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Source: HANDBOOK OF MECHANICAL ENGINEERING CALCULATIONS SECTION 2 STEAM CONDENSING SYSTEMS AND AUXILIARIES Design of Condenser Circulating-Water Systems for Power Plants 2.1 Designing Cathodic-Protection Systems for Power-Plant Condensers 2.7 Steam-Condenser Performance Analysis 2.12 Steam-Condenser Air Leakage 2.16 Steam-Condenser Selection 2.17 Air-Ejector Analysis and Selection 2.18 Surface-Condenser Circulating-Water Pressure Loss 2.20 Surface-Condenser Weight Analysis 2.22 Weight of Air in Steam-Plant Surface Condenser 2.23 Barometric-Condenser Analysis and Selection 2.24 Cooling-Pond Size for a Known Heat Load 2.26 DESIGN OF CONDENSER CIRCULATING-WATER SYSTEMS FOR POWER PLANTS Design a condenser circulating-water system for a turbine-generator steam station located on a river bank. Show how to choose a suitable piping system and cooling arrangement. Determine the number of circulating-water pumps and their capacities to use. Plot an operating-point diagram for the various load conditions in the plant. Choose a suitable intake screen arrangement for the installations. Calculation Procedure: 1. Choose the type of circulating-water system to use There are two basic types of circulating-water systems used in steam power plants today—the once-through systems, Fig. 1a, and the recirculating-water system, Fig. 1b. Each has advantages and disadvantages. In the once-through system, the condenser circulating water is drawn from a nearby river or sea, pumped by circulating-water pumps at the intake structure through a pipeline to the condenser. Exiting the condenser, the water returns to the river or sea. Advantages of a once-through system include: (a) simple piping ar-rangement; (b) lower cost where the piping runs are short; (c) simplicity of operation—the cooling water enters, then leaves the system. Disadvantages of once-through systems include: (a) possibility of thermal pollution—i.e., temperature in-crease of the river or sea into which the warm cooling water is discharged; (b) loss of cooling capacity in the event of river or sea level decrease during droughts; (c) trash accumulation at the inlet, reducing water flow, during periods of river or sea pollution by external sources. 2.1 Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM CONDENSING SYSTEMS AND AUXILIARIES 2.2 POWER GENERATION FIGURE 1 a. Once-through circulating-water system discharges warm water from the condenser directly to river or sea. Fig. 1b. Recirculating-water system reuses water after it passes through cooling tower and sta-tionary screen. (Power.) Recirculating systems use small amounts of water from the river or sea, once the system has been charged with water. Condenser circulating water is reused in this system after passing through one or more cooling towers. Thus, the only water taken from the river or sea is that needed for makeup of evaporation and splash losses in the cooling tower. The only water discharged to the river or sea is the cooling-tower blowdown. Advantages of the recirculating-water system include: (a) low water usage from the river or sea; (b) little or no thermal pollution of the supply water source because the cooling-tower blowdown is minimal; (c) remote chance of the need for service reductions during drought seasons. Disadvantages of recirculating systems include: (a) possible higher cost of the cooling tower(s) compared to the discharge piping in the once-through system; (b) greater operating Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM CONDENSING SYSTEMS AND AUXILIARIES STEAM CONDENSING SYSTEMS AND AUXILIARIES 2.3 complexity of the cooling tower(s), their fans, motors, pumps, etc.; (c) increased maintenance requirements of the cooling towers and their auxiliaries. The final choice of the type of cooling system to use is based on an economic study which factors in the reliability of the system along with its cost. For the purposes of this procedure, we will assume that a once-through system with an intake length of 4500 ft (1372 m) and a discharge length of 4800 ft (1463 m) is chosen. The supply water level (a river in this case) can vary between 5 ft (1.5 m) and 45 ft (13.7 m). 2. Plot the operating-point diagram for the pumping system The maximum cooling-water flow rate required, based on full-load steam flow through the turbine-generator, is 314,000 gpm (19,813 L/s). Intermediate flow rates of 283,000 gpm (17,857 L/s) and 206,000 gpm (12,999 L/s) for partial loads are also required. To provide for safe 24-hour, 7-day-per-week operation of a circulating-water system, plant designers choose a minimum of two water pumps. As further safety step, a third pump is usually also chosen. That will be done for this plant. Obtaining the pump characteristic curve from the pump manufacturer, we plot the operating-point diagram, Fig. 2, for one-pump, two-pump, and three-pump op-eration against the system characteristic curve for river (weir) levels of 5 ft (1.5 m) and 45 ft (13.7 m). We also plot on the operating-point diagram the seal-well weir curve. The operating-point diagram is a valuable tool for both plant designers and operators because it shows the correct operating range of the circulating-water pumps. Proper use of the diagram can extend pump reliability and operating life. 3. Construct the energy-gradient curves for the circulating-water system Using the head and flow data already calculated and assembled, plot the energy-gradient curve, Fig. 3, for several heads and flow rates. The energy-gradient curve, like the operating-point diagram, is valuable to both design engineers and plant operators. Practical experience with a number of actual circulating-water installa-tions shows that early, and excessive, circulating-pump wear can be traced to the absence of an operating-point diagram and an energy-gradient curve, or to the lack of use of both these important plots by plant operating personnel. In the once-through circulating-water system being considered here, the total conduit (pipe) length is 4500 4800 9300 ft (2835 m), or 1.76 mi (2.9 km). This conduit length is not unusual—some plants may have double this length of run. Such lengths, however, are much longer than those met in routine interior plant design where 100 ft (30.5 m) are the norm for ‘‘long’’ pipe runs. Because of the extremely long piping runs that might be met in circulating-water system design, the engineer must exercise extreme caution during system design—checking and double-checking all design assumptions and calculations. 4. Analyze the pump operating points Using the operating-point diagram and the energy-gradient curves, plot the inter-section of the system curves for each intake water level vs. the characteristic curves for the number of pumps operating, Fig. 3. Thus, we see that with one pump operating, the circulating-water flow is 120,000 gpm (7572 L/s) at 48.2 ft (14.7 m) total dynamic head. With a weir level of 5 ft (1.5 m), and two pumps operating, the flow is 206,000 gpm (12,999 L/s) at 79 ft (24.1 m) total dynamic head. When three pumps are Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM CONDENSING SYSTEMS AND AUXILIARIES 2.4 POWER GENERATION FIGURE 2 Operating-point diagram shows the correct operating range of the circulating-water pumps. (Power.) used at the 5-ft (1.5 m) level, the flow is 225,000 gpm (14,198 L/s) at 79 ft (24.1 m) total dynamic head. Using the sets of curves mentioned here you can easily get a complete picture of the design and operating challenges faced in this, and similar, plants. The various aspects of this are discussed under Related Calculations, below. 5. Choose the type of intake structure and trash rack to use Every intake structure must provide room for the following components: (a) cir-culating-water or makeup-water pumps; (b) trash racks; (c) trash-removal screens—either fixed or traveling; (c) crane for handling pump removal or instal-lation; (d) screen wash pump; (e) access ladders and platforms. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM CONDENSING SYSTEMS AND AUXILIARIES STEAM CONDENSING SYSTEMS AND AUXILIARIES 2.5 FIGURE 3 Energy-gradient diagram shows the actual system pressure values and is valuable in system design and operation. (Power.) A typical intake structure having these components is shown in Fig. 4. This structure will be chosen for this installation because it meets the requirements of the design. Trash-rack problems are among the most common in circulating-water systems and often involve unmanageable weed entanglements, rather than general debris. The type of trash rack and rack-cleaning facilities used almost exclusively in the United States and many international plants, is shown in Fig. 4. Usually, the trash rack is inclined and bars are spaced at about 3-in (76.2-mm). The trash rake may be mechanical or manual. The two usual rake designs are the unguided rake, which rides on the trash bars, and the guided rake, which runs in guides on the two sides of the water channel. If the trash bars are vertical, the guided rake is almost a necessity to keep the rake on the bars. But neither solves all the problems. If seaweed or grass loads are particularly severe, alternative trash rakes, such as the catenary or other moving-belt rakes, should be considered. These are rarely put into original domestic installations. There are many other alternative types of trash racks and rakes in use throughout the world that are successful in handling heavy Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. ... - tailieumienphi.vn