Features of assessing the service life of furnace tubes of high-pressure boilers with regard for operation-induced heterogeneity

Regular inspections of the state of boiler tubes are an indispensable condition for energy security. They are performed according to the regulations approved by the RF Ministry of Energy, which prescribe a number of tests of special samples cut from these tubes. In particular, the decision to extend the service life is to be based on measurements of tube wall thickness and on tabular data of yield strength. For additional testing, the yield strength of the metal at room temperature is determined, which must fall within the range specified in these documents. To determine the mechanical properties of tube steels when inspecting boiler tubes, the guidelines prescribe the use of samples cut along the tube axis. The paper examines samples cut from boiler tubes made of steel 20 after 200,000 hours of operation at the parameters of the internal pressure of the steam-water mixture P = 16.2 MPa and the temperature T = 350 °C with recorded signs of hydrogen embrittlement. Our studies show that the predicted value of the residual life of heating surface tubes significantly depends on the selected place of cutting the sample for testing (fire or rear or lateral tube sides). This difference leads to significant uncertainty in the results of the examination if the samples are not cut from the fire side, this being unspecified in the current company or industry standards. A relationship was found between low yield strength at the operating temperature and high hydrogen concentration in the tube wall. The presence of operational heterogeneity and its relation to hydrogen concentration are also confirmed by metallographic studies. It was found that, with the development of hydrogen damage, standard methods give unfounded predictions of equipment life. Non-standard ring-shaped samples were tested for tension at room and operating temperatures to determine the actual allowable stress. The values obtained were then used to calculate the actual residual life, which corresponded to the actual state of the tube at both room and operating temperatures.