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Damages through Corrosion/ Erosion
Damage of heat exchangers can be caused by a large number of factors. One of the main causes of heat exchanger damage – the worst case being a leak – is corrosion. Corrosion is described by DIN EN ISO 8044 as „the reaction of a metallic material with its environment, which causes a measurable change of the material and can affect its function (…)“. Amongst others, the standard de- fines 37 types of corrosion, including pitting corrosion, stress corrosion cracking and erosion corrosion.
Pitting corrosion mostly occurs in passivated materials with an electrolyte present. The occurrence is strongly influenced by high temperatures, low electrode potential of the material as well as a low pH- value of the electrolyte, i.e. the fluid, and a low oxygen concentration in the electrolyte. For the occurrence of stress corrosion cracking, three conditions must be met: On the one hand the material must be susceptible to stress corrosion cracking. Furthermore, tensile stresses in the form of residual stresses or externally applied stresses must be present. Moreover, a specific corrosive agent must be present; for austenitic steels these are fluids containing chloride and for copper zinc alloys these are ammonia, amines, nitrites or nitrates.The erosion corrosion not only depends on mechanical material characteristics, but also on current conditions, such as speed and geometry, as well as on possible specific substance conditions like dissolved solid particles in fluids or formation of droplets in gases, which can result in abrasive loss.
By definition, erosion or wear and tear is the progressive loss of material from the surface of a solid object. If mechanical forces affect this base body, i.e. contact and relative movements of a solid, liquid or gaseous counter body, this results in erosion.
Amongst others, this undesired change of the surface occurs on tubes and tubeworks. Wear and tear is one of the main reasons for tube damages and causes the associated downtimes of heat exchangers and their connected components. Erosion on pipes can be manifold, with more or less severe consequences.
Erosion at support plates
This kind of tube damage is a type of erosion that is caused by support plates and insert plates that bang against the tubes. Support plates normally support and stabilise the fitted tubes in common heat exchangers. However, under certain conditions, for example if there is clearance between boring and tube or if other thermal and mechanical influences are present, the tubes can vibrate in the support plates and get damaged due to the banging, and thus wear out. This quickly results in undesired leakages, with associated financial and possible environmental damages.
Particularly the first inches of a heat exchanger tube are subject to severe erosion stresses. This is due to the fact that the influx of media is exposed to strong velocities and turbulences on the first few inches. These in turn can result in dangerous thinning out of the inlet area, causing erosion.
Steam and wet steam erosion
Steam erosion is mostly found on the external of heat exchanger tubes, which are subject to steam or condensate. Depending on the design of the cooler or heat exchanger, there could be increased steam concentrations at the support plates or the free tube areas, which in turn results in leakages.
Erosion through deposits and inclusion of foreign objects in tubes
If deposits or foreign objects accumulate in a tube, the inflowing medium washes around these. This results in velocities and turbulences directly behind the point blocking the free flow. In combination with water or abrasive particles, this can lead to severe wear and tear.
Erosion through droplets mostly occurs on power plant condensers. Due to condensation, microscopic steam droplets form, which hit the external tube surface with tremendous speed and energy. The force of these droplets can be so high that the tube surface is damaged with pinholes that can result in leakages.