Water Distribution Systems

Transcription

1 The Islamic University of Gaza Faculty of Engineering Civil Engineering Department Hydraulics - ECIV 3322 Chapter 4 Water Distribution Systems 1

2 Introduction To deliver water to individual consumers with appropriate quality, quantity, and pressure in a community setting requires an extensive system of: Pipes. Storage reservoirs. Pumps. Other related accessories. Distribution system: is used to describe collectively the facilities used to supply water from its source to the point of usage. 2

3 Methods of Supplying Water Depending on the topography relationship between the source of supply and the consumer, water can be transported by: Canals. Tunnels. Pipelines. The most common methods are: Gravity supply Pumped supply Combined supply 3

4 Gravity Supply The source of supply is at a sufficient elevation above the distribution area (consumers). so that the desired pressure can be maintained HGL or EGL Source (Reservoir) (Consumers) Gravity-Supply System 4

5 Advantages of Gravity supply Source HGL or EGL No energy costs. Simple operation (fewer mechanical parts, independence of power supply,.) Low maintenance costs. No sudden pressure changes 5

6 Used whenever: Pumped Supply The source of water is lower than the area to which we need to distribute water to (consumers) The source cannot maintain minimum pressure required. pumps are used to develop the necessary head (pressure) to distribute water to the consumer and storage reservoirs. HGL or EGL Source (Consumers) (River/Reservoir) Pumped-Supply System 6

7 Disadvantages of pumped supply Complicated operation and maintenance. Dependent on reliable power supply. Precautions have to be taken in order to enable permanent supply: Stock with spare parts Alternative source of power supply. HGL or EGL Source (Consumers) (River/Reservoir) 7

8 Combined Supply (pumped-storage supply) Both pumps and storage reservoirs are used. This system is usually used in the following cases: 1) When two sources of water are used to supply water: Pumping HGL Gravity HGL Source (1) Pumping station City Source (2) 8

9 Combined Supply (Continue) 2) In the pumped system sometimes a storage (elevated) tank is connected to the system. When the water consumption is low, the residual water is pumped to the tank. When the consumption is high the water flows back to the consumer area by gravity. High consumption Low consumption Elevated tank Pumping station City Pipeline Source 9

10 Combined Supply (Continue) 3) When the source is lower than the consumer area A tank is constructed above the highest point in the area, Then the water is pumped from the source to the storage tank (reservoir). And the hence the water is distributed from the reservoir by gravity. Pumping HGL Gravity Pumping Station Reservoir HGL Source City 10

11 Distribution Systems (Network Configurations ) In laying the pipes through the distribution area, the following configuration can be distinguished: 1. Branching system (Tree) 2. Grid system (Looped) 3. Combined system 11

12 Branching System (tree system) Submain Dead End Main pipe Source Branching System Advantages: Simple to design and build. Less expensive than other systems. 12

13 Disadvantages: Dead End Source The large number of dead ends which results in sedimentation and bacterial growths. When repairs must be made to an individual line, service connections beyond the point of repair will be without water until the repairs are made. The pressure at the end of the line may become undesirably low as additional extensions are made. 13

14 Grid System (Looped system) Grid System Advantages: The grid system overcomes all of the difficulties of the branching system discussed before. No dead ends. (All of the pipes are interconnected). Water can reach a given point of withdrawal from several directions. 14

15 Disadvantages: Hydraulically far more complicated than branching system (Determination of the pipe sizes is somewhat more complicated). Expensive (consists of a large number of loops). But, it is the most reliable and used system. 15

16 Combined System Combined System It is a combination of both Grid and Branching systems This type is widely used all over the world. 16

17 Design of Water Distribution Systems A properly designed water distribution system should fulfill the following requirements: Main requirements : Satisfied quality and quantity standards Additional requirements : To enable reliable operation during irregular situations (power failure, fires..) To be economically and financially viable, ensuring income for operation, maintenance and extension. To be flexible with respect to the future extensions. 17

18 The design of water distribution systems must undergo through different studies and steps: Design Phases Preliminary Studies Network Layout Hydraulic Analysis 18

19 Preliminary Studies: Must be performed before starting the actual design: 4.3.A.1 Topographical Studies: 1. Contour lines (or controlling elevations). 2. Digital maps showing present (and future) houses, streets, lots, and so on.. 3. Location of water sources so to help locating distribution reservoirs. 19

20 Water Demand Studies: Water consumption is ordinarily divided into the following categories: Domestic demand. Industrial and Commercial demand. Agricultural demand. Fire demand. Leakage and Losses. 20

21 Domestic demand It is the amount of water used for Drinking, Cocking, Gardening, Car Washing, Bathing, Laundry, Dish Washing, and Toilet Flushing. The average water consumption is different from one population to another. In Gaza strip the average consumption is 70 L/capita/day which is very low compared with other countries. For example, it is 250 L/c/day in United States, and it is 180 L/c/day for population live in Cairo (Egypt). The average consumption may increase with the increase in standard of living. The water consumption varies hourly, daily, and monthly 21

22 The total amount of water for domestic use is a function of: Population increase How to predict the increase of population? Use Geometric-increase model P P ( r ) n P 0 = recent population 0 1 r = rate of population growth n = design period in years P = population at the end of the design period. The total domestic demand can be estimated using: Q domestic = Q avg * P 22

23 Industrial and Commercial demand It is the amount of water needed for factories, offices, and stores. Varies from one city to another and from one country to another Hence should be studied for each case separately. However, it is sometimes taken as a percentage of the domestic demand. 23

24 Agricultural demand It depends on the type of crops, soil, climate Fire demand To resist fire, the network should save a certain amount of water. Many formulas can be used to estimate the amount of water needed for fire. 24

25 Fire demand Formulas Q F 65 P( P) Q F = fire demand l/s P = population in thousands QF 53 P Q F = fire demand l/s P = population in thousands QF 320* Q C A F = fire demand flow m 3 /d A = areas of all stories of the building under consideration (m 2 ) C = constant depending on the type of construction; The above formulas can be replaced with local ones (Amounts of water needed for fire in these formulas are high). 25

26 Leakage and Losses This is unaccounted for water (UFW) It is attributable to: Errors in meter readings Unauthorized connections Leaks in the distribution system 26

27 Design Criteria Are the design limitations required to get the most efficient and economical water-distribution network Pressure Velocity Average Water Consumption 27

28 Velocity Not be lower than 0.6 m/s to prevent sedimentation Not be more than 3 m/s to prevent erosion and high head losses. Commonly used values are m/sec. 28

29 Pressure Pressure in municipal distribution systems ranges from kpa in residential districts with structures of four stories or less and kpa in commercial districts. Also, for fire hydrants the pressure should not be less than 150 kpa (15 m of water). In general for any node in the network the pressure should not be less than 25 m of water. Moreover, the maximum pressure should be limited to 70 m of water 29

30 Pipe sizes Lines which provide only domestic flow may be as small as 100 mm (4 in) but should not exceed 400 m in length (if dead-ended) or 600 m if connected to the system at both ends. Lines as small as mm (2-3 in) are sometimes used in small communities with length not to exceed 100 m (if dead-ended) or 200 m if connected at both ends. The size of the small distribution mains is seldom less than 150 mm (6 in) with cross mains located at intervals not more than 180 m. In high-value districts the minimum size is 200 mm (8 in) with crossmains at the same maximum spacing. Major streets are provided with lines not less than 305 mm (12 in) in diameter. 30

31 Head Losses Optimum range is 1-4 m/km. Maximum head loss should not exceed 10 m/km. 31

32 Design Period for Water supply Components The economic design period of the components of a distribution system depends on Their life. First cost. And the ease of expandability. 32

33 Average Water Consumption From the water demand (preliminary) studies, estimate the average and peak water consumption for the area. 33

34 Network Layout Next step is to estimate pipe sizes on the basis of water demand and local code requirements. The pipes are then drawn on a digital map (using AutoCAD, for example) starting from the water source. All the components (pipes, valves, fire hydrants) of the water network should be shown on the lines. 34

35 Pipe Networks A hydraulic model is useful for examining the impact of design and operation decisions. Simple systems, such as those discussed in last chapters can be solved using a hand calculator. However, more complex systems require more effort even for steady state conditions, but, as in simple systems, the flow and pressure-head distribution through a water distribution system must satisfy the laws of conservation of mass and energy. 35

36 Pipe Networks The equations to solve Pipe network must satisfy the following condition: The net flow into any junction must be zero Q 0 The net head loss a round any closed loop must be zero. The HGL at each junction must have one and only one elevation All head losses must satisfy the Moody and minor-loss friction correlation 36

37 Node, Loop, and Pipes Pipe Node Loop 37

38 Hydraulic Analysis After completing all preliminary studies and layout drawing of the network, one of the methods of hydraulic analysis is used to Size the pipes and Assign the pressures and velocities required. 38

39 Hydraulic Analysis of Water Networks The solution to the problem is based on the same basic hydraulic principles that govern simple and compound pipes that were discussed previously. The following are the most common methods used to analyze the Grid-system networks: 1. Hardy Cross method. 2. Sections method. 3. Circle method. 4. Computer programs (WaterCAD,Epanet, Loop, Alied...) 39

40 Hardy Cross Method This method is applicable to closed-loop pipe networks (a complex set of pipes in parallel). It depends on the idea of head balance method Was originally devised by professor Hardy Cross. 40

41 Assumptions / Steps of this method: 1. Assume that the water is withdrawn from nodes only; not directly from pipes. 2. The discharge, Q, entering the system will have (+) value, and the discharge, Q, leaving the system will have (-) value. 3. Usually neglect minor losses since these will be small with respect to those in long pipes, i.e.; Or could be included as equivalent lengths in each pipe. 4. Assume flows for each individual pipe in the network. 5. At any junction (node), as done for pipes in parallel, Q in Q out or Q 0 41

42 6. Around any loop in the grid, the sum of head losses must equal to zero: 0 loop h f Conventionally, clockwise flows in a loop are considered (+) and produce positive head losses; counterclockwise flows are then (-) and produce negative head losses. This fact is called the head balance of each loop, and this can be valid only if the assumed Q for each pipe, within the loop, is correct. The probability of initially guessing all flow rates correctly is virtually null. Therefore, to balance the head around each loop, a flow rate correction ( ) for each loop in the network should be computed, and hence some iteration scheme is needed. 42

43 7. After finding the discharge correction, (one for each loop), the assumed discharges Q 0 are adjusted and another iteration is carried out until all corrections (values of ) become zero or negligible. At this point the condition of : 0. 0 loop h f is satisfied. Notes: The flows in pipes common to two loops are positive in one loop and negative in the other. When calculated corrections are applied, with careful attention to sign, pipes common to two loops receive both corrections. 43

44 How to find the correction value ( ) h F n 2 kq n (1) Darcy, Manning Q Q o (2) n 1.85 Hazen William h from1& 2 f loop kq n k Q o Neglect terms contains 2 For each loop h F kq loop n kq n kq 0 n o n k Q n o nkq nq h f n1 n1 o kq n 0 n k n 1 2 Q n o Q n2 o nq 2 n1 o... 44

45 nkq kq n o n1 o n h h Q F F o Note that if Hazen Williams (which is generally used in this method) is used to find the head losses, then h f k Q 185. (n = 1.85), then h 185. If Darcy-Wiesbach is used to find the head losses, then h f k Q 2 (n = 2), then f h f Q 2 h h f f Q 45

46 Example 11.4 Solve the following pipe network using Hazen William Method C HW = L/s pipe L D 37.8 L/s m 305m 150mm 150mm m 457m 200mm 150mm 25.2 L/s 5 153m 200mm 46

47 h f C 10.7 L HW D 4.87 Q C HW 100, Q in L/s 1 h f h h f f C K 10.7 L HW Q D Q 1000 L D 4.87 Q F 1 hf n h Q o F 2 hf n h Q o 0.57 for pipe2 in loop1 1 for pipe2 in loop

48 hf 0.18 hf hf hf n Q o 1 n Q o 0.09 for pipe2 in loop1 1 for pipe2 in loop

49 Example Solve the following pipe network using Hazen William Method C HW =120 49

50 Iteration 1 50

51 for Iteration 1 n kqo Δ n1 nkq o n h h Q f f o Δ Δ Δ Δ

52 Iteration 2 52

53 Iteration 3 53

54 Example The figure below represents a simplified pipe network. Flows for the area have been disaggregated to the nodes, and a major fire flow has been added at node G. The water enters the system at node A. Pipe diameters and lengths are shown on the figure. Find the flow rate of water in each pipe using the Hazen- Williams equation with C HW = 100. Carry out calculations until the corrections are less then 0.2 m 3 /min. 54

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64 General Notes Occasionally the assumed direction of flow will be incorrect. In such cases the method will produce corrections larger than the original flow and in subsequent calculations the direction will be reversed. Even when the initial flow assumptions are poor, the convergence will usually be rapid. Only in unusual cases will more than three iterations be necessary. The method is applicable to the design of new system or to evaluate the proposed changes in an existing system. The pressure calculation in the above example assumes points are at equal elevations. If they are not, the elevation difference must be includes in the calculation. The balanced network must then be reviewed to assure that the velocity and pressure criteria are satisfied. If some lines do not meet the suggested criteria, it would be necessary to increase the 64 diameters of these pipes and repeat the calculations.

65 Summary Assigning clockwise flows and their associated head losses are positive, the procedure is as follows: Assume values of Q to satisfy Q = 0. Calculate H L from Q using h f = K 1 Q 2. If h f = 0, then the solution is correct. If h f 0, then apply a correction factor, Q, to all Q and repeat from step (2). For practical purposes, the calculation is usually terminated when h f < 0.01 m or Q < 1 L/s. A reasonably efficient value of Q for rapid convergence is given by; Q 2 H H L L Q 65

66 Example The following example contains nodes with different elevations and pressure heads. Neglecting minor loses in the pipes, determine: The flows in the pipes. The pressure heads at the nodes. 66

67 Assume T= 15 0 C 67

68 Assume flows magnitude and direction 68

69 First Iteration Loop (1) Pipe L (m) D (m) Q (m 3 /s) f h f (m) h f /Q (m/m 3 /s) AB BE EF FA S m /s 2( ) L/s 69

70 First Iteration Loop (2) Pipe L (m) D (m) Q (m 3 /s) f h f (m) h f /Q (m/m 3 /s) BC CD DE EB S m 3 /s 2(1488.7) L/s 70

71 14.20 Second Iteration Loop (1) Pipe L (m) D (m) Q (m 3 /s) f h f (m) h f /Q (m/m 3 /s) AB BE EF FA S m 3 /s 4.65 L/s 2( ) 71

72 7.84 Second Iteration Loop (2) Pipe L (m) D (m) Q (m 3 /s) f h f (m) h f /Q (m/m 3 /s) BC CD DE EB S m /s 2(1890.3) 5.32 L/s 72

73 Third Iteration Loop (1) Pipe L (m) D (m) Q (m 3 /s) f h f (m) h f /Q (m/m 3 /s) AB BE EF FA S m /s 2( ) 2.44 L/s 73

74 Third Iteration Loop (2) Pipe L (m) D (m) Q (m 3 /s) f h f (m) h f /Q (m/m 3 /s) BC CD DE EB S ( ) m /s 1.72 L/s 74

75 After applying Third correction 75

76 Velocity and Pressure Heads: pipe Q (l/s) V (m/s) h f (m) AB BE FE AF BC CD ED

77 Velocity and Pressure Heads: p/g+z Z P/g Node (m) (m) (m) A B C D E F

78 Example For the square loop shown, find the discharge in all the pipes. All pipes are 1 km long and 300 mm in diameter, with a friction factor of Assume that minor losses can be neglected. 78

79 Solution: Assume values of Q to satisfy continuity equations all at nodes. The head loss is calculated using; H L = K 1 Q 2 H L = h f + h Lm But minor losses can be neglected: h Lm = 0 Thus H L = h f Head loss can be calculated using the Darcy-Weisbach equation h f L D V 2 2g 79

80 First trial H H H H H L L L L L h f Q K'Q K' V x x x 9.81 Q A 2 2 L D V 2g 2 2 Q 2.77 x x Pipe Q (L/s) H L (m) H L /Q AB BC CD AD S Since H L > 0.01 m, then correction has to be applied. 80

81 Second trial H Q H 2 L L Q 2 2x L / s Pipe Q (L/s) H L (m) H L /Q AB BC CD AD S Since H L 0.01 m, then it is OK. Thus, the discharge in each pipe is as follows (to the nearest integer). Pipe Discharge (L/s) AB 47 BC 27 CD -13 AD

82 قال اهلل تعالى: رجال ال تلهيهم تجارة وال بيع عن ذكر اهلل 82

83 حب الدنيا اعمم أن جمود العين من قسوة القمب وقسوة القمب من كثرة الذنوب وكثرة الذنوب من نسيان الموت ونسيان الموت من طول األمل وطول األمل من شدة الحرص وشدة الحرص من حب الدنيا وحب الدنيا أرس كل خطيئة. 83

84 تهادوا تحابوا ال تبخل بالهذية ولى قل سعرها فقيمتها معنىية اكثر من مادية 84

85 تقوى هللا مفتاح كل نجاح ومن يتق هللا يجعل له مخرجا ويرزقه من حيث ال يحتسب 85