Environmental Engineering Reference
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Figure 3.9 PDD by Pathirana (2010) of the nets from Figure 3.8, - pipes: Q (l/s), nodes: p/ρg (mwc)
The same networks as in Figure 3.8 have been recalculated by the PDD adaptation of
EPANET developed by Pathirana (2010), assuming the PDD threshold of 20 mwc and the
emitter exponent of 0.5; the results can be seen in Figure 3.9.Here, the PDD calculation gives
more logical results. The drop of pressure in the node J3 to 12.55 mwc resulting from the
elevation of 25 mwc has caused the drop of the demand in that node from 30 to 23.76 l/s. The
pressure in J3 is however higher than if calculated in the DD mode, which is also logical; the
demand drop has also caused the reduction of friction losses in the network. In the extreme
case of elevation z J3 = 55 msl, which is higher than the head of the reservoir, the demand in
this node has been reduced to zero. The negative pressure of -9.32 msl, reflecting the high
elevation of the node, has been higher than in case of the DD calculation, again for the same
reason of reduced friction losses. Consequently, the pressures in the network with higher loss
of demand (on the right) will be generally higher.
Figure 3.10 PDD by Pathirana (2010), increased z of J5 - pipes: Q (l/s), nodes: p/ρg (mwc)
Repeating the same calculation by increasing the elevation of non-demand node J5 from 5 to
105 msl will give the results as in Figure 3.10. It shows that the PDD calculation affects only
the demand nodes i.e. the flow patterns will not be affected by the nodal elevations of non-
demand nodes. Equally, the supply of node J3 will not be necessarily affected by the increase
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