Collaborative Machining Equals Success in Nuclear Power Plant
When service engineers face the common but difficult maintenance challenges necessary to extend the reactor life in nuclear power plants, precision is key. Equally important is having partners who share the common goal of accuracy.
Babcock & Wilcox Canada Ltd. (B&W) is no exception. B&W is a developer and supplier of innovative solutions for global building and repairing of energy utility plants. The company provides power generation systems and equipment in more than 800 utilities and industries worldwide. Recently B&W sought a viable machining solution from a trusted expert to support its maintenance program for a CANDU 6 nuclear power plant in Asia.
When the company examined the plant during a scheduled maintenance outage it found that the steam generator man-way opening seals in an ASME Section III, Class 1 pressure boundary area needed repair. The port’s inner cover gasket, which measured 355 mm by 456 mm, was not seating properly. In addition, the stud seals were not leak-tight, causing a pressure differential between the port’s inner and outer covers. Integrity was at risk. The solution team at B&W needed to determine how to balance the geometry of the port and redesign the covers’ gaskets.
In addition, the steam generator’s hemispherical head was developed out of forged steel and the exact properties of the material were unknown. An earlier attempt by B&W to machine the opening with lathe technology was unsuccessful: The material proved more difficult to cut than originally expected, and the feed rates of the cutting inserts could not withstand the tough material. Following its evaluation, B&W determined the job required a specialized machine to accomplish the difficult task at hand.
Machine Design Specifications
B&W turned to Climax Portable Machine Tools for its engineering expertise and breadth of machining solutions. A team comprised of B&W engineers and project managers, and Climax engineering specialists developed a cutting repair solution to quickly and accurately machine the difficult material.
First, the team needed to determine the project requirements. They knew the tool had to operate in a tight space and would need to withstand cutting tough material. Second, the solution had to perform a dry cut. Third, the tool needed to be rigid, accurate, and quick to set up. Finally, with a tight 24-hour time allotment to complete the machining, the machine also needed to cut speedily.
The tool’s mounting requirements were another consideration. The team needed one stationary bracket to mount the tool because it would be in a crowded area where the primary head of the generators was surrounded by process piping and system components. This made access difficult. In addition, to accommodate unforeseen machining adjustments, on-site, the tool needed to include several interchangeable cutting heads.
A Standard Machine Redesign Provides the Solution
Meeting all these machining requirements limited the engineers’ tool options. After much assessment and analysis, the team decided to adapt a standard Climax Portable Machine Tools BB5000 Boring Machine. The tool’s ability to program precise cutter movements provided the flexibility necessary to perform onsite cutting of material with unknown qualities. The machine also offered precise control of the spindle RPM, allowing feed rates and cutter movements to be fine-tuned by the operator at the repair area.
The boring machine incorporated changeable tooling and cutting technology to provide accurate cuts and inserts. An adjustable mechanical stop and an incremental adjustment process were machined into the tool head and bit. The cutting head was automatically fed axially on a traveling bar using the standard axial feed screw with mechanical stops. The radial feed was manually adjustable using a tapered locking mechanism.
It also included a 108 mm diameter x 1219 mm long chromed bar, a rotational drive unit and an axial feed unit. The boring machine also featured an electric drive motor, 115v 50/60 Hz, with two-speed gearboxes and an 115v remote control pendant with variable speed and stop start.
To meet the job’s difficult attachment concerns, a special bearing mount was developed into a slide mechanism and attached to a modified version of the hydraulic chuck supplied by B&W. The modified chuck incorporated a slide, which held the bearing and allowed it to move from one side of the bore to the other. The chuck was mounted using stops and screws at either end of the slide. A passage bore cut into the chuck enabled the hydraulic line to connect and move to either side of the bore.
B&W engineers supplied Climax with a 3-meter hydraulic hose with a quick connect for attaching to a hydraulic pump for activating the chucking system. A B&W manifold attached to the top of the slide mechanisms, and the lines were routed away from the bar using hose brackets and looms to provide stability to the machine once it
was mounted on-site.
A special standoff bracket supported the bar’s other end and used the manway pivot block as the primary mount. The fixture was anchored against the wall of the steam generator and the plant’s structural steel. Adjustable legs with jacking feet were expandable to secure the fixture in place. The team designed a slide mechanism into the standoff bracket for positioning the bar on both ends.
Once the new boring tool was completed, B&W’s operational staff visited Climax Portable Machine Tools’ Training Center in Newberg, Oregon, for a full education on its capabilities. Machinists tested the boring tool by simulating the repair on a replication of the repair site. This testing and training process provided B&W with a level of confidence that the tool could complete the work to specification within the allotted time. Having the opportunity to learn how to use the machine before on-site work began also helped reduce any on-site guesswork by machinists.
Approval Cycles
Before deploying the tool, Climax proved the boring machine’s capabilities to a validation committee comprised of B&W and personnel from the nuclear plant. The acceptance criteria were based on set-up times, cutting rates and reliability of operation over what was required at the site. This was deemed necessary to ensure that the equipment could complete the machining functions successfully, considering the uncertainties of the vessel material properties.
The final solution met expectations and passed a test plan before any work was completed.
The Repair
On-site, B&W machinists mounted the customized boring bar to re-machine the outer seal of the existing port.
The crew enlarged the opening from 492 mm to 592 mm in an obround shape to provide a footprint of the opening size for counter-boring. Machinists removed approximately three inches radially into the primary head of the material during this first machining step and five eighths of an inch in the width.
The machinist’s second step in the job was to counter-bore off a considerable amount of material to form an obround opening from a round opening. For the major shape to match the outer seal’s opening, they completed cutting the opening from 193 mm to 211 mm.
Once the operators made the cuts, they completed the repair by using a B&W developed Computer Numerically Controlled (CNC) machine to achieve the necessary gasket face location and size. This machine also accurately produced the desired smooth finish of the obround shape.
Results
The final repair results met all of the plant mangers’ expectations. Together, the teams exceeded the exacting
cutting requirements and beat the time requirements.
By working together, B&W and Climax efficiently and collaboratively defined the requirements and customized the tool according to specifications including space and location criteria. Additionally, they shortened the development process and accounted for all potential problems during the machining process. As a result, Climax completed the machine from quote to finished job in 12 weeks — well within the targeted outage time. Together, B&W and Climax proved to be partners that could quickly and accurately get the job done.
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