Imagine trying to weld in a dark, 60-foot diameter, 160-foot tall metal tank with numerous obstacles conspiring to hamper your efforts, including water nozzles, ductwork and a narrowing of the tank as it reaches the top. These are some of the conditions that welding crews face during the wallpapering process of a power plant`s flue gas desulfurization (FGD) scrubber components.
Wallpapering adds thin stainless and nickel alloy sheets to the walls of the scrubber system`s absorber vessel(s). This lining helps the walls withstand the corrosion and pitting caused by the sulfur dioxide gases flowing through the unit. The corrosive elements of the sulfur dioxide are so severe that the sheets lining the absorber have to be replaced every 7 to 10 years. Wallpapering is further complicated by the fact that the alloys used in the process make welding more difficult-the thin, overlapping nickel sheets are at a constant risk for warpage.
The stainless sheets used to wallpaper the absorber walls are 4 x 10 feet in size and 1/16 or 1/8 inch thick. The sheet thickness depends on the area of the absorber where they will be used and what the temperature and pH levels are in that particular area. For example, at the bottom of the absorber, where the limestone slurry accumulates, 1/16 inch stainless (316L) plate is used; in the areas near the spray nozzles, where the gases are more corrosive, a thicker 1/8 inch stainless (317LMN) sheet is required. Thicker sheets could offer longer corrosion resistance, but are not cost-effective when considering the maintenance schedule and projected lifetime of the scrubber system.
To weld the stainless sheet to the walls, the sheets are first plug welded, then the edges are either seam or tack welded. The edges of the sheets overlap several inches, which requires a welded lap joint. In a normal application, the process of lining one absorber takes from 7 to 10 weeks to complete.
The stainless wallpaper sheets are difficult to weld for a number of reasons. First, because the plate is thin, burnthrough and warpage are constant threats. Second, precise heat input is required since the wallpaper is being welded to a thick substrate (1.5 to 2 inches), which consumes much of the applied heat and hampers weld penetration. Care must be taken so that the weld doesn`t burn through the thin edge where the sheets overlap. Third, a high nickel alloy wire must be used to weld because of its resistance to corrosion. This alloy, American Welding Society Designation ERNiCrMo-3, tends to be more difficult to weld than most wire electrodes.
Williams Union Boiler recently entered into a license agreement to use Lincoln Electric Co.`s newly developed Surface Tension Transfer (STT) technology for a wallpapering project at TVA`s Cumberland City plant in Tennessee. The Cumberland City plant consists of two coal-fired units each rated at 1,300 MW, placed into service in 1973. In 1994, TVA installed limestone FGD systems on each unit, three absorber vessels per unit, to reduce sulfur dioxide emissions.
Most conventional wallpapering applications use pulsed spray gas/metal arc welding technology (GMAW), which tends to show signs of chromium carbide precipitation in the heat-affected zone adjacent to the weld(Figure 1). Because pulsed spray GMAW also produces a larger weld with higher heat input, the grain structure of the base material is distorted, lessening weld quality; this distortion is lessened with the STT process(Figure 2).
With STT technology, the heat-affected zone is essentially non-existent. While most conventional welding processes have heat inputs as high as 25,000 to 30,000 joules per inch, the STT`s heat input is only 7,000 joules per inch, which ultimately leads to reduced distortion, especially in the corners of the overlapping plates. Even under high magnification, the chromium carbide precipitation cannot be detected (Figure 3).
The key to STT technology is its ability to control current independent of wire feed speed. This means that more current can be applied without adding more wire. STT is a new approach to what has been known as the short arc transfer mode or short arc welding. A high-speed inverter precisely controls the output current waveform during the entire shorting cycle, producing response times measured in milliseconds.
A comparison with conventional GMAW improves understanding of STT. The conventional, constant voltage GMAW short circuiting process is illustrated in Figure 4 (y-axis not shown to scale). In this mode, the magnitude of the current is relatively high at the moment the droplet separates from the wire, causing the fuse to explode and generating a high level of spatter. In an absorber environment, this spatter creates an increased risk of crevice corrosion since particles can easily adhere to this spatter. Therefore, this spatter must be removed, requiring extensive grinding and polishing and adding an extra labor step to the wallpapering process.
The STT waveform is shown in Figure 5. It is different in several ways from conventional GMAW. Before the electrode initially shorts, the current is reduced, eliminating the large spatter balls. This low-level current is maintained for a short period of time so that the surface tension forces can begin transferring the drop to the puddle, forming a solid mechanical bridge.
A high level of current is then applied to speed up the transfer of the drop. During the “upward tilted” portion of the waveform, the “necking down” or squeezing of the shorted electrode is monitored. When a specific value is reached, the pinch current is quickly reduced to a low value before the fuse separates. When a short breaks, it does so at a low current, producing very little spatter.
Following this, the arc is re-established and a high current is applied that is referred to as peak current. This momentary current pulse causes the arc to broaden, melting the surface area. This action eliminates cold lapping, which is the tendency of a weld to form into a ball instead of spreading out on the surface, thereby preventing good fusion.
Williams Union Boiler installed more than 4,000 sheets during the Cumberland wallpapering project, requiring more than 100,000 linear feet of weld output for the six absorber vessels. The project was completed between July 1997 and July 1998, as each vessel was successively taken out of service for the 7 to 10 week wallpapering process. Notably, Williams decided to purchase new STT process equipment for use at Cumberland even though pulsed GMAW equipment was available on-site at no charge.
Williams Union Boiler reported that the STT process produced consistently higher quality welds without sheet distortion. Substantial labor savings were documented due to increased travel speeds, decreased repair time and less clean-up. Although weld productivity figures are comparable between STT and conventional GMAW-about 150 feet of weld in an eight-hour shift-weld acceptability with the STT process is considerably higher because of reduced spatter, plate distortion and heat input.
In addition to reduced distortion, repair time was also reduced at Cumberland. Each weld was examined through one of two visual tests, a vacuum box test for flat surfaces or a dye penetration test for welds that were made in the absorber`s water nozzle areas. The vacuum box test consists of establishing a seal around a welded section of wallpaper, pulling a vacuum and then inspecting the area for visible signs of leakage. The dye penetration test involves spraying a red dye on a welded surface, wiping off the excess, applying a white chalky “developer” substance and then looking for signs of dye breakthrough. If a crack exists, the dye will be exposed by the developer substance.
The final weld quality check consisted of a non-destructive acoustic analysis in which the vessel was pressurized and sensitive instrumentation was used to “listen” for leaks. According to Williams Union Boiler, the repair rate at Cumberland was less than 1 percent, much lower than with pulsed GMAW, and despite the fact that the sheer welding output was much higher. p
Author: Darryll Dodson is Marketing Product Manager for STT at the Lincoln Electric Co. He has 12 years` experience in sales and application engineering with Lincoln Electric. Dodson holds a bachelor`s degree in electrical engineering from Texas A&M University.