Liquid drop erosion

Solid particle and liquid drop erosion are surface removal processes caused by impingement of solid particles or droplets carried in a moving gas or liquid stream against a surface. Erosion can also occur when the surface is the moving body impacting suspended particles or drops, as happens on a helicopter blade moving through rain. 

J H Brunton and M C Rochester, Erosion of solid surfaces by the impact of liquid drops . Treatise Mater Sci Technol 1979 16 185-248. 

The appearance of the eroded surface impacted by liquid drops is different from solid particle erosion. Liquid drop... 

ASTM G 40 ASTM G 73 Standard Terminology Relating to Wear and Erosion Standard Practice for Liquid Impingement Erosion Testing Definition of terms for erosion and wear. Resistance to erosion from impacts of liquid drops or jets. 

The repeated impact of liquid against a solid surface using an intermittent jet at sufficiently high speeds leads to a form of erosion. At impact, a liquid drop produces a very high compressive stress in the vicinity of the area of contact, and this is followed by outward, radial flow of liquid at very high speed which shears and erodes the surface. Erosion-corrosion in alpha-brass comprises two parts. The initial progressive plastic indentation is followed by the formation of an annulus where pits gradually form. 

In many natnral (raindrops, fogs, river water fall) and industrial systems (sprays, oil combustion engines, cleaning processes), one encounters liquid drops. The rate of evaporation of liquid from such drops can be important for the function of these systems. Raindrops contribute also major part to erosion and other related phenomena. Extensive investigations on the evaporation of liqnid drops (free hanging drops drops placed on solid surfaces) have been reported in the current literature (Birdi, 2003a 2008, 2010a Pu and Severtson, 2012 Xu et al., 2013 Yu et al., 2004). These drops have been analyzed as a function of... 

Liquid impingement erosion has been defined as progressive loss of original material from a solid surface due to continued exposure to impacts by liquid drops w jets (Ref 8). The opoative words in this definition are inipacts by liquid drops or jets. Liquid impingement erosion connotes repeated impacts or collisions between the surface being eroded and small discrete liquid bodies. 

J.H. Bnmton and M.C. Rochester, Erosion of Solid Surfaces by the Impact of Liquid Drops, Treatise on Malerktls Science and Tedmdogy, Vol 16, Erosion, CM. Preece, Ed, Academic Press, foe., 1979, p 185-248... 

In liquid ejectors or aspirators, the hquid is the motive fluid, so the gas pressure drop is low. Flow of slurries in the nozzle may be erosive. Otherwise, the design is as simple as that of the Venturi. 

Similar considerations apply to the selection of pressure drops where there is freedom of choice, although a full economic analysis is justified only in the case of very expensive units. For liquids, typical values in optimised units are 35 kN/m2 where the viscosity is less than 1 mN s/m2 and 50-70 kN/m2 where the viscosity is 1-10 mN /m2 for gases, 0.4-0.8 kN/m2 for high vacuum operation, 50 per cent of the system pressure at 100- 200 kN/m2, and 0 per cent of the system pressure above 1000 kN/m2. Whatever pressure drop is used, it is important that erosion and flow-induced tube vibration caused by high velocity fluids are avoided. 

However, cavitation may result in an even more serious condition than vapor lock. When the pressure at any point within the pump drops below the vapor pressure of the liquid, vapor bubbles will form at that point (this generally occurs on or near the impeller). These bubbles will then be transported to another region in the fluid where the pressure is greater than the vapor pressure, at which point they will collapse. This formation and collapse of bubbles occurs very rapidly and can create local shock waves, which can cause erosion and serious damage to the impeller or pump. (It is often obvious when a pump is cavitating, because it may sound as though there are rocks in the pump )... 

Typically, fluid velocities encountered in heal exchangers range belweeb 0.7 and 7 m/s for liquids and between 3 and 30 m/s for gases. Low velocities are helpful in avoiding erosion, tube vibrations, and noise as well as pressure drop. 

A liquid may be introduced into a tower as fine drops through a nozzle. This device is known as a spray chamber. The flow could be cocurrent or countercurrent. A countercurrent spray chamber is shown in Fig. 8. These towers are considerably more resistant to plugging when solid particulates are present in the inlet gas. However, difficulties with plugging in spray towers and erosion can be troublesome when the spray liquid is recycled. Particle settling followed by fine strainers or even coarse filters is beneficial to eliminate... 

For high pressure drop design, fluid velocity may be the more limiting criterion, but a liquid velocity exceeding 15 ft/s can result in erosion-corrosion in an exchanger. 

Flow direction considerations for liquid systems are somewhat different than those for vapor flow, In liquid or dense-phase flow the buoyancy force of the liquid must be considered as well as the pressure drop (see below). Thus, for upfiow adsorption the flow velocity must not cause bed lifting, As the Dow rate exceeds lifting velocity, the pressure drop increases only very slowly with increasing velocity. Because of this, sometimes liquid systems are designed with some bed en puns ion wben it is desirable to limit pressure drop. Since too much expansion wjtl cause the adsorbent io become well mixed, with a concomitant drop in removal efficiency, expension is usually limited to about 10%. Higher velocities also tend to create loo much particle turbulence, abrasion, attrition, and erosion. Upflow ndsorption is a preferred direction if the... 

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