Save Article Instructions
Close 

Using Rapid Prototyping Methods to Repair Runner Cavitation Damage

A new technique developed by Ontario Power Generation allows for accurate, quick repair of cavitation damage on a runner blade. The technique is based on rapid prototyping methods and shows promise for other hydro plant applications.

By Darrell Lewis

To overcome limitations involved in performing repairs of turbine runner blades suffering from cavitation damage, the author developed a new repair technique based on rapid prototyping methods. This technique consists of using a portable scanner to produce a three-dimensional (3D) image of the runner blade surface that can be downloaded to a computer. The data can be analyzed and used to build a replacement piece that is then welded to the existing turbine runner.

Ontario Power Generation (OPG) has applied this technique twice, with good results. The first application showed that the technique was feasible and provided good correlation between the replacement piece and the damaged runner blade. The second application, at a working hydro facility, allowed OPG to continue running the unit while the replacement parts were manufactured. The utility plans to use this new technique for future cavitation repair work at its hydro facilities.

Standard cavitation repair techniques

Small bubbles can form at a pressure less than the vapor pressure of the water. If the vapor bubble collapses near the runner surface, highly localized pressure forces can remove runner material. This phenomenon, known as cavitation, can result in significant damage to a turbine runner.

OPG operates 65 hydro stations with a total capacity of 6,963 MW. The smallest station has a capacity of 1 MW; the largest more than 1,400 MW. Repair due to cavitation erosion is the most frequent maintenance task OPG personnel must perform on turbine runners. Historically, the average interval between repairs has been about 12 years. However, this can be highly variable among units as newer runner designs have been greatly improved to reduce or even eliminate cavitation.

For runners that experience cavitation damage, repair typically begins with an inspection to identify the damaged area(s). Personnel then must grind the damaged area to remove the pitted surface. Frequently, this damaged area can be far larger than what appears upon first inspection. Once surface preparation is complete, the lost volume of the runner is built back up using weld overlay. Finally, the surface is ground back down to best approximate the original runner surface.

Many factors make this repair method time-consuming and problematic:

Developing a new technique

Based on new rapid prototyping and 3D modeling methods used in manufacturing and other industries, OPG personnel decided to investigate the possibility of utilizing some of these methods for repairing hydro turbine runners and other complex surfaces, such as spill rings and wicket gates.

OPG wanted to improve in-situ repair to achieve five goals:

The technique OPG developed is based on use of a portable stereoscopic handheld laser scanner. This scanner projects a laser beam onto a surface, captures reflected photons from this beam, and creates a geometric shape or "point cloud" that represents the 3D surface being scanned. Data point position is determined by referencing to an internal coordinate system created by adding randomly positioned targets affixed to the runner surface.

This initial graphics exchange specification (IGES) file represents the surface of a runner after it was ground to remove cavitation damage. This file is used to create another IGES file that represents a 3D plug or insert needed to restore the runner to its original profile.

The completed scan has a resolution of +/- 0.004 inch. The resultant data file also depends on the area covered and data capture rate.

Because of the large quantity of collected raw data, the data is converted to a smaller polygonal mesh stereolithography (STL) file for easier use. The polygonal mesh is a series of triangular faces and edges that represent the surface to be modeled. The smaller the mesh size, the higher the resolution and the better the creation of a smooth appearance of the model. A laptop with enhanced memory and some computer-aided drafting (CAD) capability can usually suffice to do the work.

The STL file is then converted to a neutral file format known as initial graphics exchange specification (IGES). This format is common in 3D CAD software packages. From the IGES file representing the runner surface, the software technician then can create an IGES file that represents a 3D plug or insert. The portion of the insert that mates with the runner pocket is obtained directly from the runner surface data.

The outer surface of the runner insert is determined by analyzing the surrounding non-cavitated runner surface. This surface information allows the developer to design a smooth contour for the new insert that best matches the original runner profile. Additionally, an edge bevel and other features can be designed to facilitate the final installation.

At this point, the IGES file of the runner insert is complete and can be sent to vendors to create the actual solid from the material of choice, either by casting or CNC machining.

This model of an insert (foreground) needed to repair runner cavitation damage was machined using a CNC router. The holes provide locations for plug welds when the insert is welded onto the damaged runner.

Applying the technique

OPG personnel performed the first test of the new technique on a discarded propeller runner at the Caribou Generating Station. Personnel removed a portion of the runner blade, transported it to the maintenance shop at the dam site, and selected an arbitrary region to repair. Personnel then ground this region to replicate a typical cavitation repair job.

OPG hired Agile Manufacturing in Uxbridge, Ontario, Canada to scan the runner area and existing surface surrounding the damaged area. From this scan data, an IGES file was created that provided OPG with a true representation of the repair surface and undamaged peripheral surface. This latter data is needed to develop the "missing" runner surface that was eroded. Using various computer software programs, such as Solidworks and Rhino3D, OPG personnel could then produce a 3D insert that both fit the runner void and closely matched the existing runner surface.

The outer surface contour of the insert was approximated by referencing and extrapolating scan data from the undamaged periphery surrounding the void using the 3D modeling software.

The file representing the insert was now complete, and OPG used the computer model to simulate a test fit. The completed file was used to produce a CNC machine file. A computer program was written for a five-axis milling machine and, once complete, the newly created machine file was uploaded. Agile Manufacturing machined the part from an aluminium billet for ease of milling. The completed test piece was fitted to the mock runner section, and the fit was dimensionally correct, with good correlation with the runner surface.

Building on the success of this experiment, OPG decided to use this technique to repair an in-service turbine. Personnel selected a 107-inch-diameter Morris turbine runner at the 18.5-MW Ear Falls Generating Station.

The turbine was shut down in the fall of 2008 for routine maintenance, and inspection revealed cavitation damage to the runner blades and throat ring.

Fitting a machined insert to the damaged turbine runner for which it was created shows the good correlation resulting from Ontario Power Generation's technique.

OPG personnel selected two runner blades and about a third of the throat ring section for repair. Eight locations were found to have severe cavitation-induced pitting. The damaged areas were ground to remove the pitted surfaces, then a scanning technician with Agile Manufacturing scanned the area. When scanning was complete, in about four hours, the unit was returned to service.

Once the eight runner insert IGES models were developed, the completed files were e-mailed to MA Steel Foundry in Calgary, Alberta. Site staff then created a casting mold and produced a sand casting. Details such as casting shrinkage had to be compensated for to produce an exact match with the computer model. MA Steel Foundry used a software program called Magmasoft to simulate and optimize the casting pour to eliminate any possible voids and defects. For this test, the runner pieces were cast from 316L stainless steel.

In the spring of 2009, the unit was shut down to install the new runner inserts. During the six months the unit had run, minor increased cavitation damage occurred in the areas to be repaired. Despite this, the cast inserts fit well, and only minor grinding was necessary to fit all pieces. The inserts were edge bevelled, then welded into place by OPG personnel. On larger inserts, personnel drilled a few randomly placed 1-inch-diameter holes to facilitate plug welding.

During this shutdown, OPG also decided to test the new repair method under real-time conditions. Site staff selected three runner blades with cavitation damage. The damaged surfaces were ground and scanned on May 21, 2009. New inserts were designed and were e-mailed to the foundry for casting. The pieces were then created by the investment cast process from 316L stainless steel. The completed parts were shipped from the foundry on May 28, 2009. Thus, total duration from start of scanning to completed part was just seven days. The new inserts were welded in place the following day, and the unit was returned to service

After a year in service, OPG shut down this unit to inspect the repaired runners. No visible damage was noted using a diving team equipped with video cameras. The unit was returned to service.

As a result of these tests, OPG personnel made several important observations:

OPG personnel also made the following observations, which can be used for further development of this technique:

Conclusion

OPG's experience with these two trials shows that this new technique meets the utility's original criteria and is quite versatile. OPG has greatly reduced time spent at the runner, as most of the work is done using a computer. The utility now has a relationship with vendors to provide the services required to perform this technique quickly and efficiently.

For OPG, the maintenance staff have a much simpler task in just preparing the damaged runner surfaces for scanning. A technician can complete the scan within about four hours. Then an outside vendor completes the modeling and production of new parts. And company maintenance staff formerly dedicated to the runner repair can be assigned to other work. Once the new runner inserts arrive on site (in about ten days), OPG was able to install the pieces over a period of about two days.

There may be other opportunities in the future to use 3D scanning at hydro facilities. For example, this technique can be employed in areas affected by erosion (such as head gates, penstocks, and wicket gates) and for assessing any general condition by scanning the area in question. A big advantage is the ability to rapidly capture a precise 3D representation of plant condition in difficult to access areas. The unit can then be returned to service and the collected data be transferred to the computer. Upon completion of the computer analysis, repair procedures can be developed and components fabricated in the shop, thus greatly reducing plant downtime.

Darrell Lewis, P.Eng., is a senior plant engineer with Ontario Power Generation. He developed the concept for the technique described in the article and identified companies that could perform the work.

More Hydro Review Current Issue Articles
More Hydro Review Archives Issue Articles


To access this Article, go to:
http://www.hydroworld.com/content/hydro/en/articles/hr/print/volume-29/issue-7/articles/using-rapid-prototyping-methods-to-repair-runner-cavitation-damage.html