For more than a century, a conventional, run-of-the-river hydroelectric facility on Vermont’s Winooski River has helped meet the power needs of rural homes in the state’s northwest region.
Operated by a well-known, investor-owned utility, the 8.4-MW hydroelectric plant has not changed much since it opened in 1917. The utility often hosts tours of the facility so guests can appreciate the original brick structure, wooden floor, and equipment popular at the turn of the century.
The centerpiece of the plant is its electrical room, where four original 2,223 kW, horizontal Francis-type turbines drive four electric generators. Invented in 1849, the Francis turbine was the first hydropower turbine.
Over the years, utility technicians have painstakingly worked to keep the generation system up and running. However, significant modifications have been avoided since they would alter the simple, age-gone-by ambience of the facility.
The upgrade is saving the hydroelectric plant money in seal repair and replacement costs and time spent by technicians on maintenance.
Reliability Improvements Needed
Even so, some changes have been necessary to improve the reliability of the plant and keep overhead costs down. In the mid-2000s, utility management grew concerned that at least one of the eight mechanical seals installed on the turbines would need to be replaced or repaired every six months. The seals were critical to the turbines’ reliability and used to protect the equipment from leaks and contamination.
Every time a seal needed attention, the host turbine was removed from service for at least two days, limiting the generation capability of the facility and taking two technicians away from other maintenance duties. The short mean time between repair (MTBR) rate cut into the utility’s bottom line and could potentially disrupt grid reliability.
An audit revealed two issues. First, sediment from the Winooski River clogged the seals, especially during the spring and fall seasons when water levels fluctuated. Second, the shaft connecting the turbine to the generator was out of alignment, otherwise known as runout. The runout was so severe that the seals could be seen wobbling and leaking during operation.
Since the utility could not limit the natural sediment flow―and fixing the runout problem would require removing each turbine with arcane―the utility worked with the existing seal supplier for a remedy. Unfortunately, the reliability problems persisted, resulting in broken seal parts.
New Seal, Problem Solved
In 2018, the utility tried a different route to fix the problems. Instead of repeatedly redesigning the existing seals and adding to the problem, the utility agreed to try a time-tested technology specifically developed to handle difficult-to-seal equipment, like the hydroelectric plant’s turbines.
Utility management was skeptical at first, agreeing to initially install two seals on one turbine and keep one as a backup. However, they were hopeful that a solution would soon be available because the seal was pretested to accommodate the runout at one of the supplier’s facilities.
To accommodate the runout, the split mechanical seal was designed with a flexible elastomer bellows compression ring. Runout is automatically tracked by the seal without diminishing performance. Further, the seal’s compression ring acts as a spring, eliminating the clogging problems.
To help handle the sediment issue, an external water flush was added. A throat bushing on the inboard side also helps build back pressure and prevent solids from entering the seal area.
Results
Since the 2018 seal upgrade, the seals have not required maintenance or repair― a significant improvement over the previous six-to-eight-month MTBR rate. Today, all four turbines have been upgraded with new seals or are in the retrofit process.
The upgrade is saving the hydroelectric plant money in seal repair and replacement costs and time spent by technicians on maintenance.
Equally as important as the financial and time savings, the utility has avoided invasive repairs to the antique turbines and changes to the hydroelectric plant’s infrastructure. The facility remains as it was in 1917―and will for the foreseeable future.
