Erosion corrosion prevention

For inlet or outlet end erosion-corrosion, either extend tube ends 3 or 4 inches into the water box or install sleeves, inserts, or ferrules into the tube ends. These should be a minimum of 5 inches long. The ferrules may be nonmetallic or erosion-resistant metals, such as stainless steel, if galvanically compatible. The end of the ferrule should be feathered to prevent turbulence. 

There are basically five ways to reduce or prevent erosion corrosion ... 

Environmentally the most important variables are pH, oxygen content and temperature of the water (Figure 1.96). In single phase conditions both high pH and additions of low levels of oxygen have been used to prevent erosion corrosion . However, because of partitioning effects between water and steam this is more difficult to achieve in two-phase flow. Although additions of morpholine or AMP (2-amino-2-methyl-propan-l-ol) have been successfully used to control pH. 

Erosion corrosion may be considered at the design stage when a wider range of options is available to prevent its occurrence than if it occurs after the item of plant has been built. Various options and examples for these two situations are summarised in Table 1.32. 

The term erosion-corrosion is used to describe the increased rate of attack caused by a combination of erosion and corrosion. If a fluid stream contains suspended particles, or where there is high velocity or turbulence, erosion will tend to remove the products of corrosion and any protective film, and the rate of attack will be markedly increased. If erosion is likely to occur, more resistant materials must be specified, or the material surface protected in some way. For example, plastics inserts are used to prevent erosion-corrosion at the inlet to heat-exchanger tubes. 

Problems with steam can occur in let-down valves as a result of erosion-corrosion. To prevent attack, hard facing (e.g., stellite) is commonly used when the pressure drop exceeds 150 to 200 psi (1,035 to 1,380 kPa). This limit can be raised to 500 psi (3,450 kPa) for clean, dry steam. Corrosion-erosion also occurs in wet steam. Carbon steel is unsatisfactory in wet steam when pvx, the product of the pressure (psia), velocity (ft/s), and wetness (% water) exceeds 1 x 105. Resistance to wet steam is enhanced by increasing both the metal hardness and chromium content. 

Both austenitic and super SS s have excellent resistance to erosion-corrosion in velocities up to 85 ft/s (26 m/s). Usually, copper base alloys are not considered because of poor resistance to hydrogen sulfide/10 poor resistance to erosion, and low strength. Prevention of corrosion by coatings is usually impractical in production equipment because of limited life, as described previously, and because the coating can be blown off by sudden depressurization when the operating pressure is above -650 psi (4,480 kPa). 

The original saltwater condenser tube made of admiralty brass was found to be susceptible to erosion-corrosion at tube ends. Aluminum brass containing 2% aluminum was more resistant to erosion in saltwater. Inhibition with arsenic is necessary to prevent dezincification as in the case of admiralty brass. The stronger naval brass is selected as the tube material when admiralty brass mbes are used in condensers. Cast brass or bronze alloys for valves and fittings are usually Cu-Sn-Zn compositions, plus lead for machinability. Aluminum bronzes are often used as tube sheet and channel material for exchangers with admiralty brass or titanium tubes exposed to cooling water. 

The obvious method of corrosion control of farm equipment and machinery is to keep it clean and dry after use. Prevention of corrosion under deposits such as mud or waste products prolongs the life of the equipment. Exposure of the equipment to bacteria, fertilizers, cleaning agents, and sanitizing solutions should be minimized. It is also advisable to remove the mud or adhering dirt such as sand particles to decrease wear and possible erosion-corrosion on engines and moving components. 

The mechanism is that deposits of corrosion products, or salts precipitated because of the corrosion process, are worn off, dissolved or prevented from being formed, so fliat the material surface becomes metallically clean and therefore more active. In extreme cases, erosion corrosion may be accompanied by pure mechanical erosion, by which sohd particles in flie fluid may tear out particles from the material itself and cause plastic deformation, which may make the metal even more active. 

Figure 7.47 Prevention of erosion corrosion by design. (Partly reproduced from Pludek [7.3] with permission of Palgrave MacMillan.)...

Figure 7.48 Prevention of erosion corrosion by separation of different phases.

Particular attention should be given to compatibility of the materials used with regard to the water chemistry in order to prevent corrosion phenomena. For all equipment exposed to damp steam or to fluids which can cause severe erosion, corrosion and erosion resistant materials should be used. Low alloy steel containing chromium (Cr >0.5%) may be used. 

Erosion Corrosion caused by the impingement of a corrosive fluid or fluid containing abrasive particles on a component, removing or preventing the formation of a protective coating. Corrosion resistance of galvanized and passivated metals can effectively be eliminated by erosion. 

Prevention and/or reduction of erosion corrosion can be accomplished by one or more means ... 

An erosion corrosion allowance is present in the pipeline wall. The thickness is defined also by taking into account these factors. This represents a further safety barrier mitigating material degradation and preventing worse consequences, such as unscheduled repair and maintenance, loss of production or loss of containment. 

An example is given, considering the threats of erosion/corrosion in the petroleum production. This example shows how the proposed model deals with different operative conditions and how the risk of loss of containment is affected by them. In this way, the importance of both the state of the threats and the performance of the barriers is demonstrated. An effective action of risk prevention should be then focused on both of them by giving priority to the improvement of poor barriers connected to critical threats. 

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