potable water. This represents about 7% of San Diego County’s usage, and diversifies the region’s water supply to a drought-proof and locally controlled supply. Reverse osmosis desalination has been widely used in other countries to solve water shortages. This design, however, requires a pipe material that can withstand the roughly 1000 lb/in.2 required to remove the salt with osmotic pressure through the semipermeable membrane but still allows water molecules to pass through. In addition, this pipe must be able to resist the corrosive properties of seawater and a highly concentrated brine at 7% salinity, all while maintaining a minimum design life of 30 years. In the past, super duplex was the piping material of choice; however, super duplex has its drawbacks, being much more unforgiving in weldability. It also poses a potential risk of lessening its mechanical properties, toughness, and corrosion resistance if the balance of austenite and ferrite in the welded joint is not maintained. The new choice of material is SMO 254 (UNS 31254), chosen for its ability to provide excellent resistance to corrosion (PREN>40) and pitting, f or its high impact toughness, resistance to chloride stress corrosion cracking, and excellent workability and weldability. Joint Design and Welding Procedure The process engineer dictated the welding parameters of the SMO 254 piping, with much of the procedure incorporating past lessons learned from years of desalination plants in operation as well as new research. The gas tungsten arc welding (GTAW) process was chosen for this welding. Although this required the most skill, it also provided the most control of heat input and of the welding variables. Typical joint design was a single V-groove butt joint, with parameters shown in Fig. 1. Pipe thicknesses ranged from Schedule 40S (0.300 in.) to 22 mm (0.866 in.). Figures 2, 3 show a typical fitup of the piping joint with bridge tacks used to hold the fitup at the root opening. Welding of fully austenitic stainless steels carries the potential risk of hotcracking in the weld metal, in particular if the weldment is under constraint. In addition, an SMO 254 weldment has low thermal conductivity and high thermal expansion. Thus, heat input into the weld and welding sequence must mitigate potential distortion during the welding process. The welding procedure specification (WPS) limited the heat input to 1.5 kJ/mm at the root pass, and 0.9 kJ/mm on the cold and fill passes. This required constant monitoring of temperature and speed of the welder, certainly a tedious task for a quality control program, but necessary considering the sensitivity of the material to heat input and the risk of weld failures. The amperage range was 80–125 A on the root, with 70–100 A on the cold and fill passes. Another element that affects heat input is the welding technique. The pipe manufacturer as well as the process engineer specified the use of a stringer bead on the welds. The stringer bead is not common to many pipefitter welders and required practice and training to ensure the correct technique was being applied within the required parameters specified by the WPS. Figure 10 shows a completed weld, which demonstrates the stringer technique used. SMO 254 does not usually require any preheat, provided ambient temperature is above 50°F. However, maximum interpass temperature was limited to 95°C (203°F) in the WPS, again to address the material’s sensitivity to heat input and cracking. Gas purging is critical when welding all types of stainless steel alloys to facilitate good weld pool fusion characteristics and protect the weld and heat-affected zone (HAZ) surfaces JUNE 2016 / WELDING JOURNAL 61 Joints (QW-402) Joint Design: Butt Backing or Backing Material: See Para Gas (QW-408) Root Spacing: See below Fig. 1 — Typical single V-groove butt joint used for SMO 254 piping at the Carlsbad Desalination Project. Fig. 2 — Typical fitup on an 8-in.- diameter, 11-mm pipe wall thickness SMO 254 piping at the Carlsbad Desalination Plant. Fig. 3 — Picture of a joint design and fitup on a 24-in.-diameter, 22-mm pipe wall thickness SMO 254 pipe at the Carlsbad Desalination Project. Bridge tacks were utilized to hold the root opening and fitup.
Welding Journal | June 2016
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