Chapter 2 [PDF]

Zitiervorschau

CHAPTER 2 WATER SUPPLY

Prepared by Dr. Ahmed H. Birima

Outlines • • • • • • • • •

Water demand Water supply planning Transportation of water Distribution Systems System Configurations Distribution System Components System Requirements Design of Water Distribution Systems Distribution Reservoirs and Service Storage Prepared by Dr. Ahmed H. Birima

Water demand Water demand is commonly Expressed in cubic meter Per hour (m3/h) or per second (m3/s), liters per seconds (L/s) Mega liter per day (Ml/d) or liter Per capita per day (l/c/d or lpcpd). Typical imperial units are Cubic feet per second (ft3/s) Galion per minute (gpm) or Mega gallon per day (mgd)

Flows in water supply system (source - introduction to urban water distribution)

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Water demand The design of water supply facility begins with determination of design capacity. This is a function of water demand. Determination of water demand consist of: (1) Selection of a design period (2) Estimation of the population, commercial, and industrial growth, (3) Estimation of the unit water use (4) Estimation of the variability of the demand.

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• Design period • The design period (also called the design life) is the length of time it is estimated that the facility will be able to meet the demand (design capacity). • The life expectancy of a facility or piece of equipment is determined by wear and tear. • Typical life expectancies for equipment range from 10 to 20 years. Buildings, other structures, and pipelines are assumed to have a useful life of 50 years or more.

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The number of years selected for the design period is based on the following: 1. Regulatory constrains 2. The rate of population growth. 3. The useful life of the structures and equipment. 4. The ease or difficulty of expansion. 5. Performance in early years of life under minimum hydraulic load.

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Table 2.1 Design Period for Water Works

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Unite water use • Local factors affecting unit demand • Due consideration must be given to the following local factors: 1. Climate: is the most important factor influencing unit demand. People require more water in hot climates. 2. Industrial activity: Small rural and suburban communities will use less water per person than industrialized communities. 3. Meterage : Meterage imposes a sense of responsibility not found in unmetered residences and businesses. This sense of responsibility reduces per capita water consumption because customers repair leaks and make more conservative wateruse decisions almost regardless of price. Prepared by Dr. Ahmed H. Birima

Local factors affecting unit demand 4. System management: If the water distribution system is well managed, per capita water consumption is less than if it is not well managed. Well managed systems are those in which the managers know when and where leaks in the water main occur and have them repaired promptly. 5. Standard of living: Per capita water use increases with an increased standard of living. Highly developed countries use much more water than less developed nations. Likewise, higher socioeconomic status implies greater per capita water use than lower socioeconomic status.

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Variability of demand • The unit demand estimates are averages. Water consumption changes with the seasons, the days of the week, and the hours of the day. • The variation in demand is normally reported as a factor of the average day (e.g., U.S. national average factors are: maximum day = 2.2 x average day; peak hour = 5.3 x average day). • when the proposed project is in a community with an existing community supply, the community's historic records provide the best estimate of water use.

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• Design basis for water source and treatment facilities shall be for maximum day demand at the design year. • Pumping facilities and distribution system piping are designed to carry the peak hour flow rate. • When municipalities provide water for fire protection, the maximum day demand plus fire demand is used to estimate the peak hour flow rate.

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Population forecasting • Population estimates for the operation and design of water supply and waste treatment works may be: 1. Short-term estimates – in the range of 1 to 10 years 2. Long- term estimates of 10 to 50 years or more.

• Trend Based Methods • Most short-term estimates are made using trend-based methods. They often follow segments of a typical population growth as shown on Figure 2.1.

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Population forecasting

Figure 2.1 Population growth curve Prepared by Dr. Ahmed H. Birima

Population forecasting

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•

= population at the census preceding the last census (time t1)

• Y2 = population at last census (time t2 ) •

t = the end of the forecast period

2) Constant-percentage growth rate • For equal periods of time, this procedure assumes constant growth percentages. If the population increased from 90,000 to 100,000 in the past 10 years, it would be estimated that the growth in the ensuing decade would be to 100,000 + 0.11 x 100,000, or 111,000. • This can be expressed mathematically and integrated to:

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2.2

• Where • Kp = a constant percentage increase per unit time. 2.1 • The other variables are defined as in 7.1

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Water supply planning • Basic in water supply planning: 1. Estimate the demand for water – the volume of water required for a certain period of time. 2. Locate a suitable water source. 3. Determine quantity ( this related to water demand, surface water and ground water hydrology) and quality ( this related to water pollution, quality management and water treatment). 4. Determine if water can be treated economically to meet water quality standards. 5. Design water supply, water treatment and water distribution system. Prepared by Dr. Ahmed H. Birima

Transportation of water Water is transferred by gravity pumping Combination of both

First stage Conveyance of water from the Source to the treatment plant

Second stage Conveyance of treated water from the treatment plant to the distribution System

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by pumping into an overhead tank and then supplying by gravity

by pumping directly into the water main for distribution.

Water supply facilities

Figure 2.2 water supply facilities addressed by the uniform Technical guidelines (UTG) (Source SPAN) Prepared by Dr. Ahmed H. Birima

Figure 2.3 typical external reticulation system for single Development area (Source SPAN) Prepared by Dr. Ahmed H. Birima

Example 2.1 • A reservoir has a capacity of 0.9 x 10 12 gallons. How many years would this supply a city of 100,000 population, if evaporation is neglected. • Assume a use rate of 180 gallons per capita per day.

• Example 2.2 • If the minimum flow of stream having a 12.39 x 10 6 gallons/day. What is population’s number could be supplied continuously from the stream? Assume water usage rate is 175 gallons/capita.day. Prepared by Dr. Ahmed H. Birima

Example 2.3 • A community having a population of 250,000 in 2000 estimates that its population will increase to 400,000 by the year 2020. The water treatment facilities in place can process up to 55 million gallons per day (mgd).The 2000 per capita water use rate was found to be 160 gpcd. Estimate the water requirements for the community in2020 assuming that the per capita use rate remains unchanged. Will new treatment facilities be needed to accommodate this growth in population? If revised plumbing codes were adopted during the period of growth and if these changes resulted in an overall reduction in the community’s water use by 15 %, what would the water requirement be in 2020? Could expansion of treatment facilities be deferred until after year 2020 under these conditions? Prepared by Dr. Ahmed H. Birima

Distribution Systems • Water distribution systems are designed to satisfy the water requirements of domestic, commercial, industrial, and fire fighting purposes. • The system should be capable of meeting the demands placed on it at all times, and at satisfactory pressures. • Pipe systems, pumping stations, storage facilities, fire hydrants, house service connections, meters, and other appurtenances are the main elements of the system

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System Configurations • Water distribution systems may be classified as grid systems, branching systems, or a combination of the two. The configuration of the system is influenced by street patterns, topography, degree and type of development of the region to be served, and location of treatment and storage works (Figure 2.4) • Grid systems are usually preferred to branching systems, • Since they can supply a withdrawal point from at least two directions. • Minimized loss of pressure. • Ensure uninterrupted water supply in case of maintenance requirements.

• Branching systems do no permit this type of circulation, because they have numerous terminals or dead ends. Prepared by Dr. Ahmed H. Birima

Figure 2.4.1 Types of water distribution systems (a) Branching (b) Grid (c ) combination Prepared by Dr. Ahmed H. Birima

Figure 2.4.2 Types of water distribution systems- serial and branch network configuration

Figure 2.4.3 Types of water distribution systems- looped configuration Prepared by Dr. Ahmed H. Birima

Distribution System Components • A water distribution network is a collection of links connected together at their end points called nodes (Fig.2.5). • Links may include pipes, pumps, and valves. • Nodes may be points of water withdrawal (demand nodes),locations where water is introduced to the network (source nodes), or locations of tanks or reservoirs (storage nodes).

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Figure 2.5 Network Components

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System Requirements • Pressures should be great enough to meet consumer and firefighting needs. At the same time, they should not be excessive, since the development pressure head is an important cost consideration. • For commercial areas, pressures in excess of 60 pounds per square inch, gauge (psig) are usually required. • Adequate pressures for residential areas usually range from 40 to 50 psig. • In tower buildings, it is often necessary to provide booster pump to elevate the water to upper floors. Storage tanks are usually provided at the highest level and distribution is made directly from them. Prepared by Dr. Ahmed H. Birima

• The capacity of the distribution system is determined on the basis of local water needs plus fire demands. • Once the flow has been determined, pipe sizes can be selected by assuming velocities of from 3 to 5 fps.

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Needed fire flow • Needed fire flow (NFF) is the rate of water flow required for fire fighting to confine a major fire to the buildings within a block or other group complex with minimal loss.

• Practical Limits of Fire flow • Withdrawal of a large quantity of water from a water system is not the preferred method of fire suppression. For many buildings, automatic sprinkler systems are more effective in protection of life and property than relying solely on the distribution system to provide fire protection.

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Needed fire flow • The minimum fire flow for buildings without sprinklers is 500 gpm (32 l/s) at a residual pressure of 20 psi (140 kpa). • The maximum fire flow is 3500 gpm (220 l/s) • The flow requirement for the sprinkler system is in the range of 150 to 1600 gpm (10 to 100 l/s). • Need fire flows for single family and two family are shown in table 2.2

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Table 2.2 (source: water and wastewater technology by Hammer & Hammer)

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Duration • The required duration for fire flow is 2 hr for up to 2500 gpm and 3 hr for fire flows of 3000 gpm and 3500 gpm. • The period may be five, three, or two days depending on the system component under consideration and the anticipated out- of- service time required for maintenance and repair work

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Layout of distribution system • Supply mains, arteries, and secondary feeders should extend throughout the system properly spaced-about every 3000 ft (910 m) and looped for mutual support and reliability of service. • Gridiron pattern of small distribution mains supplying residential districts should consist of mains at least 6 in. (150 mm) in diameter. • Where long lengths are necessary, exceeding about 600 ft (180 m), 8-in. (200-mm) or larger intersecting mains should be used.

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Layout of distribution system • A distribution system is equipped with a sufficient number of valves located so that a pipe-line break does not affect more than 1/4 mile of arterial mains, 500 ft (150 m) of mains in high-value districts, or 800 ft (24O m) of mains in other districts.

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Design of Water Distribution Systems 1. The flow must be segregated to individual sub areas of the system 2. A system of interlocking loops must be laid out. 3. The segregated flows are then assigned to the various nodes of the system. 4. The design then involves determination of the sizes of the arterials, secondary lines, and small distribution mains (secondary distribution system)required to ensure that the pressures and velocities desired in the system are maintained under a variety of design flow conditions. 5. These design conditions are based on the maximum daily flow rate plus one or more fires, depending on the size of community. Prepared by Dr. Ahmed H. Birima

Water reticulation design for secondary distribution system • The secondary water distribution system is used as a link between the main distribution pipes and the house connections including the fire hydrants. • The design must achieve the sufficient pressure and quantity of water in the most cost effective manner. • Sluice valve and shut-off valves are strategically located such that when the pipe repairs and maintenance are necessary, the portion of the pipe involve are isolated with minimum disruption to water supply to nearby area.

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Water reticulation design for secondary distribution system • Pressure reducing valve (PRV) is provided at the tapping point to reduce the available pressure to the allowable maximum of pressure as required by Guideline (National Water Services Commission (SPAN)).

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Steps to design water reticulation for secondary distribution system 1. Determine the tapping point and get the available pressure (this obtained from water authority). 2. Determine the design parameters from the guideline uniform technical guidelines (UTG)- water reticulation and plumbing (SPAN). Design parameters such as: peak flow factor (2.5), Hazen William Coefficient C, Min residual head ( peak and average flow case), Residual pressure at Highest Supply Level HSL, minimum residual head from platform level (average flow case), static pressure at any point on supply line, fire flow, maximum hydrant spacing, Head loss for Peak flow case, Maximum velocity in Peak flow case, Maximum velocity in Average + Fire Flow Case, allowable head loss, ……etc (refer to page 34, UTG) Prepared by Dr. Ahmed H. Birima

Steps to design water reticulation for secondary distribution system 3. Layout of water reticulation network. 4. Calculation of water consumption based on Table 2.3. for both cases - peak flow case and average flow + fire flow. 5. Analysis of reticulation network based on two cases, i.e., peak flow case and average flow + fire flow. • Determination of pipe size, flow in each pipe, velocity, and head loss for each line. William Hazen method of analysis is used for this purpose using software - EPANET. • Free download http://www.epa.gov/nrmrl/wswrd/dw/epanet.html

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Table 2.3 Tabulation of Estimated Water Demand Rate for Planning of External Water Reticulation System (SPAN)

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Table 2.3 - continue

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Example 2.4 • A development area consists of 250 units of double storey terrace houses, Surau for 200 persons and hospital with 200 beds. Determine the water demand required for the design of the secondary water reticulation system.

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Flow in pipes under pressure • Hazen-Williams equation • The most common pipe flow formula used in the design and evaluation of water distribution system is the Hazen-Williams equation. • ………………………………….2.4 • Where = flow rate, gallons per minute C = coefficient, Table 2.4 D = diameter of pipe, inches S = hydraulic gradient, feet per foot Prepared by Dr. Ahmed H. Birima

Table 2.4 values of coefficient C for the Hazen-Williams formula.

The nomograph shown in Figure 2.6 & 2.7 solve the equation for coefficient equal 100, representing 15 to 20 years old, ductile- iron pipe. Note: Head loss in pipes with coefficient values other that 100 can be determined By using the correction factors in Table 2.5 Prepared by Dr. Ahmed H. Birima

Table 2.5 Correction factors to determine head losses From Figure 2.6 at values of C other than C = 100

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Figure 2.6 Nomograph in English Unites for Hazen Williams Formula based on C= 100

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Figure 2.7 Nomograph in SI Unites for Hazen Williams Formula based on C= 100

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Example 2.5 • If a 200 mm water main (C= 100) is carrying a flow of 30 l/s, what is the velocity of flow and head loss? • Solution • (a) Using figure 2.7 , straight line extended through a discharge of 30 l/s and diameter of 200 mm intersects head loss at 8 m/1000 m (0.008 m/m). • (b) Q= 30 l/s = 0.030 m3/s, and D = 200 mm = 0.20 m • The cross-sectional area of the flow = π (0.20/2)2 m2 • V = Q/A = (0.030 m3/s)/ π (0.20/2)2 m2 = 0.955 m/s.

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Example 2.6 • An extremely simplified water supply system consisting of reservoir with lift pumps, elevated storage, piping, and load center (withdrawal point) is shown in Figure 2.8. • (a) based on the following data, sketch the hydraulic gradient for the system: • ZA = 0 ft, PA = 80 psi, ZB = 3O ft, PB = 30 psi, ZC = 40 ft, PC = 100 ft (water level in the tank) • (b) for these conditions, calculate the flow available at point B from both supply pumps and elevated storage. Use C = 100 and pipe sizes as shown in the diagram.

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Figure 2.8

Solution (a) Hydraulic head at A = 0 ft +80 psi X 2.31 ft/ psi = 185 ft at B = 30 + 30X2.31 = 99 ft at C = 40 +100 = 140 ft. The hydraulic gradient is shown as straight lines connecting these Hydraulic heads drawn vertically ( shown in Figure 2.9).

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Example 2.6 – continue (b) hL between A and B = 185 – 99 = 86 ft hL per 1000 ft = 86 /5 = 17.2 ft Using Figure 2.6, align hL = 17.2 ft/1000 ft and 12-in diameter, read Q = 2160 gpm. hL per 1000 ft B to C = (140 -99)/3 = 13.7 ft For hL= 13.7 ft/1000 ft and 10 in, Q = 1180 gpm. Hence, total Q available at B = 2160 + 1180 =3340 gpm.

Figure 2.9 Prepared by Dr. Ahmed H. Birima

Single-Path Adjustment (P) Method (Hardy cross method) • General procedure: • An initial set of flow rates that satisfy continuity at each junction node is selected. • A flow adjustment factor is computed for each path to satisfy the energy equation for that path. Continuity is maintained in this process. • Step 2 is repeated, building on improved solutions until the average correction factor is within an acceptable limit.

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Flow Adjustment Factor Flow correction can be calculated by equation 2.3 2.3

Where Q = flow through the pipe H = friction head loss in the pipe n = constant = 1.85 • Application of this equation involves an initial assumption of discharge and a sign convention for the flow. Either clockwise or counterclockwise flows may be considered positive, and the terms in the numerator will bear the appropriate sign. Prepared by Dr. Ahmed H. Birima

• The denominator, however, is the absolute sum without regard to sign convention. The correction ∆Q has a single direction for all pipes in the loop, and thus the sign convention must also be considered in applying the correction.

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Example 2.7 • For the pipe network shown below, carry out Hardy cross analysis using spreadsheet to determine the direction and magnitude of flow in each pipe. Assume that the HazenWilliams coefficient c is 100.

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Solution for Example 2.7

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Distribution Reservoirs and Service Storage • Distribution reservoirs provide service storage to meet fluctuating demands often imposed on distribution systems, to accommodate firefighting and emergency requirements, and to equalize operating pressures. • They may be elevated or below ground level. • The main categories are surface reservoirs, standpipes, and elevated tanks (Figure 2.10). • Standpipes or elevated tanks are normally employed where the construction of a surface reservoir would not provide sufficient head(figure 2.11).

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surface reservoir

Standpipe

Elevated tank

Figure 2.10 Types of reservoirs Prepared by Dr. Ahmed H. Birima

Figure 2.11 surface reservoir at sufficient head

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Distribution Reservoirs and Service Storage • Distribution reservoirs should be located strategically for maximum benefit. Normally, the reservoir should be near the center of use, but in large metropolitan areas a number of distribution reservoirs may be located at key points. • service storage must be high enough to develop adequate pressures in the system they are to serve. • Total storage should be calculated based on: • Requirements for firefighting • Emergency storage that to sustain the community’s needs during periods when the inflow to the reservoir is shut off • The equalizing or operating storage requirement. Prepared by Dr. Ahmed H. Birima

Distribution Reservoirs and Service Storage • Example 2.8 • Calculate the total amount of storage required for a residential area based on the following information: • Number of population = 8000, average use rate = 150 gpcd, expected emergency period = 3 days, fire flow = 2750 gpm for 10 hr.

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Solution • Emergency storage for 3 days: • • Firefighting storage :

• Required Equalizing or operating storage: • Determination of this volume can be obtained as shown in table 2.5 • = 1.47 mil gal • Total amount of storage = 3.6 +1.65+ 1.47 = 6.72 mil gal Prepared by Dr. Ahmed H. Birima

Table 2.5 hourly demand for the maximum day ( source water supply and pollution control, by Viessman& Hmmer)

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