Evaluation of Welding Techniques for Stainless Steels Piping Without Use of Backing Gas

Abstract Austenitic Stainless steel SS304L and SS316L are extensively used as piping material for cryogenic and corrosive services in an LNG plant. GTAW welding process is a widely used welding process for joining stainless steel pipe work in pipe fabrication yards. For pipe welding, the GTAW process uses inert backing gas or purge gas (e.g. Argon) to prevent oxidation of root pass in order to achieve the weld quality. Welding large diameter stainless steel pipes using GTAW with inert gas introduces significant risk of asphyxiation when a welder enters the pipe that has resulted in fatalities based on industry references. A large LNG project mandated that only welding techniques without backing gas were permitted for joining stainless steel pipes of size > 24”. The project team evaluated four alternative welding techniques with no backing gas and Advanced or Modified Short-arc GMAW was selected. Initial scope covered cryogenic services and later extended to include corrosive / wet (aqueous) services. This paper discusses the approach the project chose including the literature evaluation from the industry experiences, non-backing gas welding qualification and test protocols, curating technical specification, PQR testing and developing WPS’s and welder training and qualification program at the module fabrication yards. The paper also presents the knowledge, experiences gained, and challenges faced in implementing the non-backing gas welding for joining large size stainless steel pipe work in an extensive scale spanning multiple piping module fabrication yards globally.