This article explains the cavitation erosion of centrifugal pumps and its countermeasure technology. The principle of cavitation generation is water vaporization when its pressure drops below the saturated vapor pressure. In a fluid machine, cavitation occurs when the pressure drops at a point where the flow velocity increases and becomes below the saturated vapor pressure. Bubbles generated by cavitation collapse at the place where the pressure of the fluid recovers, and an impact force of several MPa to GPa acts on the material surface due to the underwater shock wave and microjet generated at the time of the collapse. Cavitation erosion is a phenomenon in which the surface of a material is spongy-like damaged by repeated bubble collapse, and is a very serious problem that causes through-holes and breakage of the machine parts. This article proposes three countermeasures against cavitation erosion of centrifugal pumps. The first is to keep the erosion stage in the incubation period for as long as possible, which can be realized by suppressing the occurrence of cavitation itself. The second is a method of reducing the erosion rate, which can be achieved by replacing the member with a material that is less eroded based on the quantified erosion resistance value of the material. The third method is a repair / shape restoration method for the part where cavitation erosion has occurred, and is a countermeasure for cases where the cavitation erosion problem cannot be solved even with the above two methods.

In oil and water-hydraulic components including positive displacement pumps/motors and control valves, cavitation erosion causes serious problems such as damage of the parts, decrease in efficiency, and degradation of reliability. From the viewpoints of reduction and avoidance of the erosion, some of the experimental results and discussion are briefly reviewed and summarized. In this article, the achievements and proposals mainly based on the jet method with hydraulic fluids including mineral and synthetic oils, biodegradable and high bulk modulus oils, and municipal water are presented; The influences of liquids, geometry, materials, and conditions on the jet cavitation erosion are discussed; The estimation methods of the jet-cavitation erosion and countermeasures using the flow-field control are also shown.

Leading edge erosion on wind turbine blade is an important issue to tackle for steady growth of wind energy generation in the world. This paper reviews the fundamental topics of rain erosion, such as physical phenomenon of liquid droplet impingement, the influence of surface roughness and liquid film on the growth of erosion. The experimental techniques for rain erosion testing are described using whirling arm and pulsating jet facilities. Furthermore, the information is added on the state of the art of ongoing research topics on rain erosion by international research committee of wind energy and perspectives on erosion protection devices.

Particle erosion is the wear caused by particle impingement, and problems caused by the particle erosion are sometimes observed in various industrial plants. To evaluate the particle erosion, mainly three kinds of testing apparatuses are used, and examples of the blast type erosion testers are introduced. And several data about influence of material hardness and microstructure of hardened overlay on erosion resistance are shown. Harder materials, for example ceramics, show higher erosion resistance. On the other hand, it should be noted that the microstructure affects the erosion resistance. Recently, methods for prediction of erosion amount by using computer simulations and machine learning technique are reported. An example of prediction method by using database of erosion tests and multivariate analysis is introduced.

Seafloor polymetallic sulphides have been be found in the deep water within Japan's exclusive economic zone. However, the development of them has not reached the commercialization stage because many technical issues still need to be solved. One of those issues is estimating pipe wear. Estimating pipe wear due to slurry flow is important for designing and operating mining systems for seafloor polymetallic sulfides. However, few studies focused on pipe wear at harsh wear environment such as slurry containing large particles which are maximum 50 mm in diameter. We considered the mining systems which consisted of a subsea production tool, a lifting unit and a support vessel. To obtain the data for estimating pipe wear in commercial mining system, two kinds of experiments were carried out. One was the experiment circulating slurry and the other was drop test. Then we introduced wear trends derived from these experiments briefly.

Cavitation is harmful phenomenon for hydraulic machineries such as pumps and valves, as cavitation normally causes severe impact at bubble collapse. However, cavitation impacts can be utilized for surface treatment to enhance fatigue strength and/or fretting fatigue of metallic materials in the same way of shot peening. The surface treatment using cavitation impact is named as “cavitation peening”. The great advantage of cavitation peening is that the increase of surface roughness caused by cavitation peening is lesser than that of shot peening, as shots are not used in cavitation peening. In typical case of cavitation peening, cavitation is caused by injecting a high-speed water jet in water and it is called as a cavitating jet. Cavitation can be generated by irradiating a laser pulse in water. In the present paper, mechanism of cavitation peening using the cavitating jet and the submerged pulse laser is explained, and the effect of cavitation peening on the fatigue properties is revealed.

The objective of this study is to experimentally investigate cavitation pressure in the sliding surface of a mechanical seal. The reversed Rayleigh-step was formed to generate negative pressure and the formation of cavities was directly observed with a CMOS camera. A pinhole was produced in the Rayleigh-step groove, and the pressure in the cavitation region was measured directly with a diaphragm-type pressure sensor placed in the hole. Water was used as the sealing fluid. It was found that the cavitation pressure depended on temperature of water, and was almost equal to the vapor pressure of water. On the other hand, the pressure of the sealed water had no effect on the cavitation pressure.