Orbital GTAW for Tube and Pipe Contribute to Record-Breaking Performance at the Angra II Nuclear Power Plant in Brazil

Orbital GTAW for Tube and Pipe Contribute to Record-Breaking Performance at the Angra II Nuclear Power Plant in Brazil

Orbital GTAW for tube and pipe contribute to record-breaking performance at the Angra II Nuclear Power Plant in Brazil.

Manufacturers of welding equipment often have difficulty compiling accurate data comparing orbital and manual gas-tungsten-arc welding (GTAW) under similar conditions. The recent installation of the Angra II Nuclear Power Plant near Rio de Janeiro in Brazil offered such an opportunity.

Although the 3,865 orbital welds comprised less than 2 percent of the project total of 200,000 welds, the increased productivity compared to manual welding was clearly demonstrated and well documented.

Initially there was some resistance to using orbital welding since manual welding procedures for all the materials had been qualified for the construction of Angra I, completed 20 years earlier.

Qualification of orbital welding procedures would take additional time and delay the start of production welding. However, UNAMON, seven Brazilian subcontracting companies joined to build nuclear power plants, had previous experience in the use of orbital welding. The consortium was eager to explore the use of the newer technology on this major installation of a Siemens KWU 1,300 Megawatt PWR.

Siemens, the German company supervising the installation, wanted manual welding used on the entire installation. UNAMON persisted and received permission by Siemens to use orbital welding on a limited basis, provided that all of the orbital welding procedures were qualified to DIN (German) Standards.

UNAMON called Arc Machines, Inc., Pacoima, CA. The firm supplied orbital welding equipment and did prequalification welds during the summer of 1966 for several types of material including Type 347 stainless (UNS S347000). This demonstrated that the orbital welds could meet the requirements for RC Grades 1 and 2, the highest level DIN certification.

The DIN Standard ranks the criticality of piping systems in Grades from 1 to 5 with Grade 1 being the most critical.

For example, utility piping for fresh water would be Grade 5 while the most critical systems, such as the Residual Heat Removal system (RHR) and the Safety Injection System (SIS), would be ranked Grade 1. The SIS removes heat from the reactor (RHR) or injects water into the reactor in case of overheating. The Instrumentation Tubing was mostly Grade 1 and 2, but included some tubing ranked Grade 3.

The most critical piping, Grades 1 and 2, require 100 percent radiography inspection. Grade 3 requires only 10 percent taken at random.

Manual welds were not required to meet such a high level of quality.

Selection of Best Sizes for Orbital Welding

Because UNAMON planned only a small percentage of orbital welds, it made preliminary tests in the autumn of 1996 to determine the most effective size of tubing and pipe to join with orbital welding.

UNAMON took ten of their best manual welders and divided them into 4 groups corresponding to the pipe sizes (see Table 1). Each manual welder did 100 welds. The tests compared the time to complete each weld and the reject rate of the best welder in each group to productivity achieved during the weld qualification procedures for orbital welding.

The tests showed manual welding reasonably efficient for the larger diameters and wall thicknesses of Groups III and IV and orbital welding significantly more efficient on the smaller sizes of Groups I and II.

Based on these test results, UNAMON chose orbital welding for all of the sizes represented by Groups I and II, including the instrumentation tubing. In addition, welding the root pass manually and using the orbital welding equipment for the subsequent fill passes resulted in a significant time saving on some of the carbon steel pipe. The weld joints on the carbon steel pipe had been pre-machined for manual welding with a "V" groove and would have required re-prepping for orbital welding.

Separate groups of welders worked on the root and fill passes for the carbon steel pipe, with manual welders doing only the roots and orbital welders doing only the fill passes. The 8 and 12 in. carbon steel pipe for the fuel lines for the emergency diesel generator in the turbine building were prepped with a modified "J" prep for orbital welding.

Table 1. Tube and Pipe Sizes

One of the 11 orbital welding operators welds a carbon steel pipe in the pipe shop with an Arc Machines' Model 79-6625 open-frame weld head. Only certified nuclear-qualified welders wore the red uniforms.

Pipe Shop

Orbital welding productivity peaks with consecutive welds of a large number of similar joints. UNAMON set up a pipe shop near the Angra II site to perform qualification welds and to prefabricate assemblies prior to installation at the site. The shop achieved assembly line-style production of carbon steel pipe-to-flanges and pipe-to-elbow weld joints by tack-welding the joints in place and arranging the assemblies in rows for welding.

For stainless steel, the pipe shop provided a controlled environment with a clean work area to plan the job and efficiently lay it out.

In-place orbital welding - the electrode moves circumferentially around the pipe while the pipe remains stationary - was easier in the pipe shop than rotating the pipe with a stationary electrode. (Orbital welding in the field is always done in-place.) Initially all of the orbital welding equipment was used exclusively in the pipe shop with no field welds being made until October 1997.

Pipe shop welders used three Arc Machines Model 15 full-function weld heads and three Model 79 open frame weld heads. Both head models have wire feed, electronic arc gap control, and torch oscillation for wire-feed applications. Three Model 227 pipe welding power supplies feed the weld heads. An autogenous welding system consisting of an Arc Machines Model 207 and a Model 9-1500 fusion weld head were used in the pipe shop for operator training and weld qualification for the instrumentation tubing. Welders completed 1,669 orbital welds.

After completing the pipe-shop welding, the contractors moved the orbital welding equipment to the job site. They relocated all of the orbital equipment by the last 5 months of the job that ended in June 1999.

Critical Stainless Steel Pipe Welds

Difficulties with manual welding of the 44 welds on Type 347 stainless steel pipe for the primary circuit, the most critical piping system on the site, brought the job to a virtual standstill between February of 1997 and late September to early October of 1997, a delay of nearly seven months.

Grinding of the manual SMAW welds revealed surface microcracking with a reject rate of 20 percent. The repair took 2-3 hours and often required repeating.

The high reject rate on Type 347 stainless steel with the shielded-metal arc welding (SMAW) electrode process prompted an evaluation of orbital GTAW for this application. The orbital welds had a flatter, more uniform crown and required very little grinding. The GTAW process eliminated the microcracking, possibly because of the more uniform controlled heat input compared to manual SMAW on this niobium containing alloy.

However, although the prequalification orbital welds on this material had been good, there were delays in obtaining a fully qualified procedure. The original weld certification had been done with pure argon gas. After several months on the job, UNAMON received permission to change to an argon-hydrogen mixture to increase penetration at the same welding current. Shielding gas type is an essential variable. Changing it requires requalification of the weld procedure. To prevent "suckback," concavity of the ID weld bead required an extension to the land on the "Modified J" pipe end-preparation. This also increased the time to obtain the qualified procedure.

A greater delay in obtaining a qualified orbital procedure resulted from difficulty in finding a certified source of filler wire since certification of all materials is considered essential for nuclear quality welding. The filler material used for the prequalification welds came from a United States supplier that was not certified by TIOV, the European standards group. After several months UNAMON found a suitable source of wire in Europe.

Thus, orbital welding on the site did not begin until October of 1997, more than one year after the start of manual welding.

UNAMON welders use an Arc Machines' Model 207 power supply with a Model 207-CW water cooling unit for cooling the weld head and cables and a Model 9-1500 fusion weld head to weld stainless steel instrumentation tubing at the job site.

Orbital Welding to the Rescue

Once orbital welding began, the 7-8 man orbital crew worked two shifts a day for a total of 24 hours a day for several months. The crew attempted to recover time lost in delays caused by rejects and repairs of the manual welds on the primary circuit.

Orbital welding proved successful in critical situations that would have been difficult, if not impossible to do successfully with manual welding.

UNAMON also used the orbital equipment when a bottleneck developed in the manual welding to bring the manual welding schedule up to speed. The total of number of orbital field welds was 2,196 joints.

Recording of the average daily DI of orbital welds and manual welds clearly demonstrated the superior productivity of orbital welding compared to manual welding. DI, or diameter inch, was the unit used to measure productivity on the job. One diameter inch can be the sum of 10 welds on one inch (nominal diameter) pipe or one weld on a 10 inch (nominal diameter) pipe. (For nominal pipe OD in mm divide by 25 to convert to inches.)

To calculate the average daily DI per welder add the total diameter inches for all of the welders for one week, divide the total by the number of welders working, and divide that number by the number of days worked in a week.

Note: Total Pipe Shop and Total Field values include data from both orbital and manual welds.

DI Data Shows High Productivity of Orbital Welds

In the pipe shop the combined DI for orbital and manual welding for weeks' 74-111 was 11.93 per welder per day. The combined DI was the unit used by UNAMON to invoice the job.

Separate data was obtained for orbital welding, but not for manual welding. However, the pipe shop DI for orbital welding averaged 50.6 per day during this time while that for manual welding was calculated to be about 3 to 4.5 at the most.

In Germany, an average DI of between 7 and 10 is the norm for manual welding.

Thus the DI for orbital welding in the pipe shop showed an extraordinary increase in productivity compared to manual welding. In the field, the average combined daily DI for all welds was 4.13. The DI for orbital welding was 11, exceeding UNAMON's goal of 10 for the job. The manual welding in both the pipe shop and the field fell short of this goal. The DI for orbital welding repairs on failed manual welds on the critical piping was 6.5 for weeks 112 to 151, better than the combined field DI.

The Repair Index, RI, used as a measure of weld defects, was lower for orbital welds in all cases with separate data. Since most of the carbon steel welds had been done with manual roots and orbital fill passes, there was no way to determine if a defect was on the manual or orbital part of the weld. For the 537 autogenous orbital welds on stainless steel instrumentation tubing, the RI was zero (0.0), requiring no repairs.

At the Angra II Nuclear Power Plant project, UNAMON welders use an Arc Machines' Model 15 full-function pipe weld head with the Model 227 Power supply to weld Type 347 stainless steel pipe for the critical Safety Injection System piping.

Conclusions

Although the job was behind schedule in October of 1997, when orbital welding began, the job was finished on time. According to Deputy Site Manager for UNAMON, Ronaldo Pavao Viera, the installation at Angra II set a world record for the speed of a mechanical and electromechanical erection.

While only a small part of the total job, orbital welding clearly demonstrated its potential for improving joining technology in nuclear power plant piping installations.

Orbital welding was advantageous to manual welding on this site in productivity and quality. This was true not only in the pipe shop under controlled conditions, but also in the field where the conditions were less predictable.

However, UNAMON's welding manager, Helsio Bravo Mosciaro, said that he miscalculated the effort required in switching from manual to orbital welding technology. He likened the switch to a cultural change. People are resistant to change even when there is clear evidence that the change would be for the better.

Angra II is expected to make a significant contribution to the electric power industry in Brazil. At this time less than 2 percent of the total power in Brazil is nuclear, with most power supplied by hydroelectric sources and the balance from a combination of gas and fuel oil.

Future nuclear power plants in Brazil are in the planning stages. The well-documented success of orbital welding on Angra II should help to pave the way for the use of orbital welding technology on future nuclear power plant construction projects.


Reprinted from Welding Design & Fabrication, September, 2002. The article was edited from a paper by Barbara K. Henon, Ph.D., and Eng. Angel Brond of Arc Machines, Inc., and Eng. Hélsio Mosciaro, UNAMON Consórcio de Montagem Nuclear, presented at FabTech International, November, 2001.