closed conduit water supply

CE 516

Design of Water Distribution Systems

In this section, we have learned how to size pipelines and pumps for pipe networks.We will now examine some of the practicalities which need to be considered whendesigning such closed conduit systems. This is a review of section 2.5 in yourtext (which you should read). The primary function of water distribution systemsare to

1. meet the water demands of users while maintaining acceptable pressures

in the system

2. supply water for fire protection at specific locations within the system,

while maintaining acceptable pressures for normal service.

3. provide sufficient level of redundancy to support minimum level of service

during emergency conditions (i.e. power loss or water main failure.)Components

The components of a water distribution system include:

• pipelines - carry water from the treatment facility to the users.

transmission mains - the largest pipes which carry flow from the water

treatment facility to the network. These pipes are often greater than

(D > 600 mm).

feeder mains - (pipelines which feed flow from the transmission main

to the individual pipe networks of every service area (D ∼ 400 - 500 mm)

distribution mains - the grid of pipelines which provides service to

all users (D ∼ 150 - 300 mm)

service lines - pipelines which go from the distribution mains to the

individual house/facility.

The individual lines are sized as per momentum equation (Darcy-Weisbach) andenergy equation (head loss equation.)

Closed Conduit Flow 1 of 12 Design of water distribution systems

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Components (continued)

• pumps - maintain required pipeline service pressure. Because of variable de-mand requirements multiple pumps or pumps with variable motors are oftenrequired.

booster pumps - maintain required service pressure along long pipelines

fire-service pumps - provide additional capacity for emergency situations

Pumps operate at the intersection of pump performance and network

system curves and must adjust to highly variable demand. As such,

multiple pumps may be required for steady, cyclical, and emergency

demands

• storage facilities - accommodates demand fluctuation by storing excesswater until it’s necessary

ground storage - ground level storage which discharge water to the

system with a pump

elevated storage - storage tank at the elevation required to deliver

water at required pressure (or head).

• Valves - required for removing components and rerouting flows

• Meters - required for monitoring flows


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Component Design Life

The preferred design life for the various components of water supply systems isgiven with the following table (Chin, Table 3.11)

Component Design Period Design Capacity

(years)

Sources of supply

River indefinite Max daily

Wellfield 10-25 Max daily

Reservoir 25-50 Average annual

Pumps

Low-lift 10 Max daily

High Lift 10 Max hourly

Water Treatment 10-15 Max daily

Service Reservoir 20-25 Working storage +

fire + emergency

Distribution system

Pipe or Conduit 25-50 Max daily + fire OR

max hourly demand

Distribution Grid


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Operating Pressure of System

The required system pressure demands on several considerations listed below:

• excellent flow to a 3 story building requires 290 kPa

• adequate flow for residential areas requires 240 kPa

• adequate flow to a 20 story building requires 830 kPa**Please note this isn’t desirable because of waste and leak, instead most tallbuildings have their own on site pumps. Generally, pressures of greater than650 kPa should be avoided.

• adequate flow to most systems recommends 410-520 kPaordinary consumption for 10 story buildingsadequate service for sprinklers in buildings of 4-5 storiesadequate fire hydrant serviceadequate margin for fluctuations due to clogging and other losses


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Water Demand

Demands of the entire population must be considered before designing a waterdistribution system. Possible demand sources include:

• residential

• commercial

• industrial

• public

The average city requires 660 liters/day/person. The distribution of demandbetween all the possible sources is given in the table below.

Typical distribution of water use for an average city (Shin, 2000, Table 3.4)

Category Average use Percent of total

(liters/day)/person

Residential 260

Commercial 90

Industrial 190

Public 70

Loss 50

Total 660


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Water Demand (continued)

The distribution of average per capita rates among 292 water supply systems inthe U. S. that serve 95 million people is given in the table below.

Average distribution of per capita water demand (Shin, 2000, Table 3.5)

Range Number of Percent of total

(liters/day)/person systems

190-370 30 8

380-560 132 34

570-750 133 34

760-940 51 13

950-1130 19 5

>1140 27 7

Please note: these figures are based on 392 US water supply systems serving 95million people (1984 Water Utility Operating Data, 1986 AWWA)


CE 516

Water Demand Projections

When planning for a water supply system, the water demand at the end of thenetwork design life is generally used as the basis for the project design. Becausethe demand of the system 20 years in the future is not known, it is necessary tomake some kind of prediction or forecasting about the municipality growth. Avariety of Forecasting models exist, including:

• aggregate models - treat the population as a whole

• disaggregate models - break up the population into groups and predict thegrowths of each group. An example of this is cohort analysis (Sykes 1995)which segregates age and gender. These models require large quantities ofdata.

• empirical models - are based solely on data.

Please note that after 10 years, empirical models are as reliable as disaggregatemodels.

Population growth is not generally steady and tends to grow at varying rates.


CE 516

Geometric growth phase - occurs when there are wide open spaces and ismodelled with

Arithmetic growth phase - occurs after the initial growth has leveled off andis modelled with

Declining growth phase - occurs when growth becomes limited by availableresources

Each of the above phases is generally limited to 10 years in duration. For situa-tions when a longer projection is required a long term projection can be approx-imated with an S-curve (most common used is a logistic curve)

Please note, use existing data to determine a & b.Please note, that for projections of less than 10 years a 10% error can be ex-pected, but for projections greater than 20 years a 50% error can be expected.


CE 516

Demand Variations

The demand in a water distribution system varies, daily, weekly, seasonally, andin the case of emergencies such as fires. Typical daily variations are given in thebelow figure (Chin, Figure 3.23)


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The range of demand conditions are specified with peaking or demand factors.An example of the demand factors is found in the below table (Chin, Table 3.6).

Condition Range of Typical

demand factors value

Daily average in maximum month 1.1-1.5

Daily average in maximum week 1.2-1.6

Maximum daily demand 1.5-3.0

Maximum hourly demand 2.0-4.0

Minimum hourly demand 0.2-0.6

In emergency situations as a result of fires the demand may increase significantly.The most common method for estimating peak demands due to fires is a methodproposed by the Insurance Services Office (ISO, 1980). Their method estimatesthe Needed Fire Flow, NFF, with

NFFi = CiOi(X + P )i

whereC is the construction factor

O is the occupancy factor

X is the exposure factorP is the proximity factori is the location where the flow is needed

The maximum needed fire flow is less than 45,000 L/min

Please see Chin for a more complete discussion of needed fire flows and the re-quired coefficients


CE 516

Required fire flow durations to satisfy insurance requirements (Chin, Table 3.10)

Required fire flow Durations

(L/min) (hours)

<9,000 2

11,000-13,000 3

15,000-17,000 4

19,000-21,000 5

23,000-26,000 6

26,000-30,000 7

30,000-34,000 8

34,000-38,000 9

38,000-45,000 10


CE 516

Example

A water-supply system is being designed to serve a population of 200,000 people,with an average per capita demand of 600 L/day/person and a needed fire flowof 28,000 L/min. If the water supply is to be drawn from a river, then whatshould be the design capacity of the supply pumps and water treatment plant?For what must be kept in the service reservoir to accommodate a fire? Whatshould the design capacity of the distribution pipes be?

Solution

assume:a) find design capacity given:

b) determine required flow duration and volume

c) determine design capacity for pipes


closed conduit water supply

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