Orbital Welding in Record Time
Orbital Welding in Record Time - A company in Holland maximizes productivity with orbital tube-to-tubesheet welding.
When Verolme Machinefabriek ijsselmonde b.v. (VMIJ), a member of Verolme ijsselmonde Holding b.v., located near Rotterdam, Holland, was awarded the contract for four carbon steel reactors in late 1996, the customer required the reactors to be orbitally welded. To meet this specification, VMIJ purchased an orbital tube-to-tubesheet welding machine (Fig. 1) from Arc Machines, Inc. (AMI), Pacoima, Calif. Manager of Procurement Rob van den Berg said VMIJ was awarded the contract because the project had a very fast time frame and VMIJ had an excellent record for on-time delivery during the last five years.
VMIJ already had experience with an orbital welding system for welding tube-to-tubesheet. However, the customer wanted their order quickly, within only about seven months. Because of this, the job required rugged equipment capable of operating 24 h a day, seven days a week for months at a time. The four reactors each required 4800 tubes, for a total of 19,200 tubes with a two-pass weld at each end - 38,400 welds that had to be qualified, completed and inspected between February and August 1997.
VMIJ was established in 1946 at the present site along the river Nieuwe Maas with a one-room workshop. Since then, one room has grown into an industrial campus housing several large machine and assembly shops and about 475 employees. With direct access to the North Sea, the company ships products to destinations worldwide. It produces turnkey systems for international corporate clients, delivering equipment mounted and installed on-site at locations around the world.
Figure 1. A VMIJ welder sets up an orbital welding machine.
The company specializes in the design and manufacture of heavy equipment, usually about 20-30 tons. Products such as heat exchangers and pressure vessels are delivered to the world's energy, petrochemical and chemical industries. These products work in tough environments, so VMIJ works with metal more than 16 mm (0.64 in.) thick; also, corrosion-resistant materials are vital. VMIJ also repairs heat exchangers, removing old tubes and refurbishing the tubesheets with new tubing.
This type of work requires a high level of engineering and fabrication expertise, as well as state-of-the-art facilities, equipment and fabrication tools. VMIJ has welders skilled in all the major welding processes. The company owns equipment for shielded metal arc welding (SMAW), submerged arc welding (SAW), manual gas tungsten arc welding (GTAW), gas metal arc welding (GMAW), oxyacetylene welding (OAW) and electroslag welding (ESW). The company also has equipment capable of surface tension transfer and has facilities including special-purpose furnaces, for heat treatment of assemblies weighing up to 120 tons.
Figure 2. Weld preparation (left) and acceptable weld penetration (3mm [0.12 in.]) for the reactor. Tubes are recessed 1 mm (0.04 in.) with a groove cut around the tube. The weld must penetrate the tube wall, fill in the groove and penetrate into the tubesheet wall.
VMIJ has a 90 x 28 m (295.2 x 91.84 ft) clean room suitable for handling alloys such as stainless steels, including duplex and super duplex materials, nickel and nickel-based alloys, copper and copper-based alloys, aluminum, chrome-moly and titanium. These materials require a clean fabrication environment to maintain their unique mechanical properties, corrosion resistance and other favorable attributes.
Figure 3. Two tube-to-tubesheet weld heads welding the carbon steel reactor tubesheet. Spare locating fixtures are in position for welding the next tubes to be welded, reducing the time required between welds.
Figure 4. Inspection personnel prepare a finished reactor for helium leak testing.
Figure 5. A tube-to-tubesheet weld head welding a Cu-Ni tubesheet for the salt industry. The qualification test sample is shown at bottom right, below the tubesheet.
Because VMIJ had only seven months to work with, the company had to acquire equipment that could weld very fast. VMIJ selected three Model 227 microprocessor-based GTAW power supplies and three AMI Model 6 tube-to-tubesheet weld heads that can add welding wire to the weld. Cooling units circulated water through the cables to prevent overheating the cables and the torch - essential for such a prolonged, continuous welding operation. The company also used a 227-RP operating pendant to remotely start and stop the weld sequence. This allowed the operators to observe the welds closely, to make adjustments and stop the sequence if a problem occurred.
This kind of orbital tube-to-tubesheet welding equipment is complex, needing trained personnel for proper setup, operation and maintenance. AMl's European Office (AMI-EO) personnel, in cooperation with VMIJ's quality control manager, Kees van de Mast, who is in charge of weld qualification and inspection, trained and qualified VMIJ welders on the orbital welding equipment in early February 1997. Fifteen qualification welds were performed and submitted to X-ray. The tubes were degreased and sanded, removing oil, grease, rust, paint and other contaminants.
Codes, Standards and Safety
When fabricating equipment for global distribution, a company must pay special attention to assure compliance with international codes and standards. The reactors were basic-engineered in Houston, Tex., detail-engineered and manufactured in Holland (at VMIJ) and delivered to the end-user in Germany. VMIJ is experienced in working with international code organizations such as AD Merkblaetter, which regulates pressure vessel standards for Germany, and the American Society of Mechanical Engineers (ASME), which sets pressure vessel standards in the United States. VMIJ Sales Manager Rob Vermeer said when equipment is designed in the United States for use in Europe, VMIJ must redo the design to comply with codes from more than one country. For instance, a product manufactured in Germany and installed in the United States must meet the engineering codes from both countries.
For this job, the specified material, though acceptable under code in the United States, was unacceptable under the German code, AD Merkblaetter. So, this material was replaced in accordance with the DIN (German) standard. The company also had to meet the quality standards of TUV, a company that specializes in international testing and certification for manufacturing.
As in any operation, safety is of prime importance when orbital welding. VMIJ is required to meet the highest safety standards by its customers. In 1995, the company received a Commendable Achievement Safety Award for one million work hours without a lost-time accident. Meeting its customers' demands the company has qualified for Lloyds of London's Certificate of Approval for its safety assurance system, which covers design, engineering and fabrication procedures for the process, petrochemical and offshore industries.
Certified by TÜV, the weld procedure qualifications for the customer's reactors were done to the TÜV AD Merkblaetter/EN288-3 Code. The company performed bend and tensile tests, as well as pressure tests, on the qualification welds. They were also sectioned for macrographic examination, which would have revealed any significant inclusions, cracks or other defects, but none was found. Vickers hardness testing was also done with a maximum Vickers 325 allowed. All the test welds had hardness values between 119 and 227 Vickers which was well within the acceptable range.
The Weld Program
Data detailing the weld program, stored in the computer memory of the Model 227 power supply, were printed out and used as part of the weld qualification procedure. The weld program controlled such parameters as welding currents, travel speed, pulse times and wire feed speed. Weld parameters were based on the tubes' diameter (25.4 mm [1 in.]) and wall thicknesses (1.9 mm [0.076 in.]). The program used STEP rotation for the first two levels of the weld program, completing the first pass. During STEP rotation, the travel was synchronized, or "in step," with the pulsation; to achieve maximum weld-joint penetration, the torch was moved only during the "low" or "background" current pulse and held stationary during the "high" or "primary" current pulse. For the third level of the weld program which completed the second pass, CONT (continuous) rotation was used, meaning that a constant travel speed was maintained.
For the first pass, welding current began at 150 primary amps, pulsing to 45 A on the background. Travel speed during the background pulse was about 4 in./min, and the average travel speed for the first pass was 2 in./min. For the second pass (third level), the welding current and wire feed rate was changed. Arc time per weld totaled only 138 s, with 5 s each for inert gas prepurge before starting the arc, and postpurge after the arc was terminated.
The inert-gas purge, using an argon (grade 4.0) shielding gas, protected the weld pool and tungsten electrode from oxidation. Purging the tubes' inner diameters (ID) was not required since the material was carbon steel, a material that usually does not require ID purges. In any case, an ID purge would have been difficult for this application due to the internal configuration of the reactors, which were sectioned off by dividers.
In addition, the welding specification called for the tubesheet to be preheated 40°C (104°F) before welding.
There are several possible basic weld preparations for tube-to-tubesheet welding. The tubes may be flush with the tubesheet, projected or recessed. Each type of weld prep requires a slightly different torch modification. VMIJ had a selection of torches to cover all basic types of preparations. For this application, a torch suitable for welding flush or recessed tubes (Fig.2) was used. The tubesheet material for the reactors was WSTE 355, which is quality forging material. The tubes were ASTM 179 arranged in a triangle pattern with a pitch (distance between tube centers) of 31.75 mm (1.27 in.). The tubes were projected and then cut flush. Then a groove was machined out around each tube. To hold the tubes in place during welding, one tack weld per tube was permitted on the back side of the reactor. Because of a steep wall around the outside edge, the weld head could not be positioned close to the edge of the tubesheet. So, welds close to the edge were manually gas tungsten arc welded.
With such a tight schedule, high productivity was vital. With manual welding, the welder typically starts and stops the arc in the middle of a weld, adding time to the operation. Not so here, however, where the machine's weld head could complete the entire operation without stopping, even for multipass welds. Taking advantage of continuous welding, the company prepared, purchasing spare parts and consumables to avoid unnecessary downtime.
VMIJ also developed proper weld sequencing to ensure smooth operation. Proper weld sequencing was extremely important to prevent distortion of the tubesheet. Six to eight weld layers were performed across the tubesheet, followed by welds in the vertical direction to balance the heat input.
Each machine performed 220 welds during a 10-h shift. The head quickly mounted and dismounted from the pneumatic loading fixtures. By using two fixtures per head, the welding operator could set up the second fixture while the head was welding, then quickly move the head from one fixture to the next as soon as the weld was finished - Fig.3. The fixture was adjusted to a specific tubesheet pitch and the sheet's triangular pattern. Then, the head was located directly onto the fixture, with the torch concentric to the tube being welded. This added to productivity because the heads mounted on the fixture directly, instead of using a mandrel that would have required additional setup time between welds.
No mandrel meant that two other kinds of AMI torches could use a watercooled chill follower; the follower is located inside the tube that contacts the tube wall directly opposite the electrode. This cooled the tube so that higher amperage could be applied for better tubesheet penetration without melting through the thinner tube wall. Another kind of torch with no chill follower also was used. Using this torch, the weld head completed the weld in a single operation without stopping between passes. The specified interpass temperature of 300°C (572°F) was not a factor, so the weld heads could operate at virtually 100% duty cycle.
The weld head's method to feed wire into the weld also contributed to overall productivity. A built-in wire straightener kept the wire from kinking. The wire, along with shielding gas and weld current, fed through a manifold in the head allowing the torch to rotate freely any number of turns, without needing to rewind cables at the end of each weld. At times VMIJ had three weld heads operating simultaneously on the same tubesheet, all operated by a single worker.
In orbital welding, once successful weld parameters have been established, it is possible to repeat the process again and again with identical welds of the same high quality. This weld repeatability comes from the precision of the power supply and the arc voltage control. The arc voltage control maintains a constant arc length around the entire weld-joint circumference, even if the tube is somewhat out of round. Since the arc length affects the arc voltage, the constant arc length maintains consistent heat input from weld to weld. A consistently good end preparation is also essential for weld repeatability.
Inspection and Testing
Once the first reactor was finished in April 1997, VMIJ's quality control team did a visual inspection of all the welds followed by dye penetrant tests to check for cracks. The team then performed a simple soap bubble test to locate any possible leaks. After determining no leaks were present, the tubes were expanded into the tubesheet.
After the inspection, VMIJ's team prepared the reactor for helium testing, taping sections of the heat exchanger to isolate individual sections so that if a leak was detected, inspectors could quickly locate it - Fig.4. Then, the reactor was submitted to an independent quality control company, which conducted helium leak testing according to ASME guidelines. Helium is used for leak testing because the helium molecule is very small, which enables it to penetrate very small cracks. To conduct the test, the unit was pressurized with helium to 1.5 bar (21.75 Ib/in.2) and subjected to a 30-min soak while inspectors used the leak-detector probe on the outside, checking for the presence of helium. If a leak was found, the soaking time would have been extended to about 3-4 h. Fortunately, no leaks were detected. The helium leak test was done both before and after the tubes were rolled into the tubesheet.
In addition to the helium leak test, hydrostatic testing was done in which the unit is first filled with water on the shell side; then the tube side is tested, with the outside tubes tested first. The reactors will operate in pressures of up to 24 bar (348 Ib/in.2) on the shell side and 32 bar (464 Ib/in.2) on the tube side; therefore, to assure safety, the reactors were hydrotested at 150% of design pressures. After leak testing, the top of the heat exchanger was finally welded on. After final inspection, the reactor was stamped to show it met all the code requirements. Then the 120-ton structure was moved 140 m (459.2 ft) and loaded onto a barge on the river.
On to Other Projects
Before finishing the carbon steel reactors, VMIJ was already using Arc Machines' orbital welding equipment for other projects. One project involved a reactor to be used as a preheater for salt (Fig. 5), so a copper-nickel tubesheet was selected because of its corrosion resistance in this type of environment. Unfortunately, Cu-Ni is very difficult to weld by hand. Therefore, a specific Model 6 torch, which provided a steeper angle on the throat of the fillet weld, was used so that the margins of the individual welds would remain separated even when the spacing between tubes was narrow. The chill follower proved particularly effective in controlling the heat input with the Cu-Ni material, achieving good penetration of the tubesheet without melting through the thinner tube wall.
The orbital qualification welds on the Cu-Ni materials were successful, and in April VMIJ had two Model 6 heads at work on the carbon steel reactors and a third head at work on the Cu-Ni tubesheet.
Several other orbital welding tube-to-tubesheet projects using higher-alloyed materials are also being planned for the future.
By Barbara K. Henon, Ph.D., Arc Machines, Inc.
The author gratefully acknowledges the kind assistance of VMIJ's Rob van den Berg, manager of procurement and subcontracting; Kees van de Mast, quality control manager, and Rob Vermeer, sales manager, for providing technical information.
The author would also like to thank Marco Pensa of Arc Machines, Inc., European Office, for his contribution in coordinating communication between the author and VMIJ.