A. Neidel, S. Riesenbeck, T. Gädicke, B. Fischer
September 28, 2017
A leakage occurred in the area of the weld of a vent nozzle in the pressure pulsation dampener of a natural gas compressor. It was caused by a crack adjacent to the weld on the side of the pipe. A metallurgical failure investigation was ordered to identify the mechanism that caused the damage. The defective pressure pulsation dampener was brought back to the welding manufacturer for repair. The crack that caused the leakage was opened and the pipe was detached from the shell by the vessel manufacturer. A sample of the pipe containing the crack edge on the side of the pipe was brought to the laboratory for examination. A fatigue crack which initiated at the outer surface of the pipe was identified as the metallurgical cause of the failure. No indications of weld defects were found. In the damaged area, the microstructure at the outer surface of the vessel's vent nozzle presents a number of damage-promoting features such as small surface ruptures and scale residues (which will lead to an increased notch effect there) as well as decarburization and coarse grain. The design of the vessel's nozzle at the shell of the pressure pulsation dampener does not seam very suitable for dynamic loading. Also taking into account the poor surface quality of the pipes, the vessel's nozzles are insufficiently dimensioned for the present loading condition which clearly comprises dynamic elements. It was recommended to review the design and to apply adapted repair measures.