Erosion-corrosion elimination

Due to the numerous factors that can contribute to an erosion-corrosion process, numerous approaches can mitigate or eliminate the problem. 

Because erosion-corrosion is directly linked to velocity and turbulence, reducing velocity and turbulence are important first steps in elimination. Reducing velocity and turbulence often requires design changes such as the following ... 

Entrainment of fine particulate matter such as sand and silt in cooling water can contribute significantly to erosion-corrosion. In these cases it is important to eliminate or reduce the amount of particulate by settling or filtration. It may also be necessary to reduce or eliminate entrained gas bubbles. 

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. 

One of the best ways to reduce erosion-corrosion is to change the design to eliminate fluid turbulence and impingement effects. Other materials may also be used that inherently resist erosion. Fmlhermore, removal of particulates and bubbles from the solution lessens its ability to erode. 

Product Quality Considerations of product quahty may require low holdup time and low-temperature operation to avoid thermal degradation. The low holdup time eliminates some types of evaporators, and some types are also eliminated because of poor heat-transfer charac teristics at low temperature. Product quality may also dic tate special materials of construction to avoid met hc contamination or a catalytic effect on decomposition of the product. Corrosion may also influence evaporator selection, since the advantages of evaporators having high heat-transfer coefficients are more apparent when expensive materials of construction are indicated. Corrosion and erosion are frequently more severe in evaporators than in other types of equipment because of the high hquid and vapor velocities used, the frequent presence of sohds in suspension, and the necessary concentration differences. 

Another fouling mechanism that can occur is corrosion of boiler tubing and erosion of refractories due to formation of acids and their buildup in the combustion units from conversion of sulfur and chlorine present in the fuel. Fortunately, the amounts of these elements in most biomass are nil to small. The addition of small amounts of limestone to the media in fluidized-bed units or the blending of limestone with the fuel in the case of moving-bed systems are effective methods of eliminating this problem. Other sorbents such as dolomite, kaolin, and custom blends of aluminum and magnesium compounds are also effective (Coe, 1993). 

It is generally difficult to eliminate all the droplets leaving with the air flow so that in induced draught designs, these droplets are likely to enter the fan system with the possibility of erosion and corrosion and damage to the fan. 

We is the pure erosion rate, i.e. mass loss rate when corrosion is eliminated, and Wc is the corrosion rate in the absence of sand erosion. Wec and Wce are both synergy effects Wec is the increase in erosion rate due to corrosion and Wce is the increase in corrosion rate due to erosion. 

It is possible to determine the four contributions to the total material loss rate by the following experimental principles the total material loss rate Wt is determined by weighing the specimen before and after exposure under combined erosive and corrosive conditions. The sum of Wc and Wce (the corrosion components) can be measured by electrochemical methods during the same exposure (the methods described in Section 9.2 can also be used under erosive conditions). We is determined by weighing the specimen before and after exposure in special tests where corrosion is eliminated by cathodic protection (or possibly by oflier means) but otherwise under the same conditions as in the former experiments. Wc can be measured electrochemically in tests like the original ones but with all solid particles excluded. Finally, the synergy components, Wce and Wec, can be derived from Equation (7.9) and the mentioned experiments. 

The transmission of stresses from piping to equipment can be eliminated by installation of manufactured expansion joints. Corrosion, erosion, and cyclic stresses, however, limit the use of mechanical expansion joints. Piping stresses on equipment is usually controlled by proper piping arrangement and use of supports and anchors. 

Use of a steam generator to separate the primary loop from the secondary loop largely confines the radioactive materials to a single building during normal power operation and eliminates the extensive turbine maintenance problems that would result from radio-actively contaminated steam. Radioactivity sources are the activation products from the small amount of corrosion that is present in the primary loop over the 12-18-month reactor cycle, as well as from the occasional (<1 in 10,000) fuel rod that develops a crack and releases a small portion of its volatile fission products. Uranium dioxide fuel is very resistant to erosion by the coolant, so the rod does not dump its entire fission product inventory into the RCS. 

Technology to maintain certain oxygen regime of lead-bismuth coolant to eliminate corrosion and erosion of stainless steel claddings in coolant flow. This technology is available in the Russian Federation, which has an 80-year operation experience with small lead-bismuth cooled reactors for nuclear submarines. 

---