Draft tube aeration systems on three turbine-generating units at the 18-MW Lloyd Shoals plant allow Georgia Power Company to meet state dissolved oxygen requirements. With the aeration systems in place, the utility was able to remove a downstream aerating weir that was decreasing head at the plant.
By Lawrence B. Moore Jr.
Lake Jackson, the reservoir that supplies water for Georgia Power’s 18-MW Lloyd Shoals hydro plant, becomes thermally stratified in early summer and remains so into September. In this situation, only the upper 5 feet of the lake contains dissolved oxygen (DO). The intakes for the six turbines at Lloyd Shoals, on the Ocmulgee River in Georgia, are about 20 feet below normal pool levels. Thus, the water used to generate electricity is drawn primarily from layers of the lake with no DO.
The Lloyd Shoals plant has two modes of operation. Normally, it either passes the required minimum flow of 400 cubic feet per second (cfs) through a single unit or is operating at full load for peaking operations. In both of these modes of operation, water discharged from the plant contains levels of DO that often are below state standards and sometimes approached zero. The state of Georgia requires DO in plant discharge of 4 milligrams per liter minimum and 5 milligrams per liter average.
Work performed to increase DO
In the early 1990s, Georgia Power underwent the Federal Energy Regulatory Commission (FERC) relicensing process for Lloyd Shoals. As a result of input from the Georgia Department of Natural Resources (DNR) during that process, Georgia Power has been working to improve DO levels downstream from Lloyd Shoals.
The first step taken was to design an aerating weir, to be located downstream from the powerhouse. The design of this labyrinth weir was based on work done by the Tennessee Valley Authority some years earlier, which was funded in part by EPRI.
The weir, built in 1991 about 200 feet downstream of the powerhouse, was 10 feet tall and made of steel sheet pile. The labyrinth design provided 1,440 feet of crest length over the 200-foot width of the discharge channel. During full operation of the plant, water falling over the weir created a zone of aeration, raising DO levels from almost zero to 5 milligrams per liter. During minimum flow releases from the plant, the weir was less effective. However, it still provided significant increases in DO levels, frequently raising it to 3 to 5 milligrams per liter.
In July 1994, a tropical storm flooded the Lloyd Shoals powerhouse to a depth of 5 feet. After this flood, plant operators noticed that flow over the weir was reduced from levels before the flood. Before the flood, flow over the weir was observable when the units were loaded. After the flood, the area behind the weir was not always full. Plant personnel also observed boils just downstream of the weir. An investigation performed in 1995 by divers with Southern Company (parent firm of Georgia Power) revealed that the boulders on which the weir was founded had shifted during the flood, resulting in significant flow beneath the weir. This water was not being aerated, so the weir was making virtually no contribution to DO.
In 1995, personnel attempted to repair the weir by using sandbag wall forms filled with concrete to seal the leaks. Although this work restored the function of the weir, it proved to be only a temporary repair. Each year after that for the next six years, Southern Company divers placed grout bags in the leaks and inflated them using a sand mix Portland cement grout. This cost about $30,000 each year. Because of the expense of repairing the weir and the fact that it was never able to raise DO enough to be in compliance with state standards, Georgia Power began exploring alternatives to the aerating weir.
The turbines at Lloyd Shoals are 1920s style double runner horizontal-shaft Francis machines. The units have vacuum breaker valves that connect to the upper part of the draft tube. These valves normally operate only as the wicket gates close, but they can be manually opened during operation. Personnel determined that four of the units would naturally draw air through the vacuum breaker system during operation. Plant personnel decided to modify one of the units and determine the results in terms of increased DO.
During a planned outage, plant maintenance staff installed a simple 45-degree baffle in the draft tube above the entrance of the vacuum breaker pipe. The goal of this work was to increase the pressure differential and the resulting air flow. Results were encouraging in that there was an increase of a couple of milligrams per liter, but the increase in DO was not sufficient to meet the state of Georgia’s water quality standards.
Further work centered on a method Alabama Power Company developed in the late 1970s for draft tube aeration at several of its plants. (Alabama Power also is owned by Southern Company.) Personnel designed a system for Lloyd Shoals that would increase DO levels from almost zero to near saturation. The design of this system is typical of those Alabama Power has used for 20 years or more, but adapted to the very different physical layout of the Lloyd Shoals units. The Lloyd Shoals units are horizontal-shaft double runner units set well above the tailwater. The previous installations primarily were on vertical-shaft units with runners set below the tailwater.
This system consists of:
— A muffler to limit noise from the in-rush of air;
— A check valve to protect against flow back into the powerhouse in the event of a broken pipe;
— A gate valve to provide isolation of the system; and
— Baffles in the upper section of the draft tube to create a zone of low pressure to draw in air.
At Lloyd Shoals, the upstream bearing gallery provided a location near the turbines from which the air could be supplied. This avoids having to drill long distances through concrete to connect the turbine with an outside air source.
Installing the aeration system
In the fall of 2004, plant personnel added an aeration system to one unit, during an outage for other repairs. Personnel used a diamond core drill to make a penetration 8 inches in diameter and 8 feet long into the water casing, which is similar to the spiral case on a vertical unit. They then placed a 6-inch-diameter air supply pipe in the hole and sealed the annular space using a cementitious grout. This air supply pipe feeds two 4-inch-diameter lines routed to the upper portion of the draft tube. A penetration of the draft tube just above the water passage floor level takes air into the draft tube. Above each pipe penetration, a steel baffle was installed to create a low pressure zone and thus induce air flow.
The system began operating in the summer of 2005, with excellent results. The modified unit was shown to raise DO levels from almost zero to near saturation. It effectively aerated at minimum flow and full flow of the unit. However, with all the units running, the low DO of the water flowing through the unmodified turbines diluted the aerated flow to the point that state water quality standards were not always met. This led Georgia Power to conclude that an additional unit would require aeration to achieve the desired results. In the fall of 2006, personnel fitted a second unit with a similar aeration system.
In 2005, a flood caused severe damage to the aerating weir at Lloyd Shoals. One of the sheet pile walls was split, allowing water to flow through, rather than over, the weir. Georgia Power was concerned that the weir was not operating adequately as a result of this damage. In consultation with the Georgia DNR, in 2006 Georgia Power designed and installed a monitoring system to test both the effectiveness of the two new aeration systems and the weir.
The monitoring system consisted of four luminescent dissolved oxygen (LDO) probes and controllers, manufactured by Hach Environmental; data loggers manufactured by Campbell Scientific; and associated power supplies. The system collected DO levels at four locations: the intake, downstream of the powerhouse above the weir, just downstream of the weir, and at a municipal water intake about 0.5 mile down river. Readings were taken every 15 minutes from late May, when the DO levels historically start dropping, through the end of August when temperatures decrease and the lake water mixes.
Monitoring in 2006 indicated that the weir was not effective in its current state and that the new aeration systems could raise DO levels to near saturation. In addition, the annual stratification of the reservoir also was evident in the data. During long periods of time, DO in the water entering the intakes was almost zero, while levels measured downstream of the powerhouse but upstream of the weir were around 7 milligrams per liter. When non-aerating units operated, DO levels were depressed by their contribution to the mix of flows. Even in this case, the target levels were met or nearly met.
The data also indicated that DO levels dropped between manual cleaning intervals for the sensing tip of the probes and recovered as much as 2 milligrams per liter immediately upon cleaning. This appears to be a factor of biofouling on the probe lenses.
The air intake system and valves for the draft tube aeration system at the 18-MW Lloyd Shoals plant take air from the upstream bearing gallery and feed it to the draft tubes to increase dissolved oxygen in the water.
To provide more assurance that DO targets can be met and allow one aerated unit to be taken down for maintenance without affecting total DO, plant personnel added the aeration system to a third unit at Lloyd Shoals at the end of 2006. This made three units available for aeration during the summer of 2007.
In cooperation with the Georgia DNR, plant personnel established a monitoring plan to test the effectiveness of aeration using three units. This monitoring plan called for two intake probes and one downstream of the powerhouse. Probe cleaning intervals were shortened to one week, and the effect of cleaning was monitored. Each monitoring station collected both temperature and oxygen levels because of the strong correlation between the two. Warmer water from the top levels of the reservoir contains more oxygen than cold water from the bottom of the reservoir.
Monitoring showed that the three aerating units were capable of raising DO levels. However, it also showed some limitations of the monitoring system. For example, the LDO probe downstream of the powerhouse required weekly cleaning to provide accurate readings.
A telephone call from the operators of a water intake and treatment system about half a mile downstream of the dam revealed an additional benefit to the aeration system. Before the aeration system was installed, manganese levels in the river, measured at the water treatment plant during the treatment process, had spiked as high as 1 milligram per liter during the summer months. Water treatment plant personnel typically had to treat the water heavily during the summer months to reduce the manganese levels. However, the increased DO levels produced at Lloyd Shoals reduced the amount of dissolved manganese in the water. This meant less manganese needed to be removed during treatment, resulting in a significant cost savings for the water treatment plant. The highest manganese level recorded after the installation was 0.58 milligram per liter, during testing in which the aeration system was cycled off and on.
After the successful results in 2007, Georgia Power proposed to Georgia DNR that the draft tube aeration system be adopted as the only system for discharge aeration and that the damaged weir be removed. In January 2008, Georgia DNR agreed with the plan. Work is being planned to remove the weir.
Installation of draft tube aeration at Lloyd Shoals has proven to be an economical and effective means of meeting DO standards. The total cost to install the system was just over $200,000 for all three units. The annual cost to repair the weir and the loss of energy (1 to 5 percent) from the weir have been eliminated. The lost energy alone cost Georgia Power more than $100,000 each year.
The only cost of operation is a slight reduction (about 2 to 3 percent measured at other plants with this aerating system) in efficiency of the aerated units, which only occurs when the aeration system is on. This reduced efficiency is more than offset by the increased head — 5 to 6 feet at minimum flow and 1 foot at full load — that results from removal of the weir. This equates to about 1,600 megawatt-hours of electricity each year.
Larry Moore, senior engineer with Southern Company (parent firm of Georgia Power), designed the physical layout of the draft tube aeration system and managed construction and installation.