problems that will be addressed here will be hydraulic failures cavitation

Problems that will be addressed here will be

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problems that will be addressed here will be hydraulic failures (cavitation, pressure pulsations, radial thrust, axial thrust, suction and discharge recirculation), mechanical failures (bearing failure, seal failure, lubrication, excessive vibrations, fatigue), and other types of failure (erosion, corrosion, excessive power consumption). Each problem will be outlined including its cause and effect, symptoms, and pertinent mechanical corrective procedures. Figure 1: Cross section of a Flygt centrifugal pump with sensors (with permission from ITT Corporation) [2]
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2 2 HYDRAULIC FAILURES Hydraulic failures arise from changes in pressure either in the volute or the pipes leading to the pump due to changes in factors such as temperature, velocity of the fluid flow, and volumetric flow rate of the fluid. This section will cover the main hydraulic problems, reasons behind them, and solutions, if any. 2.1 Cavitation Cavitation is the formation of vapour bubbles in a moving fluid where the pressure of the fluid falls beneath its vapour pressure. In essence, cavitation results from a reduction in suction pressure, an increase in suction temperature, or an increase in the flow rate above that for which the pump has been designed [3]. Designers of pumps attempt to take into consideration the fact that pumps do not always run at peak efficiency, and try to take into account the operating range of the system. A centrifugal pump is usually operated comfortably within the range of 85% to 110% of its best efficiency point (BEP). However, many pumps are forced to operate outside of this range [4]. As a result, designers go to great lengths to ensure that cavitation bubbles do not collapse in the pump, but rather in the main piping system, far away from the impeller vanes . There are several causes for cavitation in a pump and piping system, such as: high volumetric flow; a large decrease in the amount of fluid in the system which results in an abnormal increase in the temperature of the fluid; decrease in suction pressure due to changed conditions on the suction side of the pump; heating the fluid in the suction system, which leads to a higher fluid vapour pressure at the pump inlet; flow instability within the pump, which normally occurs at flow rates well below the pump’s best efficiency point (BEP) flow rate; flow close to zero which results in rapid fluid heating in the pump casing, and quickly results in vapour-locking in the system; poor distribution of the fluid in pumps operating in parallel; oversized pumps operating at high capacity; pumping warm water with high vapour pressure; it is also hypothesised that cavities formed between the fluid and the vibrating parts of the pump that are in contact with the fluid; and a high percentage of leakage flows may lead to an increase in the temperature at the eye of the impeller, which would then possibly cause localized flashing [4-9].
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