Oil Refinery Relies On Orbital Welding
Oil refinery relies on orbital welding
Few defects despite restricted access, challenging material, labor shortage.
by Gregory P. Erickson
When David Stine at CB&I Howe-Baker reviewed the welding requirements for a new hydrogen reformer, he knew a portion of it would be difficult if performed by conventional manual welding. Difficult material, restricted access, and limited labor created the perfect opportunity for automated orbital welding equipment to shine.
Hydrogen reformers in oil refineries extract hydrogen from natural gas or waste gases. The refinery uses the hydrogen to remove sulfur from, and improve the quality of, the hydrocarbon products the refinery produces.
Although CB&I Howe-Baker designed and built its own hydrogen reformers for many years, this project was a little different. This reformer was designed by Haldor-Topsoe for Air Liquide’s gas processing plant in El Segundo, California, and CB&I Howe-Baker was the primary construction contractor for the job.
Concerns and Limitations
One concern was the 5.3-inch diameter catalyst tubes. These tubes, which are mounted vertically, have a wall thickness of 0.6 in. and are nearly 50 ft. long. Welders would have to field-weld approximately 250 of these tubes to 5.3-in. stubs on two 26-in.-dia., 120-ft.-long cold collector headers. The stubs were not a concern because they already were welded to the header. The base material of the tubes was centrifugally cast 20Cr32NiNb (essentially cast INCONEL® alloy). The base material of the stubs was static-cast INCONEL (also 20Cr32NiNb).
INCONEL castings can be difficult to weld because of casting inclusions and porosity.
Because of the critical nature of catalyst tubes, welders had to make all of the catalyst tube welds in accordance with ASME Section VIII Div. 1UW-51 Code with a 100 percent RT (radiological test, or X-ray) nondestructive examination (NDE) final exam.
Figure 1. A welder monitors the orbital welding process as it joins a 1-in. expansion loop pipe to an outlet header weldolet.
Figure 2. Physical access to the catalyst tubes was limited. The clearance around the tubes was about 4-1/2 in. and structural supports and plates made straight access difficult. The fabricator used orbital welding to resolve the access difficulty.
Inlet distribution headers were located approximately 60 ft. above each cold
collector header. These 14-in.-dia. Headers had 125 outlets. Welders had to
join a 29-ft.-long, 1-in., Sch. 40 expansion pipe loop to each of the pipe
outlet stubs (see Figure 1). They joined the other end of each expansion loop
to an inlet stub on the top end of the catalyst tube (see Figure 2). The base
material of the pipe was seamless 321 stainless steel, and the stub material on
the catalyst tube and inlet header was a 321 stainless steel forged weldolet.
The 1-in.pipe-to-header stub welds were 7 in. off of the sixth-floor supports near the edge of the structure itself. During plant operations, this header expands about 14 in. along its horizontal axis, and the catalyst tubes expand about 10 in. along the vertical axis.
The expansion loop pipes had to be fitted and welded with a significant off-set so they could be aligned during operation. All of the 1-in. pipe welds had to be performed in accordance with ASME B31.3 Normal Service with a 100 percent RT NDE final exam.
The final concern was the labor pool. In recent years Southern California has experienced a shortage of welders, especially qualified gas tungsten arc (GTAW) pipe welders. CB&I Howe-Baker needed many welders for this project, and anything that would reduce the number of welders needed would be a great benefit.
Manual weld estimates for the 5-in.- catalyst tube were about one to 1-1/2 joints per welder per 11-hour shift. Estimates for automatic orbital welding were from four to eight welds per shift per machine. These estimates, in addition to predicting a labor savings, also meant that the welding would not have to be critical path- in other words, unknown problems were not likely to delay the entire project.
Making Decisions, Developing Procedures
CB&I Howe-Baker had previous experience using orbital fusion tube welding equipment from Arc Machines, Inc. (AMI), so CBI & Howe-Baker personnel contacted AMI’s local representative, R&B Welding Applications to investigate developing welding procedures and leasing equipment for the job.
Based on R&B’s recommendations, CB&I Howe-Baker selected an AMI model 227 power supply, a model 79-6625 weld head for he 5-in. tubes, and a model 79-3500 weld head for the 1-in. pipe. R&B set up the equipment in its shop and provided a week of training for Stine and four welders. Stine brought weld samples to be used for developing schedules and training the welders in using the equipment.
For the 5-in. tube-to-stub welds, the procedure called for a 37-1/2 degree bevel and a 1/16-in. land. The two components were fitted together with a UNS N08810 backing ring. Because the inner portion of the tube was filled with insulating materials, it was important to penetrate the backing ring without harming the i8nsulation.
The 1-in. expansion loop piping came prepared with a 37-1/2-degree bevel. The team that developed the weld procedure changed this to a J-prep for more consistent results. The weldolet end preparation was 37-1/2 degrees with a 1/16-in. land.
A Georg Fischer BRB 4.5 unit was used to reprep the pipe. The J-prep had a 20-degree bevel, 1/16-in. radius, and a 0.050=in. land and extension. The tool was able to square-face and prep these pipes in one operation.
Overcoming Welding Problems On-Site
The first welds were on the 5-in. catalyst tubes, and the installation crew immediately ran into problems. One of the risks is that welding disturbs the microinclusions and porosities in cast material. The material sometimes blows up and gets on the tungsten and the gas lens.
In manual welding, this may or may not bother the welder significantly. Automated systems with arc voltage control (AVC), such as the ones used on this job, are more sensitive to this type of problem. After a blowup the operator has to stop welding, replace the tungsten and repair or replace the gas lens, and reprep the area if necessary.
Although blowups occurred far more often than anyone anticipated, the project proceeded on schedule. Of 250 welds, none were rejected in the final X-ray. The crew averaged five welds per shift, with a high of eight welds in one shift.
The local weather also caused some welding problems The El Segundo, California plant is a few hundred yards from the beach. Mornings bring dew and fog, and afternoons bring wind. Any air movement faster than a few miles per hour disturbs the gas coverage and upsets the weld puddle. Excessive wind causes severe gas coverage problems and AVC instability.
The crew had to erect wind barriers for every weld. The 5-in. welds near ground level were protected easily with tarpaulins and other shields. The 1-in. pipe welds were far more difficult to protect-the crew had to reduce, and in some cases eliminate, afternoon welding. These measures helped to keep rework to a minimum. Of the 1-in. pipe welds, which numbered 500, 15 rejects were identified in the final RT. This equates to a 3 percent rejection rate. Although less than 1 percent is common in automated welding, 3 percent is an acceptable rate when wind is a favor.
The biggest maintenance problem was the unusually large number of gas lenses and gas cups that needed to be replaced because of the cast material problems.
Even with the delays caused by blowups and prevailing weather conditions, the crew experienced relatively few delays with the automated welding process. Most of the holdups were in fit-up, tacking, waiting for backpurge, and waiting for other tradesmen. In fact, the decreased weld times made it possible to absorb these delays and still finish the project on time.
Arc Machines, Inc., 10500 Orbital Way, Pacoima, CA 91331, 818-896-9556, fax 818-890-3724, www.arcmachines.com.
INCONEL is a registered trademark of Huntington Alloys Inc.
CB&I, One CB&I Plaza, 2103 Research Forest Drive, The Woodlands, TX 77380, 832-513-1000, www.cbi-nv.com
Georg Fischer AG, Amsler-Laffon-Str. 9, 8201 Schaffhausen, Switzerland, 41-52-631-1111, www.georgfischer.com
R&B Welding Applications, 21828 Lassen, Suite L., Chatsworth, CA 91311, 818-727-7972, www.rbwelding.com
Reprinted from The Fabricator, August, 2005