Orbital Welding of Dissimmilar Boiler Tubes Saves Contractor $375,000

Orbital Welding of Dissimmilar Boiler Tubes Saves Contractor $375,000


Paper Presented by J. Mark Frederick, Carolina Power & Light Company Southeastern Electric Exchange Production Section, Perdido Beach, Alabama

The purpose of this paper is to describe the process of automatic welding of boiler tubes at the Roxboro Plant of Carolina Power & Light Company. The major benefits of the process over manual welding methods are: quality of welds, reduced personnel resources, time reduction, and cost savings. 

power plant
Roxboro Electric Generating Plant

Carolina Power & Light System Background

CP&L is an investor-owned, tax paying, public electric utility whose service area covers about half of North Carolina and one-fourth of South Carolina with approximately 950,000 customers.

Roxboro Electric Generating Plant Background

The Roxboro Electric Generating Plant is the largest fossil generating plant of Carolina Power & Light Company. Its net generation is 2477 megawatts.

Roxboro Unit 2 Background

Unit No. 2 at the Roxboro Plant, where the automatic welding was performed, is a Combustion Engineering steam generator and a General Electric 670 MW net capacity turbine generator.

The unit was placed in operation in 1968 and has a 4.6 million pound per hour steam generating capacity.

Dissimilar Weld Repair Program Roxboro Plant

Management at the Roxboro Plant formalized a program to repair the approximate 3,000 tubes with dissimilar welds on Unit No. 2 over a period of 6 years due to problems identified on the unit along with problems occurring elsewhere in the industry. The objective was to grind or machine the carbon steel side of the welds on each tube down to the root and leave the stainless material. All tubes would then be manually rewelded with an "Incoweld A" filler. The reason was that the actual separation of dissimilar welds occurs on the carbon side while the stainless remains intact. All the dissimilar welds in the furnace area were repaired during outages lasting less than 2 weeks in length.

repairs
Welding operator makes fine adjustments while the Model 81 Weld Head is mounted in position.

We planned to perform manual repairs to the superheater and reheater tubes in the penthouse in 1988. This outage was scheduled for 12 weeks. The scope of work had to be modified over what had been done in the furnace due to the physical restrictions in the area which would require us to cut our way in and weld our way out. Dutchmen, including the dissimilar weld, were fabricated before and during the outage utilizing both manual and automatic welding processes.

A contract had been negotiated with a contractor to make the repairs to the 1,500 tubes with dissimilar welds in the reheater & superheater area utilizing manual welding processes. (750 reheater tubes & 750 superheater tubes). Approximately 4 weeks into the outage, the contractor experienced a 16% reject rate on welds, along with a shortage of manpower to complete the project on schedule. The management of the Roxboro Plant negotiated and contracted Power Cutting, Inc. who originally aided in the fabrication of the dutchmen, to utilize the automatic welding process on the repairs of the 750 superheater tubes. The superheater tubes were 2" in diameter with a .420" wall thickness on 4" centers. The actual welds would total 1,500 which included T-22 and 347H material. The reheater tubes would continue to be repaired manually by the original contractor.

Method of Repair Utilizing Automatic Welding

Power Cutting, Inc. utilized a Model 115 Power Source with a Model 81 Compact weld head manufactured by Arc Machines, Inc. of Pacoima, California. The following plan was developed to utilize automatic welding:

  • All 750 tubes with dissimilar welds would be cut out of penthouse in 2' lengths.
  • All tube ends remaining in the penthouse would be prepped.
  • New dutchmen would be tacked in place by qualified welders.
  • 2 certified welders would be required to operate each of the 2 automatic machines to weld root, filler and cover passes.

 

Summary of Utilizing Automatic Process

By utilizing the automatic welding process, we were able to complete all of the superheater dissimilar welds in the penthouse. Listed below is the specific data comparing the automatic process for the superheater vs. the manual method we had intended to use. Due to the reheater and superheater jobs being similar in scope, the information compiled from the manual reheater process was utilized for the superheater. Both jobs had to be 100% completed during the outage due to impact on future reliability and outage schedules.

Comparison of Utilized Automatic Welding vs. Manual Welding

 AutomaticManual
 

Tube Description2" OD x .420" wall2" OD x .420" wall
Quantity of Welds347H & T-22347H & T-22
Personal Requirements2075
% of Welds Completed100%100%
Reject Rate by Radiography7 of 1,500 welds  0.47%240 of 1,500 welds or 16%
Labor cost including equipment$550,000$925.000


As we identified during the outage, automatic welding is cost effective with high quality results. It can be utilized in areas, such as in boilers, where there is limited accessibility and where there is a shortage of qualified manpower. CP&L has also used the automatic welder on feedwater piping at our nuclear plants with great success.

J. Mark Frederick, Carolina Power & Light Company