September 29, 2009
Working on a project at the world's largest nuclear waste treatment plant, Bechtel National found that it could simplify the operation, enhance weld quality, and promote safety by using inverter technology.
Nuclear warfare and the development of ultra destructive weapons played a major role in World War II and the Cold War. Although these wars are over, the legacy left by the specter of nuclear war still threatens U.S. citizens. Millions of gallons of radioactive waste remain buried beneath the ground at the Hanford Nuclear Reservation near the Tri-Cities area of south-central Washington state. Nearly one-third of the underground tanks containing this waste have leaked, threatening the ecosystem of the mighty Columbia River and hampering this region's effort to grow out of the shadow of its nuclear past.
In July 2002 Bechtel National Inc., an engineering, construction, and project management company, and Washington Group International began major construction on the Department of Energy's (DOE) Office of River Protection Waste Treatment Plant (WTP) Project. This is the world's largest nuclear waste treatment plant. By the time it was finished, more than 985,000 ft., nearly 187 miles, of pipe were welded using hundreds of gas tungsten arc welding and shielded metal arc welding inverters.
During the nuclear power plant work of the 1960s and '70s, the preferred welding machines were eight-arc, multioperator systems that weighed about 4,500 lbs. While very rugged (which is why these system remains popular today), such a heavy system is relatively immobile. To reach work sites hundreds of feet away, up to 4,000 ft. of welding lead (cable) per system would be strung out over the floor or the ground or hung overhead. This increased safety hazards related to tripping, as well as increased the potential for stress and strain injuries from carrying thousands of pounds of cable. It also increased upfront purchase costs, and it may have hampered productivity.
After consulting with the Plumbers and Steamfitters Local 598 from Pasco, Wash., which supplied the pipefitter labor for the WTP Project, Bechtel selected Miller Maxstar® 200 welding inverters for the job. The lightweight, inverter-based machines helped the company fulfill a primary value: safety.
Bechtel has earned an industry-leading record of achieving zero lost-time incidents on 90 percent of its projects worldwide, representing more than 100 million work hours per year. Year after year its safety performance is right at the top of its industry. Recently in one calendar year Bechtel U.S. had 0.16 -lost workday incidents per 100 workers, compared to a U.S. industry average that is historically near 4.0 incidents.
The purpose of the WTP Project was to solidify nuclear waste through a process known as vitrification. This mixes nuclear material with molten glass and creates a solid glass product that, when placed in stainless steel canisters and containers, isolates the waste from the environment (visit www.waste2glass.com for more details).
The vitrification process requires miles of pipe that must safely contain hazardous materials. To weld to very stringent nuclear-quality level No. 1 standards, Local 598 intensified its training program so that it could provide hundreds of qualified journeymen and apprentices.
The inverters selected for the job each weigh 37 lbs.; produce 1 to 200 amps of DC welding power (175 amps at 60 percent duty cycle); and have a built-in, high-frequency (HF) GTAW arc-starting function. They can connect to single- or three-phase, 50- or 60-Hz, 120- through 460-V primary power. The inverters continue to work through voltage spikes and dips, and draw primary amperage of 5.2 amps on a 460-V, three-phase line.
Because the inverters are lightweight and portable, Bechtel eliminated the need to purchase, transport, and manage tens of thousands of feet of welding lead.
Less mobile welding systems often need 300 to 600 ft. of welding cable per arc (round-trip distance) to guarantee accessibility to distant work locations. For a heavy, eight-arc system, this could easily add up to 4,000 ft. of cable. As shown in Figure 1, 4,000 ft. of cable could weigh more than 3,200 lbs. and cost more than $8,000 to purchase.
A typical inverter system at the WTP Project included a heavy-duty extension cord, SMAW electrode holder, and work cable (2/0 diameter) that were each 15 or 50 ft. long, as well as a GTAW torch with a 12- or 25-ft. cable. Supplemental lengths of welding lead were rarely needed (see Figure 2).
"Running lead is nonproductive. We want welders making welds, not running lead,"said Terry George, WTP Project labor relations manager for Bechtel National. "And welding lead is heavy. Anytime you are dragging, pulling, or pushing extra welding lead, you increase the opportunity to strain, sprain, and twist the body."
Back pain is one of the most common musculoskeletal injuries American workers sustain today, according to OSHA's Advisory Committee on Construction Occupational Safety and Health. Many of these injuries are caused by improperly lifting heavy objects and can lead to costly downtime and workers' compensation claims.
As with any system, removing complexity eliminates potential sources of error, and the same is true with cable clutter. To keep cable off walkways as much as possible, operators put up hangers just for that purpose. Over a couple of weeks, hundreds or thousands of feet of unused lead gets left on the hangers, and the company ends up ordering even more lead.
By lifting the burden of carrying excess cable off the shoulders and backs of operators, Bechtel reduced the chances of lost-time accidents related to lower back strain or tripping.
Welding codes in critical applications demand that every single weld pass visual and ultrasonic (UT) or X-ray inspection (see Figure 3). With heavy, immobile welding machines, operators sometimes make do with one weld parameter setting rather than walking back to the machine to fine-tune the arc. The proximity of welder to machine made possible by the inverters made it easier for welders to change settings and processes, for example, from GTAW to SMAW (Figure 4).
George summed up Bechtel's philosophy behind using lightweight inverters on large job sites: "Inverters put our pipe welders and fitters closer to the machine controls, so they can control their own destiny. Inverters make the workplace safer, easier, and more productive."