Utility Self-Performs on Boiler Tube Replacement

Utility Self-Performs on Boiler Tube Replacement

Consolidated Edison's Power Generation Maintenance Department recently completed a major maintenance outage at their Astoria 50 boiler in Queens, N.Y. This boiler repair and repairs at their Ravenswood facilities were done using advanced technology orbital pipe welding equipment manufactured by Arc Machines, Inc., of Pacoima, California.

This investment of capital was part of a well-thought-out plan to save money on this job as well as on future outages.

Part of this plan involved the use of machine welding to improve weld quality and productivity. Improved weld quality would result in better welds, reduce the frequency of future repairs, and extend the operating life of the boiler.

Consolidated Edison operates 8 different power plants in the 5 boroughs of New York City. These are primary fossil fuel plants, but they also operate the Indian Point Nuclear Facility in Westchester County. The Astoria 50 boiler, on which the present repairs were done, is a 380 MW Combustion Engineering twin furnace, radiant reheat controlled circulation boiler. The boiler was designed as a pressurized unit, but was later converted to a balanced draft operation. Both furnaces are supplied with a tilting tangential firing system capable of burning oil or natural gas through four elevations of burners. The boilers produce 2.4 million pounds of steam per hour to run the generators to make electrical power. The repairs on Astoria 50 were done as part of a Major Maintenance Outage which are scheduled every 5-7 years. The boiler was placed in service in 1962 and had 202,566 operating hours at the time of this overhaul. This outage began in September of 1991, and the boiler was back in service in May, 1992. Con Edison made the decision to purchase the orbital welding equipment and to train their own maintenance personnel to operate the equipment. Alternatively, they could have hired an outside contractor that owned orbital welding equipment and paid them to do the tubing repairs and replacements. This would have avoided the initial investment in capital equipment for Con Edison, but would have been more expensive in the long run.

Con Edison purchased four Arc Machines, Inc. Model 215 pipe welding power supplies and leased another two during the summer of 1991. The welding systems included the power supplies and the smaller, Model 81 pipe welding heads.

Certified Welding Inspectors from Con Edison who had the responsibility for overseeing the field operations during the shutdown, came to Arc Machines, Inc. in California for initial training on the welding systems. They were trained by Arc Machines' Pipe Welder Product Manager. Additional training of their welding operators was done in August of 1991 by John Cushing, Arc Machines' Manager of Customer Service, who trained two groups of 8 welders each from Local 1 and 2 of Utility Workers Union, in two 5-day classes on-site at Con Edison's facility.

Training and Weld Procedure

Fig. 1. The AMI Model 81 Weld Head is shown mounted in position for welding the side wall tubes where they meet the upper rear header at Astoria Station. The welding operator makes fine adjustments to weld parameters with the Model 215 Program Operator Pendant shown in the foreground.

During the first part of the training, the welder/mechanics learned to set up the equipment and how to operate it. When they were comfortable handling the equipment, they were taught how to develop weld schedules or programs for the different size and type of tube. These schedules are the numbers for weld parameters such as welding current, travel speed, automatic arc voltage control (which maintains a constant arc gap), wire feed speed, torch oscillation, pulse times, etc. These values are entered into the microprocessor-based Model 215 power supply from a portable program operator pendant (POP) and stored in memory. Each weld program and each pass of a particular weld program can easily be recalled for immediate use. This permits the use of weld schedules which can be precisely repeated for each similar weld joint and provides a high degree of repeatability from weld to weld.

Before beginning the welding for the replacement project, the weld procedure for each different type of weld had to be qualified to ASME Section IX of the Boiler and Pressure Vessel Code. This requires three documents, the Weld Procedure Specification (WPS) and the Procedure Qualification Record (PQR) which qualify the welding process and welding equipment, and the Performance Qualification Record (PQR) which qualifies the operator who performed the test weld. These tests require the documentation of test conditions and weld parameters for the welding of test coupons which are subjected to tensile tests to test the strength of the weld and to bends of the root and face of the welds to assure that the welds are ductile.

The orbital pipe welding equipment was used on the terminal tube legs of the superheater intermediate pendents, the basket bottom replacement, and the front pendent loops of the Astoria 50 boiler.

Superheater Terminal Tube

Failure of the dissimilar metal welds (DMW) in the terminal tube legs of the superheater intermediate pendents made it necessary to replace them. Replacement of the DMWs required removal of the existing terminal tubes. In removing the terminal tubes, both the old bottom DMWs, and upper T-22 low alloy steel welds were removed. This was accomplished by making field cuts with a low clearance track saw 1/2 inch below the DMWs and 1/2 inch above the T22 bw alloy steel weld, completely removing the weld and its heat-affected zone (HAZ).

Both the existing tube ends and the replacement tube ends were prepped with standard 37-172 degree bevels. The replacement terminal tubes included DMWs which had been fabricated by Combustion Engineering in their facility.

Combustion Engineering used the EPRI (CS-, 4252)guide lines for this fabrication which had a 60 degree included bevel angle on the low steel side of the joint. The DMW roots were welded with automatic GTAW and completed with GMAW. The similar metal welds were made in the field by Con Edison on the T-22 and 321 stainless steel weld joints. The stainless tubing was 2.125" OD with a wall thickness of 0.480".

Fig. 2. Welding being performed on the upper rear drum at Astoria Station. The operator is observing the arc through a welding helmet and, if necessary, can make small corrections to the weld program via the Program Operalor Pendant.

Since an ID purge was not feasible in this application, a backing ring was used on the stainless tubing to minimize oxidation of the weld ID. The bottom stainless steel welds were made using automatic orbital GTAW. A total of 395 welds out of 790 welds at this location were done by machine as the remainder had only 1 inch radial clearance while 1.75 inches of radial clearance is required for the Model 81. The welds were of excellent quality as shown by the clarity of the X-rays.

Now that Con Edison has had experience with the orbital GTAW equipment, they plan to fabricate the DMWs in their own shop on their next outage.

The lower stainless steel welds were approximately 3 inches off the floor of the penthouse making it impractical to weld manually. With the orbital welding equipment, there was an improvement of approximately 50% in productivity for welding in this difficult location.

Weld sequencing is a critical part of planning for an outage such as this. The order in which the tubes were welded with orbital equipment was different from manual welding. For example, the terminal tubes are arranged in five rows of 79 elements from North to South. With the orbital head, they welded one row of 79 straight across the furnace to get the most favorable positioning of the weld head, while on a manual job they would have welded all 5 tubes from each row, and then done the next 5, etc.

Basket Bottom Replacement

basletFig. 3. Typical completed welds on the upper rear drum where the side wall meets the header nipples at Astoria Station. Tight radial clearances make these weld joints difficult to access for manual welding, but present no problem for the M-81 weld head.

Also at Astoria, boiler water from the water wall leaks had caused cold-side (external) corrosion of the basket bottom tubing which mandated its replacement. This job consisted of 318 tubes on each front and rear wall, and 204 tubes on each side wall (Fig. 1). There were also 130 tube welds on the four intermediate side walls. This replacement required a total of 1,064 welds on the lower drum and 322 welds on the upper drum for a total of 1,386 welds (Figures 2-5).

When they were ready to weld the tube bend configurations for the lower front drum and side wall drum connections, Con Edison found that they had an axial clearance which was limited to only 5.5 inches. Since this was less than the 6.325 inches axial clearance required for the water-cooled M-81 weld head, they had to engineer a special clamp to adapt the M-81 to fit the reduced clearance. They changed the rollers and did what was necessary to make the head work on the reduced clearance welds.

The clamp was developed under a research and development contract for the Ravenswood outage and was subsequently used for the basket bottom tubing at Astoria, since both boilers are identical in design. The new clamp design greatly increased the number of welds that could be done orbitally on the boilers.

The first welds coming off the drum were made between SA 192 carbon steel, Grade B, 1-1/2 to 1-5/8 inch OD, 0.185 wall tubing to SA 106 carbon steel, Grade C nipples, with a maximum carbon content of 0.025 wt%, and a wall thickness of 0.440". These small diameter welds were done in 3 or 4 passes with stringer beads.

Superheater/Reheat Pendent Welds

Replacement of the 78 front pendent elements, 39 elements in each furnace, was predicated by recent overheat failures. Nondestructive oxide film thickness evaluations coupled with sample tube analysis confirmed the need for replacement. The pendent loops consist of five inlet-outlet leg connections coupled to their respective headers. The pendent elements were upgraded to include stainless steel, and T-22 (2-1/4 Cr, 1 Mo) in place of T-11 sections. ER-9OS wire was used as filler.

For welding the pendent loops, the M81 weld head was mounted upside down on the tubes with the torch pointing downwards. The operators found it easier to handle the cables with the weld head in this position, and the torch was protected from falling objects. ER-9OS filler was used to make the weld after a 400-600 degree F preheat.

In spite of the admittedly rough environment inside the boiler, the welding equipment performed well. The welders had some basic maintenance training at AMI, they maintained an inventory of spare parts and were able to make most repairs themselves. They had some problems with electrical spikes in the supply voltage, so Con Edison purchased a power surge protection unit to electrically isolate the Model 215 Power Supply. For their next outage Con Edison plans to have additional training for their maintenance technicans in order to be completely self-sufficient. They feel that the 2 weeks that were allotted for the initial welder training & troubleshooting was not enough to fully develop the maintenance crew.

Fig. 4. The welding operators are shown using a mirror to observe the weld joint 4" from the floor being performed on the lower front drum nipples. The programming operator pendant is used to fine-tune steering of the torch during welding.

They intend to visit AMI to learn to disassemble and reassemble weld heads in order to become more proficient as periodic maintenance-and-repair welders/mechanics.

Results and Conclusions

Field welds were all subjected to radiography to test for porosity, lack of fusion, cracking, and other potential defects. Of the 2,500 field welds done with the orbital equipment which were tested by radiography, only 0.2% failed to pass the ASME PW-51 acceptance criteria and had to be repaired.

The use of the orbital equipment resulted in an improvement in efficiency which resulted in a higher level of productivity than could be achieved on a similar application done with manual welding technology. Production rates varied depending on location, tube OD and wall thickness. The terminal tubing and front pendent areas had peak production rates of 8 welds per 12 hour shift. The average production rate on the lower front drums was 10 welds per 8 hour shift with a peak of 16 welds per 8 hour shift. Production rates were somewhat higher on the upper drum with an average of 16 welds per 8 hour shift and a peak of 20 welds per shift. On two of the three jobs done by Con Edison with the orbital welding equipment, the jobs were completed in about half the time that a similar job manually would have taken.

Fig. 5. Model 81 We/d Head mounted on one of the lower front drum nipples where the axial clearance was only 5.5". The cables are prewrapped in preparation for welding. A special clamp, designed and built by Con Edison, permits the Model 81 to be mounted within this reduced axial clearance.

On the present outage, the overall time for the outage was comparable to a similar job done with manual welding technology. However, there was a saving in man-hours since, where amanual job would require two welders, a similar orbital job could be done in the same time frame with one welding power supply and weld head with a single operator. Con Edison has a talented welding crew which is dedicated to producing a quality job. They also give careful consideration to safety on the job.

On the Astoria 50 site they did a total of 6,000 welds. Of these about 2,500 were done with the orbital equipment. On the Ravenswood Number 10 boiler they did 6,000 boiler tube welds of which about 1,000 were done orbitally between October of 1991 and January of 1992. The welds that were not done with the orbital equipment were those where the radial clearance was less than 2 inches or otherwise consisted of tangent tubes.

Con Edison now owns their own equipment, they have a trained crew, and expect to do their own repairs during a planned outage at their Waterside, East River and Astoria 30 locations sometime next year. They are planning to purchase a larger, fullfunction, Model 15 weld head from AMI to extend their size capabilities for their next outage. This head will be used for welds on heavy-walled 8 inch, 12 inch, and 16 inch pipe.

The power industry has been slow to computerize their operations. Con Edison's Power Generation Maintenance Organization is leading the way in modernizing their welding operations with equipment which will save them money in the long run by making them self-sufficient in their maintenance operations. This attests to their efficiency, resulting in a longer service life of the boiler which will result in improved service to their customers, and is consistent with their cost-containment goals.

By Michael J. Jirinec, Technical Engineer,
Eugene V. Sanquini, Chief Certified Welding Inspector
and Richard Slote, Certified Welding Inspector,
Consolidated Edison Company of New York, Inc.,
Power Generation Maintenance Department, Bronx, New York
and B.K. Henon, Ph.D., Arc Machines, Inc.