When the U.S. Department of Interior’s Bureau of Reclamation began overhauling the turbine-generator units in its 6,809-MW Grand Coulee’s Powerhouse No. 3, it was determined that rough concrete surfaces in the draft tube supplying water to Unit 23 were impacting water flow though the draft tube.
The concrete draft tube for this unit is a roughly 40-foot by 40-foot square tube structure located at the base of the dam, where water exiting the turbine flows into Washington’s Columbia River. The draft tube sees a tremendous rate of flow and, as a result, over time the concrete surface became deeply ridged and worn. In fact, striations averaged between 1/2 inch to 3/4 inch deep.
An improvement in the draft tube’s flow, as stipulated by the Hazen-Williams formula, would potentially decrease the amount of friction loss and create an easier flow pattern for water exiting the tube. The turbine would therefore experience less loss and, potentially, a higher rate of generation.
Selecting a solution
After a Reclamation site survey found the rough wear lines in the tube, the agency decided to coat the tube’s entire surface with an epoxy gel. USBR had previously examined more than 50 candidate concrete repair materials for thin repairs at its Concrete, Geotechical and Structural Laboratory in Denver for criteria that included tensile and compressive strengths, adhesiveness to damp aggregate, ability to cure at low temperatures and ease of application.
|Located on the Columbia River in Washington, Grand Coulee is the largest power-generating facility in the United States.|
For this project, the selected product had to exhibit good flow characteristics (very smooth surfaces), while being volatile organic compound (VOC)-free per ASTM C881 specifications, and be able to be applied up to a 1-inch thickness in both vertical and overhead applications.
Reclamation would go on to select Milamar Coating LLC’s medium-cure Epoxy-Gel MC over a cementitious option as it was expected the epoxy coating would only be a half-inch thick and require relatively little surface preparation. In addition, the epoxy was expected to provide better bond and wear resistance than a concrete-based option.
Worksite challenges and site prep
Access to the draft tube, located 230 feet down from the turbine deck, was provided by an overhead crane lowering a six-man basket. All tools, man-lifts, epoxy product and other equipment had to be lowered daily as needed.
Fortunately, the temperature of the draft tube and ambient air – a steady 55 degrees Fahrenheit – is a perfect environment for Epoxy-Gel MC, which is rated to 50 degrees.
Stipulations dictated by both the physical constraints of the draft tube and Reclamation’s environmental practices also contributed in large part to how the tube’s concrete substrate was prepared to receive the epoxy gel. The work was conducted by a crew from Andritz Hydro, trained specifically for the project by field personnel provided by the Process Equipment Corp. (PEC) and coatings manufacturer Milamar.
Normally, a combination of acid etching, sand blasting and diamond grinding might be used prior to the epoxy’s application, though in this instance, only high-pressure water jets were used to clean and prepare the surfaces. This method was both relatively easy and environmentally benign, producing only water and minimal solids as waste.
Finally, an air lance was used to eliminate any remaining standing and pooled water.
Applying the epoxy
It was determined early on by the installation crew and PEC that the process of using automated mixing and pumping equipment would be unfeasible for a number of reasons. First, the constraints of the six-man lift would have made hauling the equipment difficult, while environmental considerations, space limitations and a regard for safety meant any cleaning agents and solvents required by the application equipment could only be used 230 feet up at the turbine deck.
This would have made it difficult to maintain a wet edge between batches of epoxy as crews would have had, at most, 90 minutes to haul equipment up from the draft tube, clean it, prepare the next batch, and send it back before the previous section had cured.
Therefore, it was decided that a lack of sufficient finishers and challenges posed by the work environment would make a small, hand-mixed batch method preferable. The epoxy gel was mixed in three-gallon units that would cover, on average, 40-square-feet at applications of 1/8 inch thick, or 10-square-feet at 1/2 inch thick.
|Grand Coulee’s first powerhouses were completed in 1942. The third was finished in 1974.|
This proved to be a more manageable means of delivering the epoxy to the application crew. Material was placed using typical sheetrock taping tools, and further placed and finished using hand trowels over the prepared walls. Areas covered per batch were more reasonable, and the end product was much more smooth and finished to desired specifications. Also, there was little to no waste, and the crew was able to cope with product mixing and cleanup procedures with greater ease.
Although Reclamation had initially thought it would need to cover the entire draft tube with a half inch of the epoxy, it was found that the vertical fissures could simply be filled before a 1/8 inch skim coat was applied to the overall surface. Also present were some deeper holes and limited spalling, which was also smoothed and filled with epoxy.
It took just over six weeks to complete work on the draft tube’s 6,400-square-feet of surface area, due in large part to challenging field conditions, confined spaces and the learning curve necessitated by such an application.
The coating of the concrete surface in the draft tube with an epoxy prevents further wear, limits degradation and erosion, and improves surface flow coefficients. Data shows the Hazen-Williams flow factor was improved by the application, with a surface smoothness equal to 150.
The epoxy gel’s bond strength to prepared concrete surfaces was excellent as demonstrated by pull tests. These tests indicate that, with an average pull test of 480 psi, the failure plane was the concrete substrate rather than the interface between the epoxy and concrete.
Reclamation was so confident in the results of the epoxy’s application below Unit No. 23 that it chose to coat a second draft tube beginning in 2016. Materials are currently on-site and work has begun. Reclamation said it will coat a third draft tube in 2017-18.
– By Norm Klapper, sales engineer and manufacturer’s representative, Process Equipment Corp. Kurt F. von Fay, concrete repair specialist for the Bureau of Reclamation, reviewed this article.