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WEEK 04: GENERATION: AUXILLIARIES Sections: Circulating-Water | Flue & Waste | Diesel Plant Purpose. The primary purpose of the circulating-water system is to provide the cooling water to condense the steam exhausted from the turbine for reuse as feedwater by the steam generator. This system is also sometimes used as a source of cooling water from various auxiliary services in the turbine building. Types. The circulating-water system may be either a once-through (open) or a closed-loop system. The open system continuously draws the required flow from a source such as a river, lake, or ocean and returns the heated water to the same source. The source then becomes a natural heat sink. Hybrids (once-through systems containing a manmade heat sink on the discharge line to reduce the heat load on the system's water source) have been built to meet regulatory requirements which limit the temperature of thermal discharge. However, the effectiveness of such arrangements is limited. During periods when the ambient air temperature is high relative to the temperature of the natural water body into which the system is discharging, the cooling capacity of the "supplementary" heat sink becomes negligible. The circulating-water temperature rise for this system is approximately 15 to 25°F (8 to 14°C). The closed-loop system recirculates its cooling water in a continuous loop which contains a manmade sink. This sink maybe a reservoir, cooling pond, spray pond, wet cooling tower, or dry cooling tower. In addition to these, this system will have a makeup-water supply system, and a blowdown system. Sources of makeup water normally include rivers and lakes, and seawater. Some stations use groundwater, sewage treatment plant effluent, and irrigation drainage for makeup water. Cooling in the closed-loop is accomplished primarily by evaporation (with the exception of dry cooling water). In addition to the evaporation loss, a certain portion of the circulating water must be disposed of (blowdown) and replaced by fresh water to limit the concentration of dissolved solids in the water. Makeup water requirements for evaporative losses, blowdown, and other minor losses typically amounts to less than 5% of the total circulating-water flow. The circulating-water temperature rise for this system is approximately 20 to 30°F (11 to 17°C). Flows of modern circulating-water systems range from approximately 200,000 to 1,000,000 gal/min (12.6 to 63 m³/s). As a general rule, fossil and nuclear power stations require approximately 500 and 750 gal/min (0.03 to 0.04 m³/s) per megawatt of generating capacity, respectively. This is because of the higher steam flow requirements of the turbine in a nuclear power station. Major Components. A system typically consists of an intake structure which contains screening equipment and from two to six circulating-water pumps; several throusand feet of pipe, with diameters of up to 12 ft (3.7m); a surface condenser; a discharge structure in the form of a flume or submerged diffuser; and associated valves, gates, expansion joints, etc. The following are the major components found in open- and closed-loop circulating water systems. a. Intakes. The intake of a once-through circulating-water system, or makeup system for a closed-loop circulating-water system, generally consists of a shoreline or bankline structure which houses screening equipment and pumps. When a power plant has a shoreline inlet, equipment which requires maintenance, such as mechanical screens and pumps , are located in an easily accessible structure on shore. Water is conveyed to this onshore screen.pumpwell via a pipeline or tunnel leading from the offshore inlet. The offshore inlet is normally a submerged concrete structure with vertical openings protected by bars spaced so as to keep out large debris. b. Trash Rakes. The entrance to the conventional shoreline intake is protected by trash racks. The racks keep large, cumbersome debris from reaching and damaging the traveling screens. The rack bars are generally spaced every 3 in (76 mm) and may be cleaned by a mechanical rake. Racks can be vertical or sloped. A single rake usually traverses several screens via a track. The rake hose frame contains a cart into which the rake dumps the debris. Traversing motors range from 2 to 5 hp (1.5 to 3.7 kW) and hoist motors from 10 to 15 hp (7.5 to 11.2 kW). c. Traveling Water Screens, usually located several feet downstream of the trash racks, screen out the finer debris to protect the condenser from clogging. The criterion for sizing the openings of a traveling screen is approximated as one-third to one-half the diameter of the condenser tubes, resulting in 3/8-in² (10-mm²) openings. The screen panels are made up of cross-woven 10- or 12-gage wire. There are three basic types of traveling screens: the through-flow type, the dual-flow type and the center-flow type. The through-flow consists of a series of panels, up to 14 ft (4.3 m), in length, which span the intake bay and travel on a continuous pair of chains around two sets of sprocket shafts. The direction of travel is upward on the upstream face and can vary from a few feet to 30ft/min (0.15 m/s). Debris is flushed from the panels by a high-pressure backwash spray system consisting of a series of nozzles on a horizontal header. The debris is washed in to a trough, which extends over the width of the intake surface, and is sluiced to a disposal area. The dual-flow (no-well screen) is similar in structure to the through-flow system, however, the axis of the sprocket shafts is parallel to the flow, and the screen is normally directly connected to the pump via a transition piece and elbow leading into the pump section. This is an economical design, but only practical in warm climates where ice is not a concern. Because the pump takes suction from the center of the screen, debris carryover is not possible. The center-flow system has the ability to remove fish and other organisms. In principle, it is comparable to the dual-flow system with its flow paths reversed. Flow enters the central area of the screen and exits outward through the screen panels. It is oriented with its axis parallel to the flow path. The screen panels are semicircular in cross section and the debris trough is located in the central area of the screen. As the screen rotates, much of the debris falls from the screen into the debris trough abd there is no carryover. A backwash spray system is also provided to remove the remainder of the debris. |
