Erosion high-velocity

Erosion High velocities erosion a concern Low velocities erosion not a... 

Erosion is the deterioration of a surface by the abrasive action of solid particles in a liquid or gas, gas bubbles in a liquid, liquid droplets in a gas or due to (local) high-flow velocities. This type of attack is often accompanied by corrosion (erosion-corrosion). The most significant effect of a joint action of erosion and corrosion is the constant removal of protective films from a metal s surface. This can also be caused by liquid movement at high velocities, and will be particularly prone to occur if the solution contains solid particles that have an abrasive action. 

Flow nozzles are commonly used in the measurement of steam and other high velocity fluids where erosion can occur. Nozzle flow coefficients are insensitive to small contour changes and reasonable accuracy can be maintained for long periods under difficult measurement conditions that would create unacceptable errors using an orifice installation. 

Titanium resists erosion—corrosion by fast-moving sand-laden water. In a high velocity, sand-laden seawater test (8.2 m/s) for a 60-d period, titanium performed more than 100 times better than 18 Cr—8 Ni stainless steel. Monel, or 70 Cu—30 Ni. Resistance to cavitation, ie, corrosion on surfaces exposed to high velocity Hquids, is better than by most other stmctural metals (34,35). 

It is often localized at areas where water changes direction. Cavitation (damage due to the formation and coUapse of bubbles in high velocity turbines, propellers, etc) is a form of erosion corrosion. Its appearance is similar to closely spaced pits, although the surface is usually rough. 

Impingement or erosion attack can occur when Hquids or gases impact metal surfaces at high velocity. The corrosion rate is high under such circumstances because any corrosion product films that can be protective if adherent are swept away as quickly as they are formed to leave exposed fresh surface. 

Corrosion was caused by carbonic acid. A film of condensed moisture and dissolved carbon dioxide formed the acid. The erosion was caused by high-velocity movement of air across the tubes. Attack occurred intermittently. Deepest metal loss was 33% of the 0.040 in. (0.10 cm) wall thickness. 

When very high velocities are encountered, metal loss from erosion-corrosion can be general. T ically, however, erosion-corrosion produces localized metal loss in immediate proximity to the disrupted flow. Smooth, rolling, wavelike surface contours are often produced, or distinct, horseshoe-shaped depressions (Fig. 11.1) or comet tails... 

Erosion-corrosion problems on the outside of tubes are frequently associated with impingement of wet, high-velocity gases such as steam. This typically involves peripheral tubes at the shell inlet nozzle (Fig. 11.9). Baffle and tube interfaces may also be affected. 

The metal loss and resulting split were caused by erosion from impingement of high-velocity steam in which droplets of water were entrained. The conical pinnacles in the affected region are typical of deterioration by this mechanism. 

Erosion-corrosion of these components was caused by high-velocity turbulent flow resulting from incomplete opening of the valve. In this case, erosion is the dominant factor in the metal loss, corrosion being a minor contributing factor. 

Exposed ends of the bolts and nuts have also suffered severe localized deterioration, resulting in smooth surface contours. Adjacent areas of each component show no deterioration. Erosion resulting from the impingement of high-velocity slab-cooling spray is the predominant factor in this metal loss. 

Close examination of each circular spot revealed a small forest of erosion cones (Fig. 11.35). Erosion was caused by the impingement of high-velocity steam probably mixed with droplets of condensate. The circular, equally spaced erosion sites apparently reproduced corresponding, equally spaced circular openings in the exhaust inlet. Compare Case History 11.3. 

Turbulence and high fluid velocities resulting from normal pump operation accelerated metal loss by abrading the soft, graphitically corroded surface (erosion-corrosion). The relatively rapid failure of this impeller is due to the erosive effects of the high-velocity, turbulent water coupled with the aggressiveness of the water. Erosion was aided in this case by solids suspended in the water. 

Metal loss in these areas had produced a smooth surface, free of deposits and corrosion products. The rest of the internal surface was covered by a thin, uniform layer of soft, black corrosion product. The graphitically corroded surfaces of the pump casing provided soft, friable corrosion products that were relatively easily dislodged by the abrasive effects of high-velocity or turbulent water (erosion-corrosion). 

Drill Pipe Wear. Erosion can occur between the hard band and the tool joint metal when the box end is hardbanded. This erosion is due to the high-velocity flow of cuttings in the annulus section of the borehole. 

The appropriate size of pipe to carry the required weight of steam at the chosen pressure must be selected, remembering that undersized pipes mean high-pressure drops and high velocities, noise and erosion. Conversely, when pipe sizing is unduly generous, the lines become unnecessarily expensive to install and the heat lost from them will be greater than it need be. 

However, movement at appreciable rates can result in another form of attack that is brought about by the conjoint action of erosion and corrosion hence the term erosion-corrosion that includes all forms of accelerated attack in which protective films, and even the metal surface itself, are removed by the abrasive action of movement of a fluid (gas or liquid) at high velocity. In general, the higher the velocity the more abrasive the solution. 

Impingement attack, as is implicit in the name, is a form of erosion-corrosion in which the solution strikes the metal surface at a high velocity—a situation that can occur at bends, tees and sudden changes in section in a... 

Sulphuric acid is frequently made, stored and conveyed in lead. The corrosion resistance is excellent (see Figure 4.15) provided that the sulphate film is not broken in non-passivating conditions. Rupture of the film may be caused by erosion by high velocity liquids and gases containing acid spray. 

Special devices have been used to study erosion-corrosion by boiler water moving at high velocity, and an example is the method used by Wagner, Decker and Marsh . 

All tests are designed to provide high erosion rates on small specimens so that the test can be conducted in a reasonable time, and although vibratory and high-velocity jet methods may not simulate flow conditions they give rise to high-intensity erosion and can be used, therefore, for screening materials. 

A type of metal wastage caused by the mechanical action abrasion of the metal surface by high velocity steam, air bubbles or solid particles. Often part of a larger erosion-corrosion process. 

---