NPSHr scaling in centrifugal pumps

Abstract

One classic challenge in pump operation is the occurrence of cavitation. Cavitation is the forming of vapour bubbles in a fluid due to a local drop in pressure, followed by a subsequent implosion of those bubbles due to an increase in pressure. The vapour formed can significantly decrease pump efficiency and the implosion can be detrimental to the pump and damage components. Centrifugal pumps play an essential role in dredging, here cavitation can be problematic, both due to loss of efficiency and because of damage to the pump. The condition related to the onset of cavitation is called the Net Positive Suction Head (NPSHr). Knowing the NPSHr conditions of a pump is important for its design and to define the conditions for operating a pump. Unfortunately it is difficult to predict NPSHr properties of a newly designed pump and it is expensive to test a full scale dredging pump. Therefore it is relevant to be able to predict NPSHr conditions of a full scale pump by performing small scale experiments in a lab environment. In order to do this a lab setup was designed for centrifugal pumps with an inlet diameter of 100 [mm]. In this setup the inlet conditions (flow rate, inlet pressure, water temperature) of a centrifugal pump were controlled to determine the NPSHr properties of the small scale pump for a range of shaft speeds and flow rates. The relation between the flow, shaft speed and inlet pressure were investigated for the lab scale experiment. Also the data was compared with available full scale data and analysed to find an expression for NPSHr conditions as a function of flow, shaft speed, inlet pressure and pump diameter for geometrical identical centrifugal pumps. The relation of the NPSHr with shaft speed and diameter appears to be quadratic, provide the flow profile is comparable (i.e. the specific capacity is constant). This relation is optimal at the Best Efficiency Point .