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Radiographic Testing of Hydroelectric Turbine Cases

by Alex Bagarry III*

Radiography is a widely utilized technology for nondestructive testing of structural components. This paper provides a good example for general consideration when conducting radiographic testing, discussing test standards, procedures for performing testing and analyzing data, and radiation safety regulations. Production engineers considering this technology will find this of interest, as will radiography professionals reviewing their practices. J.G. Sun Reviewer

 

Our company was recently contracted by a manufacturer of hydroelectric systems to perform Ir-192 gamma radiographic testing of the assembly welds for several newly constructed hydroelectric turbine cases. The turbine cases in question were of various sizes, and ranged in output levels from 100 to 300 kW. Each of the turbine cases, when finally assembled, will contain a Francis turbine water wheel used to convert the motion of river water into rotational motion, which, when close-coupled to an electrical generator, will produce electricity. Figures 1 and 2 illustrate different types of hydroelectric water wheels.

Figure 1 - Francis wheel runner.

Figure 2 - Pelton wheel runner.

Technique Development

We began our technique development on the smallest case, and, with the capable technical assistance provided by our associates, were able to determine the most appropriate approach to this complicated radiographic testing project.

This project was a success, as well as a great learning experience.

The drawings from the manufacturer called for radiographic testing process controls in accordance with the requirements of Section V, Article 2 of the ASME Boiler and Pressure Vessel Code (2007a). In addition, the acceptance criteria noted were for compliance to Section VIII, Division 1 of the same code, for unfired pressure vessels (2007b). The radiographic acceptance criterion in this reference document is given at Paragraph UW-51, and is a relatively tight criterion for this water transport application.

Initial Efforts

We began our efforts by taking test shots at the larger (inlet) end of the welded spiral case turbine assembly. Initially, we used the single wall exposure/view technique, as sufficient source to film distance could be achieved while remaining below the maximum geometric unsharpness allowed for this thickness of material (5 mm [0.020 in.]). We did this by placing the end of the guide tube, with the collimator inside the tube, against the inner wall opposite from the film positioned on the outer diameter (Figure 3a).

Figure 3 - Collimator and film position: (a) for single wall exposure/view technique used for larger diameter butt welds; (b) for double wall exposure/single wall view technique used for smaller diameter butt welds.

Each turbine case resembled a snail shell, and as we progressed down the large inlet and around the snail shell, the butt weld diameters got smaller and smaller. At a certain point, this necessitated a change in technique; also, the placement of the source collimator became more of a challenge (and more time consuming), so a double wall exposure/single wall view technique was now needed for the rest of the assembly weld radiography (Figure 3b).

Figure 4 - Welded turbine spiral case assemblies: (a) small spiral case in gamma shooting vault; (b) medium spiral case in gamma shooting vault; (c) turbine spiral case inner diameter and guide tube access to internal chamber.

Interpretation of Results

A close look at these welded turbine spiral case assemblies reveals what a challenging welding project this was (Figure 4). A review of the initial radiographs taken showed very high quality images, with 1T sensitivity evident on many of the views, and always at least 2T (the minimum required). The techniques used showed a good balance between quality and productivity.

Once the film package was complete, the film interpretation phase began. The welds radiographed were of exceptionally high quality - only a few repair welds on each assembly were necessary, and these were quickly done using the shielded metal arc welding process in the laboratory shop, by a nuclear power plant outage circuit welder with many years of experience welding to the ASME Boiler and Pressure Vessel Code's requirements.

Radiation Safety

The radiation safety factors used combined a walk-in gamma vault with 1.8 m (6 ft) thick shielding walls; these walls were made of cinderblock inside and out, with reinforcing bars and concrete filling. The intervening space was filled with sand. The roof of the gamma vault was reinforced with steel plating and also filled with sand to keep any reflected radiation from escaping out the top of the shooting area (a phenomenon known as "sky-shine").

We used an isotope source of 3.7 TBq (100 Ci) of Ir-192, and the resulting transmission factor for the gamma radiation barely allowed enough radiation through the shielding wall to register on our radiation survey instruments when our surveys were conducted on contact with the outside wall of the shooting vault. This, in combination with our 7.6 m (25 ft) crank assembly and labyrinth entrance passage to the shielded shooting room kept all radiographic personnel's radiation exposure for the project extremely low, and in compliance with the US Nuclear Regulatory Commission ALARA principles. Of course, the requisite magenta and yellow ropes and controlled "Radiation Area" signs were used, as is required by our radioactive materials license and our operating and emergency procedures.

Conclusion

This project was a success, as well as a great learning experience. Being a relatively complex radiographic testing problem, we determined our initial techniques with care and patience. This was all the more important as we had to work within a very tight set of constraints as far as codes went. By experimenting with different setups and configurations, we were able to determine the best setup for this particular application, taking care to see that all relevant safety precautions were being followed. The results were very good. The radiographic techniques employed used conventional radiographic film, and provided good definition and contrast, with 2T sensitivity met or exceeded on all views. Repairs were able to be carried out as needed, with little difficulty.

Acknowledgments

This paper was written in reference to a contract from Canyon Hydro. The author would like to thank John Griffith and Torsten Pundt, of Anvil Corporation, for their assistance with this project.

References

ASME, ASME Boiler and Pressure Vessel Code, Section V, "Nondestructive Examination," Article 2, "Radiographic Examination," New York, American Society of Mechanical Engineers, 2007a.

ASME, ASME Boiler and Pressure Vessel Code, Section VIII, Division 1, "Rules for Construction of Pressure Vessels," New York, American Society of Mechanical Engineers, 2007b.

 

* GKN Aerospace Chem-tronics, 1150 W. Bradley Ave., El Cajon, CA 92020-1597; (619) 258-5193; fax (360) 755-9578; e-mail alex.bagarry@usa.gknaerospace.com

 

Copyright © 2008 by the American Society for Nondestructive Testing, Inc. All rights reserved.

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