inlet of the pump.
The vapor bubbles pass through the impeller, and when they arrive in the output section, the high pressure present here will cause a violent implosion, creating the characteristic sound, as if a gravel is passing by the pump.
This implies a significant erosion of the pump intake, resulting in a decrease in hydraulic properties.
The tendency of a pump to show cavitation phenomena is expressed by the concept of NPSH (Net Positive Suction Head), usually expressed in meters (in technical system).
In particular, the NPSHR (acronym for NPSH required) is characteristic of the pump and expresses the fluid energy required for crossing the portion between the flange of the pump inlet and the first impeller (pressure drop, energy kinetic, potential rise).
The NPSHA (acronym for NPSH available), closely linked to the circuit, expresses the energy of the fluid at the level of energy intake flange.
To ensure operation in the absence of cavitation NPSHA> NPSHR is required.
21. Liquid Mixing