Water and Wastewater Engineering
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16-48 WATER AND WASTEWATER ENGINEERING
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Systems Security, McGraw-Hill, New York, p. 1.6.
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Available,” in L.W. Mays (ed.) Water Supply Systems Security, McGraw-Hill, New York, 3.1–3.42.
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Water
Works, unpublished manuscript.
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pp. 56–60.
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Association, vol. 79, no. 9, pp. 93–106.
MWH (2005) Water Treatment: Principles and Design, John Wiley & Sons, Hoboken, New Jersey,
p. 1860.
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Washington, D.C.
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Vulnerability,” in L.W. Mays (ed.) Water Supply Systems Security, McGraw-Hill, New York,
p. 5.8–5.9.
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and Organic DBP Precursors,” Journal of American Water Works Association, vol. 99, no. 12,
pp. 61–71.
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5) “Minneapolis Water Works’ Ultrafiltration Plant Gets Off to a
Big Start,” Journal of American Water Works Association, vol. 97, no. 12, pp. 56–63.
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17-1
17-7 NETWORK ANALYSIS
17-8 SANITARY PROTECTION
17-9 CHAPTER REVIEW
17-10 PROBLEMS
17-11 DISCUSSION QUESTIONS
17-12 REFERENCES
STORAGE AND DISTRIBUTION SYSTEMS
17-1 INTRODUCTION
17-2 DEMAND ESTIMATES
17-3 SERVICE PRESSURES
17-4 PIPE NETWORK DESIGN
17-5 STORAGE TANK DESIGN
17-6 PUMP SELECTION
17
CHAPTER
17-2 WATER AND WASTEWATER ENGINEERING
17-1 INTRODUCTION
Planning and design of a typical distribution system includes the following elements:
• Demand estimates including fire demand.
• Service pressures.
• Pipe network design.
• Storage tank design.
• Pump selection.
• Network analysis.
• Sanitary protection.
These are the topics
of discussion for this chapter.
17-2 DEMAND ESTIMATES
The terms used in water distribution planning and design are defined in Table 17-1 .
D e mand estimating techniques were discussed in Chapter 2. These apply to the distribution
system as well as the water treatment plant. The demand estimate for the distribution system
differs in that, in addition to the total de
mand of the water distribution system, the individual
demand of segments of the community must be estimated. Where the type of d emand is simi-
lar, for example, domestic or c ommercial or industrial, broad zones or districts may be
defined
to establish the demand. If a decision has been made to provide water for fire protection, this
demand must also be cons idered as part of the dem and estimate. In larger distribu tion systems,
Terms Definition
Average day demand The total annual quantity of water production for a municipality
divided by 365.
Maximum day demand The highest water demand of the year during any 24-hour period.
Peak hour demand The highest water demand of the year during any
one-hour period.
Peaking factor The increase above the average annual demand experienced during
a specified time period. Customarily, these are multipliers of the
average day demand. Some examples are maximum day/average
day and peak hour/average day.
Distribu
tion main A smaller diameter pipe that serves a relatively small area.
Trunk line A large diameter pipe that serves a relatively large area.
Transmission main A larger diameter pipe that is designed to transport large quantities
of water. Water services for small individual customers are
normally not placed on trunk lines.
TABLE 17-1
Terms used in water distribution planning and design
A dapted from Ysusi, 2000.
STORAGE AND DISTRIBUTION SYSTEMS 17-3
the additional requirement of providing fire protection has a marginal effect on s izing decisions,
but in smaller systems, the requirement to provide fire protection will have a significant impact
on sizing.
Fire Flow Requirements
Community Governance. The decision of whether or not to size distribution system compo-
nents for fire protection must be made by the governing body of the community. There is no legal
requirement that the governing body must size the distribution system for fire protection. In some
instances, it may be prohibitively expens
ive (AWWA, 1998).
An approach that has proven more economical for large cities is to establish codes and ordi-
nances that require condemnation or upgrading of unsafe buildings to meet fire codes, including
mandatory automatic sprinkler systems (AWWA, 1998). This approach works for two reas
ons.
The first is that if buildings that have activities conducive to rapidly burning fires are eliminated,
the water demand to provide fire protection is reduced. The second is that the fire flow require-
ments for the municipal system are less if buildings are provided with automatic sprinklers. This
approach is not viable for suburban residential communities.
Automatic Fire Sprinkler Systems. Automatic sprinklers have been the single most impor-
tant system for automatic control of fires in buildings for more than a century. Among the ben-
efits of automatic sprinklers is the fact that they operate directly over a fire. S
moke, toxic gases,
and reduced visibility d o not affect their operation. In addition, much less water is used because
only those sprinklers fused by the heat of the fire operate, especially if the build ing is compart-
mented (Coté and Linville, 1997). Of all the tools available to facilitate and promote fire protec -
tion, none offers such a wide variet
y of benefits to the building owner, developer, fire service,
water supplier, and the general public as the widespread us e of automatic sprinkler systems
(AWWA, 1998).
Sprinklers are designed to control fires, but to completely extinguish a fire, hose streams are
required. The National Fire Protection Association (NFPA) recomm
ended automatic sprinkler
flow rate per unit area is shown in Figure 17-1 . The total hose-stream allowance and fire flow
duration are shown in Table 17-2 . The determination of the hazard group noted in the figure and
table is a multifaceted process that is described in the several publications of the NFPA. A broad
average description of the hazard group
s is as follows (AWWA, 1998):
• Ordinary group 1: retail stores, offices, hotels, and institutional buildings.
• Ordinary group 2 or 3: warehouses and manufacturing activities.
• Extra hazard groups: occupancies with highly flammable products or processes.
The pressure required to operate the sprinklers varie
s depending on the manufacturer.
The NFPA pressure may be specified at every sprinkler, for example, 50 kPa, or at the base of
the sprinkler riser, for example, a pressure equivalent to 100 kPa at the highest sprinkler. For
domestic sprinkler systems, the required operating pressure ranges from 140 to 275 kPa. For all
of these sprinkler sys
tems, the pressure at the street may have to be considerably higher to over-
come friction and elevation losses.
Fire sprinkler designers must be appraised of the minimum pressures that may be available
at the water main in the street. The selection of the pressure to be used for the sprinkler system
17-4 WATER AND WASTEWATER ENGINEERING
0.05
1,500
2,000
2,500
3,000
4,000
5,000
2.0 4.1 6.1 8.1 10.2 12.2 14.3
0.10
Light
Ord
inar
y
hazar
d
gro
u
p 1
Or
d
inary
hazar
d
gro
u
p 2
Or
d
inar
y
hazar
d
gro
u
p
3
E
x
tra hazar
d
grou
p 1
E
x
tra hazar
d
g
ro
u
p 2
0.15 0.20
Density, gpm/ft
2
Area of sprinkler operation, ft
2
Area of sprinkler operation, m
2
Density, (L/min)/m
2
0.25 0.300.35
139
186
232
279
372
465
557
FIGURE 17-1
Sprinkler system design curves. Reprinted with permission from NFPA, 13-1987.
( Source: NFPA, 1987.)
Hazard
classification
Hose-stream allowance,
m
3
/min
Duration,
min
Light 3.8 30
Ordinary group 1 9.5 60–90
Ordinary group 2 9.5 60–90
Ordinary group 3 19 60–120
Extra hazard group 1 19 90–120
Extra hazard group 2 38 120
TABLE 17-2
Hazard classification schedule for hose-stream allowance
Adapted from AWWA, 1998.
depends on the level of risk that the owner is willing to accept and the related cost. The lower
the design pressure us ed to size the sprinkler, the lower the risk of failure due to inadequate
pressure.
STORAGE AND DISTRIBUTION SYSTEMS 17-5
In designing the water system, the design engineer does not design the individu al sprinkler
sys tems for every building. Instead, repres entative requirements for portions of the town are
used. For example, one hotel sprinkler system may be designed to represent a hotel district, or
one commercial es
tablishment’s sprinkler system may be designed to represent a district. In spe-
cial cases, the client may require a more detailed design, as in the design of a mall water supply
system.
E xample 17-1 illustrates the estimate of fire flow for a sprinkler syste
m and a hose-stream in
a commercial establishment.
Example 17-1. A representative sprinkler plus hose-stream fire flow is desired for a motel
district. Based on a review of the plans for several motels, the following assumptions have been
made:
1 . Area of a representative motel 1,500 m
2
.
2. Four compartments are used for the sprinkler system.
3. The motels are classified as Ordinary group 1.
Solution:
a . Because four compartments are used, the floor area for sprinkler demand is esti-
mated as
1 500
4
375
2
2
, m
m
b. Using Figure 17-1 and 375 m
2
, the flow rate per unit area is estimated to be
3.8 L/min · m
2
or 0.0038 m
3
/ min · m
2
.
c. The sprinkler flow rate is then estimated to be
Q
sprinkler
mm/min m ()( )375 0003814
2 3 2
..m /min
3
d. From Table 17-2 , hose-stream rate is 9.5 m
3
/ min.
e. The total fire demand is then estimated to be
Q
fire demand
m /min m /min m /14 95 10 9
33 3
.. .mmin
Comments:
1 . The fire demand would apply to the entire motel district because there is a basic assump-
tion that only one fire will occur at a time.
2. As noted in the next section, the fire flow requirement is added to the maximum day
demand in the design of the system for the motel dis
trict.
17-6 WATER AND WASTEWATER ENGINEERING
Needed Fire Flow (NFF). The range of fire flow requirements is quite broad ( Table 17-3 ).
A r ule of thumb is that the minimum amount of water that can safely and effectively con-
trol a fire when a sprinkler system is not available is 115 m
3
/h at a residual pressure of 140 kPa.
This represents the amount of water for two stand ard hose streams on a given fire. Above this
minimum, it is rec ommended that at any given point in the distribution system, the sys tem be
able to provide the required design flow by using one of three methods for determining the NFF:
(1) Insuranc
e Services Office, (2) Illinois Institute of Technology Research Institute, or (3) Iowa
State University. The Insuranc e Services Office (ISO) method is most likely to yield realistic
requirements (AWWA, 1998). GLUMRB (2003) recommends that the requirements of the State
Insurance Services Office be followed.
The ISO metho
d was developed for use during an evaluation for insurance purposes. It is
based on the following empirical equation:
NFF
iii ii
CO X P()()( )10.
(17-1)
where N FF
i
needed fire flow
C
i
construction factor
O
i
occupancy factor
X
i
exposure factor
P
i
communication factor
The construction factor ( C
i
) takes into account materials of construction (e.g., frame, masonry, or
fire resistant). In SI units, it is approximated as
CFA
ii
18
0 5
()( )
.
(17-2)
where F construction class factor
A
i
effective area, m
2
The effective area is the total area of the largest floor, plus a percentage of other floors.
The occupancy factor ( O
i
) takes into account the combustibility of the materials or processes
inside the structure. The exposure factor ( X
i
) depends on the construction and the length-height
value (length of wall times height in stories) of the exposed building, the distance between facing
walls of the subject building (the one to have a sprinkler system), and the exposed building (a
nearby building that might catch fire). The communications factor ( P
i
) depends on the protection
for communicating wall openings and the length of the communication between fire divisions.
ISO has prepared tables for selection of these variables.
Land use Fire flow requirement, m
3
/h
Single family residential 115–455
Multifamily residential 340–680
Commercial 570–1,100
Industrial 800–2,300
Central business district 570–3,400
TABLE 17-3
Typical fire flow requirements
A dapted from Ysusi, 2000.
STORAGE AND DISTRIBUTION SYSTEMS 17-7
For one and two family dwellings, the ISO procedure is simplified to the requirements listed
in Table 17-4 .
E xample 17-2 illustrates the calculation procedure using the ISO equations and tables.
Example 17-2. For comparison, compute the NFF for the representative motel in Example 17-1
assuming s
prinklers are not provided. Use the following assumptions:
1 . Area of a representative motel 1,500 m
2
.
2. One-story high.
3. Joisted masonry construction.
4. Occupancy classification C2.
5. Distance to exposure building 20 m.
6. Exposure building is a blank wall.
7. Exposure building wall is masonry with no windows.
Solution:
a . Calcu
late C
i
. For joisted masonry construction, F 1.0.
C
i
18 1 0 1 500 697 14
2053
()( )., .
.
mm/h
By rule, this is rounded to nearest 60 m
3
/h, so C
i
720 m
3
/h.
b. For the occupancy classification of C2, O
i
0.85.
c. Because the exposure building is a blank masonry wall, the values for X
i
and P
i
are both
zero.
d. The NFF is then
NFF m /h m /h()()( )720 0 85 1000 612
33
..
B y rule, this is rounded to nearest 60 m
3
/h, so NFF 600 m
3
/h or 10 m
3
/ min.
Distance between buildings, m Needed fire flow, m
3
/h
Greater than 30115
10–30 170
3–10 230
Less than 3340
TABLE 17-4
Needed fire flow (NFF) for one- and two-family dwellings
a
a
Dwellings not to exceed two stories.
A dapted from AWWA, 1998.
17-8 WATER AND WASTEWATER ENGINEERING
Comments:
1 . This is slightly less than the required sprinkler plus hose-stream fire flow found in
Example 17-1 . Given the approximations and assumptions, these estimates are the same.
This is because this building was treated as a “stand-alone” building.
2. If the proximit
y of another building with less than fire resistant construction was within
10 m, the sprinkler system would require less water.
3. Regardless of the economy or lack thereof in supplying water for fire suppression, some
places require fire suppression. For example, motels and similar structure
s, where a high
fraction of the occupants are not employees, will be provided with automatic sprinklers
for preservation of human life and reduction of human injury.
It is not unusual to determine the NFF in consultation with the local fire marshal or fire chief.
In the absence of a realistic estimate, the ISO (for structures without sprinkler
s) or NFPA (for
structures with automatic s prinklers) is used to estimate the NFF. As is in the case of sprinkler
systems, every building is not evaluated for NFF. Instead, a representative structure for a district
is evaluated.
Good engineering judgement is
required in establishing the NFF. In general, it would not be
reasonable for the fire demand of a few isolated structures in a residential community to drive the
selection of the NFF for the whole community. An exception might be if that structure provided
most of the jobs in the community. Involvement of the community in the dec
ision in these special
cases is warranted.
Peak Hour Demand
Most systems are quite capable of meeting the average day conditions. It is only when the system
is stressed that deficiencies become apparent. The peak hour demand estimate is used to evaluate
the distribution system response to s tress. It does
not include fire demand, which is considered
localized rather than systemwide. The peak hour demand may be considered as communitywide
demand because of normal community use under special cir
cumstances such as widespread lawn
watering on a hot summer day.
The range of peaking factors u sed in the United States varies from 2.0:1 to 7.0:1 (Ysusi,
2000). Figure 2-1, in Chapter 2, provides a means of selec ting an appropriate peaking factor
based on the community population. Example 17-3 illustrates the calculation of the peaking fac-
tor when no actual data are available.
Example 17-3. Estimate the peak hour demand for a community of 8,000 people. Assume the
average demand is equal to the United States public supply use of 580 Lpcd.
Solution:
a. The estimated average day demand is
()( )( )580 8 000 10 4 640
33
Lpcd people m /L m,,
33
/d
b. Using Figure 2-1 and the population of 8,000, the peaking factor is about 3.2.
STORAGE AND DISTRIBUTION SYSTEMS 17-9
c. The peak hour demand is then estimated to be
(.)( )3 2 4 640 14 848 15 000
33
,,,m /d or m /d
Comment. For volume calculations, it should be recognized that the calculated peak hour
demand has a duration of one hour, that is, the volume of water is about (15,000 m
3
/ d)
(1h/24h/d) 625 m
3
Maximum Day
The range of peaking factors used for the maximum day in the United States is from 1.5:1 to 6:1
with the common range being 1.8:1 to 2.8:1 (Ysusi, 2000). The U.S national average in 1967
was 2.2 (Linaweaver et al., 1967). The distribution system is tested (by calculation) at various
locations with the community maximum day estimate set a
s a “background” plus the NFF for the
zone or district.
Determining the Design Flow
The design flow is usually determined in the following stepwise fashion:
1 . Estimate the average daily demand.
2. Estimate the maximum daily demand.
3. Estimate the maximum hourly demand.
4. Estimate the required
fire flow if fire protection is to be provided.
5. Select a design flow.
Normally the distribution design flow rate is either the sum of the required fire flow rate for the
most stringent situation plus the maximum daily demand or just the peak hour demand, which-
ever is greater. Current design practice a
ssumes that the coincidence of fire demand and the peak
hour demand are sufficiently rare, and that the cost of meeting such an event is so high that it is
not a reasonable criterion for the design of the distribution system.
17-3 SERVICE PRESSURES
Water pressures in the mains must fall into a range of values. If the pressure is too low, fixtures
will not operate satisfactorily. If the pressure is too high, fixtures may leak, valve seats will wear
out, and hot water heater pressure relief valves may discharge.
Design Criteria
GLUMRB (2003) specifies that the system shall be designed to maintain a minimum pressure of
140 kPa at ground level at all points in the distribution system under all conditions of flow. The
normal working pressure in the distribution system should be approximately 410 to 550 kPa and
not less than 240 kPa.