Emitter Based Algorithm for Pressure-Driven Demand Calculations of Water Distribution Networks - Pattern Recognition for Reliability Assessment of Water Distribution Networks - page 45

network on the left. Eventually, the surplus of head of exactly 9.32 mwc, is to be deducted
from all the pressures. More usual and simple approach is to remove the emitter coefficient in
node J3 and set its demand to zero, as it has been the case in the network on the right. Both
procedures will be of course more difficult in complex networks with multiple sources of
supply and nodes with negative pressures.

Figure 3.14 DD simulation, elimination of emitter supply in J3 , - pipes: Q (l/s), nodes: p/ρg (mwc)

3.4 EMITTER BASED PDD ALGORITHM

The PDD algorithm based on the EPANET emitters can be relatively easily developed using
the EPANET toolkit functions. Essentially, it is a computational loop consisting of three
steps:
1. Removing the node baseline demand and introducing emitter coefficients in all the nodes
where the pressure is lower than the PDD threshold.
2. Reinstating the node baseline demand and removing the emitter coefficients in all the
nodes where the pressure is equal or higher than the PDD threshold.
3. Running a snap-shot DD hydraulic simulation.

Preliminary hydraulic simulation will be executed and the emitter coefficients calculated for
all the nodes, as in Equation 3.9, before the loop has been executed. Furthermore, the loop
execution will stop when acceptable difference in the results in two consecutive iterations has
been reached.

Based on the selected exponent, the introduced emitters will start reducing the node demand
which has as a consequence the pressure increase in the rest of the network. In the event that
an emitter operates in the node with negative pressure, resulting from extreme topographic
conditions, such emitter will generate negative demand i.e. a dummy inflow that reduces the
source supply. This peculiarity can be mitigated in two ways:
1. by removing the emitter while retaining the baseline demand equal to zero;
2. by disconnecting i.e. closing all the pipes connected to that node; this action can be
expanded to all the pipes that have at least one connecting node with negative pressure,
which will then also include non-demand nodes with negative pressure.