PSI - Issue 31

Goran Vukelic et al. / Procedia Structural Integrity 31 (2021) 28–32 Goran Vukelic et al. / Structural Integrity Procedia 00 (2019) 000–000

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1. Introduction Research presented in this paper is concerned with the failure of an air pressure vessel. Pressure vessel, Fig. 1, was in a form of a thin-walled steel cylinder and was used as a part of an air compressor unit. Vessel was manufactured by „Prva petoletka”, Trstenik, former Yugoslavia, in 1984. It has a volume of 60 l, declared mass of 21.3 kg, but the actual weighted mass was 21.6 kg. Working pressure of the vessel is 8 bar and test pressure is set at 13 bar. Overall length of the vessel is 820 mm, outer diameter 306 mm and shell thickness 3 mm.

Fig. 1. Inspected pressure vessel. Failure occurred during hydrostatic test when two cracks were noted on the shell of the vessel and test fluid leakage from them observed. Hydrostatic pressure test performed with the aim of reaching and holding test pressure of 13 bar. Testing liquid was water. Analogue pressure gauges and digital pressure transducer were connected to the vessel to monitor pressure drop. Pressure loss was noticed on the pressure gauges and computer display connected to digital pressure transducer. Also, two leaks were visually identified on the outer surface.

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Fig. 2. a) Hydrostatic pressure test setup; b) water leakage from two cracks. In order to determine possible causes of failure, experimental investigation was performed, comprising of acquisition of technical specifications, non-destructive testing (NDT), material characterization and visual and microscopic observations. Methods used and results obtained are given in following sections of this paper. Conclusions about the failure causes and recommendations about the avoidance of similar scenario are given the end. Also, directions for future research based on numerical procedures are outlined. 2. Experimental failure analysis Visual inspection of the outer surface revealed light surface corrosion uniformly spread at the contact between the vessel and the saddle holdings of the compressor installation. Also, sporadic localized corrosion points were noted, Fig. 1. As for the inner condition, uniformly heavily corroded surface with pitting corrosion points was noted using inspection borescope, Fig. 3. This type of defect presents a serious threat to the integrity of the pressurized components (Chmelko and Biro 2019), especially when the corrosion is coupled with mechanical loads (Kelil et al. 2019) that can have alternating nature also (Baragetti and Arcieri 2018), so further investigations followed.