Repair of a Riveted Steel Penstock in the Mormon Flat Dam with Carbon FRP

Mormon Flat Dam is one of four hydroelectric projects on Salt River, Arizona. The smaller hydroelectric generating unit on this dam utilizes a 120-in. (3048 mm) diameter 5/8-inch (16 mm) thick riveted steel penstock. Ultrasonic testing results indicated that the penstock had lost as much as 50% of its thickness in certain areas. After considering various alternatives, SRP’s engineers chose to repair the pipe with carbon FRP. The repair area included two 96-inch (2440 mm) pipes about 15 feet long (4.57 m) that merged into a single 120-inch (3048 mm) 45-ft long (13.72 m) segment. The geometry of the Y-connection complicated the repair. A drop in elevation of nearly 20 feet (6.1 m) halfway through the repair section provided additional challenges to the project. The pipe surface was prepared by removal of nearly ½-inch (13 mm) thick coal tar before blasting the surface to a near-white condition. The rivets proved to be longer than anticipated, adding further difficulty in smoothing those regions for a gradual transmission in profile. The strengthening of the pipe included application of 5 layers of FRP based on the design-build contract. A total of 11,000 ft² (1022 m²) of carbon FRP was installed within the allotted 3-week time frame. A final abrasive resistant coating was then sprayed over the FRP for additional protection against any small debris that may enter the penstock. The repair is expected to provide a long-lasting maintenance-free penstock for the dam.