Initial Stress State Of Boiler Tubes For Structural Integrity Assessment

Due to detected deficiencies, additional tests were requested for checking the stress-strain state in accepting the new boiler. Residual stresses were measured by hole drilling method in critical zones of boiler drum and damaged tubes. The change of state during and after water pressure proof test was monitored by strain gauges and acoustic emission. In this way, initial (nil) stress state was established before placing the boiler into service.The initial level of residual stresses (39% to 44% of yield stress) had reduced to 32% after pressure testing, as a consequence of stress redistribution. Stresses calculated based on measured strains are elastic, lower than 99.3 MPa in boiler drum and less than 66MPa on critical tube 52. This was confirmed by acoustic emission measurements.Performed tests and analyses showed that boiler can be accepted as fit for service. During exploitation the boiler state can be evaluated comparing to the initial stress state.