Orbital Welding Saved $750,000 at Pulp Mill while Increasing the Life of Its Boiler
Orbital Welding Saved $750,000 at Pulp Mill while Increasing the Life of Its Boiler
James River used orbital welding technology to increase the life of its boiler at its Marathonmill in Canada. The result: A job well done in less time and for less money than any manual welding
Technicians aligned the new economizer header assembly to the old header tubes in preparation for welding.
A welding operator observes an orbital weld in progress. Green markings indicate that the welds have passed X-ray examination.
A welding technician holds the Program Operator Pendant in case torch steering is needed during boiler tube weld.
Completed orbital welds on economizer header tubes. Welds are identified by operators’ initials.
JAMES RIVER'S MARATHON pulp mill in Ontario, Canada, recently saved itself a lot of money. It extended the life of its boiler without replacing the unit's economizer headers and without having to use manual welders. Rather than replace the back economizer header bundles, which would have been very expensive, the mill replaced only the lower back headers and 2.1 m of tubing stubs using orbital welding technology. The resulting cost saving compared to manual welding is estimated at around $750,000.
Saving on economizers
The nine lower economizer headers of the boiler are arranged three deep and three across. Each of the lower headers is connected by vertical rows of tubing to the upper economizer header some 18 m above. Each upper and lower header and the tubing joining them comprise an economizer header bundle. The back three headers and attached tubing on the bottom were most in need of repair, and it was felt that if these were replaced, along with the bottom 2.1 m of tubing on each header, that the boiler life could be extended without the need to replace the entire set of back header bundles. This would have been very difficult to do within the time-frame of the planned 25-day shutdown and would have required removing the roof of the building and using a huge crane to raise the economizer bundles to the top of the boiler building. Replacement of the lower back headers was a much simpler, and considerably more economical solution.
However, this header replacement would not have been practical with manual welding techniques because of the limited radial clearance between the header tubes; the distance separating the rows of tubing was only 5 cm. Furthermore, there was no access from the other side of the back header tubes since this was blocked by the placement of the front header tubing.
ABB Combustion Engineering Services was contracted to carry out the project using orbital TIG welding equipment supplied by Arc Machines, USA. The company used a model 81 orbital pipe weld head which has a radial clearance of less than 5 cm. It can reportedly be mounted onto and removed from the tubes easily and makes a weld of this type in about eight minutes.
To make the repairs, the three lower headers and attached tubing were hoisted into place on the fifth floor of the boiler building. Since each header assembly weighs about 2,780 kg, this was not a simple operation, but still easier than replacing the header bundles would have been. Each header has six rows of tubes from back to front and is 26 tubes across. With the three header units placed side by side, this was a total of 78 tubes across and six tubes deep.
To replace the headers, the old headers and worn-out tubing were removed and the new assemblies hoisted into position for welding. The welds on the sixth row of tubing in from the back were done first. These were simple tube-to-tube welds joining the old tube ends to the new header tubes. To allow access of the weld head to the other weld joints, spool pieces were cut from new tubing. On the fifth row in, the spool pieces measured just under 23 cm long, and on each successive row out, the spool pieces were made 7.5 cm longer than on the row before.
The extra length on the spool pieces added to the length of tube which was replaced by new tubing. Thus, on the rows without the spool pieces, there was one weld per replacement tube, while on the rows with the spool pieces, there were two welds each, for a total of 858 field welds to be done in less than three weeks. Similar manual welds had previously needed about 10 hours to weld a small spool piece in place.
Welders train on the job
Welding supervisor, Ted Jedruch of ABB Combustion, selected the welders for the project from the International Brotherhood of Boilermakers in Thunder Bay, Ontario. The men were trained for two weeks in groups of four using a mock-up of the economizer. The mock-up was also used to develop weld programs or schedules to be used on-site, and to estimate the time for each weld, including set-up time to be used in planning the job.
Weld schedules are the numbers used to program the power supply, specifying the values for welding current, arc voltage control which determines the arc gap, travel speed of the torch around the weld joint, wire feed speed, pulse times, oscillation if used, and so on. These numbers are based on the tube diameter and wall thickness for the material being welded, which in this case was 5 cm outside diameter with a wall of 4.8-5.0 mm. Weld parameters are entered into the Arc Machines 215 microprocessor and stored in memory. The program parameters are entered via a program operator pendant (POP). The weld programs may also be modified from the POP as required. Any program can be called up from memory and used without affecting any of the other stored programs.
The new tubing material was the same as the old tubing which was 178 mild steel. "J" type insert rings and 0.8-mm diameter ER 80SD2 wire were used as filler. The "J" rings were made to order by the Weldring Company and with consistent dimensional tolerances, saved a lot of unnecessary adjustments during fit-up on the job.
The ends of the old header tubes were prepared for welding and the ends of the new and old tubes were cut and prepared with a 37.5 degree bevel for welding. A cut-off saw was used to prepare the spool pieces and the ends of these and the tubes were beveled with either TriTool or Protem end-preparation equipment. Prior to welding, the ends of the tubes were cleaned with acetone to remove grease. This was especially important for the old tubing to remove accumulated contaminants. The tubes were aligned to get a straight weld joint and pretacked in place using a Miller or Cannox power supply with a manual TIG torch to make the tack. Welds were generally done in two passes, a root pass and a fill pass which also served as a cap pass. If the fit-up between the tubes was good, two passes were adequate. If the gap was slightly wider, an additional pass was made, and a small amount of torch oscillation was programmed in if the gap was especially large. According to Jedruch, weld parameters remained consistent from weld-to-weld except that the arc voltage control (AVC), used to maintain a constant arc gap, had to be set at a higher level during the night shift due to the increase in humidity in the air. (The mill is on the edge of Lake Superior and the weather tends to turn foggy at night).
To provide a better view of the weld joint during welding, the weld head was modified slightly to provide a half torch extension of 3 cm. Operators used a mirror positioned behind the tube to visualize the weld joint. This was necessary to see whether the electrode remained centered on the weld joint or whether cross-seam adjustments were needed. If steering corrections to the torch were required, this was done via the POP. The root pass was welded in the anti-clockwise direction while the cap and fill pass were done in the clockwise direction. Passes were done in alternating directions to avoid having to rotate the weld head. Model 215 power supplies were placed on both ends of the headers at 6th-floor level where the weld heads were connected by standard cable lengths without the need for extensions. For quality-control purposes, the welders marked each weld with their initials.
Welds let go on green
The target for the project had been for a total of 60 orbital welds/day using two machines on two 10-hour shifts. The figure was somewhat less than this at the start of the job, but later in the project the welders achieved up to 35-38 welds per shift.
Normal specification for such a boiler repair requires only 10% of the welds to be examined by radiography. In James River's case, all the welds were subjected to radiography as an on-going part of the job to prevent any possibility of a bad weld joint in the tubing. Each weld joint was painted green after passing the radiography test so that no joint would be missed. A leak during hydrotesting would have made it necessary to cut out bundles of tubing to access the faulty joint for repair which would have been unacceptable.
The only cut-outs that were done were due to poor alignment after tacking. In this case, it was easier to cut the tacks and re-weld, rather than to try to weld a poorly-aligned joint. The repair rate was about 1.9%.
Upon completion of the project, the boiler was hydrotested. The system was filled with water, heated to operating temperature, and subjected to up to one and a half times the design operating pressure. The design operating temperature and pressure for the economizer headers was 371 degrees C and 5,516 kPa, so it was tested at 8,270 kPa. There were no leaks.
This article was written by Barbara Henon of Arc Machines, USA, in collaboration with Richard Jedruch and Ted Jedruch of ABB Combustion Services, Canada.