Erosion-corrosion failure

Many erosion-corrosion failures occur because turbulent flow conditions exist. This type of failure occurs in the inlet ends of tubing in condensers and similar shell-and-tube heat exchangers and is designated inlet-tube corrosion. ... 

Erosion-corrosion is a fairly complex failure mode influenced by both environmental factors and metal characteristics. Perhaps the most important environmental factor is velocity. A threshold velocity is often observed below which metal loss is negligible and above which metal loss increases as velocity increases. The threshold velocity varies with metal and environment combinations and other factors. 

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. 

Pump and compressor seal areas are by far the most common areas where vapor releases may occur. This is followed by instrumentation sources, valve seals, gaskets and sample points and the most rare but usually catastrophic erosion and corrosion failures of process piping. 

Specific segments of process piping handling flammable liquids or gases in mixtures known to promote pipe failures through erosion, corrosion, or embrittlement. These include hydrocarbons, streams that may contain entrained catalyst, caustics, acids, hydrogen, or similar materials where development of an appropriate scenario envelope is feasible. 

Formation of bubbles in rapidly flowing or turbulent water, causing risk of pumping failure and erosion/corrosion. 

One report stated thickness measurement a short distance away from the rupture showed the line was a nearly full design thickness. Investigators concluded the line failure was the result of the thinning of the Schedule 120 carbon steel 90-degree elbow due to long-term erosion/corrosion. [21] Another story stated the piping was originally or nominally 0.625 inches thick, but had worn down to 0.085 inches. [27] That represents an 86-percent wall loss. 

The combination of wear or abrasion and corrosion results in more severe attack than with either mechanical or chemical corrosive action alone. Metal is removed from the surface as dissolved ions, as particles of solid corrosion products, or as elemental metal. The spectrum of erosion corrosion ranges from primarily erosive attack, such as sandblasting, filing, or grinding of a metal surface, to primarily corrosion failures, devoid of mechanical action. 

All types of equipment, exposed to fluids in motion, are subject to the erosion-corrosion phenomena. This can include pipeline networks (particularly curves, elbows and T-squares), floodgates, pumps, centrifugal fans, helixes, wheels of turbine, tubes of intersections of heat exchangers and measuring devices. In many cases, failures due to erosion-corrosion occur in a relatively short time.16 Most metals are susceptible to erosion-corrosion in the liquid phase under specific conditions. Resistance of metals depends on the physical and chemical properties of the corrosion product and/or the... 

High solids (carbonates and bicarbonates) BFW will result in significant formation of carbon dioxide. The resulting carbon dioxide-laden condensate causes erosion-corrosion attack on carbon steel. Failure occurs by deep pitting, furrowing, or channeling. Corrosion inhibitors can be added to minimize this attack. 

Any action in mitigating flow maldistribution must be preceded by an identification of possible reasons that may cause the performance deterioration and/or may affect mechanical characteristics of the heat exchanger. The possible reasons that affect the performance are [131,147] (1) deterioration in the heat exchanger effectiveness and pressure drop characteristics, (2) fluid freezing, as in viscous flow coolers, (3) fluid deterioration, (4) enhanced fouling, and (5) mechanical and tube vibration problems (flow-induced vibrations as a consequence of flow instabilities, wear, fretting, erosion, corrosion, and mechanical failure). 

Materials problems in newer coal gasification processes accrue generally from operating temperatures of 1500 -2800°F and pressures of 150-1200 psi. Added to these conditions are low oxygen activity and high sulfur activity in the product gas atmosphere. Finally, coal ash and sulfur sorbents present in the system can cause materials failure by corrosion and/or erosion-corrosion. Current metallic alloys that were developed... 

Random failure (see Chapter VII) Random failures are project specific in the sense that they depend on the process and its use. From lEC 61508 it is found that a failure occurs at a random time, which results from one or more degradation mechanisms. Random failures are mainly caused by physical damage/changes such as wearout, thermal stress erosion/corrosion, etc. These are applicable for hardwires of E/E/PEs in automation systems. The rate of failure of random failures normally cannot be reduced instead for random failures focus should be on their detection and handling. Statistical data handling and treatments can be applied to random failures, hence risks associated with random failure can be calculated. This is not possible in the case of software with systematic faults. Common cause failure (see Chapter VII) This is a kind of fault that causes multiple devices/systems to fail simultaneously. Common cause failure may be random or systematic. This is discussed in Fig. 1/8.3-1 in Chapter I, Chapter Vll, and in Ref. [9]. 

The attack takes the form of grooves, i.e., scooped-out rounded areas, horseshoe-shaped depressions, gullies, or waves, all of which often show directionality. At times, attack may be an assembly of pits. Ultimate perforation due to thinning or progression of pits, and rupture due to failure of the thinned wall to resist the internal fluid pressure are common. All equipment exposed to flowing fluid is subject to erosion corrosion, but piping systems and heat exchangers are the most commonly affected. 

Finally, the main causes for metallurgical failures include fatigue, erosion, corrosion (internal or external), vessel designed/constructed to an inadequate specification, use of incorrect or inadequate materials of construction, failure to satisfy specified construction codes, embrittlement by chemical action, and vessel used for purpose not covered by specification. 

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