Global measurements and hysteresis-based NPSH acoustic approach for the experimental study of cavitation in a centrifugal pump

Abstract

One of the main drawbacks of centrifugal pumps is the reduction of their operation range to avoid cavitation. This undesired phase change phenomenon takes place as a result of high flow velocities attained in the impeller, which locally reduce the static pressure below the saturation level. Also, certain design features intended to improve pump efficiency in non-cavitating conditions may tend to add even more restrictions to the working range. In the present article, a global measurement of the cavitation characteristics is performed on a test pump, following the traditional performance drop approach, at three different impeller rates of 1800 rpm, 2100 rpm and 2400 rpm. Special stress is made in the discussion of the required Net Positive Suction Head (NPSHr) curves obtained, deviating from common values in centrifugal machines and resembling those of an axial pump at low flow numbers due to the axial inlet with high wrap angle (high angular span of a given blade, from inlet to outlet section) featured by the tested impeller. Afterwards, the study of airborne noise signature is conducted focusing on three working points (namely QN, 0.4QN and 1.7QN, where QN stands for the nominal point flow rate) revealing the suitability of both tonal (blade passage frequency) and high-pitch acoustic emission to characterize flow behavior. In addition, cavitation hysteresis is introduced as a novel method to double-check NPSHi determination. Finally, the acoustically measured NPSHi and the performance-based NPSHr are compared to establishing the required safety margin arising from the application of the proposed methodology.