EI 11C103
CEMP-E
1 March 1997
or changing existing storage facilities solely for the purpose of increasing residual pressures to 140 kPa (20
psi). Refer to TM 5-813-6/AFM 88-10, Vol. 6 for additional guidance on minimum residual pressures for fire
flow.
(b) Maximum pressure. Maximum water pressures in distribution mains and service lines should
not normally exceed 520 kPa (75 psi)at ground elevation. Static pressures up to 670 kPa (100 psi) can be
tolerated in distribution systems in small, low-lying areas. Higher pressures require pressure reducing
valves on feeder mains or individual service lines to restrict maximum service pressures to 520 kPa (75 psi).
(c) Multiple pressure levels. If an extensive area has pressures higher than 520 kPa (75 psi) or
lower than 280 kPa (40 psi) under a single pressure level configuration, it may be appropriate to divide the
system into two or more separate areas, each having different pressure levels. Within each level,
pressures within the distribution system should range from 280 to 520 kPa (40 to 75 psi) at ground
elevation.
(2) Pressure distribution with elevated storage.
(a) Elevated storage within the distribution system permits distribution pumps at the treatment
plant to operate at uniform rates.
(b) The usefulness of elevated storage is shown in figure 1-1. The system illustrated in figure 1-1
(A) (without elevated storage) requires storage at the plant sufficient to provide for system demand rates in
excess of the plant production rate, assuming the plant is operated at a uniform rate. The pump station
forces water into the service main, through which it is carried to three load areas: A, B, and C. Since all
loads on the system are met without the use of elevated storage, the pump station must be capable of
supplying the peak rates of water use to Areas A, B, and C, simultaneously, while maintaining the water
pressure to Area C at a sufficient level. The minimum recommended pressure in the distribution system
under peak nonemergency flow conditions is 280 kPa (40 psi). Figure l-l (B) assumes the construction of
an elevated storage tank on the service main between Areas B and C, with peak loads in Area C and part of
the peak load in Area B being satisfied from this tank. The elevation of the tank ensures adequate
pressures within the system. The storage in the tank is replenished when water demands are low and the
pump station can fill the tank while still meeting all flow and pressure requirements in the system. The
figure 1-1 (B) arrangement reduces required capacity of the distribution pumps.
(c) Most elevated storage tanks "float" on the distribution system. That is, the elevated tank is
hydraulically connected to the distribution system, and the volume of water in the tank tends to maintain
system pressures at a uniform level. When water use is high and pumping facilities cannot maintain
adequate pressures, water is discharged from elevated tanks. Conversely, when water use is low, the
pumps, which operate within a reasonably uniform head-capacity range, supply excess water to the system
and the elevated storage is refilled.
e. Provision of emergency water supplies.
(1) Firefighting flows. This demand can occur at any time, but may well coincide with other large
water demands on the system. Necessary flows for fire fighting purposes are as given in TM 5-813-6/AFM
88-10, Vol. 6, and TM 5-813-7/AFM 88-10, Vol. 7. Storage and distribution facilities will include capacity for
required firefighting flows at adequate pressures at any point of the installation.
(2) Other emergencies. Water storage must provide an emergency supply of water in the event the
water treatment plant, distribution pumps, or a principal transmission main is out of service. The amount of
emergency storage required depends on the reliability of the system and the extent of other safeguards
incorporated into the system, i.e., finished water interconnections with a municipality (for either normal or
emergency use).
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