Environmental Engineering Reference
In-Depth Information
3.1
INTRODUCTION
Water distribution modelling practice still relies mostly on the demand-driven (DD)
calculations of steady and uniform flows in pressurised networks. For the scenarios
describing regular conditions with sufficient pressures, this approach is accurate enough
while providing very fast and robust algorithm. In situations when the pressure in the network
drops, either due to a pipe- or pump failure, or as a result of 'regular' intermittent supply
caused by inadequate source capacity, the low pressures can affect the demand. Demand-
driven models are unable to capture this reduction and the only indicators of the failure they
are able to produce are negative pressures. When this is about to happen, the hydraulic
simulation should switch to more computationally intensive pressure-driven demand mode
(PDD).
PDD models have become essential tools for hydraulic analysis of water distribution
networks under stress conditions as well as they are applied for modelling of leakages. No
universally accepted method exists here, due to inability of mathematical equations to
precisely describe the hydraulic complexity of irregular supply, on one hand, and practically
impossible monitoring of the data that would enable full model calibration, on the other hand.
In simplified approach, the PDD models can be based on the principle of emitter coefficient
available in EPANET software (Rossman, 2000). Common approach assumes the definition
of pressure threshold as an indicator of sufficient service level, which is then used to switch
between the DD- and PDD mode
.
3.2
PRESSURE-DRIVEN DEMAND CONCEPT
The concept of pressure-driven demand can be compared to the discharge through an orifice,
as shown in Figure 3.1 (Trifunović, 2006).
Figure 3.1
Analogy between discharge through orifice and pressure-driven demand (Trifunović, 2006)
The pressure,
p/ρg
, above the water tap eventually becomes destroyed and can therefore be
considered as the minor loss,
h
m
. Equation 3.1, describes the flow relation in both cases.
2
Q
1
Q
=
CA
2
gh
⇔
h
=
ξ
⇒
Q
=
A
2
gh
3.1
m
m
A
2
2
g
ξ
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