Proceedings of The Fourth International Zebra Mussel Conference Madison Wisconsin

Proceedings of The Fourth International Zebra Mussel Conference, Madison,Wisconsin, March 1994 Zebra Mussel Control Using Thermal Treatment for Electric Utility Stations Roy I. Marcus Staff Engineer Commonwealth Edison Company Steven L. Wahlert Senior Structural Project Engineer Sargent & Lundy ABSTRACT When electric utilities and other water users on the Great Lakes were confronted with operating problems due to zebra mussels, Commonwealth Edison Company (CECo) established a task force to develop a plan to counteract the threat at the CECo electric generating stations. A monitoring program was initiated at the stations and an evaluation of control options was started. At State Line Station, located on the southern tip of Lake Michigan, CECo experimented with thermal treatment of the circulating water and service water systems. The station design allows recirculation of the cooling water with minimal modifications. The trial at State Line Station proved successful in controlling the zebra mussels with minimal impact on operations. Based on the successful trial and the task force’s assessment of other control options, CECo determined that, for their coal-fired generating stations, thermal treatment was the most cost-effective approach, with the least impact on station operation and the environment. Of the 10 coal-fired generating stations operated by CECo, 8 have been selected for modifications. The other 2 stations have not yet been affected by zebra mussels. Before performing detailed design, a study was performed for each station to evaluate the operation of the equipment at elevated temperature and to determine the operating limits needed at the target treatment temperature of 95° F. Conceptual designs for the modifications were developed, and the most cost-effective arrangement was selected for detailed design. Case studies of the modifications being constructed at several stations are presented. The modifications to the circulating water systems are described. Initial results of the treatment are reviewed. 371 INTRODUCTION Commonwealth Edison Company (CECo) has found a simple approach to control zebra mussels at the utility’s stations: heat. Thermal treatment is accomplished by recirculating the condenser discharge to raise the circulating water (CW) and service water (SW) temperature to 95° F. The effectiveness of thermal treatment was demonstrated at CECo’s State Line Station in 1990 and 1991. At the State Line Station, the plant arrangement (see Figure 1) allows recirculation of the heated discharge back to the crib house and CW and SW pumps by opening the division wall and north and south recirculation gates. Thermal treatment took 10 hours to raise the water temperature and an additional 6 hours to maintain the 95° F treatment temperature. A diver’s exam showed 100% zebra mussel mortality. In parallel with the State Line demonstration, a CECo task force assessed treatment options for all their stations. The task force determined that thermal treatment was the least-cost control approach considering construction costs and long-term operating and maintenance expenditures. Compared to other alternatives, thermal treatment minimizes unit deratings and outages and minimizes impact to the environment. Based on the task force’s assessment and the successful treatment at State Line Station, thermal treatment was selected as the preferred, least-cost method to control zebra mussels at their fossilfueled stations. This paper discusses the steps Commonwealth Edison Company took to develop a control strategy and why thermal treatment was selected as the preferred approach for their stations. The paper also provides examples of the station modifications and the results of thermal treatments performed to date. BACKGROUND Commonwealth Edison Company provides electric service to the northern one-fifth of Illinois, a service territory covering 11,500 square miles. CECo has over three million customers and eight million residents in their territory. CECo owns and operates 10 fossilfueled and 6 nuclear stations. These 16 stations have a combined net generating capacity of 22,522 megawatts. Three stations (two fossil and one nuclear) are located next to Lake Michigan and get their cooling water from the lake. The other stations are located inland from Lake Michigan and obtain cooling water from either (1) a river or canal or (2) a man-made cooling lake. Zebra mussels have been sighted at all but one of the stations. Figure 2 shows Commonwealth Edison service territory and their fossil stations in Illinois. Zebra mussel colonies can affect the generating station operation by clogging the vital circulating water and service water systems. The mussels can attach to power plant equipment including water pipes, crib houses, tunnels, traveling screens, bar grills, pumps, 372 heat exchangers, and condenser tubes. The mussel colonies can reduce or cut flow to critical plant equipment causing unit deratings or unplanned outages. In addition, the operability of the fire water system can be threatened. To respond to this threat to the operation of their stations, CECo established a task force to evaluate control alternatives and to develop a system-wide zebra mussel control strategy. DEVELOPING A TREATMENT STRATEGY In 1991, CECo established a zebra mussel task force comprised of representatives from the generating stations, engineering and support services, and environmental services. The task force was charged with reviewing treatment options and with developing a recommended treatment plan for all stations. The task force approached the problem in the following steps: • First, the task force researched zebra mussels to understand their life cycle and options for controlling them. The task force reviewed available literature, information from other utilities, and reports from EPRI (Electric Power and Research Institute), and attended seminars and conducted tests at the stations. • Next, each control option was evaluated, including an assessment of the effectiveness, impact on station operation, and impact on the environment. For each control method, budgetary estimates of capital and operating and maintenance costs were developed. • Finally, after weighing these factors, a control strategy was selected. Priorities for implementing the modifications were established based on results of monitoring activities at the stations and the task force’s determination of the stations most susceptible to the zebra mussel threat. There are several treatment methods that have been tried to control zebra mussels. The task force evaluated three proven methods: mechanical cleaning, chemical treatment, and thermal treatment. A brief discussion of each alternative and CECo’s assessment of each approach follows. Mechanical Cleaning Cleaning methods typically include manual labor and hand tools. High-pressure water jets and mechanical cutting devices have also been used to dislodge the tightly adhered mussels from large surface areas such as found in cribhouses and circulating water tunnels. Cleaning heat exchangers, coolers, strainers, and other equipment includes disassembly, manual cleaning, and reassembly of the equipment. 373 The advantage of mechanical cleaning is its low capital cost. The disadvantages are that (1) it is labor intensive and must be repeated periodically as the zebra mussel colonies are reestablished and (2) station outage time or unit deratings are required to clean cribhouses and the circulating water and service water systems. Chemical Treatment Chemical treatment involves use of oxidizing chemicals, primarily chlorine or bromine, or nonoxidizing chemicals and molluscides. Chlorination is a common method that has been used for water treatment for many years by industry and municipalities. Chlorination includes the injection of chlorine, usually in the form of hypochlorite, and then dechlorination before the water is returned to the waterway in order to meet EPA limits on residual chlorine. In addition, the EPA requires that the discharge be analyzed and monitored to demonstrate that the system is working properly and that the EPA limits are met. Researchers continue to examine the potential health effects of chlorine and other chemicals used for control of zebra mussels. CECo’s experience with the EPA and other regulatory bodies has been to reduce the amount of chemicals used at its stations and to closely monitor the station discharge. EPA regulations place additional burdens on the utility staff to keep records and file reports on chemical usage. Advantages of chemical treatment are that (1) modifications can be constructed without interruption of operations and (2) treatment can be performed without limiting operations. Disadvantages of this approach are (1) the cost (which includes the capital cost for initial installation, the daily cost of chemicals to chlorinate and dechlorinate, and other operating and maintenance expenses); (2) a greater potential environmental impact; and (3) the high probability of future, more stringent, limits on use of chemicals. Thermal Treatment Thermal treatment uses heated water to control the zebra mussels. Experience has shown that the mussels are killed when exposed to a water temperature of 95° F for approximately 4 hours. Thermal treatment for circulating and service water systems at electric generating stations is accomplished by recirculating the heated condenser discharge back to the circulating water intake. With each pass of the recirculated water through the condenser, additional heat is provided by the steam turning the turbine, thereby raising the temperature of the cooling water. The time needed to raise the temperature to 95° F depends on the starting temperature of the cooling water, the amount of thermal losses, and the volume of water to be heated. These factors vary with station arrangement and environmental conditions, such as lake temperature, at the time of treatment. After initial treatment, periodic thermal treatment is performed to control reestablishment of the 374 mussels. Other methods for supplying the heated water, such as electric heaters, can be used to treat remote systems such as makeup pumps for cooling lakes and fire pumps. Advantages of this approach include (1) the minimal environmental impact, (2) the low operating and maintenance cost, and (3) the minimal impact on operations. Disadvantages are (1) the high capital cost and (2) that some outage time may be needed to construct the modifications. Selection of Preferred Treatment Method The task force eliminated mechanical cleaning from consideration because of the disadvantages mentioned earlier. For the remaining approaches, chemical and thermal treatment, the task force developed estimates of capital, operating, and maintenance costs for several stations. The estimates, summarized in Tables 1 and 2, showed that thermal treatment is the least-cost option. The task force concluded that thermal treatment (1) was an effective method to control zebra mussels, (2) has little impact on station operation, and (3) has minimal impact on the environment. Based on their assessment and the successful treatment at the State Line Station, thermal treatment was selected by the task force as the preferred, least-cost control approach, to control zebra mussels at their fossil-fueled stations. MODIFICATION DESIGN CECo contracted Sargent & Lundy to assist in developing the conceptual design and then in preparing the detailed design for the modifications for thermal treatment at their fossil stations. The design process included the following steps: q Assess station operation at the 95° F treatment temperature. Develop alternate arrangements considering the station configuration and operating parameters. Prepare construction cost estimates for the most promising alternatives. Select the arrangement for detailed design. Prepare detailed design documents for construction. q q q q The operation assessment was an important aspect of the design to determine any operating restrictions required during the treatment period with the cooling water at 95° F. Critical parameters that were evaluated for each station included turbine back pressure, 375 maximum service water temperature for operation of equipment such as the turbine lube oil coolers, and permit limits on the discharge water temperature. Thermal treatment involves recirculation of the large flows from the circulating and service water systems. To reduce the cost of the modifications, they were designed for reduced flow, one half of the maximum flow. This stipulation meant that treatment could be performed on weekends or at night when unit loads can be less than full load. Operating at lower than full load also helps keep the discharge temperature within permit limits. The existing plant facilities were considered in developing the arrangement for the modifications. Where possible, existing and abandoned piping and structures were incorporated into the design to reduce the cost. The modification details were designed to minimize the outage time required for installation. In addition, the construction schedule was coordinated with the existing outage schedule. This arrangement ensured that station availability was not affected. CASE STUDIES The following case studies illustrate the modifications being constructed at several CECo stations. As these examples show, the modifications vary significantly depending on the plant arrangement. Waukegan Station Waukegan Station has three operating units with total net generating capacity of 625 MW. It is located in northern Illinois near the Wisconsin border. Cooling water is provided from Lake Michigan via an open channel, and the discharge is returned to the lake in a parallel channel separated from the intake by a sheet piling divider wall. Figure 3 illustrates the modifications, which included a new gate structure across the discharge channel with eight 10-foot-wide gates to block the flow. A new gate structure, with three 10-foot 6-inch-wide gates, was added on the divider wall; these gates and the three existing 8-foot-wide gates are used to recirculate the cooling water. The modifications were designed based on the station operating at low load on a weekend. The gates were sized for circulating water flow of 323,000 gpm, one half of the maximum circulating water and service water flow. The initial thermal treatment was performed on a Saturday in October 1993 when Lake Michigan temperature was 56° F. The divider wall gates were opened and the discharge 376 gates were closed. In less than 8 hours the water temperature was raised to the 95° F treatment temperature. Because of the long path that the recirculated water travels before reaching the pump structures CECo was concerned about thermal stratification. CECo feared that the lower areas of the structures and equipment might not see the heated water, reducing the effectiveness of the treatment. To monitor the temperature profile, station personnel took temperature readings across the height and width of the pump structures and verified that the desired treatment temperature was achieved. During treatment station personnel closely monitored operation of critical plant equipment such as the turbine generator and lube oil coolers. One concern arose due to differential thermal growth between the turbine low-pressure casing and the turbine shaft which reduced critical clearance dimensions. To avoid this concern in the future, CECo plans on reducing unit load during future treatments to reduce the rate of temperature increase and the differential movements. After treatment, the remains of the zebra mussels from the structures, equipment, and piping were found in the tubes of heat exchangers and the condensers. Ten cubic yards of mussels were removed. Since this was the initial treatment, the mussels had grown for several years to a size large enough to plug the l/2-inch to 3/4-inch diameter tubes. Future treatments are planned at least twice a year to kill the mussels when they are small enough to pass through the plant equipment. Joliet Station 9 & 29 There are two stations at Joliet: Station 29 on the north side of the Des Plaines River and Station 9 on the south side. Joliet is located 55 miles southwest of downtown Chicago. Both stations get cooling water from the Des Plaines River. Joliet Station 29 has two operating units with a net generating capacity of 1025 MW. The recirculation system at Joliet was designed for maximum circulating and service water flow. The turbines at Joliet have restrictive limits on turbine back pressure when operating with 95° F cooling water, limiting operations at reduced load. This restriction means that treatment cannot be performed with reduced flows as at the other stations. Therefore, the recirculation system was designed for the full circulating water flow of 920,000 gpm. Figure 4 illustrates the modifications being constructed at Joliet Station 29. A 70-foot long concrete tunnel with two 14-foot-wide by 10-foot 6-inch-high sections connects the discharge flume to the intake. Two gates are provided at the entrance to the recirculation tunnel and four 11-foot-wide gates are provided in the discharge flume to block the flow. These isolation gates will regulate the amount of water recirculated to the intake. 377 At Joliet Station 9, only Unit 6 is operating with a net generating capacity of 302 MW. Unit 5 is retired. The cost of the modifications at this station was minimized by incorporating the abandoned Unit 5 circulating water pipe and pumphouse into the recirculation flow path. The modifications were designed for 168,000 gpm, one half the maximum flow. Figure 5 shows the modifications at Joliet Station 9. The 1 l-foot diameter Unit 6 circulating water discharge pipe was connected to the 7-foot diameter Unit 5 supply pipe to recirculate the discharge water to the Unit 5 crib house. A sheet piling wall and gate structure were provided in front of the Unit 5 & 6 cribhouses to isolate the forebay from the river and to complete the recirculation path. A 12-foot square roller gate was added at the end of the discharge pipe to block the discharge and initiate the recirculation flow. Will County Station Will County Station has four operating units with a total net generating capacity of 1092 MW. It is located 40 miles southwest of Chicago. Cooling water comes from the Chicago Sanitary and Ship Canal. Each unit has a pumphouse that supplies water from the canal to the station, The discharge goes to a common tunnel serving all four units which returns the heated water to the canal, 1500 feet from the intake. Because of the plant configuration, the supply and discharge could not be economically connected. Instead, the thermal treatment scheme established the recirculation circuit by interconnecting the circulating water systems for two units. This arrangement takes advantage of the existing piping configuration designed to allow back washing the condensers. Figure 6 illustrates the recirculation paths for the paired Units 1 & 2; the arrangement for Units 3 & 4 is similar. The Unit 1 & 2 scheme includes the following features: • Connection of the circulating water pipes for the two units near the condenser with isolation and crosstie valves. •Connection of the adjacent circulating water pumphouses by cutting openings in the divider walls. Gates were added to the new openings. •Use of existing bulkhead gates at the pumphouse inlet to isolate the pumphouse from the canal. These modifications provide a closed circuit for the cooling water. Thermal treatment will be performed with one unit operating at reduced load and the other unit not operating. 378 Cooling water will be supplied from one of the operating unit’s circulating water pumps (75,000 gpm) pumping water through the condenser. The valve in the discharge pipe at the condenser outlet is throttled and most of the condenser discharge flows through the new crosstie pipe then through the condenser of the nonoperating unit. The water is then returned to the crib house through the circulating water supply pipe (flow opposite to normal operation), through the pumps of the nonoperating unit. Finally the water flows through the openings in the divider walls to the pumphouse with the operating CW pumps, completing the recirculation path. The modifications for Units 3 & 4 are similar except that the units are larger and the recirculation flows are greater, making the modifications larger in size. SUMMARY These examples illustrate the modifications being constructed at the CECo stations to accomplish thermal treatment. The concept of recirculating the condenser cooling water is the same at all stations; however, the different plant arrangements and other station limitations called for unique and innovative designs. Table 3 summarizes the status and total cost of the thermal treatment modifications at the CECo fossil stations. By spring 1994, CECo will be using thermal treatment at six stations; the thermal treatment system will be operational at two other stations by the end of 1994. At the remaining two stations, CECo is presently monitoring for zebra mussel infestation. Commonwealth Edison Company has been proactive in responding to the zebra mussel threat to their operating stations. Thermal treatment, using waste heat from the station, (1) is an effective control method, (2) is the least-cost control option and (3) has minimal effect on the environment. For Commonwealth Edison, thermal treatment has proved to be the most effective means to control zebra mussels at their fossil stations. 379 Table 1 Estimated Capital Costs Zebra Mussel Treatment Options Station Crawford Fisk Waukegan Will County Chemical Treatment Thermal Treatment $2,400,000 $900,000 $2,600,000 $3,400,000 $1,800,000 $500,000 $1,100,000 $3,500,000 Note: Costs based on projections from estimate for State Line Station. Table 2 Estimated Operating and Maintenance Costs Zebra Mussel Control Options Thermal Treatment $6,000 Table 3 Capital Budget and Completion Dates Zebra Mussel Control Modifications Capital Budget Note 1 $2,000,000 $1,500,000 $400,000 $1,100,000 $2,000,000 $0 $1,100,000 $3,500,000 Scheduled Completion Date October 1994 March 1994 November 1994 March 1994 March 1994 Note 2 January 1993 May 1994 Station Collins Crawford Fisk Joliet 9 Joliet 29 State Line Waukegan Will County Note 1: Budget based on preliminary design for each station. Note 2: Recirculation capability was included in original design of station. 380 Water Arrangement Figure 2--Commonwealth Edison Service Territory and Fossil Station Locations Figure 3-Waukegan Station--Modifications to Circulating Water System Unit 7 Unit 8 Figure 4—Joliet Station 29--Modifications to Circulating Water System 382 Figure 5—Joliet Station 9-Modifications to Circulating Water System Figure 6-Will County Station Units 1 & 2— Modifications to Circulating Water System 383