Erosion damage results

In practice it is expensive, and therefore uneconomic, to produce a pump which operates completely free from cavitation. As a result it is usual for commercial pumps to operate in the NPSH range between inception and a point where erosion damage is unacceptable. The extent of this range may be increased by using impellers made from the more resistant materials shown in Fig. 8.77. The subject of cavitation in pumps has been dealt with extensively by Pearsall and Grist... 

Erosion corrosion results in an increased rate of corrosion attack attributable to the velocity of a corrodent over the exposed surface. The movement of the corrodent can be associated with mechanical wear. The increased corrosion is usually related to the removal or damage of a protective surface film. The mechanism is usually identified by localized corrosion, which exhibits a pattern that follows the flow of the corrodent. 

In addition to the four main types of wear, related mechanisms can occur in certain design applications. Erosion by a stream of sharp particles is analogous to abrasive wear. Cavitation involves damage to a surface caused by the collapse of a bubble in an adjacent liquid. The surface damage results from the mechanical shock associated with the bubble s sudden collapse. 

Irregular flow patterns resulting from erosion damage. 

Erosion corrosion is mainly observed in hydraulic installations (pumps, turbines, or tubes). This form of corrosion appears at a point where the flow velocity of the bulk solution exceeds a critical limit, or where this limit is exceeded by local turbulence. Erosion corrosion results from an interaction between mechanical and chemical influences. Fig. 1-26. One model describing the mechanism of erosion corrosion assumes that local shear forces acting on the metal surface as a result of the high flow velocity forms pores or unprotected areas. Accelerated mass transfer then occurs in these areas and aggravates corrosion damage. 

Most metals and alloys are susceptible to erosion-corrosion damage. Many depend upon the development of a surface film of some sort (passivity), for resistance to corrosion. Examples are aluminum, lead, and stainless steels. Erosion-corrosion results when these protective surfaces are damaged or... 

Incorporation of nanosized SiC particles in the EL NiP matrix increased the hardness from 627 43 to 704 60HVqqj. However, incorporation of micronsized SiC particles increased the hardness to 1669HVqqj. The cumulative mass loss versus time (see Fig. 8.26) due to cavitation erosion corrosion in 3.5% NaCl is found to be very low for EL Ni-P-nano SiC composite coating compared with its coimterpart incorporating micron-sized SiC particles. The EL Ni-P-nano SiC coated specimen exhibits a smooth and uniform surface after the cavitation erosion corrosion test with no visible pits (see Fig. 8.27). The uniform distribution of the nanosized SiC particles in the EL NiP matrix (R 0.95 0.060 pm) inhibits the formation of pits. However, the higher surface roughness (R 1.72 0.051 pm) promotes bubble formation and causes detachment of the SiC particles, which results in the formation of small cavities on the surface of EL Ni-P-micro SiC coated steel (see Fig. 8.27). Hence, it is evident that incorporation of nanosized particles in EL NiP matrix could provide a better cavitation erosion corrosion resistance and inhibit the onset of erosion damage near surface defects. 

Exploration activities are potentially damaging to the environment. The cutting down of trees in preparation for an onshore seismic survey may result in severe soil erosion in years to come. Offshore, fragile ecological systems such as reefs can be permanently damaged by spills of crude or mud chemicals. Responsible companies will therefore carry out an Environmental Impact Assessment (EIA) prior to activity planning and draw up contingency plans should an accident occur. In Section 4.0 a more detailed description of health, safety and environmental considerations will be provided. 

Erosion and cavitation both can degrade materials simply by mechanical means or by combining the effects of mechanical deterioration and corrosion to produce a synergistic result. However, the mechanisms by which erosion and cavitation operate, and the resulting damage, are quite distinct. 

U-Tube U-Bundle Only one tube sheet required. Tubes bent in U-shape. Bundle is removable. High temperature differentials which might require provision for expansion in fixed tube units. Clean service or easily cleaned conditions on both tube side and shell side. Horizontal or vertical. Bends must be carefully made or mechanical damage and danger of rupture can result. Tube side velocities can cause erosion of inside of bends. Fluid should be free of suspended particles. 1.08... 

Under cavitating conditions a pump will perform below its head-performance curve at any particular flow rate. Although the pump may operate under cavitation conditions, it will often be noisy because of collapsing vapor bubbles and severe pitting, and erosion of the impeller often results. This damage can become so severe as to completely destroy the impeller and create excessive clearances in the casing. To avoid these problems, the fol-iotving are a few situations to watch ... 

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