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
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