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Repair technique extends rotary dumper life

Issue 9 and Volume 101.

Repair technique extends rotary dumper life

By Justin Alberty and Johnnie Zibert, Grand River Dam Authority

Rehabilitation project reduces maintenance downtime and avoids costly equipment replacement

In mid-1996, after 15 years and nearly 500,000 rotations to unload 50 million tons of coal, the Heyl & Patterson-manufactured rotary dumper at the Grand River Dam Authority`s (GRDA) two-unit, 1,010 MW Coal-Fired Complex in Chouteau, Okla., began to show its age. Cracks appeared on the spill truss side of the end rings, along a T-section where the base plate joins the web plate.

Plant personnel suspected that severe stress on the fatigued metal of the end rings caused the cracking. The cracks forced GRDA`s coal yard maintenance crew to spend more and more time making repair welds, thereby decreasing the dumper`s availability.

The failures were running in cycles and were becoming a major problem for GRDA. Every six months employees had to spend a week or two making major repair welds. About June, GRDA began to realize that further steps had to be taken because it was not going to be able to keep the dumper reliable and safe. The metal was just too fatigued.

In addition to metal fatigue concerns, GRDA engineers worried about the stress on the spill truss side of the end rings; if not reduced, this stress would continue to cause premature failures because of the existence of only one trunnion support wheel. Each dumper rotation cycle concentrated a tremendous amount of stress at the point where the single trunnion wheel made contact with the rail. Simply put, there was neither enough support beneath the assembly nor enough stability within it to withstand the generated stress levels.

Repair options

At a materials-handling seminar in Pittsburgh in late 1996, GRDA learned that many other coal-fired power plants were experiencing similar problems with rotary car dumpers. Unfortunately, these plants were all practicing the same solution as GRDA: repair welds. GRDA identified two viable repair options–a complete replacement of the rotary barrel assembly or a complete replacement of the end rings–to remedy the problem, but both were expensive and time-consuming. A third option available to GRDA, which some other dumper operators were trying, involved conversion to a dual-trunnion wheel assembly to reduce the stress concentration on the end rings.

In December 1996, as maintenance crews once again made major repair welds, Johnnie Zibert (GRDA coal yard maintenance supervisor) and Dan Hudnall (GRDA assistant superintendent of maintenance at the facility) carefully considered each repair option. Somewhat unwilling to “bite the bullet” and proceed with costly repairs that would require extended downtime, the two intently studied the situation, hoping to find another repair method. When they couldn`t identify a viable technical alternative, they did the next best thing and invented one.

This whole procedure was based on theory. There was not a lot of real experience to go on. It had to be theoretical.

Starting from simple sketches of the end rings, the GRDA project team completely re-evaluated the situation by focusing on the failure symptoms of the problem. Because an overwhelming majority of the cracks were concentrated in one area along the spill truss side, it did not seem necessary to replace the whole end ring; therefore, it was certainly not necessary to replace the whole barrel assembly. The GRDA maintenance staff soon conceived a repair method (see figure)–involving the replacement of only the weakened sections of the two end rings–that, in theory, would cost less and require less downtime, while also greatly extending the life of the dumper.

Project implementation

Although it was a method that had never been attempted before, GRDA management approved the unique approach, providing the maintenance staff with three months to prepare for the repair project, which would occur during a routine spring maintenance outage at the facility.

GRDA drafting technicians transformed the conceptual sketches for the repair into appropriately detailed design drawings for internal technical review. GRDA engineers modified the design in a few areas to ensure the stability of the heavy-duty dumper. After final engineering approval, GRDA began the bid process for project materials and mechanical/construction contractors.

Through the bid process, GRDA selected Commercial Fabricators Inc. (CFI)–a GRDA electricity customer and a neighbor in the MidAmerica Industrial Park–to fabricate the new end-ring sections. GRDA officials visited the fabrication shop to confirm that CFI had the adequate equipment and space to complete this portion of the project and, most importantly, to ensure that it could be completed in time.

CFI was responsible for the construction of web plate sections across nearly one-half the circumference of each end ring. Per GRDA design, CFI fabricated these pieces from three-fourths-inch steel, providing greater structural strength than the original one-half-inch web plates. Also per GRDA design, CFI drilled a new bolt pattern into the end-ring rails to standardize the rail system all the way around the end rings. In the past, rail sections had not been interchangeable, leading to long periods of downtime when rails had to be replaced.

For installation of the new end-ring sections, GRDA enlisted the services of Williams Construction Co. Williams` familiarity with GRDA`s rotary dumper–having performed some repair work on the unit in past years–accelerated the project schedule and significantly contributed to the project`s success. Williams used two crews, working 10-hour shifts, to complete its portion of the project in only one week.

Following Williams` portion of the project, Heyl & Patterson supervised the installation and alignment of a set of two-wheel trunnions on the spill truss side of each end ring. GRDA “split the difference” with its trunnion wheel design. Manufacturers typically offer either a single-trunnion wheel at each corner of the dumper (four wheels total) or an upgraded dual-trunnion wheel system (eight total wheels). After analyzing past problems and discussing the current situation with Heyl & Patterson, GRDA elected to upgrade to dual-trunnion wheels on only the spill truss side corners (six total wheels). This helped disperse the weight from the relatively small point of contact between the rail and the single-trunnion wheel into a much broader area across the two wheels. Also, with each end ring strengthened with thicker steel, there would be much greater structural integrity within the barrel assembly itself.

Payback

GRDA has received favorable results from its rotary dumper rehabilitation project. The rejuvenated rotary dumper now possesses a decade-long life expectancy, and downtime for weld repair has been eliminated. Furthermore, GRDA realized significant savings from the project. Total costs for the repair project, including labor, parts and materials amounted to approximately $185,000. For a complete barrel assembly replacement–which would have required two new end rings, labor for two weeks and a new eight-wheel trunnion support system–GRDA engineers estimated costs at $540,000, resulting in a net cost savings of $355,000. z

Authors:

Justin Alberty has been the corporate journalist for GRDA`s Media Services Department for three years. Alberty is the author of several company newsletters and magazines and has written several articles about GRDA projects for various trade publications.

Johnnie Zibert is the coal yard maintenance supervisor for GRDA`s Coal-Fired Complex. Zibert has been with GRDA since the Coal-Fired Complex`s construction in the early 1980s and has gained extensive experience in the design and operation of coal-handling equipment.

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Williams Construction Co. employees work to install new end-ring sections on the rotary car dumper at GRDA`s Coal-Fired Complex in Chouteau, Okla.

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The old and the new. On the left is the original single-wheel trunnion; on the right is the new, two-wheel model. GRDA decided to install two-wheel trunnions on the spill truss sideof the dumper tobolster support.