[ safety ]
Protection Association , which will be used as a starting point for this article . However , additional equipment is recommended for hydrogenspecific applications . 3 The main components of a hydrogen gas train system are moisture and particle filtration , flow and pressure regulation , emergency shutoff and venting , as well as purging for maintenance . Equipment physically surrounding the gas train will also be considered , including hydrogen gas detectors and flame detectors to identify leaks and hazardous areas .
Material selection
Particularly important to ensuring a robust storage vessel is mitigating the effects of the five types of hydrogen-induced damage : hydrogen-induced blistering , cracking from the precipitation of internal hydrogen , hydrogen attack , cracking from hydride formation , and hydrogen embrittlement . Each of these begins when gaseous hydrogen dissociates on a metal surface , and the atomic ( nascent ) hydrogen can be absorbed into the metal structure , at which point various types of hydrogen-induced damage can occur . The first four types have been extensively studied and are concerned with the influence of a phase transformation ( precipitation of a hydrogen gas bubble , production of an insoluble gaseous product , or the precipitation of a hydride ) on the metal ’ s mechanical properties . However , hydrogen embrittlement is less understood .
A generalized process of delayed failure , hydrogen embrittlement describes absorbed hydrogen migrating to dilated locations randomly distributed in the metal lattice . When stress is applied ( with the pressure vessel in service ), hydrogen localizes in some dilated regions , increasing its concentration . When the hydrogen concentration reaches a critical level , a crack will nucleate and propagate , moving the lattice dilation region to a new zone . Hydrogen will then relocate to the newly dilated zone and the crack nucleation / propagation process will repeat until the crack reaches a critical size and the component fractures . 4
To mitigate this effect , austenitic ( 300 series ) stainless steels are recommended , as they are relatively immune to high-pressure hydrogen embrittlement , with 316 being the preferred grade . Carbon and alloy steels can also be used for low-pressure hydrogen vessels (< 20 MPa working pressure ) above 29 ° C . However , grey , ductile , or cast irons and plastic should not be used for storing or transporting gaseous hydrogen . 5 , 6 A complete list of acceptable materials for hydrogen service can be found in the American Society for Mechanical Engineers ( ASME ) code B31.12 . The valves and instruments selected for use in the gas train should match the material selection for the piping .
Within a hydrogen gas train , the only other chemical compounds that could be present are an inert gas such as nitrogen ( often used for purging purposes ) and / or other residual constituents in trace amounts that arise from the production of hydrogen ( e . g ., water vapor , oxygen , carbon dioxide ). These constituents depend on the specific production process . In a hydrogen-rich gas ( i . e ., > 98 vol % H 2
), the presence of other trace compounds should not affect the material selection for the gas train and associated components . Nonetheless , a thorough analysis of the chemical composition of the hydrogen gas should be performed to ensure material selection compatibility . The operation of an inert gas purging system will be discussed later in this article .
Operating conditions and instrumentation
The gas train inlet arrangement is designed to ensure that the supplied hydrogen is free from any contamination originating from the gas source or the upstream pipeline that could interfere with the burner system ’ s operation . This begins with a manual inlet isolation valve that is
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