PSI - Issue 17

V. Chmelko et al. / Procedia Structural Integrity 17 (2019) 520–525 Chmelko, V., Bíro, D./ Structural Integrity Procedia 00 (2019) 000 – 000

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The using of standards for assessment of safety of pressure pipes with corrosion defects leads to conservative results. The current state of the art is that there is not yet a uniform and coherent methodology for more accurate assessment of destructive pressures of pipelines with corrosion defects. This article is dealt with a rate of conservativeness confronted with experimental results and computational simulations, which facilitate reliably evaluation of safety of pressure of pipes with corrosion defects.

2. Using of Standards

The most widely used standards for assessment of safety of piping systems with corrosion defects are ASME B31G a DNV-RP-F101. These standards determine the destructive pressure of a critical place of pipeline by simplifying the defect geometry defined by depth and length, the defect being oriented in the direction of the pipe axis. These standards consider a pure load applied as internal overpressure. The ASME B31G burst pressure calculation is based on comparison with the yield strength ( σ y ) of used material. This standard is used for steels of older types, i.e. more brittle and with higher ratio between yield and ultimate strength. Only the length and depth of the defect (Fig. 1), which are oriented in the longitudinal direction of the pipe axis are taken into the calculation. The standard considers the parabolic shape of the defect profile with 0.85 times of the depth of the actual defect. The maximum possible depth of defect is 0.8 times of the remaining wall thickness.

Fig. 1. Longitudinal (axially oriented) corrosion defect on the pipe

The burst pressure of pipe with defect defined by ASME B31.G (2009) is calculated by: = ( + 68.95 ) 2 ( 1 − 0.85 1 − 0. 8 5 ) where

(1)

= √1 + 0.6275 ( √ 1 ) 2 − 0.003375 ( √ 1 ) 4

(2)

Calculation of destructive pressure defined by DNV-RP-F101 is based on ultimate strength of material ( σ u ). Generally, this method predicts a higher destructive pressure than the previous one. The DNV method considers the rectangular defect profile with maximum length and depth of the real defect (Figs. 2). Using of this method is suitable for modern tough materials. The maximum possible depth of defect is 0.85 times of wall thickness.