The development of compact, full-function pipe weld heads with low axial and radial clearances that can fit between closely spaced rows of piping in areas that are often inaccessible to manual welders has greatly extended the field of orbital pipe welding. These compact heads, used in combination with portable microprocessor-controlled power supplies, have made orbital welding of boiler tubes and furnace tubes practical.
Types of Full-function Orbital Pipe Weld Heads
Full-function pipe weld heads for use with the gas tungsten arc welding (GTAW) process are made in several configurations and have a variety of features:
- Functions used in orbital fusion welding such as pulsed current and travel
- Provisions for adding filler wire into the weld puddle
- Torch oscillation to permit weaving of the torch back and forth across the weld joint
- Electronic control of the arc gap (AVC), which maintains a constant distance between the electrode and the work when the torch travels over uneven surfaces
Although compact heads are the focus of this article, there are three types of full-function weld heads of which welders should be aware.
Full Size. For large-diameter pipe, full-sized orbital weld heads that mount on a guide ring are practical. The entire head and torch move around the guide ring, orbiting the joint circumference to complete the joint. Typical applications for this type of head include orbital welding of large-diameter pipe in nuclear and fossil power plants; welding of titanium, duplex, and copper-nickel pipe for offshore applications; welding of stainless steel pipe in shipyards; and many other applications requiring welds of very high quality.
Full-sized, full-function orbital pipe weld heads can be used for multiple passes on very heavy-wall pipe and for standard pipe sizes from 3 to more than 30 inches in diameter, as well as for welding of vessels or welds requiring flat track for special applications. The full-sized heads also have been used for fusion welding of Schedule 10 stainless steel pipe in sizes up to 12 inches for high-purity semiconductor applications.
Open Frame. If radial clearance is not limited, open-frame pipe weld heads that clamp directly onto the pipe next to the weld joint can be used. This type of head is suitable for pipe shop applications, as well as for in-place, on-site use in many industrial applications.
These heads come in several sizes, and each can weld a range of pipe sizes. Sizes within the range of the head can be changed very quickly, requiring only a slight adjustment of the clamp.
Other types of full-function orbital pipe weld heads include heads that mount inside the pipe and heads for spiral cladding of pipe or valve inside diameter (I.D.).
Compact. Compact weld heads, which are the focus of this article, are designed for low-profile orbital welding of pipe with a 2- to 6-inch O.D., radial clearances of 2 inches or less, and a nominal axial clearance of 6.325 in the standard configuration.
Generally, orbital welding is used for a particular quality requirement, but compact heads often are specified because of clearance restrictions in applications such as boiler tube replacement in utilities and furnace tube installation or replacement in chemical plants or refineries where the tubes are arranged in closely packed rows. Such spacing does not provide good access for manual welders, who often must use mirrors to locate the torch on the weld joint.
All manufacturers of compact orbital pipe weld heads provide water cooling of the torch, but some provide water cooling of the housing as well, making them more suitable for work in higher-temperature operating environments.
Materials and Applications
Any material that can be welded with the manual GTAW process can be orbitally welded successfully. Orbital welding often is used on stainless steel (including duplex) and nickel-based alloys because quality welds help to retain the favorable mechanical properties and high corrosion resistance of these materials.
The compact weld heads frequently are used for carbon steel and chrome-molybdenum boiler tubes, as well as for more exotic metals such as INCOLOY® 800 alloy, which was used by Swinerton and Walberg (S & W) for the pigtails in a hydrogen furnace, or INCONEL® 600 alloy, used by Dow Chemical Canada, Inc., Western Division, for the upper convection coils in a vinyl chloride monomer (VCM) furnace.
INCOLOY alloy requires careful control of the heat input and thus cannot tolerate stopping and starting of the arc, which, in this application, would have been unavoidable in the tight spaces between the furnace tubing if they were manually welded. It might have been possible on this site to use two manual welders welding at once to prevent stopping and starting the arc, but it would have been hard to get a torch in position, because a welding helmet could not fit between the rows of tubing. In this area, reject rates of manual welds had been more than 15 percent.
Welding of the 800 alloy was more critical than welding of carbon steel because a maximum interpass temperature of 500 degrees Fahrenheit had to be maintained. If the interpass temperature were not observed, the weld would scallop badly, and the puddle would become uncontrollable. Carbon steel, on the other hand, can get cherry red with no consequences.
The 800 alloy (33 percent nickel, 21 percent chrome, and 46 percent iron) was welded with a shielding gas of 95 percent argon 5 percent hydrogen, which provided a hotter arc than pure argon. Argon was used to purge the I.D. of the joints, because a poor purge or lack of purge can cause loss of corrosion resistance and/or unsatisfactory service performance in the higher alloys. The filler wire used in this application was 0.035-inch-diameter ERN/Cr-3.
INCONEL alloys are used in furnace applications because of their superior high-temperature properties and weldability, but they are difficult to weld by hand. They are relatively expensive materials, but the costs were justified in this case because their use would reduce replacement and shutdown frequency.
Before the 600 alloy upper convection coils were welded, the joints were manually tack welded in place. An I.D. purge was provided during tacking to ensure that the arc did not deviate around the tacks, as it might if the tacks were oxidized.
Furthermore, the I.D.s of both field-welded and shop-welded joints were purged during orbital welding. An oxygen meter was used to indicate when the purge was adequate before the next segment was joined, and care was taken not to overpressurize the weld, which would alter the weld profile or possibly blow out the weld.
Although I.D. purging is important to prevent oxidation, it is not always possible on welds with restricted access. In such cases, backing rings have been used with some success to protect the pipe I.D. from oxidation.
Orbital welds done with compact, full-function pipe weld heads have had to meet the most stringent welding code requirements. The INCONEL 600 upper convection coil orbital welds, for example, had to meet American National Standards Institute (ANSI)/American Society of Mechanical Engineers (ASME) B31.3 code for severe cyclic chemical piping, as well as ASME Section IX of the Boiler and Pressure Vessel Code, because of the severe service environment in the VCM cracking furnace. Although not required by these codes, the welds were subjected to 100 percent radiography, and no linear indications were found.
Consolidated Edison of New York, Inc., welded more than 6,000 joints on a boiler repair and was able to do 2,500 of these orbitally using a compact orbital pipe weld head. Of those orbital welds subjected to radiography, only 0.2 percent, or 12 joints, failed to meet the ASME PE-51 acceptance criteria and had to be repaired.
Modified Head Configurations
Compact weld heads are versatile and can be modified for many different types of industrial applications. For example, they can be modified with AVC tilt, putting the torch motion in the same plane as the torch tilt for applications that include socket welds and flanges. They can be set up for welding smaller pipe sizes, equipped with a direct-view video camera for welding in hazardous environments, or mounted on a mandrel and equipped with AVC/oscillator interchange for welding tubes on mud drums and economizer drums.
Clamp Modification. Boiler tube arrangements in utilities, with their closely spaced rows of tubing, have not been designed with orbital welding in mind. However, manual welding often is difficult and results in high rejection rates.
Consolidated Edison engineered a special clamping mechanism to allow mounting of a compact pipe weld head on boiler tubes with axial clearances as small as 4 inches (standard nominal weld head configuration clearance is 6.325 inches). The redesigned clamp greatly extended the number of welds in boilers that could be done orbitally.
AVC/Oscillator Interchange. To speed up the welding of replacement tubes on upper and lower economizer drums for a Canadian paper mill, a compact orbital pipe weld head was mounted on a mandrel. This allowed the head to remain in position on the mandrel while the mandrel was moved from tube to tube, which greatly facilitated mounting and dismounting of the head.
A compact head with AVC/oscillator interchange was used, reversing the AVC and oscillator functions on the weld head so that the oscillator motor controlled the arc gap. In this application, the welds took about four minutes each. With two welders working in the same drum at once, more than 80 welds could be completed in each 10-hour shift, which compared favorably to production rates for manual welds.
 Figure 1 For pipe with walls from about 1/4 to 1/2 inch thick, a modified “J” preparation with an extended land to prevent the puddle from climbing the side walls has proven successful. |  Figure 2 A special weld joint preparation, guideline CS-4252, was designed to improve the longevity of the bimetallic weld joints in power plants. |
Joint Preparation
Modified “J” Prep. A consistent machined joint preparation is essential for achieving success with orbital welding. For pipe with walls from about 1/4 to 1/2 inch thick, a modified “J” preparation with an extended land to prevent the puddle from climbing the sidewalls has proven successful (see Figure 1). When the puddle is allowed to move up the sidewall, it pulls the molten metal away from the joint and results in concavity of the inner weld bead surface, known as “suckback.”
Typically for stainless and carbon steel, the land is about 0.65 inch thick, but for materials such as the INCOLOY 800 alloy used for furnace tubes at the Arco refinery, a “J” bevel with 1/8-inch land and 0.050-inch extension was found to be practical.
Insert Rings. If the pipe walls are out-of-round, achieving consistency with a modified “J” prep can be difficult. In this case, especially with thinner-wall materials, a 37 1/2-degree bevel with an EB insert ring or a K ring may be used. The K ring is a more forgiving arrangement if the pipe is badly out-of-round.
A weld joint preparation with a 45-degree/25-degree compound bevel with 1/8-inch-O.D. INCONEL 600 insert rings was used to weld the 3.5-inch-O.D. 600 alloy furnace tubes at the Dow Chemical plant. The insert rings were fusion-tacked in place and welded with 600 alloy 0.035-inch-O.D. filler wire (ERNICR-3).
Bimetallic Welds. A special weld joint preparation, guideline CS-4252, was designed by Electric Power Research Institute (EPRI) to improve the longevity of the bimetallic weld joints on superheater header tubes in power plants (see Figure 2).
After about 75,000 operating hours, these dissimilar-metal weld joints would begin to fail, with the failure propagating internally from the weld heat-affected zone (HAZ). Failure of these weld joints, occurring between stainless steel internal elements and the chrome-molybdenum external elements that connect the header to the rest of the piping system, has been extremely costly to power plants, resulting in losses of up to $50,000 per day.
Rather than the typical 37 1/2-degree bevel used on manual weld joints for boiler tubes, the EPRI prep features a 37 1/2-degree bevel on the stainless steel side and a 60-degree bevel on the T-22 chrome-molybdenum side. INCONEL alloy is used as filler instead of E309 welding rod, and the weld profile is altered and has a wide cap.
The new weld joint design showed a fourfold improvement on laboratory creep tests, indicating that it is more able to withstand the stresses of high operating temperatures and pressures, as well as the added stresses attributed to varying expansion coefficients between the dissimilar elements that occur during periodic shutdowns.
EPRI predicts that the combination of the improved joint preparation and orbital welding will increase the useful life of the dissimilar-metal welds to about 200,000 operating hours, or about 25 to 30 years, compared to six to eight years for manual welds.
Although utilities were quick to use orbital welding on the similar-metal superheater welds (stainless to stainless and T-22 to T-22), they still sent the more critical dissimilar-metal weld joints off-site for manual welding, or they did orbital GTAW roots with GMAW fill passes. However, after experiencing success with orbital welding, Consolidated Edison and TransAlta Utilities began using orbital welding for the dissimilar-metal weld joints on subsequent shutdowns.
Weld Head Setup
The clamping assembly on some compact orbital pipe weld heads minimizes the time required for mounting and dismounting the head. In areas with reasonable access, this can be done in as little as 30 seconds.
The latches on the guide ring and the clutch are released to open the clamp assembly. The head is then placed around the pipe with the torch roughly centered on the weld joint (see Figure 3A). The latches on the clamp assembly are placed into position and firmly latched in the closed position (see Figure 3B). A spring-loaded tension arm is pushed against the pipe and latched into position. With the head mounted on the pipe, the torch is adjusted manually so that the tungsten is over the center of the joint.
Adjustments to the wire feed are made using an Allen wrench (see Figure 3C). AVC is used to jog the torch to an arc gap of about 1/16 to 1/8 inch before arc start if radio frequency (RF) arc start is to be used. Touch start also can be used if available. Weld head travel speed should be calibrated to the power supply on which it is installed, if the equipment permits.
Before welding, the weld head cable may be prewrapped on the pipe to avoid interference during welding. Many orbital equipment operators prefer to unwind the cable during welding rather than wind it because there is less possibility of jamming the cable and stopping the weld head rotation.
When welding in the 2G (vertical) position, the head usually is mounted with the torch pointing up. Operators welding the pendant loops on Con Edison’s Astoria boiler, however, preferred to mount the weld head with the torch pointing downward because it was easier to handle the cables in this position, and the torch was protected from falling objects.
 Figure 3A The weld head is placed around the pipe with the torch centered roughly on the weld joint. |  Figure 3B The latches on the clamp assembly are placed into position and firmly latched into position. |  Figure 3C Adjustments to the wire feed are made using an Allen wrench. |
Programming and Welding Techniques
Once the head has been adjusted properly, a weld schedule must be entered into the power supply or recalled from memory before welding can commence. Weld schedules, or programs, consist of values for the various weld parameters such as weld current, travel speed, and arc time:
1. Weld current. Weld currents determine, to a large extent, the amount of heat input into the weld. The current must not be set so high during the second pass that it remelts the root. The wire feed usually is increased during the third or other fill passes to fill up the joint, and this requires more amps to melt.
Weld currents can be synchronized with travel for stringer beads, or they can be synchronized with torch oscillation, with the torch swinging back and forth across the joint to create a weave bead. In this “sync” mode, the high current pulses are concentrated on the sidewalls to achieve good tie-in, and the lower current levels are applied as the torch crosses the center of the weld to prevent overmelting of the previous weld bead.
Current values based on previous similar welds can get the welding operator “in the ballpark,” but actual current values must be adjusted based on results obtained during weld development to get satisfactory results.
2. Travel speed. Travel speed is the speed at which the torch moves along the surface of the weld joint. This usually is set between 3 and 5 inches per minute (IPM). For a 2-inch-diameter tube at a travel speed of 5 IPM, it will take about one minute and 15 seconds to travel the 6.28 inches of circumference with travel in "continuous" mode. If travel is in "step" mode during which the travel is stopped or slowed during the high-current pulse, the actual travel speed is reduced accordingly.
When the torch is oscillating, the torch speed across the weld joint is determined by the oscillator amplitude and the excursion time, while the speed at the sidewalls is determined by the dwell time, travel mode, and the programmed travel speed. During welding, the travel speed may be adjusted from the operator pendant. Increasing or decreasing the travel speed sometimes is used as a technique for adjusting the size of the puddle and the rate of fill. Slowing the travel speed will increase the fill rate, filling the joint more quickly.
3. Timing. During weld development, the timing usually is set to manual mode. The operator watches the progress of the weld and, after the weld has tied-in (traveled more than 360 degrees), stops the weld by selecting the stop switch, which initiates the current downslope and extinguishes the arc. When the weld program is finalized and the travel speed has been determined, timing can be done automatically so that after the level has timed out, the power supply initiates downslope.
Even with a proven weld schedule, however, welding cannot begin until the arc gap is set using the AVC. To accomplish this, an arc is started on a test piece or the joint, and the arc gap is adjusted with the AVC up or down button on the remote pendant. The AVC is a programmed weld parameter, but its effect is modified by environmental conditions such as gas type and purity, temperature, atmospheric pressure, etc.
The important thing to remember is that the AVC may need to be adjusted to give the correct arc gap from day to day. The AVC value that results in the desired arc gap may even change from morning to afternoon on the same day.
On the first (root) pass of a weld, the AVC mode is set for “sample primary,” which checks the AVC during the primary current pulse only. On subsequent passes, AVC mode is continuous, which means that the AVC is active during both primary and background current pulses. The AVC must be balanced so that the torch moves in a fairly level position and does not bounce up and down during the current pulses.
Once the weld has started, the welding operator views the weld through a welding lens in the heads-up display (HUD) and may make small adjustments to the weld parameters directly from the HUD (see Figure 4). During the weld, the operator must observe to see if wire entry into the puddle is correct. If the wire is set too low, it can drag. If it is set too high, it will melt before entering the puddle. If the wire angle was set before the arc start, the entry angle usually can be corrected with minor AVC adjustments or by a mechanical adjustment of wire position.
 Figure 4 Once the weld has started, the operator views the weld through the welding lens of a heads-up display and may make small adjustments to the weld parameters. |
The welding operator also must check for cross-seam adjustment and, if necessary, center the torch with the oscillator jog switches as the torch progresses around the joint. The operator also must visually verify that the weld puddle is washing into the sidewall(s).
As the weld nears the end of the first pass, the welder visually monitors the tie-in, turns off the wire after 360 degrees of travel, and allows the torch to travel about another 1/2 inch before hitting the sequence stop button to initiate downslope, during which the welding current is gradually reduced until the arc is extinguished.
Depending on the material and the welding specification, the welder may brush the first pass before beginning the next pass. If an interpass temperature is specified, this must be checked before the next pass is begun. The location of the beginning of each pass is staggered somewhat so as not to start all the arcs in the same area.
Because of the generally heavier wall thicknesses, variation in joint preparations, and material differences, there is much more variability in the program parameters used for orbital pipe welding than for orbital tube welding, and the formulas for calculating weld parameters are not as applicable. However, a number of welding techniques can be used to provide good control of the puddle and a good weld profile.
For example, the root pass, which is almost always a stringer bead with no oscillation, may be done with the travel mode set for continuous or step mode. In step mode, the welding current is synchronized with the travel so that the electrode is stopped (or slowed) during the higher, primary pulse time and moved during the lower or background pulse. This technique is used when welding stringer beads to achieve maximum penetration for a given weld current.
The step technique also helps to keep the weld bead from sagging when welding in the vertical (2G) position. The resulting weld bead appears similar to a series of overlapping spot welds around the circumference of the tube. Wire feed and torch oscillation are used for subsequent passes to create a weave bead, with the width of oscillation increasing from pass to pass. A maximum weave of about 5/8 inch is observed, mainly because this limit has been commonly specified for manual welding.
Weld Sequencing for Orbital Welding
Weld sequencing is a critical part of planning for a major outage. For Consolidated Edison’s boiler repair, the order in which the tubes were welded with orbital equipment was different from the order in which they were manually welded during previous shutdowns.
For example, the terminal tubes were arranged in five rows of 79 elements from north to south. With the orbital head, one row of 79 was welded straight across the furnace to get the most favorable positioning of the weld head. On a manual job, each row of five tubes would have been welded one after another.
Differences in weld sequencing greatly improved Con Ed’s welding efficiency. In several of its locations, jobs done with orbital welding were completed in about half the time as similar manual jobs. In one location, although the time to completion was similar for orbital and manual welding, two welders working together would have been required with manual welding, while the orbital welding was done with one machine and one operator.
On a bimetallic weld replacement project at the TransAlta Utilities power plant in Canada, the company was faced with the task of completing 684 superheater header welds within a three-week time frame. The header tubes were arranged in 114 rows stacked three tubes deep, with about 2 inches of radial clearance.
Using a compact orbital pipe weld head, the tubes at one end of the header were welded first, with the operators working toward the center on two of the three machines. With the third machine, they worked from the center toward the other end, leaving a window of unwelded tubes to provide access for welding. The center weld in each row would have been difficult to access for manual welders working in an uncomfortable position and would have required two welders working on the same weld joint at once.
Similarly, S & W changed the sequencing of welds on the furnace pigtails. Welders made root passes for a set of three welds to allow time to reach the specified interpass temperature, and then they returned to the first joint to do the first fill pass. The total arc time for the pigtail welds was about seven minutes for the five passes. The welding operators averaged about 20 complete welds per shift, with a maximum of 44 welds completed in a 10-hour shift.
Conclusion
Compact orbital weld heads have proven useful for welding in locations that are difficult or impossible to access manually. In such locations, the reject rate is much lower for orbital welds when compared to manual welds. These lower reject rates, combined with more efficient job layout, can increase productivity and cost savings.
The proper planning, including training of orbital welding operators and allowing sufficient time for hands-on experience, is essential for success with orbital welding. When major projects are approached in a professional manner with the essential details worked out in advance, orbital pipe welding has proven to be an effective welding method.
(source : http://www.arcmachines.com/news/case-studies/limited-clearance)
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