Orbital Welding Delivers Treatment Plant on Time

Orbital Welding Delivers Treatment Plant on Time

Switching to automated orbital welding helped a major water treatment project stay on schedule in spite of a 30% increase in scope.

Switching from shielded metal arc welding (SMAW) to orbital welding was a key factor in meeting original project delivery deadlines after a 30% increase in the scope of a water treatment facility.

The $100 million Scottsdale, Ariz., Water Campus project is considered the largest ever in the United States, and it represents a departure from traditional treatment methods in the western United States. This project will pump a large portion of the treated water into the ground to recharge an aquifer rather than use it for irrigation.

New Approach to Welding

Fig. 1 - The contractor conducted a series of tests before orbital welding was approved for the project.

While orbital welding is frequently used in the semiconductor and pharmaceutical industries, this represented one of its first uses in construction of a water treatment plant. This automated method increased productivity from 3 to 4 welds per day to 13 to 16 on the 6- to 8-in. pipe used for air distribution to the microfiltration system.

Meeting Water Needs

Designed by Black & Veatch, the Water Campus is the largest U.S. facility for treatment of raw sewage to potable-quality water. In the summer months, this plant is projected to treat 12 million gallons per day of wastewater conventionally for golf course irrigation. In the winter, when irrigation requirements are lower, 10 million gallons per day of the water treated by the plant will receive advanced treatment, microfiltration and reverse osmosis to exceed potable standards. This purified water will be reinjected into local groundwater aquifers. Once completed, the Water Campus is designed to help ensure the city of Scottsdale's supply of drinking water for the next 100 years and to comply with Arizona's 1980 Groundwater Management Act, which prohibits communities from extracting more groundwater than is naturally or artificially recharged.

Process Flow at Treatment Plant

At the purifying facility, wastewater undergoes primary, secondary and tertiary treatments and advanced water treatments. The primary and secondary treatments are sedimentation basins that remove larger particles from the waste stream. The tertiary stage is a sand filter that removes finer particles. Chlorination is also performed at several stages of the treatment process. The final stage is reverse osmosis through a semipermeable membrane that raises the water to drinking quality standards. A portion of the water will then be used to recharge the aquifer in the Carefree/Cave Creek Basin. Without this recharge, concerns have been raised that with the present rate of usage this aquifer could run dry by the year 2007.

The piping contractor on the project was University Mechanical, a mechanical contractor that specializes in the installation of sophisticated process piping. High-purity piping is used in the facility for gases in the laboratory and the air distribution system that performs backwashing operations on the microfiltration system.

Laboratory Gas System

The laboratory gas distribution system involves a network of piping with many complicated twists and turns, providing a high potential for leaks. Since most of the lines are concealed in ceilings, it would be very difficult to identify and repair leaks in the fittings. It was suggested to the owner that orbital welding be used to create a completely seamless system. He purchased a Model 9-750 orbital welding head from Arc Machines, Inc., Pacoima, Calif., and used it mostly in controlled off-site conditions. When welding was completed, testing showed there were no leaks in the laboratory gas network.

The air distribution system represented one of the most challenging parts of the entire project. It consists of 6- to 8-in. headers used to reverse the flow through the system in order to remove particles from the microfiltration filters. Using traditional shielded metal arc welding would have been a very labor-intensive task and one that would have been challenging from a quality standpoint. While the company had a team of experienced welders, their productivity was limited by the tools they used. As this phase of the project neared, it was realized the scope increases would make it impossible to meet the project deadline using the methods that had originally been envisioned. The owner began investigating alternative technologies in an effort to keep the project on track.

Increasing Productivity

Fig. 2 - AMI Model 79 obital weld head that uses an adjustable clamp on the pipe.

"We have used orbital welding for a number of years on projects for semiconductor and pharmaceutical manufacturers in order to make identical highquality welds in small bore piping such as that used in the lab gas system," said Tom Gusich, project manager for University Mechanical. "But we had never tried it on anything with this large a bore in the past." A series of tests (Fig. 1) revealed each head and machine outperformed the company's best welders by a factor of 3 or 4. "The tradespeople learned the new technology very quickly and did a great job. Each welder was able to produce an average of 15 welds per day on the large bore welds in the air distribution system. Three weeks were saved on the overall schedule, and the quality of the welds on the larger bore pipes was as good as previously seen in smaller bores. As with the lab gas system, there wasn't a single leak in the air distribution system," said Gusich.

Orbital Welding Basics

Orbital welding is an automated version of the gas tungsten arc welding (GTAW) process. In manual GTAW, the welder moves the welding torch and controls the welding current. In orbital GTAW, the tungsten is installed in a weld head that clamps on the tube or pipe - Fig. 2. The tube remains in place while the weld head rotor revolves around the weld joint circumference to complete the weld. Welding is done in an inert atmosphere to protect the metal from oxidation as it is heated to melting temperature.

The welding power supply controls weld parameters such as welding current, primary and background amperes, travel speed, wire feed speed, weld bead overlap, delay of rotation at the start of the weld and current downslope at the end of the weld. The enclosed weld heads used for small-diameter tubing welds provide a chamber filled with inert gas that encloses the entire joint during the weld. A timed prepurge and postpurge are usually used to time the flow of inert gas into the weld head before arc initiation and to continue the purge for a timed period after the arc has been extinguished. This allows the weld tool to cool sufficiently to prevent oxidation before the weld head is opened to remove the welded tube.

Welding Process Steps

Model 15 uses a track and Model 79 uses an adjustable type of clamp on the pipe during welding. The welder starts the operation by aligning the tungsten to the bevel in the joint, then adjusts the wire feed and centers the oscillation with the actual weld. Tabs on the welding machine hold the cable out of the way as the welding head wraps around the bore to complete the pass in about four minutes on a 7-in.-diameter pipe. The welder then removes the head. The only cleanup that is required is brushing the weld to remove any excess discoloration. The orbital welding systems used in this project provided a display for the operator to view the welding operation along with a screen that shows welding current and other programmed functions.

Will Use It Again

Orbital welding was so successful on this project that the contractor plans to use the technology in water supply and wastewater treatment projects on a regular basis. With the addition of new processes and new chemical additives, piping installation in wastewater plants is continually getting more challenging. "We intend to respond by increasing the use of orbital welding to provide the same productivity gains that we have already achieved in the semiconductor and pharmaceutical industries," said Gusich.

Reprinted from the Welding Journal, March, 2001

By Jerry Fireman, Structured Information, Arc Machines, Inc.