CPFP's Gold River Mill Uses Automatic Orbital Welding to Repair Its Boiler
CPFP's Gold River Mill Uses Automatic Orbital Welding to Repair Its Boiler
The Gold River Pulp Mill of Canadian Pacific Forest Products Ltd., located in Gold River, B.C., recently underwent a planned shutdown for the purpose of overhauling its recovery boiler. The mill hired the design/engineering firm of Asea Brown Boveri (ABB Combustion Services Div.) for the project.
ABB has used high-technology orbital pipe welding equipment for a series of projects in Canada. The success of these projects is largely the work of welding specialists Steve Chambers of ABB in Edmonton, Alta., and Ted Jedruch of ABB in Thunder Bay, Ont. They utilized two Model 215 microprocessor-controlled pipe-welder power supplies, manufactured by Arc Machines of Pacoima, Calif., for the replacement of boiler tubes on the upper and lower economizer drums of Gold River's recovery boiler.
The team also used an Arc Machines' Model 81 pipe welding head with AVC tilt to hold a constant arc gap at a 45° tungsten angle during welding. The head was mounted on a special mandrel that locates inside of the tubing ends to support the weld head while making seal welds between the base of the tubes and the inside of the drums. The Model 81 is water-cooled to allow for a high-duty cycle.
PROJECT BENEFITS. Although ABB has used orbital MIG on previous boiler repairs, this was the firm's first application of orbital GTAW [tungsten-inert gas (TIG)] welding equipment for recovery boiler tubing replacement. Canadian Pacific's and ARE's reason for using this type of equipment for the boiler tube replacement project was primarily to obtain the quality of welds that can be achieved with this process. The difficult operating conditions under which welders must work to perform these welds manually tends to result in welds of inconsistent quality, which has led to problems in the past.
The two economizer drums have a 30-in. inside dia (i.d.), so the position in which a manual welder would work is very cramped. The tubes being welded in the upper economizer drum, located on the seventh floor of the boiler building, exit from the lower half of the drum and enter the upperhalf of the lower drum 42 ft below.
Finished welds of boiler tubes to the upper economizer drum. The drum has seven rows of 70 tubes each and two rows of 35 tubes each for a total of 560 welds per drum. Orbital welding technology produced excellent welds of consistent high quality.
The welder would either have to kneel or lie on his/her stomach to weld the tubes into the upper drum. The welder would have to lie on her/his back and weld in the overhead position, which is much more difficult, to install tubing in the lower drum.
A high-temperature working environment, resulting from the necessity of preheating the carbon steel drums and tubes to 200 degrees F prior to welding, adds to the welder's discomfort. With ambient temperatures outside the drums reaching 95 degrees F on some days, the heat inside the drums becomes unbearable. These working conditions are incompatible with the requirement that a manual welder achieve consistent high-quality welds.
However, sound welds are very important in this dual-purpose boiler system. The two functions of the recovery boiler are recovery of inorganic chemicals from black liquor to be recycled in the pulping process and the combustion of the organic constituents in the black liquor to produce valuable steam.(1) Water to be heated into steam circulates through the tubing in the economizer when the boiler is in operation.
Arc Machines Model 215 pipe welder power supply in the foreground and the upper economizer drum of the Gold River boiler in the background. Five-hundred sixty, 2-in.-dia boiler tubes extend between the upper and lower economizer drums. Frank York of Arc Machines observes the prepared tubes.
Explosions have occurred in recovery boilers in the past when water from various sources has leaked onto the smelt (inorganic salts recovered from the pulping process). In a survey of recovery boilers in the U.S. between 1948 and 1990, boiler leaks were classified as "critical" and "noncritical," where a critical leak is defined as one that has the potential to cause an explosion. While some critical leaks have developed in economizers and a few of these were weld-related, the economizer is not a primary site of critical leaks, and the majority of economizer leaks were classified as noncritical.(2)
However, weld-related defects in the boiler as a whole accounted for about half of all the critical leaks that occurred. Attachment welds were involved about three times as often as butt welds. The recovery boiler inspection survey stated that the most difficult potential problem to detect is a one-of-a-kind condition, such as a faulty weld. Thus, the consistent high-quality welds that are possible with orbital welding technology offer a practical solution to this longstanding problem.
WELD QUALIFICATION AND OPERATOR TRAINING. Prior to shutdown, ABB sent a mock-up drum section and some boiler tubes to Arc Machines to determine the feasibility of doing the welds with orbital TIG equipment. Frank York, Arc's pipe welder product manager and welding specialist, did the welds. The mock-up was shipped back to ABB for evaluation and testing.
Once the welds were found to be acceptable, the next step was to train operators to use orbital welding equipment. York went to Gold River to provide extensive welder training. Four days were devoted to training five welders from the International Brotherhood of Boilermakers, Local 359 of Vancouver, B.C.
In the course of the training, the operators learned to set up and operate the power supply with its cables and the weld head, as well as how to determine the values for programmable welding parameters, including welding current, travel speed and time, arc voltage, pulsation rate, wire-feed rate, etc. These numbers comprise the weld schedule for the particular weld. Once a repeatable program or weld schedule has been established, it is stored in the Model 215 Power Supply memory.
To make a weld, the operator calls up the weld schedule from the power supply memory, installs the weld head onto the weld joint, and pushes a button on the program operator pendant (POP) or auxiliary pendant to start the weld sequence. During the sequence, the welding operator may be required to make minor adjustments, via the POP, to steer the torch in order to achieve an ideal weld. The AVC may also require some adjustment during the weld to keep the torch a uniform optimum distance from the weld surface.
SCOPE OF WORK. The upper and lower economizer drums are each about 30 ft. long and 30 in. (i.d.) The upper economizer drum has nine rows of tubes projecting from both sides and underneath the lower half of the drum. Each row except the top row on either side has 70 tubes, while the spacing on the top rows is twice that of the lower rows. The holes (and the tubes) are 2 in. dia and most of them are 4 in. from center to center, providing 2-in. radial clearance between the tubes. There is a total of 560 welds for each economizer drum.
A small manway on both ends of the drum provides access for the welder. In preparation for welding, prior to their insertion into the drum, the tubes are cleaned and faced to make a smooth end and the oxide layer removed to about 6 in. back from the tubing ends. Each tube has fins welded in place along both sides of the vertical axis, which serve to direct the air flow when the boiler is in service.
The tubes have a 2 in. outside dia (o.d.) with a 0.165-in. wall and are stubbed into position and peened to hold them in place. The tubes are then rolled to expand them into a groove cut into the hole drilled in the drum. The tube ends project a distance of 3/8 in. to 1/2 in. on the inside of the drums. Formerly, the tubes were merely rolled into place and not required to be welded; but welding, especially orbital welding, will serve to prevent leaks and provide safer operating conditions.
Tubing ends, secured in place first in the upper economizer drum, being stubbed in place in the lower economizer drum. Tubes are peened and then rolled to expand them in place prior to welding and rolled again after welding.
The weld was a two-pass fillet weld with filler wire provided by a two-pound wire spool mounted on the weld head. The wire composition was ER-80S-D2 containing 0.5% molybdenum to prevent creep cracking and for added corrosion resistance. After welding, the rolling procedure was repeated for stress relief and grain refinement.
WELDING. The power supply was positioned on the floor outside the upper economizer drum for welding that drum. The cables were routed over the top of the drum when welds were being done on the opposite side of the drum from the power supply. The POP was held in a wooden box outside the drum with the POP display exposed. The welding operator had the auxiliary pendant with him in the drum to make minor steering corrections during welding.
Each weld took less than four minutes to complete. Time for mounting and dismounting of the weld head was minimal because the head remained in position on the mandrel while the mandrel was moved from tube to tube.
The drum's internal environment was closely monitored by safety personnel during the welding operation to ensure that the concentration of oxygen did not fall below minimal levels required to sustain life. Argon gas, although nontoxic, is used for purging and could displace the air in the drum, reducing the amount of oxygen present.
An air mover was used to suck air out of the drum to allow fresh air to enter, and the CO, CO2, and oxygen concentrations were checked on an hourly basis. Isopropyl alcohol was used to clean the tubes prior to welding because fumes from stronger solvents could not be tolerated in such a confined space.
Once a good welding procedure was achieved in the field, repeatable, high-quality, visually attractive welds became routine. There were two 10-hour shifts, seven days a week during the boiler shutdown. At the start of the job, the day shift welding operator shared space inside the drum with personnel who were fitting and rolling the tubing into place. Even in this situation, the day shift welder made 10 good welds in the first two hours of the day and up to 58 welds/shift.
Later in the job, when all the tubing had been rolled into place, two welders operated two Model 81 weld heads at the same time inside the drum. In this situation it was possible to get more than 80 welds/shift, which compares favorably with manual welding production rates.
The recovery boiler is very important to mill operation. It provides steam for mill processes and recovers chemicals used in pulping. The economizer drums comprise part of one of the water/steam circuits. Water enters the circuit at the lower economizer drum and rises in tubes to the upper economizer drum. It leaves the drum at more than 400 degrees F when it enters the steam drum. Oxygen is removed from the water to reduce the corrosiveness of the fluid environment, but the water and temperature cycles still contribute to a severe service environment. Downtime for repairs is very expensive. Canadian Pacific and ABB are convinced that the improved quality of the orbital TIG welds will keep future repairs to an absolute minimum.
This project was successful because of proper planning by both the owner and the engineering firm. Orbital welding equipment was selected far enough in advance of the shutdown to develop good welding procedures and to train welding personnel. Factory support by the welding equipment manufacturer was also important for the successful boiler replacement within the planned time frame. The successful use of orbital pipe welding equipment for this project suggests that this equipment will help eliminate the problems that have often been associated with welds on small drums.
By Barbara K. Henon, Ph.D., Arc Machines, Inc.
1. Joseph G. Singer, P.E., editor, Chapter 8: Boilers for Process Use/Power Production, Combustion Fossil Power, Fourth Edition, pp. 8-42 to 8-64, published by Combustion Engineering Inc., Windsor, Conn., 1991.
2. D.G. Bauer and W.B.A. Sharp, The inspection of recovery boilers to detect factors that cause critical leaks, Tappi Journal, 74(9):92 (1991)