Unit 2. Water Distribution System Design Criteria and Planning

Transcription

1 Unit 2. Water Distribution System Design Criteria and Planning Contents: 1. Types of distribution systems and Network Configurations 2. Water Supply and demand 3. Useful models for water distribution system design (WaterCad) 4. Exercise 5. References

2 Types of distribution systems The most common types are: 1. Gravity supply: The source of supply is at a sufficient elevation above the distribution area (consumers) No energy costs. Simple operation (fewer mechanical parts, independence of power supply,.) Low maintenance costs. No sudden pressure changes 2. Pumped supply: Used whenever: 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. 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.

3 3. Combined Supply (pumped-storage supply): Both pumps and storage reservoirs are used. This system is usually used in the following cases: a) When two sources of water are used to supply water b) 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 c) 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 (reservoir). And hence the water is distributed from the reservoir by gravity.

4 Distribution Systems (Network Configurations ) In laying the pipes through the distribution area, the following configuration can be distinguished: Serial, Branching system (Tree), Grid system (Looped) and Combined system Serial network looped network Branched-tree Connected parallel Branched parallel combined network

5 Water Supply Water can be supplied to the consumers by the following two systems: Continuous system. Intermittent system. v Continuous system In this system of supply, water is supplied to the consumers all the twenty- four hours. This system is possible only when there is sufficient quantity of water available from the source. Advantages of continuous system: Consumers don t have to store water, since it is continuously available at the tap. Water always remains available for fire fighting.

6 v Intermittent system In this system, water is supplied only during certain fixed hours of the day, which are normally morning and evening hours. This system is provided when the quantity of available water from the source is not sufficient to meet the demands of continuous supply. Disadvantages of intermittent supply Consumers have to store water for non- supply hours. A large number of valves and other fittings will have to be installed. During hours of non-supply, the pressure in the mains falls below atmospheric pressure, this creates partial vacuum in the pipe. At the time of fire breakout, it is possible that water may not be available at the fire hydrants. This may cause huge damage before the supply could be turned on.

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

8 Preliminary Studies: Must be performed before starting the actual design: 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.

9 Master plan

10 It is defined as the amount of water drawn of within a certain period of time; the demand is usually expressed as a flow in m³ / h, l/s, or l/c/d. Water consumption is ordinarily divided into the following categories: Domestic demand. Industrial and Commercial demand. Agricultural demand. Fire demand. Leakage and Losses. Water Demand Studies

11 Factors affecting water demand Climate - Size of the city - Habits of people - Cost of water - Quality of water - System of supply etc 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

12 The current and the planned future domestic Water Consumption in Gaza Strip Year Domestic Water Consumption l / c / d Public Demand Purpose Schools Hospitals Public Offices Restaurants Social centers Gardens Consumption 10 l /c / d 300 l / bed / d 40 l /employee / d 70 l /c / d 10 l /c / d 25 m 3 / Donume / week Cafeterias 35 l / c / d Mosques 15 l /c / d (الھیدرولیكا وشبكات المیاه زاھر كحیل المرشد الھندسي ( Source : Consultancy service for Gaza Governorate Water Facilities Master planning.

13 Tourist water demand Accommodation l/c/d Touring caravan and camping site 68 Unclassified hotels 113 Gust houses and 2 star hotels 168 3, 4, and 5 star hotels 269

14 Los Anglos-USA Daily demand Pattern Freetown-Sierra Leone Fes- Morocco Hodeidah-Yemen

15 Water Demand Pattern in the Gaza Strip Multiplier hour

16 The 24-hour Maximum Day Peaking Pattern considered for Gaza Strip Time ( hr ) Multiplier Time ( hr ) Multiplier Source : Consultancy Services for the Gaza Governorate Water Facilities Master Planning Report

17 Water demand calculation Simultaneity diagram example Daily demand pattern No. of consumers Weekly demand Pattern Yearly demand Pattern peak factor peak factor Max. peak factor peak factor Mon Tue Wed Thu Fri Sat Sun. Jan feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

18 When the consumption patterns as shown in the previous figures are available, the demand flow can be calculated from the following formula: Q d a * L 100 Q d = water demand of certain area at certain moment Q a = average water demand Water demand calculation-continue = (1 Q Pf o = overall peak factor. This is combination of peak factor values from the daily (or simultaneity), weekly and yearly demands. Thus Pf o =pf h *pf d *pf m L = leakage expressed as percentage of water production f c = unit conversion factor (if f c 24 h/d, the demand will be expressed as L/h) Average water demand (Q a ) = number of inhabitants x consumption per capita. Q a = population density x area of the distribution x coverage of the area x consumption per capita. Q a = area of the distribution x coverage of the area x consumption per unit area. Pf ) o * f c

19 Water demand calculation-continue a) Pf h = 1, pf d = 1, pf m = 1: Q d can be understood as hourly demand at average hour of average day if expressed as volume per hour, or as daily consumption of average day if expressed as volume per day, etc. This is total average whatever is the observed period: day, week or year b) Pf h = max, pf d = 1, pf m. = 1: Q d is hourly demand at max. peak hour of average day (in one year) expressed as volume per hour. c) Pf h = 1, pf d = max, pf m = max: Q d can be understood as hourly demand at average hour of the day of max. daily consumption in one year (as volume per hour), or as daily consumption of the same day. d) Pf h = max, pf d = max, pf m = max: Q d is hourly demand at max. peak hour of the day of max. daily consumption in one year. e) Pf h = min, pf d = min, pf m = min: Q d is hourly demand at min. peak hour of the day of min. daily consumption during year. Obliviously, case d) and e) will produce extreme demands and pressure in the system during one year. Therefore their analysis is important for design purposes.

20 Storage and balancing reservoir volume Clear water storage facilities are part of any sizable water supply system. Storage serve two main purposes: -meeting variable in water demand -providing a reserve supply in emergency situations. With respect to their elevation, storage can be classified as: a. Underground b. Ground level c. Elevated Selection of the optimal storage elevation for particular situation depends upon topography, pressure situation in the system, economical aspect, climate condition, security, etc. The volume of the storage is related to the size of the distribution area, period, magnitude of water demand variation, and requested reserve. Most of reservoirs is designed to meet demand variations during 24-hours, thus the balancing volume should be determined from the daily demand pattern taking into account the shape of the curve (daily demand pattern)

21 Storage volume usually equals 10-30% of total daily consumption. The storage capacity needed for emergency reserve depends upon the danger of interruption of water supply and the length of time necessary to make repairs About 5% of daily demand can be planned for each hour of emergency supply. Some reserve has to be considered for fire fighting based on municipality estimation of potential risk. For larger system this capacity can be shared between few smaller reservoirs. The water towers are almost always designed with considerably smaller volume capacity for balancing of sudden, short changes only and not as daily accumulations. This changes could normally be met by additional pumping capacity but unless switch-on and off very frequently the pump will produce substantial waste of energy. Therefore, the water towers are very often constructed as surge tanks of pumping stations.

22

23 Example: Gaza City has a total population of 700,000 inhabitants. The proposed water demand per capita is 100 l/c/d. Use the water demand pattern to determine the storage capacity of the main reservoir to serve the whole population assuming average pumping at 24 hours. Solution: The total daily consumption = 700,000 X 100 l/c/d = 70,000 m 3 /d The average hour pumping = 70,000/24 = 2917 m 3 /hr. Time ( hr ) Multiplier Time ( hr ) Multiplier Multiplier Water Demand Pattern in the Gaza Strip hour

24 Total storage based on demand pattern curve

25 Storage time start from 21 hour (9 evening) till 8 in the morning. The total storage required = 13,900 m 3. The percentage of storage volume to the total demand = 13,900/ 70,000 = 20%

26 Time- hours

27 Population Estimation Many methods are used to forecast the population in the future. Each method has it s own assumptions 1. Arithmetic increases method: Assumption: The rate of change is constant dp dt = K Pt dp = K dt (P = population t, = time) Po t 0 P t = P 0 + Pt = population after time (t). Po= present or initial population Kt Validity: valid only if the curve is close to the real growth of the population in previous years Population Population Projection Arithmatic increase method Time (year)

28 2. Uniform percentage of increase: ( Geometric Increase ): Assumption: Uniform rate of increase dp K / P dt = By integration lnp / t = lnp + K ( t t ) 0 0 Where, K / = ln( 1+ k ), ( t t0 ) = n, (number of years), and k, population growth rate. P P k n t = (1 + ) 0 Population (Ln Pt) Population Projection Geometric increase method (Equation 1) Time (year) Population Population Projection Geometric increase method Equation Time (year)

29 3. Curvilinear method: It is a method of comparison of the city under consideration with similar cities lager in size. 4. Saturation method: In this method, the maximum possible density of population is estimated according to the number of apartments and stories per unit area and the maximum family members.

30 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. In Gaza Strip, and according to the PWA studies, the industrial and commercial demand is taken as 10 % of the domestic demand. Agricultural demand It depends on the type of crops, soil, climate

31 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. 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* C A Q 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;

32 John R. Freeman's Formula Kuickling's Formula : P Q = Q = 3182 P The American Insurance Association Formula Where: Q = Fire Demand in l/m. P = Population in thousands. Q = 4637 P P Buston's Formula Q = 5663 P Q = Fire Demand in l/d. P = Population in thousands. The above formulas can be replaced with local ones (Amounts of water needed for fire in these formulas are high).

33 Note In Gaza Strip The Egyptian Code for water distribution systems is used and it states that the fire demand should be 20 l /s from each fire hydrant with the duration of fire of 2 hours. It was considered that one fire hydrants will be use to cover each fire demand. The number of expected fires that may occur simultaneously is calculated using the following formula : No. of fires = P 3, Where P is the population in thousands

34 Example (Fire demand) In Al-Awda city, the expected population in 2035 is around 36 thousand, so No. of fires = Fire It is two fire for design. So the total required quantity for the two fires = 2 ( 20 / 1000 ) = 288 m3 / 2 hours, so there were two nodes have a fire demand of 72 m3 / h.

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

36 Design Criteria Are the design limitations required to get the most efficient and economical water-distribution network. The design criteria for water distribution system can be divided in nonhydraulic and hydraulic design consideration. One of the non-hydraulic criteria can be the ability to isolate part of the system especially during emergency operation. Hydraulic design criteria are primarily related to the flow and pressure in the network. Moreover, criteria for minimum and maximum pipe capacities, flow velocities, pressure fluctuations and pressure gradients are relevant factors. Velocity Pressure Head Losses Design Period Average Water Consumption Pipe Sizes

37 Velocity Not be lower than 0.6 m/s to prevent sedimentation Not be more than 2 m/s to prevent erosion and high head losses. Commonly used values are m/sec. Instead of pressure gradient, the velocity can also be used as a design criterion (both parameters are correlated by friction loss calculations). Design Criteria ( Velocity ) Diameter (mm) Velocity ( m/sec ) Source: Standards Handbook Design Criteria ( Head Losses ) Diameter (mm) Head losses ( m/km ) Source: Water Supply ( 4th Edition )

38 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

39 The pressure criterion can be formulated as a min. /max. In general 5 mwc above the highest tap is sufficient. For urban areas this means min mwc above street level. In case of high building, internal posting system has to be installed. As mentioned in the Standard Handbook, in multi-storied structures the following pressures are satisfactory. Design Criteria ( Pressure ) No. of Floors Pressure Required ( kg / cm 2 ) Up to and below 3 stories 2 Source : Standards Handbook Above Maximum pressure limitations are required to reduce the additional cost of the pipe, strengthening necessary due to the high pressure.

40 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 cross-mains at the same maximum spacing. Major streets are provided with lines not less than 305 mm (12 in) in diameter.

41 General requirement for pipe network 1. Mains should be divided into sections and valves should be provided so that any section may be taken out of operation for repair. 2. Dead ends are to be avoided. If a dead-end is must, a hydrant should be provided for cleaning. 3. Air valves at summits and drains at the lowest point between summits should be installed. 4. Mains should follow the general contour of the ground. 5. Pipe should not rise above the hydraulic gradient. 6. The minimum cover under roadway should be 90cm and under paths 75 cm.

42 General requirement for pipe network (continue) 7. Proper installation and operation of water supply system requires that a number of appurtenances be provided in the pipeline; 8. Gate valve: they are used at summits and to isolate a particular section. 9. Sluice gate: They are used in pipelines laid at steep grades or in openings into wells 10. Check valves: (non return valve): to allow flow in one direction only. 11. Pipes constructed of steel and other flexible material must have valves that automatically allow air to enter when the pipeline is emptied in order to prevent a vacuum, which will cause the pipe to collapse.

43 Useful models for water distribution system design

44 Most distribution networks are now analyzed using digital computer programs. In writing a computer program to solve network flow problems, the following equations must be satisfied simultaneously throughout the network : Or Q in = Qout Q = 0.0 For each complete loop: Loop h f = 0.0 For each pipe: h = f kq x

45 There are some of the computer programs, used for hydraulic analysis for water networks: Loop-Alied- Flow- Wesnet-Epanet- WaterCAD-Piccolo. Water distribution system simulation A network simulation implies the calculation of all the network pressure heads and flows together with reservoir levels, and for known pump and valve controls and consumer demands. Network simulations, which replicate the dynamics of an existing or proposed system, are commonly performed in the following situations : 1. When it is not practical for the real system to be directly subjected to experimentation, or for the purpose of evaluating a system before it is actually built. 2. To predict system response to events under a wide range of conditions without disrupting the actual system. 3. To anticipate problems in an existing or proposed system and solutions can be evaluated before time, money and materials are invested.

46 Types of simulation Steady State Simulation: It represents a snapshot in time and are used to determine the operating behavior under static conditions. This type of analysis can be useful in determination of the short-term effect of fir flows on the system or the size of pipes in the network for the water demand required. Extended Period Simulation ( EPS ): It is used to evaluate system performance overtime. This allows the user to check if the used design criteria is satisfied (Pressure, Velocity ) throughout the system in response to varying demand conditions, model the tank filling and draining and regulate valves opening and closing

47 Assembling a model 1. Reservoirs : A reservoir represents a boundary node in a model that can supply or accept water with such a large capacity that the hydraulic grade of the reservoir is unaffected and remains constant. It is an infinite source, which means that it can theoretically handle any inflow or outflow rate for any period of time, without running dry or overflowing. In reality, there is no such thing as a true infinite source. For modeling purpose however, there are situations where inflows and outflows have little or no effect at all on the hydraulic grade at a node. Reservoirs are used to model any source of water where the hydraulic grade is controlled by factors other than the water usage rate. Lakes, groundwater wells and clear wells at water treatment plants are often represented as reservoirs in water distribution system models. For a reservoir, the two pieces of information required are the hydraulic grade (Water Surface Level) and the water quality. Storage is not a concern of reservoirs, so no volumetric storage data is required.

48 2. Tanks: A storage tank is also a boundary node, but unlike the reservoir, the hydraulic grade line of the tank fluctuates according to the inflow and the outflow of water. Tanks have a finite storage volume, and it is possible to completely fill or completely exhaust that storage. Storage tanks are present in most real-world distribution systems, and the relation between a tank and its model counterpart is typically forward. For steady-state runs, the tank is viewed as a known hydraulic grade elevation, and the model calculates how fast water is flowing into or out of the tank given that HGL. In Extended Period Simulation (EPS) models, the water level in the tank is allowed to vary over time.

49 3. Junctions: At the term implies, one of the primary uses of a junction node is to provide a location for two or more pipes to meet. Junctions, however, do not need to be elemental intersections, as a junction may exist in an end of a single pipe (Typically referred to as a dead-end). The major role of a junction is to provide a location to withdraw water demanded from the system or inject the inflows into the system. Junction nodes typically do not directly relate to real-world distribution system components, since pipes are usually joined with fittings and flow are extracted from the system at any number of customer connections along the pipe. Most water users have such small individual impact that their water withdrawal can be assigned as a sum to nearby nodes without adversely affecting the model.

50 4. Pipes: A pipe conveys flow as it moves from one junction to another in a network. For modeling purposes, individual segments of pipe and associated fittings can all be combined into a single pipe element. A model pipe must have the same characteristics (size, material, etc) throughout its length. 5. Pumps: A pump is an element that adds energy to the system in the form of increased hydraulic grade. Since water flows "downhill" (that is, from higher energy to lower energy), pumps are used to boost the head at desired locations to overcome piping head losses and physical elevation differences. Unless a system is entirely operated by gravity, pumps are an integral part of the distribution system

51 6. Valves: A valve is an element that can be opened and closed to different extents to vary its resistance to flow, thereby controlling the movement of water through a pipeline. 7. Controls: Operational controls, such as pressure switches, are used to automatically change the status or setting of an element based on the time of the day, or in response to conditions with the network. For example a switch may be set to turn on a pump when pressure within the system drop below a certain value. Or a pump may be programmed to turn on and refill a tank in the early hours of the morning. Models can represent controls in different ways. Some consider controls to be separate modeling elements, and others consider them to be an attribute of the pipe, valve or pump being controlled.

52 What is WaterCad? WaterCAD is a program used to build and analyze the water distribution network. Stand-Alone, Microstation and AutoCAD environments Quick model building from any data source Easy-to-use layout and editing tools Unrivaled hydraulic analysis features Stunning result presentation tools

53 WaterCAD Features Steady-State Analysis Extended-Period Simulation (EPS) Constituent-Concentration Analysis Source Tracing Criticality Analysis Tank-Mixing Analysis Water-Age Analysis Fire-Flow Analysis Variable-Speed Pumping Pressure-Dependent Demands Scenario Modeling-Based Unidirectional Flushing Procedures 1. Layout the network and input required data for every component. 2. Analysis. 3. Browse the results of analysis.

54 Network Elements Nodes. Pipes. Valves Tanks. Reservoirs. Pumps. Reservoirs Are modeled as constant water level sources Can supply any demand! Obey conservation of mass Have a finite size Tanks Water level moves up and down and thus pressures in system change! Need to define tank geometry

55 Pumps Require a Pump Curve (discharge vs. head) Initial setting Controls for extended time analysis Water Distribution System Reservoir - used to model a clear well. Pump to lift water to elevated storage tank. turns on and off based on water level in tank. Tank feeds distribution grid. Demands applied at junctions.

56 Equations used by WaterCad The friction can be calculated by: 1. Darcy-Weisbach. 2. Hazen-Williams. 3. Mannings. Scenarios Management Calculate multiple What if situations Alternatives Parent child relationship Reporting Results Reports Tabular Reports w/ Flex Tables Profiles Contouring Thematic Mapping Property-Based Annotation Property-Based Color Coding and Symbology

57 Exercise

58 Problem 1: Hydraulic performance of the following distribution system has to be evaluated. The system is supplied by gravity from reservoir near node 2 (hydraulic losses between the reservoir and the node can be neglected). Variation of water level in the tank is insignificant for pressure situation in the network and fixed surface level of 50 msl can be assumed in all calculations. supply mm 200 mm 100 mm

59 Each node in the network supplies local area with average demand presented in the following table. Node Q (m 3 /h) Hg (msl) Node Q (m 3 /h) Hg (msl) Typical consumption pattern during 24 hours is shown. Variation of the day consumption during the week is within the range of of average value and week variation during year are between of average consumption.

60 The network is built of PVC pipes with average roughness of 1.0 mm that includes effect of turbulence losses in the system. The leakage level is assumed to be 35% of average water delivery into the system, at a constant rate. a) Calculate average delivery in the system b) Calculate delivery in the system on the max. hour of max. day consumption. c) Calculate delivery in the system on the min. hour of min. day consumption. d) Is the delivery on the max. hour of average day consumption greater than the delivery on average hour of max. day consumption. e) How many inhabitants are supplied from node 4 if the average consumption per capita is 80 l/c/d. f) The reservoir near node 2 is supplied from the pumping station by constant flow rate equal to the average water delivery. Determine the required volume that can provide balancing of the demand within 24 hours for present situation.

61 Solution: a) From the table, the sum of average consumptions equals m 3 /h. The water delivery includes leakage as a percentage of water production. Thus Qavg, avg = = m3 / h 35 (1 ) 100 b) Average consumption on max. day consumption equals: Qavg, max = *1.10*1.15 = m3 / h Maximum hourly consumption appears to be at 8 pm with peak factor Hence the maximum delivery on maximum delivery on maximum day consumption is *1.47 Q max, max = = m3 / h 35 (1 ) 100 c) *0.95*0.90*0.50 Q min, min = = 99.22m3 / h 35 (1 ) 100

62 d) *1.47 Q max, avg = = m3 / h 35 (1 ) *1.10*1.15 Qavg, max = = m3 / h 35 (1 ) 100 Yes e) N 62.88*24* = = inhabitant

63 References 1. Water Distribution Handbook. Larry W. Mays, Editor in Chief Department of Civil and Environmental Engineering Arizona State University Tempe, Arizona. McGraw-Hill Water supply and distribution. Institute for Infrastructure, Hydraulics and Environment (IHE-2001Lecture notes). 3. Khalil El-Astal. Hydraulics lecture notes. Islamic University of Gaza Palestinian Water Authority. Consultancy Services for the Gaza Governorate Water Facilities Master Planning Report زاھر كحیل المرشد الھندسي الھیدرولیكا وشبكات المیاه.٥ 6. WaterCad manual