Environmental Engineering-I

Transcription

1 Environmental Engineering-I Prof. Dr. Muhammad Zulfiqar Ali Khan Engr. Muhammad Aboubakar Farooq Water Distribution Systems & Analysis 1

2 Water Distribution Systems & Analysis References Water Supply & Sewerage (P-143 to 152) by TERENCE J. McGHEE Elements of Public Health Engg.(P-159 to 162) by K.N. Duggal 2

3 Water Distribution Systems & networks Content Water Distribution Systems Design of Water Distribution Systems Pipe Network Analysis 3

4 Water Distribution Systems & networks Part A Water Distribution Systems 4

5 Water Distribution Systems 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 5

6 Water Distribution Systems 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 6

7 Water Distribution Systems 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 7

8 Water Distribution Systems 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) 8

9 Water Distribution Systems 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 Source (2) City 9

10 Water Distribution Systems 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 Source City Pipeline 10

11 Water Distribution Systems 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 City 11

12 Water Distribution Systems 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 12

13 Water Distribution Systems Branching System (tree system) Submain Dead End Main pipe Source Branching System Advantages: Simple to design and build. Less expensive than other systems. 13

14 Water Distribution Systems Disadvantages: 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 14 as additional extensions are made.

15 Water Distribution Systems 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. 15

16 Water Distribution Systems 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. 16

17 Water Distribution Systems Combined System Combined System It is a combination of both Grid and Branching systems This type is widely used all over the world. 17

18 Water Distribution Systems & networks Part B Design of Water Distribution Systems 18

19 Design of Water Distribution Systems 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. 19

20 Design of Water Distribution Systems The design of water distribution systems must undergo through different studies and steps: Design Phases Preliminary Studies Network Layout Hydraulic Analysis 20

21 Design of Water Distribution Systems 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. 21

22 Design of Water Distribution Systems 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. 22

23 Design of Water Distribution Systems 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. It varies from 150 to 600 lpcd. The average consumption may increase with the increase in standard of living. The water consumption varies hourly, daily, and monthly. 23

24 Design of Water Distribution Systems The total amount of water for domestic use is a function of: Population increase How to predict the increase of population? Use P P ( r) n 0 1 Geometric-increase model P 0 = recent population 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 24

25 Design of Water Distribution Systems 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 total water supply (10-30%). 25

26 Design of Water Distribution Systems 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. 26

27 Design of Water Distribution Systems Fire demand Formulas Q F 1020 P( P) Q F = fire demand US Gallon/min P = population in thousands Q F *C A Q F = fire demand flow lit/min A = areas of all stories of the building under consideration (m 2 ) C = constant depending on the type of construction; Minimum and Maximum Storage Required are 4 hours & 10 hours Respectively. 27

28 Design of Water Distribution Systems Leakage and Losses This is unaccounted for water (UFW) It is attributable to: Errors in meter readings Unauthorized connections Leaks in the distribution system 28

29 Design of Water Distribution Systems Design Criteria are the design limitations required to get the most efficient and economical water-distribution network Pressure Velocity Average Water Consumption & Future Demand 29

30 Design of Water Distribution Systems Velocity Not be lower than 0.25 m/s to prevent sedimentation - WASA Not be more than 2 m/s to prevent erosion and high head losses near large fires. Commonly velocity is kept in between (1-1.5 m/s). 30

31 Design of Water Distribution Systems Pressure Pressure which is required to be maintained depends upon: Height of Highest Building Distance of locality from the distribution reservoir Supply is metered or not Pressure required for Fire Hydrants Funds available for project 31

32 Design of Water Distribution Systems Pressure Pressure in municipal distribution systems ranges from kpa in residential districts with structures of three stories or less and 500 kpa in commercial districts. Also, for fire hydrants the pressure should not be less than 150 kpa (15 m of water) to ensure proper flow in other buildings and to avoid infiltration in system. Moreover, the maximum pressure should be limited to 70 m of water. 32

33 Design of Water Distribution Systems Pipe sizes Primary feeders form skeleton of distribution system. They convey water from Pumping station to & from OHR to various parts of city. They should be provided with Air-Relief & Blow-off Valves. Size is generally 300 mm (12 in to 60 in). Secondary feeder carry water from primary feeder to various parts of city to provide normal supplies. These are located at a few (2-4) blocks apart to provide water for fire fight without excessive loss. Sizes are 200 mm, 250 mm, 300 mm. The size of the small distribution mains is seldom less than 150 mm (6 in) and dictated by fire flow. Sizes of Domestic Supply Lines are generally 100mm, 80mm. 33

34 Design of Water Distribution Systems 34

35 Design of Water Distribution Systems Head Losses Optimum range is 1-4 m/km. Maximum head loss should not exceed 10 m/km. 35

36 Design of Water Distribution Systems 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. 36

37 Design of Water Distribution Systems 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, manually) starting from the water source. All the components (pipes, valves, fire hydrants) of the water network should be shown on the lines. 37

38 Water Distribution Systems & Analysis References Water Supply & Sewerage (P-143 to 152) by TERENCE J. McGHEE Elements of Public Health Engg.(P-159 to 162) by K.N. Duggal 38

39 Water Distribution Systems & networks Part C Pipe Network Analysis 39

40 Pipe Network Analysis Pipe Network Analysis 40

41 Pipe Network Analysis Pipe Networks A hydraulic model is useful for examining the impact of design and operation decisions. However, more complex systems require more effort, 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. 41

42 Pipe Network Analysis Pipe Networks The equations to solve Pipe network must satisfy the following condition: The net flow into any junction must be zero. Inflow should be equal to Outflow. Q 0 The net head loss a round any closed loop must be zero. 42

43 Pipe Network Analysis Node, Loop, and Pipes Pipe Node Loop 43

44 Pipe Network Analysis 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. 44

45 Pipe Network Analysis Hydraulic Analysis of Water Networks The following are the most common methods used to analyze the Grid-system networks: 1. Hardy Cross method. 2. Equivalent Pipe Method. 3. Sections method. 4. Computer programs (EPAnet Software,...) 45

46 Pipe Network Analysis Hardy Cross Method In this method, the rate of flow is ASSUMED and the ERROR in the assumed flow is computed on the account of IMBALANCE OF HEAD LOSSES IN THE CIRCUIT/LOOP. After correcting the flows, final head loss through various pipes is checked to assess the adequacy of the design. If needed, the pipe diameters are changed to result in REQUIRED PRESSURE AT THE OUTLET. 46

47 Pipe Network Analysis Hardy Cross Method This method based on: Junction Q 0 Loop h f 0 1- A distribution of flows in each pipe is estimated such that the total inflow must be equal to the outflow at each junction throughout the network system The interflow in the network has +ve sign The outflow from the network has -ve sign 47

48 Pipe Network Analysis 2- Neglect Minor loss 3- In each loop h f Loop 0 4- If the direction of flow is clockwise it take +ve sign, otherwise it take ve sign h f 0 5- If the flow is correct other wise, the assumed Loop flow must be corrected as the flowing: 48

49 William Hazen n kq h n F 1.85 (1) (2) o Q Q & f n o n o n o n o n Q n n nq Q k Q k kq h from 1 f n o n o n nq Q k kq h n n o n loop n loop F nkq kq kq kq h Neglect terms contains 2 For each loop Pipe Network Analysis 49

50 Pipe Network Analysis nkq kq n o n1 o n h h Q F F o f Loop 6- After calculation correct Q o and check h 0 50

51 Pipe Network Analysis 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. 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 51

52 Pipe Network Analysis Assumptions / Steps of this method: 6. Compute head loss in each pipe. 7. Find out SUM of total Head loss in pipe having CLOCKWISE DIRECTION of flow---call it as H1. 8. Find out SUM of total Head loss in pipe having ANTICLOCKWISE DIRECTION of flow---call it as H2. 9. Find out H1 - H Around any loop in the grid, the sum of head losses must equal to zero: 0 loop h f 52

53 Pipe Network Analysis Assumptions / Steps of this method: 10. Around any loop in the grid, the sum of head losses must equal to zero: 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. 53

54 Pipe Network Analysis Assumptions / Steps of this method: 11. Calculate h f / Q 0 for each pipe and sum for loop S h f / Q Calculate using: NOTE: hf hf 1.85 Q For Common members between two loops: 0 for common member in loop for common member in loop

55 Pipe Network Analysis Assumptions / Steps of this method: 13. 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. Q = Q Repeat the procedure till: <0.2m3/min or <10% of flow in that pipe 0. 0 loop h f is satisfied. 55

56 Pipe Network Analysis kq nkq 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 hf 185. hf k Q (n = 1.85), then h 1.85 Q f 0 If Darcy-Wiesbach is used to find the head losses, then h f k Q 2 (n = 2), then h h 2 Q f f 0 56

57 Pipe Network Analysis 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 = KQ If h f = 0, then the solution is correct. If h f 0, then apply a correction factor, Q, to all Q. For practical purposes, the calculation is usually terminated when Q < 0.2 m3/min. A reasonably efficient value of Q for rapid convergence is given by; Q 1.85 H L H L Q 57

58 Pipe Network Analysis 11.4 Example 1 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 153m 200mm 150mm 200mm 25.2 L/s 58

59 Pipe Network Analysis Example 2 Solve the following pipe network using Hazen William Method C HW =120 59

60 Pipe Network Analysis 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. 60

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63 Pipe Network Analysis 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 evaluation of proposed changes in an existing system. 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 diameters of these pipes and repeat the calculations. 63

64 Pipe Network Analysis 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. 64

65 Assume T= 15 0 C 65

66 Assume flows magnitude and direction 66