During winter operation of the 28-MW Sheldon Springs facility on the Missiquoi River in northern Vermont, frazil ice clogs the cooling water strainers. This decreases water available to cool the generators and bearings, limiting production from the facility. To melt the ice, Enel North America modified the cooling water system to recirculate “warm” water that otherwise would be discharged to the tailrace. This water, pumped back to the inlet of the strainers, melts ice that enters the facility and allows the units to operate at full capacity.
Situation at Sheldon Springs
Sheldon Springs is the site of an operating paper mill that contains four small hydro units with a total capacity of 4.16 MW. In 1987, two units totaling 24 MW were added in a new powerhouse. The two new vertical Kaplan units operate at 115 feet of head and 1,300 cubic feet per second (cfs) of flow each. A 16-foot-diameter, 3,000-foot-long concrete penstock connects the intake to the powerhouse, crossing under the riverbed in the bypass section. The plant is designed for unmanned operation.
Each turbine-generating unit has its own once-through cooling water system. This system sends pressurized water from the penstock through a self-cleaning rotary strainer, then to the generator air coolers and generator and turbine bearing oil coolers. The cooling water then flows out to the tailrace. Metering on each cooler outlet provides alarms for deficient flow. Resistance temperature detectors (RTDs) in bearings, oil sumps, cooling water lines, and air passages have alarm and trip setpoints to provide protection in the event of high temperatures.
Under frazil ice conditions in the winter, the self-cleaning rotary strainers cannot handle the extremely high solids load. During these times, manual duplex basket strainers are used. These strainers are continuously swapped out and emptied by plant personnel. This practice allows a small increase in output, but at the cost of a large increase in labor. Operators have cleaned the strainers at Sheldon Springs non-stop for many hours, dumping the contents of the baskets on the floor drain and producing a pile of “snow” 3 feet high!
Labor scarcity and the limited effectiveness of manual operation led Enel to try several solutions to rectify the situation.
First, on the theory that the frazil ice would mostly be floating at the upper invert of the penstock, the cooling intake piping location was moved to a lower point on the penstock. However, this brought no appreciable difference.
Next, Enel adjusted the strainers’ backwash timing. The strainers were on timed intervals to backwash collected debris into a discharge line. Personnel changed this mode of operation to continuous manual blowdown, which bypasses the timer to allow continuous backwash. This method still could not remove the solids quickly enough, and the strainers continued to overload.
Enel investigated several other solutions, including using well water for cooling and constructing a closed-loop system with a cooling tower.
In the end, the most cost-effective solution was to recirculate “warm” cooling water after it had passed through the coolers and before it exited the facility.
Designing and implementing the modification
In 2000, the author designed a modification to the cooling water system to take the warmed water and recirculate it to the inlet of the strainers. “Warmed” is a relative term. When the inlet temperature is just above freezing, the temperature of the discharge water is about 42 degrees Fahrenheit (F). This warmer water supplies enough low-grade heat to melt the ice in the strainers. In addition, this solution reduces the amount of cooling water required from the penstock.
The traditional way to set up such a system would consist of a pump with a control valve to regulate the flow. The pump would run at maximum output and throttle the flow using the valve. However, this setup wastes electricity, which can be sold if it is not used.
Instead, Enel chose a PumpSmart system from ITT. This system consists of a variable frequency drive to control the pump speed and thereby regulate the cooling water temperature.
This system allows pump speed to be controlled by any analog signal. Enel uses an RTD in the cooling line with a controller that outputs a signal of 4 to 20 milliamps. The system was set up so that the pump runs at full speed when the temperature of the mixed cooling water is 40 degrees F or lower and at minimum speed when the water temperature is 53 degrees F or higher. It is important to determine the minimum speed at which the pump moves water. If operated slower than this speed, the pump will only churn the water and overheat.
In addition to protecting the cooling system, the PumpSmart unit will indicate any system abnormalities, such as high amperage or pump stall, and help protect the equipment by shutting down the pump.
The RTD controller, in a wall-mounted panel, displays water temperature. The PumpSmart controller also can be configured to display actual values (such as revolutions per minute, water temperature, and amperage). Using a stored model of the pump, many inferred values (such as flow rate) can be displayed and configured to sound an alarm at high or low levels.
To ensure tailwater is not drawn into the system, Enel installed a check valve in the cooling water discharge line. Tailwater is not warm and will not melt the ice, and it may contain debris that would be drawn into the pump.
The warmer discharge water is piped to a point just upstream of the self-cleaning strainer. The RTD was installed in the piping 8 feet after the strainer to give enough time and turbulence for the different temperature streams to mix well.
Because the pumps only run on rare occasions, Enel instituted a test program in which the pumps are run for one hour once a month. This keeps the bearings lubricated and ensures the pumps will operate when needed.
Enel installed the system on one of the two units at Sheldon Springs in 2001. However, Enel had to wait for more than three years for frazil ice conditions to occur and prove the effectiveness of the system. During that winter, the system ran twice, for several days each time. The system worked as expected and allowed full production from the plant. This increased generation paid for the system in less than a year.
Once this test period demonstrated the capability of the installation, Enel provided funding to install the system on the second unit. This installation was completed in 2006, at a cost of $15,000 for the pump and controls and $10,000 for the piping and fittings.
– By George Moskevitz, regional supervisor, Enel North America, P.O. Box 7, Sheldon Springs, VT 05450; (1) 802-933-2570; E-mail: george. firstname.lastname@example.org.