Orbital GTA Welding Used to Replace Tubing at Generating Plant
Orbital GTA Welding Used to Replace Tubing at Generating Plant
Each year, TransAlta Utilities Corp. (TAU) of Calgary, Alberta, Canada, shuts down one of the six coal-fired units at its Sundance Generating Station for a major turbine inspection/overhaul. The six units at the station, located 70 kilometers west of Edmonton, Alberta, have a total capacity of 2100 MW. This extended outage also provides the opportunity to undertake major boiler repairs. This year, the 342 superheater bimetallic welds were scheduled for replacement. These welds, which join the stainless steel internal elements with the chromium-molybdenum external elements, had reached the end of their useful life.
Bimetallic Weld Replacement
Fig. 1 - A weld head positioned on a superheater header tube in preparation for making a weld. When the weld head is mounted on the center tube as shown, clearance is just sufficient for locating the weld head. A finished weld can be seen in the foreground.
Fig. 2 - An orbital welding machine operator uses a program operator pendant to adjust the position of the weld head in preparation for welding the superheater header tube. Note the finished chromium-molydenum-to-chromium-molybdenum welds and the layered arrangement of the tubing entering the superheater header
The superheater design conditions are 2475 psig at 1005° F. The bimetallic welds absorb thermal stresses attributed to varying expansion coefficients between dissimilar materials, in addition to normal pressure and temperature considerations. These stresses are accumulated due to periodic unit shutdowns for maintenance, resulting in a creep life weld limitation of nominal 100,000 operating hours. EPRI (The Electrical Power Research Institute) has carried out a number of studies into this problem, and has estimated that the first weld failure could be expected at an accumulation of 75,000 operating hours.The failures typically have been shown to propagate internally from the heat-affected zone (HAZ). Unit One at Sundance is now 20 years old, and running with an availability factor of 88%. It has accumulated more than 150,000 operating hours, and numerous bimetallic welds had already been repaired manually. It was clearly the time for replacement.
Floyd Mulligan, plant manager at the station, was looking for welding quality that would extend the life of the tube replacements. The plant had experienced several bimetallic weld failures in the past and these failures typically cost the company $50,000 per day in lost revenue. Floyd wanted these replacements to allow the plant to operate with high reliability for up to an additional 200,000 hours. A new joint design was utilized for the bimetallic weld.
The new design used Inconel1 as filler, instead of the original E 309 welding rod, and featured an altered weld profile with a wide cap. This design has shown a fourfold increase in the life of the weld on laboratory creep tests. Originally done as field welds, the replacement bimetallic welds for this project were shop fabricated off-site and TransAlta believes they won’t require replacement during the remaining life of the plant. The shop-fabricated assemblies were four feet long, comprising two feet each of stainless steel and chromium-molybdenum tubing, and one central bimetallic weld. The work scheduled for the unit overhaul consisted of removing the existing bimetallic welds (and adjacent tubing) and installing the replacement bimetallic weld assemblies.
Orbital Pipe Welding
Traditionally, this work was carried out by several skilled craftsmen. The welding portion of the work, stainless steel (304-H) to stainless steel, and chrome-moly (T22) to chrome-moly, would require 20 to 26 qualified gas tungsten arc welders. On a similar project the previous year, the contractor working for TAU was unable to hire enough skilled workers to perform the work, and TransAlta was forced to recruit additional resources of manpower and orbital welding equipment, manufactured by Arc Machines, Inc. of Pacoima, Calif. During that project, the potential of the orbital welding equipment was recognized by Steve Thomas, TAU senior engineering technologist.
TransAlta wanted to use the orbital welding equipment for the bimetallic weld replacement on the Sundance Unit #1. Working with Asea Brown Boveri, Inc. (ABB Combustion Services), planning was done to incorporate the use of the orbital welding equipment into the project. TAU selected ABB partly because of its considerable experience with the use of orbital welding equipment at other major utilities throughout Canada.
Personnel Selection and Training
For the job at the Sundance Generating Station, a reliable source of trained manpower was again needed. In the Canadian power industry, major maintenance overhauls are scheduled for the summer months when the demand for power is less. During this time of year there is usually a shortage of manpower. ABB Combustion Services, at the time of the Sundance Unit #1 project, did not have a large enough staff of trained machine operators, and there was no available pool of trained operators. TAU, working closely with ABB, invested in the training of eight welders employed with ABB on a regular basis from the International Brotherhood of Boilermakers, Local 146. Local 146 is ABB’s manpower supplier in the Alberta area for boiler maintenance. The local was very interested in orbital welding technology and felt that obtaining training for its members would give them a higher-technology profile. The training took place at ABB’s Edmonton office. Frank York, pipe welding product manager and welding specialist from Arc Machines, spent two 32 hour sessions, training four welders on the equipment in each session. Upon completion of their training, each welder had 64 hours of experience working with the equipment. Half of this time was spent on welding and half on performing joint preparation, all of which was under the supervision of ABB’s senior orbital welding technician, Steve Chambers.
The Scope of Work
The scope of work was carefully planned with a detailed estimate of the actual man-days required for each phase of the job. Daily goals were set for tube dressing, setup, fitting, welding the roots, x-ray, etc. The actual job was begun on August 2 and scheduled for completion on September 16. Everything possible was done to assure that the job would be done efficiently, smoothly and on schedule. TransAlta provided tools and essential equipment, including facing equipment to prepare the tubing ends for orbital welding.
The human factor was considered as well. In order to minimize the time that the men were away from the job, special air-conditioned lunchroom and washroom facilities were constructed on the seventh floor of the boiler building at the same level as the superheater header. Since the only elevator had a limited capacity and was rather slow, these specially built rooms contributed to the efficiency of the project.
A total of 684 welds had to be made in three weeks. The header tubes are arranged in 114 rows stacked three tubes deep with about 2 in. of radial clearance provided between the tubes. All tubing to be welded was 2 in. OD. The chrome-moly tubing had a wall thickness of 0.460 in., while the stainless tubing had a wall of 0.240 in. Before welding could begin, ABB’s welding engineering technologist, Larrie Hermans, had to develop a qualified welding procedure that conformed to Sections I and IX of the ASME Boiler and Pressure Vessel Code. Weld Procedure Specifications (WPS), and Procedure and Performance Qualification Records (PQRs) were required to certify that both the process and the welding operators satisfied the requirements of the code.
The work was carefully planned by ABB and TAU. Steve Thomas (TAU), Brad Herczeg, ABB maintenance manager, and Spencer Allen, ABB site supervisor, worked to generate a viable plan that gave a realistic picture of the actual man-days needed to perform the job.
Fig. 3 - The welding operator controls the welding operation from the pendant while the weld head, which is positioned on the bottom row of the superheater header tubing, executes the weld.
Fig. 4 - Partially completed chromium-molybdenum-to-chromium-molybdenum welds on superheater header tubes. The root pass and first fill pass are done first and then must pass radiography inspection before the final fill and cap passes can be done.
Fig. 5 - Finished welds show three-deep arrangement of superheater heading tubing as they enter the header at left.
Three Model 215 full-function microprocessor-controlled pipe welding power supplies were used on this project - Fig. 1. The qualified weld schedules listing the weld parameters for travel speed, arc voltage control, welding currents, oscillation and wire feed speed were entered into the unit via the program operator pendant and stored in memory - Fig. 2. During the weld, slight steering of the torch was accomplished using the smaller auxiliary operating pendant. Four Model 81 water-cooled pipe welding heads were present on the site; one was kept to be used as a spare. The M-81 had a 1-1/2-in. extension for the end of the torch, which provided better visibility of the joint during welding and protected the weld head from excessive heat. The working space was very tight with a less than 2-in. clearance between the tubes - Fig. 3. In a few places, the tubes had to be slightly spread apart to accommodate the weld head, which has a nominal radial clearance of 1.75 in.
The tubes at one end of the header were welded first with the operators working toward the center on two of the machines (Fig. 4), and from the center toward the other end with the third machine, leaving a window of unwelded tubes to provide access for welding - Fig. 5. There was good crawl space under the lower row of tubing, but the center row of tubing was the hardest to reach. If these welds were done manually, the center row weld would have to be done with the welder reaching down from the top or up from the bottom. With the orbital equipment, it was possible to have a single operator on each machine, whereas if the job were to be done manually, two welders would be working on the same weld joint at the same time. Thus, the use of the automatic equipment eliminated some of the work that would be done with the manual welder in an uncomfortable position or in difficult-to-reach places.
Production Techniques Lead to Increase in Productivity
During training, goals were set for the level of welding productivity that would be required to complete the project successfully. Although a learning curve was expected, the operators were productive from the start of the replacement project. With three eight-hour shifts in operation five days a week, the productivity level averaged about eight welds per shift per machine. Some operators were able to double this rate.
On previous projects done with manual welding technology, radiography was a constraint to productivity and a time window to perform the radiography was required. On this project, they were able to save time by scheduling the radiography to be done on the weekends.
ABB developed a number of time-saving fabrication techniques for cutting and purging. This was done to give TAU the best possible cost, as this project was done on a time and materials basis. Arc Machines cooperated by having Frank York on site to advise them and keep the welding operation going. After the start of the job, they were able to save a significant amount of welding time by reducing the number of passes on the stainless-to-stainless welds from four to three. The chrome moly-to-chrome moly weld required eight passes to complete. The stainless-to-stainless welds were done first so that these welds could be purged with inert argon gas during welding. Purging with argon protects the weld surface from oxidation and provides a cleaner, higher quality weld. The soluble purge dam material that was used was removed during hydrotesting. The welding operations were consistently below the estimated man-days for welding, but these improvements were, to some extent, consumed by extra time needed for precision fit-up.
ABB estimates that this job, done with orbital welding, was about 25% more efficient than a similar job done two years before with manual GTAW. In addition, there is now a core of trained personnel that can be tapped for future jobs.
By Barbara K. Henon, Ph.D., Arc Machines, Inc.
1. Inconel is a trademark of the Inco family of companies.