Innovative Machining and Welding Solution Results in Major Time and Cost Savings for Hydroelectric Plant Repairs

Innovative Machining and Welding Solution Results in Major Time and Cost Savings for Hydroelectric Plant Repairs

New York Power Authority (NYPA), one of the country’s largest state public power organizations had a 50” metal shaft sleeve that had premature wear, horizontal grooving and cycle fatigue cracking at their Blenheim-Gilboa (B-G) Pumped-Storage Hydroelectric Power Plant. The shaft is located at the base of Brown Mountain at subterranean depths making repairs tricky.

NYPA’s primary contractor stated that they would have to remove the entire 55-ton shaft in order to replace the shaft sleeve, thereby incurring a costly replacement program.

Plant Information

The Blenheim-Gilboa plant generates over 1,160 megawatts of electricity. At times of peak demand, water from B-G’s upper reservoir plunges 1,200 feet through a 28-foot diameter vertical shaft within Brown Mountain to drive the four pump-turbine generator motors. Above each pump-turbine is a shaft sleeve, surrounded by a seal, or “packing box”, all of which controls the leakage flow rate. The shafts and corresponding seals must provide high wear resistance to the abrasive contaminants in the reservoir water.

The Challenge
It was found that during a Life Extension & Modernization (LEM) evaluation that a more abrasive seal material had been used. This combined with reduced shaft sleeve hardness and extreme particulate (silt) velocities, created excessive wear in the seal area of the new shaft sleeves. It was determined that all four pump-turbine units would need to be replaced or repaired. Not only was cost a concern, but also the tight confines around the shaft and the toxic build-up of argon gases during welding posed a real health risk to project personnel.

During the repair feasibility phase, NYPA in addition to the welding processes determined that the machining equipment, mechanized welding equipment, mockup simulation and equipment training would be keys to the successful completion of the very difficult welding repair project.

Equipment Solution Parameters
During the repair planning process NYPA’s Senior Mechanical Engineer, Larry Spiro and Chris Brown, Mechanical Maintenance Manager, discussed several plans with their team. Based on multiple team discussions, they determined that due to the location of the shaft sleeve physical access would be demanding and with the GTAW processes being used, an IDLH atmosphere potential was almost certain in the subterranean area.

Larry and Chris’s team decided the repair process should be completed by NYPA maintenance personnel, after thorough training, and in doing so, the repair implementation effort would benefit from the maintenance personnel’s plant familiarity and knowledge.

As the shaft sleeve repair planning moved forward, and as previously stated, the welding processes, mockup simulation, equipment and training quickly all became project priorities. Based on the complexity of the repair, the planning team decided to reach out to an OEM they had previously worked with. Larry Spiro, Senior Mechanical Engineer, NYPA had successfully worked with Tri Tool in the past and called to find out if Tri Tool had a solution to their challenge.

Based on Larry’s description of the machining/welding repair objectives, physical access limitations and a host of other difficult machining/welding repair project attributes; Tri Tool’ engineering team developed a custom equipment and service solution for NYPA’s review.

Using predominately “off the shelf ” equipment, Tri Tool’s engineers worked closely with the staff at NYPA to design the perfect solution. The resulting design included a custom 655SB clamshell lathe, an OD turning tool module and the AdaptARC® mechanized welding system. Tri Tool manufactured the complete system, and then “mocked” up a metal sleeve and trained B-G’s mechanics on the hydraulic clamshell and orbital welding tools. After extensive training with Tri Tool, Blenheim-Gilboa scheduled a September outage for the unit that showed the most wear.

This integral machining/mechanized welding system design also allowed for remote operation of the welding process. This feature was very important to NYPA for the safety of their maintenance personnel based on the potential for an IDLH atmosphere while using the GTAW repair welding process. Additionally, this integral machining/welding system approach added value to the repair project effort due to the confined physical location/access to the weld repair location on the shaft sleeve.

Training, Mockup (repair simulation) and Weld Process Development
Equipment training, mockup (repair simulation) and weld process development were initially identified as key components to repair project performance objectives. The NYPA team “cut no corners” when it came to these objectives.

Factory acceptance testing (FAT) was witnessed by NYPA’s repair team personnel at the OEM’s manufacturing facility prior to acceptance and shipment of the integral Machining and Mechanized Welding System. Upon receipt of the system onsite at the Blenheim-Gilboa Power Plant, NYPA requisitioned the OEM’s Field Service personnel to mobilize and perform initial system training.

Once NYPA completed the fabrication of the mockup fixture at the plant’s onsite shop facility, they once again, requisitioned the OEM’s Field Service personnel to mobilize and assist with the mockup (repair simulation) activities. The mockup proved very valuable in that it allowed a familiar set of eyes (OEM’s Field Service personnel) to assist and critique the mockup portion of the project.

Per Chris Brown, Mechanical Maintenance Manager, the training and mockup effort represented over 800 man-hours of work/budget expenditure. Chris also said this provided valuable information and “lessons learned” that were integrated into the Shaft Sleeve Machining and Welding Repair Plan.

Based on weld process and procedure qualification records provided by NYPA, the intent of the repair welding was to provide a new hard facing surface to the OD of the shaft sleeve seal area. As discussed earlier, the increased wear on the original shaft sleeve was determined to be primarily a result of lower hardness material in the range of 135 BHN. (Brinnell) The original shaft sleeve material had hardness readings in the range of 200 BHN (Brinnel) and provided longer life expectancy in-service.

Procedure qualification testing showed (as detailed below) that weld deposit of ER309LMo filler material onto a 410 SS base material provided an average BHN (Brinnell) reading of 176 in the weld, 222 in the HAZ, and 168 in the unaffected base material.

Assumptions:

  • Hardness travers was assumed as shown below, however this information was not provided in the lab reports.
  • Original test lab readings were provided in HV10 (Vickers) and converted to BHN (Brinnell) for the purpose of discussion.

PQR thickness utilized was 0.280” (6.625” OD, Schedule 40) providing a qualified thickness range of 0.28” – 1”.

The production material was 50” diameter, 0.875” thick which would provide a considerable increase in heat sink. This additional heat sink should have provided a higher cooling rate, given that the same heat inputs as qualification were used, and subsequently should result in increased hardness reading in the HAZ. Hardness in the production weld should not increase substantially with a higher cooling rate, given the same heat input, depth of weld deposit, and filler material from the PQR was utilized in production.

The Repair Process and Project Outcome

First, the 655SB clamshell, known for its ability to perform within tight clearances and tolerances, was used to machine out the degraded area using the OD turning tool module, on the steel shaft down to .300”. Next, the mechanized AdaptARC welding system was mounted to the clamshell and performed a multi-layer GTAW deposition/cladding overlay. Due to the tight confines and the safety of the mechanics, work was performed utilizing a remote camera where they could control the quality of the welds. The welded area was then machined to ensure an acceptable surface finish and precision seal within a tolerance of no more than a .002” at a 63RMS, restoring it to original specifications.

Two shaft sleeve repairs have been successfully completed. According to NYPA plant management the projects went flawless. The stellar project performance according to the plant management was greatly attributed to the significant teamwork achieved between the plant and the OEM.

Machining tolerances (dimensional and final machined finish) requirements were met or exceeded on the repair project. Welding operations and processes parameters were monitored and in compliance with all requirements.

Project safety and IDLH atmosphere concerns were eliminated by the integration of the remote vision capabilities included in the final design for the mechanized welding repair system and scope.

Because of the success of the first shaft sleeve repair, Blenheim- Gilboa recently completed the second shaft sleeve and intends
to schedule outages for its other two shafts to eliminate the problem of the carbon and resin seals causing wear to the stainless steel sleeves.

“I was very impressed with Tri Tool’s design group customizing a tool for our unique application and the excellent customer support service they provided when the work was taking place. The project came off exactly as it was planned and I totally thank the Tri Tool staff with all their knowledge, cooperation and professionalism that was provided.”  Lawrence M Spiro, P.E. Senior Mechanical Engineer, NYPA

The integral system design of the Machining and Mechanized Welding System played a major role in the viability, execution and performance of the Shaft Sleeve Repair.

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