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1、P A R T C PIPING SYSTEMS CHAPTER C1 WATER SYSTEMS PIPING Michael G. Gagliardi Manager of Projects Raytheon Engineers and Constructors, Inc. Lyndhurst, NJ Louis J. Liberatore Supervising Discipline Engineer Raytheon Engineers and Constructors, Inc. Lyndhurst, NJ INTRODUCTION General Description Water
2、-distribution systems that serve populated areas and industrial complexes, including offi ces and light and heavy industry, are classifi ed broadly as being of the loop, gridiron, or tree types. Figure C1.1 describes these three types. Within the broad concept, there may be a combining of all three
3、types used as the building blocks for the overall system. In the loop system, large feeder mains that surround areas many city blocks square serve smaller cross-feed lines connected at each end into the main loop. See Fig. C1.1a. In the gridiron (or grid) system, the piping is laid out in checkerboa
4、rd fashion, with piping usually decreasing in size as the distance increases from the source of supply. See Fig. C1.1b. In the tree system, there is a single trunk main, reducing in size with increasing distance from its source of supply; branch lines are supplied from the trunk. See Fig. C1.1c. The
5、 grid and loop systems provide better reliability because of their multiple paths. Grid and loop systems are often backed up with feeder pipes leading directly from the pumping station to remote distribution centers serving to bolster the supply to meet increased demands with growth of population. W
6、ater distribution systems are made up of pipes, valves, and pumps through whichwater ismoved fromthesource tohomes, offi ces,andindustries thatconsume the water. The distribution system may include facilities to store treated and un- C.3 C.4PIPING SYSTEMS FIGURE C1.1(a) Loop system, (b) grid system,
7、 (c) tree system. treated water for use during periods when demand is greater than the source can supply and when special service requirements must be satisfi ed. The distribution systems are subject to the requirements of local ordinances and state laws and health regulations. Two important require
8、ments of any water distribution system are that it supply each user with a suffi cient volume of water at adequate pressure; and for treated water systems, that the quality of the water be maintained by the treatment facility and distribution system. Types of Water Piping Systems There are four gene
9、ral types of piping systems in water distribution utilities: trans- missionlines,in-plantpipingsystems,distributionmains,andservicelines.Transmis- sion linescarry waterfrom asource ofsupply tothe distribution system. Distribution mains are the pipelines that carry water from transmission lines and d
10、istribute it throughout a service area, e.g., a community or industrial complex. Service lines are small diameter pipes that run from the distribution mains to the user. The prime objective of a distribution network is to supply a suffi cient quantity of water to all parts of the system, at pressure
11、s adequate for the requirements of the users at all times and under all conditions of their demands, including suffi cient fl ow and pressure for fi re-fi ghting purposes. Therefore, the selection of pipe sizes, material, geometry, and confi guration in distribution networks is infl uenced more by t
12、he necessity of maintaining adequate water pressure than by the economics of pumping costs. The common industrial or power applications of water systems are condenser- circulating water and service cooling-water systems. A condenser uses circulating watertocondensesteamexhaustedfromtheplantsturbines
13、.Inalargesteam-power plant this requires a considerable amount of water to be continuously circulated. Consequently, since the circulating water directly affects the plants effi ciency and reliability, an effi cient, reliable, and economical circulating water system is required. Service water system
14、s provide cooling water to a plants components, heat ex- changers, and other services required by the plant. Due to current environmental regulations, recirculation-type systems in which the same water is used repeatedly must be applied in most cases. Means of cooling the water is provided in the fo
15、rm WATER SYSTEMS PIPINGC.5 of cooling towers, spray ponds, or cooling ponds. Initial fi ll and makeup water, to compensate for evaporation, leakage, and blowdown, has to be provided from a river, lake, sea, or other large natural body of water. Some service water systems and, in nuclear plants, emer
16、gency service water systems may be once-through- type systems. A siphon system is one in which the siphon principle is employed to carry the water through elevated parts of the system, such as the condenser, in order to reduce the pumping power required. These elevated portions of the water system o
17、perate under a partial vacuum. A pressure system is one in which the water fl ows under a positive head throughout. This system is generally used with recirculating systems, such as with cooling tower installations. Vertical pumps set in an intake basin are usually the most suitable for circulating
18、water and service water applications. The complexity of the intake structure is naturally affected by the number of pumps necessary for the system. Reliability points to the use of at least two pumps. The design criteria for the plant will dictate the fi nal choice, whether it will be two pumps at t
19、wo-thirds capacity each, or one- half capacity each, or some other number of pumps and load distribution. The capacity selection is the subject of a careful analysis, taking into account site space and hydraulic conditions, water requirement, variation of pumping head, the best effi ciency range of
20、the pumps, and the costs of various layouts and options. The intake chambers for vertical pumps require careful design for good pump operation. The design must bring about a uniform and undistributed fl ow of water to the pump without whirl. Most pump manufacturers and Hydraulic Institute have desig
21、n suggestions for intake chambers for their particular pumps. There are no standard solutions to vertical-pump intake problems, so each vertical-pump installa- tion should be studied individually. Booster pumps may be required to ensure pressure to most distant higher eleva- tion points without over
22、pressurizing the lowest components. Horizontal pumps are generally suited for this application. The intake piping to the suction of horizontal pumps should be designed so as to avoid air pockets. Also, the water-fl ow velocity should be made uniform over the suction inlet area by placing bends as fa
23、r as possible from the pump inlet. Discharge Structure. On the discharge end of once-through cooling water sys- tems, an underwater (or sealed) discharge must be provided to prevent entry of air into the piping, which would otherwise break the siphon action at the condenser. For complex systems an e
24、xtensive load analysis is performed to establish a seal elevation that is adequate for all operating conditions. Refer to the section on hydraulic grade lines. One means of providing this seal is through the use of a seal well, that is, a basin with a water level controlled by an overfl ow weir. The
25、 seal- well water level regulates the height of the siphon recovery, and it is the fi nal elevation to which the system circulating pump delivers the water. Beyond the seal well, the discharge into the river or other body of water must be done in such a way that the discharge velocity is dissipated
26、without washing away banks, tearing up the bottom, undermining the discharge piping, or permitting uncontrolled recirculation to the intake. These systems require attention to problems such as air binding and water hammer, as discussed in this chapter. A concern about maintaining reliability in plan
27、ts utilizing raw water for cooling is the accumulation of microbiological growth and sediment accumulation (silting). A critical concern in nuclear plants is keeping piping and components free of clogging or bacterial attack. Strong prevention and maintenance programs are the norms for important sys
28、tems. C.6PIPING SYSTEMS The design of high-temperature, high-pressure piping such as boiler feedwater (FW)systemsrequireconsiderableexperienceandstudy.RefertoChap.B2.Besides those typical hydraulic problems inherent in lower pressure, lower temperature systems, concern for fl ashing cavitation and t
29、he problems associated with handling two-phase fl ow and large-system transients are encountered. Refer to Chap. B8 and App. E9. Velocities ranging from 10 to 25 ft/sec (3 to 7.6 m/sec) in high-pressure and high-temperature water systems are normal, as the fl uid is usually high-quality, low- solids
30、 water. Piping material can range from carbon steel, such as ASTM A106 on the low-temperature end, to carbon, molybdenum and chrome, and molybdenum alloy steels such as ASTM A335 after the high-pressure heater temperature above 750?F (399?C). FW piping is usually seamless and employs welded joints.
31、Flanged connections, where required, must use a temperature-resistant gasket. Refer to Chap. A7. Network Analysis of Distribution Systems The complexity of the analysis required for a well-designed water-distribution sys- tem is comparable to that of utility electric power networks. There are severa
32、l procedures that may be used for the analysis of fl ow in complex piping networks, such as the Hardy-Cross method. All such methods involve the solution of a fl ow problem considering head losses of a complex distribution network resulting in extremely tedious and time-consuming trial and error cal
33、culations. With the devel- opmentofstate-of-the-artcomputerhardwareandsoftware,complexnetworkprob- lemsinvolvinghundredsofbranchescanbesolvedinarelativelyshorttime.Illustra- tive Example C1.1 presents a sample problem using the Hardy-Cross method of fl ow-network solution. Illustrative Example C1.1
34、1. Make a skeleton drawing of the network. Indicate by appropriate arrows the pointsofconstantfl owinputoroutput,constantheadinputoroutput(seeFig.C1.2). 2. Number all loops in the system in arbitrary sequence. Do not include loops around loops. For example, in Fig. C1.3 there are two loops, not thre
35、e. The large loop(abcdefg)isnotnumbered.Thetwobasicloops(abfgandbcdef)arenumbered. 3. Number each line. A line has two ends. An end may be a point at which water is drawn from or added to the system, one at which pipe characteristics FIGURE C1.2FIGURE C1.3 WATER SYSTEMS PIPINGC.7 FIGURE C1.4FIGURE C
36、1.5 change, or a tee joint. For example, in Fig. C1.4, the point x is the meeting of three lines, not two; point y is the meeting of two lines where an NPS 8 (DN 200) pipe joins a NPS 10 (DN 250) pipe; point z is simply a bend in the single pipe and is not the end of any line, although it could have
37、 been specifi ed as one, if desired. Figure C1.4 shows the complete numbering of the system shown in Fig. C1.2. Note that each line is numbered once and only once, even though it may be in more than one loop. Also note that the numbering is serial; that is, if there are n branches, each of the numbe
38、rs from 1 to n must be used in the numbering. 4. Assign a base direction. Put an arrow on each line in loop 1, indicating the clockwise direction (as shown in Fig. C1.5). Then put an arrow on each line in loop 2, indicating clockwise direction, except where a line which previously has been assigned
39、a direction is encountered. Then the original assignment is not changed. In Fig. C1.5, line 4 is a member of loop 1 and also of loop 2 but has been given a base direction of loop 1. The line 4 assignment is not changed. This process is continued for every loop in the network, an arrow being assigned
40、 in a clockwise direction whenever it has not been assigned previously. 5. In water-distribution systems,the situation often isencountered where system pressure must be raised by the use of booster pumps in series with the supply pipeline. If the higher pressure area is connected to the remainder of
41、 the system at one point only, the two pressure-zone networks are hydraulically independent problems. If the pressure zones are connected at two or more points, the booster pumps must be included in the appropriate loops. For all loops containing booster pumps, an unbalanced or residual head H0must
42、be determined. This is done by algebraically summing the assumed constant head changes at the boosters in a clockwise direction. Note that head losses are considered as positive in sign, so proceeding from the suction side of a pump to the discharge side gives a negative head loss. Following the hyd
43、raulic analysis, a check should be made to assure that the pumping head assumptions are suffi - ciently accurate. The resulting fl ow-rate values should allow optimum hydraulic design of the booster-station installa- tions. 6. Additional pseudo-loops must now be added to the list if there is more th
44、an one constant head input (see Fig. C1.6). If the number of such inputsFIGURE C1.6 C.8PIPING SYSTEMS is m, trace (m ? 1) paths between inputs in the same manner in which the loops were traced, making sure that each constant head input is used at the end of at least one of these loops. If the direct
45、ion of procedure is from the lower to the higher input in each path, H0will be the positive difference in the head loss between the two inputs. If booster pumps are encountered, the head change across such pumps must be algebraically added to the head difference between the inputs in order to obtain
46、 the H0for the pseudo loops. When the listing of all the loops has been completed (including the consideration of booster pumps), the work should be carefully checked, preferably by a second person, since any errors will completely upset the calculations. Note that pseudo loops do not introduce any
47、new lines. Note also that each pseudo loop must be assigned its own number. 7. The only remainingtask is to supplyinitial fl ow values andpipe characteristics which the computer can use as starting values for the calculations. The only restric- tion on these values is that they satisfy the mass bala
48、nce condition at each junction. That is, the sum of the fl ow into a junc- tion must equal the sum of the fl ows out of the junction. For example, Fig. C1.7 shows the junction of lines 3, 4, and 6; fl ows of 50 gpm in line 3 and 100 gpm in line 6 would satisfy the condition. Proceeding in this manne
49、r, balance every junction in the network, working FIGURE C1.7 towardthe variable-fl ow(constant- head)inputswhichcantakeuptheslack. When all fl ows are specifi ed, check the accuracy of the work by summing the inputs and outputs. If these sums are unequal, some computational error has been made and must be corrected. The complete schematic for this system is shown in Fig. C1.8. This schematic includes the assumed starting values of the fl ows. Several personal computer (PC)based and main-frame computer software pro- grams are available that handle steady-state and transient