To reduce water leakage within the turbine-generating unit of its 5-mw Foyers Falls hydro plant, Scottish & Southern Energy PLC in Scotland retrofitted the 12-inch-diameter main shaft with a split-type HydroSele cartridge seal from James Walker & Co. The seal – which features two elastomer-based rotary sealing elements working back-to-back with flush water introduced between them instead of a face sealing arrangement typically used by mechanical seals – solved the leakage problem.
The problem and solution at Foyers Falls
When the Foyers Falls unit was constructed in 1968, the shaft was outfitted with a pair of internally mounted mechanical seals of the spring-retained segmented carbon ring type, plus an outboard labyrinth system. Over time, sand and peat in the water in the Foyers River caused excessive wear to these seals. In addition, the turbine shaft was becoming eroded and scored around the labyrinth system. Water was spraying out of the seal housing toward the generator. The utility was replacing the carbon seals every 12 months.
By 1996, the utility decided to replace the mechanical seals with the HydroSele system. The innovative feature of the system is the way its two sealing elements operate within their housings. The two elements work back-to-back, with filtered water flushed between them at 30 pounds per square inch above the water pressure at the sealing gland. This ensures that river water does not enter the area between the sealing elements and the shaft.
The cartridge is a bolt-on unit that incorporates the housings for the two sealing elements and the flush area between them. Because the system is modular, each component can be designed and precision manufactured to fit together around a specific turbine shaft. For example, the complete split-type assembly of outer housing ring, flush ring, inner housing ring, and two sealing elements can be installed without stripping down the housing. This can reduce the time needed for the turbine to be off line during both installation and refurbishment. Another benefit is that, once installed, the seal is totally adustment-free.
Scottish & Southern chose the HydroSele system, in part, because its modular concept and custom design minimizes installation time. In addition, by installing the seal system, the utility could reduce seal maintenance requirements to intervals greater than five years.
Installing the system, results
Two engineers with James Walker installed the sealing system at the Foyers Falls plant in less than two days without mechanical lifting gear.
The new sealing system reduced leakage to three-quarters of a gallon per hour, thus eliminating the problem of water spraying toward the generator.
In 2004, during an annual maintenance shutdown at the plant, Scottish & Southern replaced two worn sealing elements in the cartridge.
The sealing system continues to work satisfactorily today, with the plant running constantly 48 weeks a year at full load speed of 756 revolutions per minute.
– By Douglas Smillie and Tim Smith. Mr. Smillie may be reached at Scottish and Southern Energy, Foyers Power Station, Foyers, Inverness IV2 6YB United Kingdom; (44) 1456-486676; E-mail: firstname.lastname@example.org. Mr. Smith may be reached at James Walker Sealing Products & Services, Lion House, Oriental Road, Woking, Surrey GU22 8AP United Kingdom; (44) 7970-007432; E-mail: email@example.com.