PSI - Issue 31

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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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Fig. 6. Ultrasonic thickness measurement orientation and zones.

Table 1. Shell thickness (in mm).

Orientation

0°

90°

180°

270°

Zone 1

2.5 1.9 2.7

2.7 2.1 2.6

2.6 2.7 2.6

2.7 1.9 2.8

2 3

Obtained results clearly show that nominal thickness of 3 mm is reduced, in some locations by even more than a third. These locations appear to be in a lower position of the vessel, near the drainage opening at the bottom. Chemical composition of vessel material was determined using glow discharge spectrometer, Table 2.

Table 2. Chemical composition of pressure vessel steel ( w t %).

C

Si

Mn

P

S

Cr

Ni

Nb

0.094

0.0376

0.401

0.0194

0.0122

0.0249

0.0191

0.0039

Cu

Al

Sn

W

Co

Pb

As

Mo

0.086

0.0631

0.0069

0.0079

0.0044

0.0003

0.0013

0.0043

Composition of tested material is adequate to steel EN 10028-2 grade P235GH (1.0345, ASTM A285). EN10028-2 is a standard for the non-alloy and alloy steel plates with specified elevated temperature properties. This is a non-alloy steel with good plasticity, toughness, cold bending and welding properties even under high temperature. Steel P235GH is mainly used for pressure equipment (boilers, heat exchangers, steam tubes and pressure vessels). It has application in chemical industries, power stations, instruments and processing utilities in fertilizer and food industries. Additionally, hardness test was performed using with found mean hardness value of 590 HV (Vickers hardness number). Maximum tensile strength of the steel derived from the hardness value (Boyer and Gall 1985) is σ TS = 3.2 HV = 410.88 MPa. According to the standard, for steel plate thickness less than 6 mm maximum tensile strength is σ TS = 360-480 MPa and yield strength σ YS = 235 MPa. 3. Conclusion Considering obtained experimental results, root causes of failure can be summarized as inadequate maintenance of the vessel without regular draining, cleaning, NDT inspection and hydrostatic pressure testing. This inadequate maintenance led to excessive pitting corrosion with through-wall multiple damage locations on the shell of the vessel. Leak-before-break scenario developed with vessel failing to pass hydrostatic pressure test well below the set test pressure value. Simple engineering assessment (Zaidi et al. 2020) was performed in order to determine the state of the vessel and to discover causes of failure. Possible recommendations to avoid such scenario include adequate maintenance of remaining undamaged vessels and appropriate choice of inspection technique (Radu et al. 2020). Also, for already damaged vessels, hoop wrapping with composite patches is proposed (Behera et al. 2019) as multilayer design can reduce the corrosion incidence (Baragetti et al. 2020) and composites have proven as a reliable material for pressurized components (Hastie et al. 2020). However, composite patches need to be optimized in the terms of their length, thickness and the orientation of the layers (Takacs et al. 2020) and their interaction and subsequent damage in contact with water