Decision Support Tool for Design and Reliability Assessment of Water Distribution Networks - Pattern Recognition for Reliability Assessment of Water Distribution Networks - page 237

Trifunović (2006) states two postulates of water distribution network hydraulic design
(quote):
1. Water flows to any discharge point choosing the easiest path: either the shortest one or
the one with the lowest resistance.
2. Optimal design from the hydraulic perspective results in a system that demands the least
energy input for water conveyance.

In practice, this means:
- maximum utilisation of the existing topography (gravity),

- use of pipe diameters that generate low friction losses,

- as little pumping as necessary to guarantee the design pressures, and

-

valve operation reduced to a minimum. (end quote)

Hydraulic design parameters mostly concern pressures, hydraulic gradients and pipe
velocities, the latter having the largest impact on water quality in distribution networks under
regular supply conditions. Acceptable pressures must be maintained throughout the system
while satisfying the demand. This is needed to protect the water from pollution entering
through damaged sections of the pipes. The pressures in water distribution networks may
vary from case to case; they are largely dependent on the availability of water and conditions
of the network. Therefore, there is no universally acceptable pressure range. For instance,
The Office of Water Services (OFWAT, 1996) in England specifies that pressures of seven
meters water column (mwc) above street level can be considered as the minimum acceptable
standard, below which the consumers may be entitled to compensation for unsatisfactory
service. Typically, the water companies in developed countries set the minimum pressures
between 15 and 20 mwc, measured in the most critical part of the system (usually at the
boundary of the network), which applies during a peak daily demand. The practice of many
water companies in The Netherlands is to maintain the minimum pressures around 20 mwc
allowing temporary drops in irregular situations to around 10 to 15 mwc. Tanyimboh et al.
(1999) indicate the required minimum service pressure to be as high as 25 mwc, to allow for
possible increases in the demand. This is more or less sufficient to supply a 'standard-high'
building without internal boosting installation, which in most urban areas is three to five
floors high. The maximum pressure in the network should normally be around 60 to 70 mwc.
According to Chase (2000), the pressures during normal operations should be kept above 20
mwc and below 70 mwc. Typical range of pressures observed in some distribution networks
worldwide is shown in Table 9.1 (Source: Kujundžić, 1996).

Table 9.1 Pressures in world cities (Source: Kujundžić, 1996)

City/Country Min. - Max. (mwc)

Amsterdam / NL
Wien / Austria
Belgrade / Serbia
Brussels / Belgium
Chicago / USA
Madrid / Spain
Moscow / Russia
Philadelphia / USA
Rio de Janeiro / Brasil
Rome / Italy
Sophia / Bulgaria

±25
40 - 120
20 - 160
30 - 70
±30
30 - 70
30 - 75
20 - 80
±25
±60
35 - 80