Critical Hydrogen Service Piping Project Met With Orbital Welding
Critical hydrogen service piping project met with orbital welding
Although orbital welding technology has been available for about twenty years, and has become the standard method of joining for high-purity applications, such as the welding of semiconductor process gas lines and hygienic piping for the pharmaceutical industry, it is still an underutilized technology in the petrochemical industry. Recently, however, the availability of small compact weld heads that can mount on closely spaced boiler tubes and furnace tubes, and the development of smaller, more portable power supplies with the capability of feeding filler metal to the welds has made this a growing industry segment for orbital welding.
Fig. 1- Hydrogen furnace under construction.
The advantages of orbital gas tungsten arc (GTA) welding for a refinery application was recently demonstrated by SW Industrial, Inc., formerly Swinerton & Walberg Co. of San Francisco, Calif. (SWI), with the welding of furnace tubes in new construction of a hydrogen furnace (Fig. 1) in Wilmington, Calif., near Long Beach. Many refineries have furnaces that extract hydrogen from hydrocarbons at very high heat. This technology will be more prevalent in the future because of regulatory pressure.
Automation Is the
An industrial contractor in business for over 100 years, SW Industrial was confident it could weld the small-bore heavy-walled piping on the hydrogen furnace manually, however, when the drawings were examined, it became apparent that the welding for this job was more complex than expected and the company would have to reconsider its position. The furnace tubes at the bottom of the state-of-the-art hydrogen furnace were to be spaced at one-foot centers making it impossible for manual welders' helmets to pass. This would mean that at least for part of the welding on the critical hydrogen service piping, they would be welding blind. Furthermore, the material specified for the 1-1/4-in. pigtails was Incoloy 8001 because of the severe operating temperature of the furnace. This material has a chemical composition of 33% nickel, 21% chrome, with the balance being iron. Such material is used because of its superior high-temperature properties and weldability, but it would be difficult to weld by hand in this application, and the job was to be done under a tight construction schedule.
The company thought of automatic orbital welding as a solution to this welding problem. The company's past experience with orbital welding was with thin-wall GTA fusion welding for the nuclear and food processing industries, and high-purity pharmaceutical and computer chip cleanroom operations. The materials and wall thicknesses for the furnace required the addition of filler metal to the weld and more advanced orbital welding equipment, which demanded more operator training and skill.
The company leased a Model 227 microprocessor-based power supply and a compact Model 81 full-function pipe weld head from Arc Machines, Inc. (AMI), in Pacoima, Calif., and sent three people to the AMI factory for training. The men selected for the training were a pipefitter, a boilermaker, and a welding engineer. At first they practiced on carbon steel samples because they were easier to learn on, and less costly than the Incoloy alloy. The welders were skeptical at first, thinking that orbital pipe welding would never be practical. After they saw it work, they thought it was impressive, but was probably too difficult for them to learn. Nevertheless, by the second day of training, they had mastered the basics and were amazed by the quality of welds made by machine. By the third day of training, each welder was developing weld schedules and making x-ray quality welds on his own.
Welding parameters for Incoloy 800 were established during training and stored in the memory of the power supply. Values for welding currents, both primary and background; arc voltage control, which provides electronic control of arc length; electrode travel speed and wire feed speed were entered for each pass of the weld for later recall.
Fig. 2- Modified “J” preparation for orbital welding of furnace tubes.
Fig. 3- Sequence of weld passes.
The preparation was a J-edge shape with a 1/8-in. land with a 0.050-in.
extension on the land - (Fig. 2). The first, or root pass, was controlled by the
STEP mode in which the electrode was stopped for higher, primary pulse time to
achieve penetration and moved during the lower or background pulse to create a
series of overlapping welds around the circumference of the tube. Maximum
interpass temperature of 500 degrees F was maintained. It is the nature of this
material that if the interpass temperature is exceeded, the weld scallops
badly, and the molten pool becomes uncontrollable. This is in contrast to
carbon steel, which can become cherry red with no consequences. The fill and
final passes were done with travel in the continuous mode. All passes were done
as stringer beads (Fig. 3) without oscillation of the welding torch.
The primary welding current varied between 190 A during the first pass and 140 A during passes two and three, while the background current was 85 A during the first two passes and reduced to 80 in the last three passes. Travel speed was about 4 in./min. The arc time was 123 s for the first pass, 72 s for pass two and 75 s for each of the last three passes for a total arc time per weld of 7 min exclusive of the time to reduce the interpass temperature to less than 500 degrees F. ERNiCr-3 filler metal 0.035 in. in diameter and 100% argon shielding gas at 30ft3 were used. To reduce the "wait time" for the required cool down of the interpass temperature, the welding procedures were set up to do one pass on each of three joints before returning to the first joint to do the fill pass. The breakdown time, which is the time to remove the weld head from one joint and set it up on the next, was so fast that this time was not even factored into the estimated time per weld.
Getting Ready to Weld
Fig. 4- Welding of these 1-1/4-in. pigtails at the bottom of the furnace is an example of the hard-to-reach locations that make it difficult for manual welding.
As the welding operators were being trained to do the welds, the furnace tubes at the job site were being fit-up in-position and the ends prepped for orbital welding. Four days after the completion of the training, welds were being made on-site in positions inaccessible by manual welding - Fig. 4. While the 1-ft centers of the tubes at the bottom of the furnace did not pose a problem for the low-profile weld head with its 1.75-in. radial clearance, it would create a problem for manual welding since a welder's helmet would not fit between the tubes. For a manual weld in this situation, two welders, each with his own torch, might be required to complete a pass without breaking an arc. Excessive starting and stopping of the arc would be detrimental to the properties of the Incoloy 800 material, which requires careful control of the heat input in order to achieve the necessary quality. A single manual welder might be able to accomplish a complete pass using a mirror to position the torch, but this is very difficult.
Quality manual welds under these conditions
are usually few and far between, with reject rates of over 15% expected.
Furthermore, to facilitate the tight construction schedule in effect on this
site, it would have required six manual welders to do the job of the single
orbital welding operator who performed the welds on this job.
At first, progress on the job was slower than expected. There were a couple of delays during the first week that worried the project manager, but by the second week they were ahead of schedule. Once comfortable with the equipment, the average was about 20 complete welds per shift including fit-up and tacking and actually peaked at 44 welds done in one 10-h shift. When the operator was asked how many acceptable welds he could have done per shift if he had done it manually, the reply was "three."
Quality of welds in this service is paramount and the welds were subjected to 100% radiography per ANSI/ASME Code for Pressure Piping B31.3. No defects were found in any of the 160 pigtail welds. Besides passing 100% of the welds by radiography, it was reported that the visual examination and appearance of the welds were beyond the client's expectations.
The difficulty of welding these materials to the required quality standards by hand, coupled with the limited access for welding, makes the welding of furnace tubes an ideal candidate for orbital welding.
By Barbara K. Henon, Ph.D., Arc Machines, Inc.
Bruce Winship, SW Industrial, Inc.