from atmospheric contamination and oxidation. Backing gas utilized was a 98% argon/2% nitrogen blend to a purity of 0.001%. Inflatable purge gas bladders were used in the pipes to provide purge dams for the backing gas, to confine the pipeline inside purge volume to a more localized and controllable level versus purging whole pipeline sections, reducing the amount of backing gas used on the project and reducing overall cost. Shielding gas was also of the same composition, and flow rate was 20–35 ft3/min (9.34–16.4 L/min). Flow rate was controlled by the welder, and a calibrated oxygen sensor was utilized to analyze the oxygen content (<0.05% per WPS) within the argon purge gas environment. Backing gas flow rate was ranged 5–60 ft3/min (2.3–28.3 L/min). Purge gas was left in place on the weld until a weld thickness of 8 mm was achieved, at which time only shielding gas was required to finish the weld. EW-Th-2 thoriated tungsten with DC Straight Polarity (DCSP or DCEN) was used during the GTAW process with Avesta P12 Polarit (1.6–3.2 mm diameter) ER NiCrMo-3 as the filler metal. Figure 4 shows a welder performing the root pass on the SMO 254 pipe. Training and Union Craft Support San Diego is certainly home to many skilled workers, on account of the Navy shipyards, numerous power plants, and other heavy industry. But welding on Alloy SMO 254 was new to the local union craft force. This presented a potential challenge to the contractor as the welder qualification is a key part of a successful project. But several steps were taken to overcome this hurdle. Job managers and supervisors met with the local union to discuss the job requirements, with local union agreeing to utilize their local training center for training and testing on the SMO 254 coupons. The contractor sent a quality control manager to the facility to assist, ensure the highest standards were held, and provide instruction during the training process. Testing was performed on 3- in.-diameter, Schedule 160 (thickness 0.436 in. or 11 mm) SMO pipe coupons in the 6G position to meet the thickness requirements for the pipe being welded at the jobsite. 62 WELDING JOURNAL / JUNE 2016 Fig. 4 — Welder performing a root pass on 36-in.-diameter SMO 254 pipe utilizing the GTAW process at the Carlsbad Desalination Project. Fig. 5 — A 3-in.-diameter, Schedule 160 SMO 254 pipe coupon that was welded as part of training at the local union training center and used for training and qualification.
Welding Journal | June 2016
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