Over time, water flowing through the three penstocks at the 90-MW Cushman No. 2 powerhouse resulted in pits and eroded areas on the butterfly valves used to shut off water supply to the turbines. The damage caused leakage, which represents lost energy production, and accelerated erosive deterioration of the valve seat. Project owner Tacoma Power used a silicon carbide-filled epoxy to restore the valve seats by building up pits and eroded areas, then applied a urethane elastomeric compound as a top layer for the sealing surface. Water savings from these repairs are eligible for renewable energy credits, and the payback is a matter of months.
|Repair of the three penstock butterfly valves at the 81-MW Cushman No. 2 plant is achieved using liquid silicon carbide-filled epoxy material to fill holes, covered by a surfacing layer of the more flexible two-part urethane compound.|
The long story
The use of epoxy-based repair materials on hydro components is not new. Tacoma Power sets all-thread anchors into concrete with Hilte epoxy, and high-solids epoxy paints have worked well in penstock applications. The utility's water division uses TNEMEC epoxy for wetted applications. But Tacoma Power's primary success using this material has been on large butterfly valve seats.
About 30 years ago, Tacoma Power's mechanical engineering group worked with Belzona Molecular, applying the company's "supermetal" to damaged pockets on sand cast plain steel turbine runners at the 50-MW Cushman No. 1 powerhouse. As these runners wore, void pockets of sand opened in the castings. The supermetal material provided better adhesion and longer service life than other comparable metal-filled epoxies. This material also cost significantly more than Devcon's metal-filled epoxy, but Tacoma Power considered it worth the cost due to the superior performance and longevity in hydro applications. The Belzona product has lasted 30 years, while the Devcon material used at that time would erode within a year or two.
Tacoma Power also occasionally used the Belzona metal-filled epoxy material to fill small damaged seat areas on the three 96-inch butterfly valves at the Cushman No. 2 powerhouse. Surface preparation was minimal, but the erosive scrubbing action at the leak points and the very clean water of the Skokomish River resulted in little surface contamination.
In the early 1990s, Tacoma Power participated with Bonneville Power Administration credits for efficiency improvements on runner replacements at the two-powerhouse Cushman project. The Voith Hydro runners selected were chosen for their peak performance, but runners designed for peak performance often have off-peak cavitation issues. Thus, because of the wide variation of power requirements on the turbines, there has been significant runner cavitation. Cushman No. 1 runs with large variations in power and head. Cushman No. 2 maintains a high head but also ranges in power. Cavitation occurs at both projects, but it is more severe at Cushman No. 1.
These newer turbine runners were fabricated from CA6NM, a tough martensitic stainless steel. Although these units can be repaired, the procedures involved at Cushman No. 1 are costly mainly due to the difficulties in accessing the repair area. The cavitation pattern at Cushman No. 1 is due to low load operation and begins at the leading edge on the back side of the blade near the bottom ring connection radius. There is not enough room between the wicket gates to access the area, and there is too much curvature to the bucket to reach it from below. Personnel must perform a partial teardown to lift the head cover and remove enough wicket gates to provide a repair area. With the wicket gates fully open, there is only 9 inches of width to reach through, and it is at arm's length.
Thus, Tacoma Power chose to experiment with use of two-part urethane compounds and a silicon carbide-filled epoxy on a more accessible turbine at Cushman No. 2 that was suffering from cavitation damage. The utility wanted to find a material and application technique that survived cavitation, then transfer this approach to Cushman No. 1.
Tacoma Power personnel experimented with urethane compounds and silicon carbide epoxies from Enecon, Belzona and Devcon. All three of these companies provide high-quality products and excellent technical support. The utility followed each manufacturer's recommendations as much as possible, using silicon carbide blasting for surface preparation, then completing dust removal and de-greasing before applying the urethane compound. Site advisors were present from the three companies during this testing and experimentation process.
The conclusion was that the material chemistry has been perfected and all three firms' materials are quite adequate. Similarly, all the materials seem quite expensive (compared to a tube of 5-minute epoxy). However, compared to trying to weld build up the bronze or stainless steel or replacing these large components, the material cost is quite low. Still, at hundreds of dollars for a four-pack of 1-pound containers, Tacoma Power personnel doled it out in small batches using disposable plastic cups and a digital scale for assuring the proper mixing ratio.
Material adhesion was good, but the cavitation environment on a turbine runner is just too severe. What Tacoma Power personnel proved was that regardless of material or brand, a cavitation environment that can dismantle CA6NM will also rapidly strip any surfacing material. As a result, the utility decided to continue relying on weld repair of the CA6NM, using E309LSi for the weld metal.
A success story
The success of this experiment is a side story from the runner cavitation. The Cushman plant manager asked if something could be done about the butterfly valve leakage. Because the penstock was dewatered and the butterfly valve open, Tacoma Power personnel addressed the eroded seats using the same materials mentioned above. Silicon carbide blasting provided surface roughness for adhesion, a surfacing template was prepared by a utility machinist using the original equipment manufacturer's drawing, and the seat shape was cut with a numerically controlled mill into a plastic squeegee.
As the crew worked on the first of the three butterfly valves at Cushman No. 2, they realized some of the Belzona material from the early 1980s was still in place, with the bronze around it more eroded from the additional 30 years of life. The crew built up both seats using sequential layers of liquid silicon carbide-filled epoxy material provided by Enecon and templated the seat to get it close to the original size and shape. A surfacing layer of the more flexible Belzona urethane was used as a top coat once the profiles were close. A lithium-based grease was used as a parting agent on the seat to keep from gluing the valve shut.
The hopes for a perfect seal soft-seated valve (urethane seal) were dashed when the penstock was watered up and still leaked in places. However, considerable improvement was accomplished and the leakage was significantly reduced.
Follow-up inspections a week or two after the work was completed and on an annual basis have shown the materials used to be adequate for the environment. The soft rubbery urethane top layer has not blown out or failed.
Based on this initial success, plant personnel proceeded with buildup repair efforts on the other two penstock butterfly valves. The intent is to inspect them on an annual basis and continue to work on leaky areas to improve sealing.
It was difficult to quantify leakage because it was well below the accurate range of penstock multi-path ultrasonic flow metering, and the reading from a clamp-on flow meter on the scroll case drain was erratic. Based on a combination of flow metering and engineering estimation, leakage on the three butterfly valves improved from 5.1 cubic feet per second to 4.71 cfs for Unit 31 and 11.8 cfs to 5.89 cfs for Unit 32. For Unit 33, a newer turbine with a different design, leakage improved from 7 cfs to 2.67 cfs.
Tacoma Power's success with this repair approach has dramatically reduced leakage on these aging valves. The cost in labor and materials is low in comparison to the value of the water savings and really minor compared with the value in restoring the condition of the valve. For example, the utility spent about $250,000 for a contractor to rebuild the seats on the two valves at Cushman No. 1, and less than a decade later these valves leaked again. Time and material cost for repairing a valve with epoxy may be $2,000 to $3,000.
Tacoma Power is pleased with this approach and is adding touch-up of these valve seats to its annual maintenance program.
- By Lew Simpson, principal engineer, Tacoma Power