PSI - Issue 13

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

211

2

1. Introduction Hydrogen is foreseen as an alternative energy carrier as it’s considered to be a clean energy source compared to oil and gas. Currently, hydrogen is becoming widely used in several industrial applications (petroleum refineries, fuel cells, power plants, etc.). Nevertheless, the use of hydrogen, especially as a gas, is always accompanied by several risks that have to be well predicted in advance (overpressure, leak, explosion, etc.). Hydrogen embrittlement of material is a major concern for scientists and gas installation designers to avoid process failures. In fact, the small size of the hydrogen atom enhances its diffusion through the lattice structure of the material (Barrera, O. et al., 2016). The latter is likely to result in hydrogen leakage and thus the escaped highly flammable gas may lead to an explosive mixture depending on its pressure and the temperature of the surrounding air. Hydrogen embrittlement of material has been extensively studied throughout the last two decades in power plant installations, nuclear reactors, as well as fuel cells (Grabke, H.J. and Riecke, E., 2000; Dayal, R.K. and Parvathavarthini, N., 2003; Brass, A.M. et al., 2008; Bhadeshiaa, H., 2016; Cendales et al., 2016; Chatzidouros et al., 2018). Nevertheless, failure of gas pipelines installations due to hydrogen embrittlement is still not well investigated. Actually, such problem arises once using existing natural gas pipelines to transport hydrogen gas in large scale infrastructures. The alternative of using natural gas pipelines for hydrogen transportation was recently adopted as an economic and efficient choice (Tabkhi, F. et al., 2008). Hence, this work is devoted to numerically study the diffusion phenomena of hydrogen gas into the internal wall of a pipeline designed for natural gas conduction. The diffusion is enhanced by the internal pressure that varies during transients. The numerical model reposes on solving gas motions equations coupled with Fick’s second law describing unsteady diffusion equation. Results showing the amount of hydrogen diffused through the pipeline wall are presented for API X52 steel (both nitrided and not treated) and API X80 steel, commonly used to replace old X52 pipelines. 2. Governing equations 2.1. Hydrogen Gas flow in steel pipelines Isothermal one-dimensional gas flow through an horizontal pipeline of axis z is described by a set of two hyperbolic partial differential equations viz. mass and momentum equations (Norazlina, S., Norsarahaida, A., 2015).

0   z V  

t  

(1)

2

  

  

z V p    

D V V



t V

 

(2)



 

2

where p is the pressure of the gas, V is the gas axial velocity,  is the gas density, D is the pipe internal diameter and  is the friction factor. 2.2. Nonsteady state Hydrogen diffusion in steel microstructure: Fick’s second law By neglecting the trapping phenomenon of hydrogen in the interstitial lattice sites, non-steady mass transfer of hydrogen atoms through the metal is described by the general diffusion equation given by Fick’s second law

t C 





 0

(3)

.     D C

where C is the molar concentration of the diffuser and D stands for the diffusion coefficient. Under a pressure p (in bar) and a temperature T (in Klevin), the solubility of hydrogen gas in steel is expressed through an Arrhenius type law written as (Elaoud, S., and Hadj Taieb, E., 2011)