2.1 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. 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. Source: HANDBOOK OF MECHANICAL ENGINEERING CALCULATIONS 2.2 POWER GENERATION River flow River flow 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 ϩ5 ft (1.5 m) and ϩ45 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 ϩ5 ft (1.5 m) and ϩ45 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 ϩ 4800 ϭ 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 ϩ5 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. STEAM CONDENSING SYSTEMS AND AUXILIARIES 2.6 POWER GENERATION FIGURE 4 Intake structure has trash rack, traveling screen, pumps, and crane for dependable operation of the circulating-water system. (Power.) loads. Log booms, skimmer walls, channel modifications, and specialized raking equipment can sometimes alleviate raking problems. Traveling screens follow the trash racks. These usually are of the vertical flow- through type. European practice uses alternative screens, such as center-flow, dual- flow, and drum screens. Traveling screens may be one- or two-speed. Most two- speed screens operate in the range of 3 to 12 fpm (0.9 to 3.7 m /min) but speeds as high as 30 fpm (9.1 m /min) have been used. Wear is much greater at higher speeds. Depending on the type of piping used in the circulating-water system—concrete or steel—some form of cathodic protection may be needed, in addition to the trash racks and rakes. Cathodic protection is needed primarily when steel pipe is used for the circulating water system. Concrete pipe does not, in general, require such protection. Since the piping in once-through systems can be 10 to 12 ft ( 3 to 3.7 m) in diameter, use of the cathodic protection is an important step in protecting an expensive investment. Cathodic protection methods are discussed elsewhere in this handbook. Related Calculations. Designing a condenser circulating-water system can be a complex task when the water supply is undependable. With a fixed-level supply, the design procedure is simpler. The above procedure covers the main steps in such designs. Head loss, pipe size, and other considerations are covered in detail in separate procedures given elsewhere in this handbook. 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 2.7 Construction of the operating-point diagram and the energy-gradient chart are important steps in the system design. Further, these two plots are valuable to op- erating personnel because they give the design assumptions for the system. When pressures or flow rates change, the operator will know that the system requires inspection to pinpoint the cause of the change. The design procedure given here can be used for other circulating-water ap- plications, such as those for refrigeration condensers, air-conditioning systems, internal-combustion-engine plants, etc. Data given here are the work of R. T. Richards, Burns & Roe Inc., as reported in Power magazine. SI values were added by the handbook editor. DESIGNING CATHODIC-PROTECTION SYSTEMS FOR POWER-PLANT CONDENSERS Design a cathodic-protection system for an uncoated 10,000-tube steam condenser having an exposed waterbox/tubesheet surface area of 1000 ft 2 (92.9 m 2 ). Deter- mine the protective current needed for this condenser if the design current density is 0.2 amp/ft 2 (2.15 amp/ m 2 ) and 95 percent effective surface coverage will be maintained. How many anodes of magnesium, zinc, and aluminum would be needed in seawater to supply 50 amp for protection? Compare the number of anodes that would be needed in fresh water to supply 50 amp for protection. Calculation Procedure: 1. Determine the required protective current needed Cathodic protection of steam condensers is most often used to reduce galvanic corrosion of ferrous waterboxes coupled to copper-alloy tubesheets and tubes. Sys- tems are also used to mitigate attack of both iron-based waterboxes and copper- alloy tubesheets in condensers tubed with titanium or stainless steel. Cathodic protection is achieved by forcing an electrolytic direct current to flow to the structure to be protected. The name is derived from the fact that the protected structure is forced to be the cathode in a controlled electrolytic circuit. There are two ways this current may be generated: (1) Either an external direct- current power source can be used, as in an impressed-current system, Fig. 5a,or (2) a piece of a more eletronegative metal can be electrically coupled to the struc- ture, as in a sacrificial anode system, Fig. 5b. The first step in the design of a cathodic-protection system is to estimate the current requirement. The usual procedure is to calculate the exposed waterbox and tubesheet area, and then compute the total current needed by assuming a current density. In practice, current needs are often estimated by applying a test current to the structure and measuring the change in structure potential. Table 1 lists actual current densities used by utilities to protect condensers made of several different combinations of metals. The values given were taken from a survey prepared for the Electric Power Research Institute ‘‘Current Cathodic Pro- tection Practice in Steam Surface Condensers,’’ CS-2961, Project 1689-3, on which this procedure and its source are based. With a design current density of 0.2 amp/ ft 2 (2.15 amp /m 2 ), the total protective current need ϭ 0.2 (1000) ϭ 200 amp. With the 95 percent effective surface cov- 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 2.8 POWER GENERATION Power Supply Auxiliary anode Protected structure (cathode) Sacrificial anode Protected structure (a) (b) FIGURE 5 a. Impressed-current cathodic protection system uses exter- nal source to provide protective current. Fig. 5b. Sacrificial-anode cathodic protection uses piece of metal more electronegative than the structure for protection. (Power.) erage, 5 percent of the surface will be exposed through coating faults. Hence, the required protective current will be 0.05(200) ϭ 10 amp. Clearly, gross miscalcu- lations are possible if the effectiveness of the coating is incorrectly estimated. The value of 0.2 amp /ft 2 (2.15 amp/ m 2 ) is taken from the table mentioned above. Another problem in estimating protective-current requirements occurs when con- densers are tubed with noble alloy tubing such as stainless steel or titanium. In this case, a significant length of tubing (up to 20 ft—6.1 m) may be involved in the galvanic action, depending on the water salinity, temperature, and the tube material. This length dictates the anode /cathode area ratio and, thus, the rate of galvanic corrosion. Protective-current needs for this type of condenser can be unusually high. 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 2.9 TABLE 1 Current Densities Used for Various Condenser Materials* Condenser materials Waterbox Tubesheet Tubes Design current density amp/ft 2 amp/m 2 Average water salinity ppm Carbon steel Aluminum bronze 90-10 Cu Ni 0.05 0.54 1000 Cast iron Muntz AL-6X stainless steel 0.1 1.08 35,000 Epoxy-coated carbon steel Epoxy-coated copper-nickel Titanium 0.07 0.75 35,000 Carbon steel Muntz Aluminum brass 0.06 0.65 1000 Carbon steel Muntz 90-10 Cu Ni 0.06 0.65 1000 Carbon steel Muntz Aluminum brass 0.2 2.2 30,000 *Power TABLE 2 Current Output that can be Expected from Typical Sacrificial Anodes Materials* Current range seawater, amp Current range fresh water, amp Magnesium 1.4–2.3 0.014–0.023 Zinc 0.5–0.8 0.005–0.008 Aluminum 0.5–0.8 0.005–0.008 *Power 2. Select the type of protective system to use Protective-current needs generally determine whether an impressed-current or sac- rificial-anode system should be used. For a surface condenser, the sacrificial-anode system generally become impractical at current levels over 50 amp. For a sacrificial-type system, the current output can be estimated by determining the effective voltage and the resistance between anode and structure. The effective voltage between anode and structure is defined as the anode-to-structure open- circuit voltage minus the back-emf associated with polarization at both anode and structure. This voltage depends primarily on the choice of materials, as shown in Table 2. Resistance of the metallic path is usually negligible for an uncoated structure and the electrolytic resistance is dominant. For a coated structure, this resistance may become significant. The maximum achievable current output can be estimated by considering the case of an uncoated structure. 3. Determine the number of anodes needed for various sacrificial materials Table 2 gives a range of current outputs estimated for different sacrificial materials with an anode of the dimensions shown in Fig. 6. Thus, for any sacrificial material, number of anodes needed ϭ (required protective-current output, amp)/(current out- put for the specific sacrificial material, amp). Since the condenser being considered here is cooled by seawater, we will use the values in the first column in Table 1. For magnesium, number of anodes required 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 2.10 POWER GENERATION 6 IN. (152 CM) 10 IN. (254 CM) 1 1/4 IN. (32 CM) FIGURE 6 Typical sacrificial anode consists of a flat slab of the consumable metal into which fastening straps are cast. (Power.) ϭ 50/ 2.3 ϭ 21.739; say 22 anodes. For zinc, number of anodes required ϭ 50 / 0.8 ϭ 62.5; say 63 anodes. For aluminum, number of anodes required ϭ 50 / 0.8 ϭ 62.5; say 63. From a practical standpoint, 63 sacrificial anodes is an excessive number to install in most condenser waterboxes. The respective service of these anodes at 50 amp are about three months for magnesium, six months for zinc and aluminum. This short service further reduces the practicality of sacrificial anodes at high protective current levels. However, in fresh water, the current output is lower and is limited by the higher resistance of the water. Corresponding service lives are 5 to 10 years for magne- sium, and 40 to 60 years for zinc and aluminum. Protective coating further reduces the effective wetted surface area and lowers the required protective current at the same time as it limits the current output of the anodes. 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