Abstract This paper focuses on evaluation of differences in the newly manufactured U-tubes and ones after operation. Differences in geometry and internal surface roughness were investigated and a flow simulation was carried out in the Ansys Fluent software, where properties such as flow nature, maximum velocity and pressure drop were examined. A numerical model validation process was carried out with an application of PIV experiments. The digital models of the U-tube were obtained with an application of computer tomography. As a result of the tests, it has been noticed that the U-tube after operation has a much more distorted cross-sectional shape comparing to the shape of the new one. Moreover, the U-tube after operation is characterized by a 3-fold higher average surface roughness Sa value than one obtained in the new tube. The maximum flow velocities for the tube after operation are about 3.4% higher than ones reached for theoretical tube.