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Boiler tubes in a steam power plant are protected from corrosion by a metal oxide layer, which grows on internal surfaces during the first startup. On top of this layer grows a harmful deposit during operation of the boiler as a result of localized corrosion or accumulation or crystallization of impurities in the boiler water. The deposit impair heat transfer from the furnace through the pipe wall into the boiler water. Rise of the tube temperature to higher level than designed increase the risk of tube failure and internal corrosion. Therefore the goal is to remove thick deposit layers with acid treatment before failures emerge. Traditionally the deposit thickness has been determined from sample tubes by a microscope. The aim of this study was to study the theory of a new kind of ultrasonic measurement and evaluate its performance in boiler tube deposit thickness measurements. The aim was also to study the formation of the inner deposit layer in the boiler tubes and the impact of the deposits as well as commonly used criteria for acid cleaning were reviewed. Deposition rate in evaporator is most affected by the power plant type, water chemistry and the amount of impurities in boiler water. The deposition rate varies widely between power plants and also vary in different points on the furnace walls. Concentration of impurities in boiler water and deposit thickness affect both in the probability of corrosion damage. Acid cleaning recommendations should take in consideration the boiler operating pressure, the boiler type and the risk of boiler water quality deterioration. Based on the measurements carried out from tube samples and in power plants the new ultrasound technique produces reliable results when the deposit has a typical structure. Only in one case, in which the deposit was loose sludge, the deposit could not be detected. Results obtained from the measurements provide a good base for the assessment of acid cleaning needs.