PSI - Issue 13

Zahreddine Hafsi et al. / Procedia Structural Integrity 13 (2018) 210–217 Hafsi et al. / Structural Integrity Procedia 00 (2018) 000–000

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After establishment of a steady state regime, transient is created through periodic variation of the gas demand in the downstream side over 24 hours as depicted in Fig.2.

Fig. 2. Varying hydrogen consumption at the outlet of the pipeline

Starting from the steady state condition of hydrogen flow in the pipeline as shown by Fig.3.a and due to the transient evolution of the gas demand (Fig.2), the obtained pressure variation with time in the outlet node of the pipe is illustrated by Fig.3.b. A periodic oscillatory pressure evolution is observed. The oscillation of the pressure in the outlet is likely to enhance the hydrogen diffusion through the lattice structure of the steel pipeline wall.

Fig. 3. (a) Initial pressure distribution; (b) transient pressure evolution

3.2. Hydrogen gas diffusion model: main results and discussion Considering obtained internal pressure in the outlet side of the pipeline, diffusion of hydrogen gas through the wall is studied by following up the evolution of hydrogen concentration in the lattice structure. Three cases of steel materials are considered viz. unnitrided API X52 steel (base metal), nitrided API X52 steel and API X80 steel (base metal). The diffusion of hydrogen is assumed to be one directional (along the radial direction r ). The 3D geometry is simplified to take into account only hydrogen embrittlement phenomenon in the outlet side of the pipe (in x=L ) as the effect of the transient is firstly detected at this section and it’s the most critical zone in which the overpressure is maximum. Actually, the amplitude of the pressure wave will be dumped progressively travelling from the downstream to the upstream side of the pipe. Hence, a 2D model is considered to study hydrogen diffusion in the outlet side of the pipeline. The 3D geometry of a 1 m length of the downstream part of the pipe (longitudinal cut view) as well as simplified 2D geometry of the outlet section of the pipe along with the boundary conditions of hydrogen concentration in the inner and the outer surfaces are reported in Fig.4.