Monitoring of Bubble Formation during the Boiling Process Using Acoustic Emission Signals

The most widely recognised effect of the cavitation process known as cavitation erosion. Cavitation erosion can be defined simply as the removal of metal from the surface of pumps and valves caused by stresses associated with the collapse of vapour bubbles in the fluid. All types of metallic solid whether hard or soft metal, brittle or ductile are susceptible to damage by cavitation erosion.

Bubble cavitation is an undesirable phenomenon because causes increase in maintenance costs, reduction of production and revenue and decrease the life of the equipment. It is also a dynamic phenomenon that occurs in fluid flows when local pressure is lower than the saturated vapour pressure at ambient temperatures [1]. The growth and collapse of cavitation bubbles lead to the erosion or pitting of metal surfaces and causes high vibration and noise [2]. Operating a pump under cavitation conditions for a long time causes impeller vane pitting. The amount of metal lost depends on the material of the impeller, the degree of cavitation and the time between two successive pressure waves [3]. Noise and vibration phenomenon are an index of cavitation, caused by the implosion of bubbles near a solid surface. This phenomenon causes component damage of pumps and valves. The intensity of the noise and the vibration depends on the number and size of bubbles. In other words, large numbers of small bubbles produce a high-frequency noise and vibration, while a limited number of large bubbles create a low-frequency noise and vibration [4]. 

Several studies for developing the application of the Acoustic Emission technology for bubble cavitation monitoring have been undertaken over the last 20-years. These studies showed that AE technique could be used, in different types of hydraulic components such as valves and centrifugal pumps, for monitoring and diagnosis of cavitation phenomena at an early stage, before it can be detected by other means. Acoustic Emission (AE) is defined as the range of phenomena that results in the generation of structure borne and fluid-borne propagating waves due to the rapid release of energy from localised sources within and/or, on the surface of a material [5]; typical frequency content of AE is between 100 kHz to 1 MHz. 

Alfayez et al. [4]found the AE method is a useful technique for incipient detection of cavitation with the RMS value of AE signal. In addition, there is a high possibility of determining the BEP (Best Efficiency Point) of a centrifugal pump or system.In his work, Masjedian et al. [6] used two methods; Characteristic diagrams and acoustic analysis in detection of cavitation phenomena in globe valves, where found a good agreement results between two techniques on acceptable levels of accuracy. In another investigation, Neill et al.[7]employed the AE method for monitoring the cavitation phenomenon in a centrifugal pump and got a more accurate result than vibration signal.