classified based on the location of the cavities inception and the location of

Classified based on the location of the cavities

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classified based on the location of the cavities inception and the location of implosion of the vapour bubbles, and each is accompanied with its own range of acoustic radiation. Sheet cavitation, which is the first type, forms cavities across the vane surface when pumps operate close to their design flow with low suction pressure. It creates a broad band noise, with low amplitude, in the range of 2 kHz to 40 kHz. Cloud cavitation, the second type, forms cavities downstream of the cavity sheet when the pump operates away from its design flow at low suction pressure. This is the loudest of the three types of cavitation. It generally appears at high frequencies, such as 20 kHz to 40 kHz, and gives the familiar sound of “pumping gravel”. Vortex cavitation, the third type, is a highly unstable form of cavitation when pumps operate at very low flows and in the inlet backflow regime. Although it is a bubble collapse phenomena, like the previous two, it is less damaging because the collapse of the vapour bubbles occur well away from solid surfaces. This type of cavitation is characterized by random bursts of noise accompanied by the typical cavitation sound. When NPSHA gets very close to the 3% head decay line and the pump operates in the back flow regimes simultaneously, vortex cavitation generates a low frequency beat in the region of 1Hz to 4 Hz. This is known as cavitating surge[12]. c) Vibration – Pump vibrations due to cavitation are characteristically high amplitude and low frequency, usually found in the 0 to 10 Hz range [9]. d) Reduction in pumping efficiency – Vapour bubbles created in the passages around the impeller impede the flow of the fluid being pumped, thus resulting in a reduction in output [13]. A drop in efficiency of the pump is a more reliable sign of cavitation occurring, since noise is not prominent until cavitation has progressed to the point where the efficiency of the pump is poor. On some occasions it has been found that the pump’s efficiency may slightly increase moments before cavitation begins. This may be due to a reduction of friction at the beginning of the separation in the flow, just before the cavities start to implode [5]. Cavitation can be detected using a suction gauge or manometer to help determine if the NPSHA is equal to or less than the NPSHR by the manufacturer or detected by microphones capturing the acoustic radiation associated with the cavitation damage [14]. Since cavitation is a common phenomenon, corrective procedures have been devised in order to avoid or control the damage. Some of these corrective procedures are: replacing the impeller in the pump with one made of a more cavitation resistant material, such as stainless steel; reducing the flow of the pump so that the NPSHR will be equal to or less than the NPSHA; completely redesigning or altering the design of the impeller by reworking its geometry or surface finish to reduce losses, improve flow characteristics, or increase the
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