Emitter Based Algorithm for Pressure-Driven Demand Calculations of Water Distribution Networks - Pattern Recognition for Reliability Assessment of Water Distribution Networks - page 41

The PDD algorithm of Pathirana (2010) deals with negative nodal pressures in a
straightforward way, by setting the nodal demand to zero. Nevertheless, high altitude nodes
without the demand, and with negative pressure, will keep the same conveyance of the
connecting pipes, regardless their elevation i.e. the (negative) pressure value. This resembles
hydraulic conditions of a siphon, which in reality is possible, but very likely causes the
reduction of capacity, and in more extreme conditions can disable the route for conveyance,
entirely. To which extent the water distribution becomes affected will certainly depend on the
nodal elevations, next to the supplying heads and pipe resistance.
Figure 3.7 Simple network ( L j = 1500 m, z i = 5 msl) - pipes: D (mm), Q (l/s), nodes: Q (l/s), p/ρg (mwc)
These phenomena have been illustrated on a simple network shown in Figure 3.7. The figure
shows the network under normal operation. For the sake of simplicity, all the pipe lengths
have been set at 1500 m, the nodal elevations at 5 msl, and the k-values at 0.5 mm. The water
level in the reservoir has been kept at 50 msl.
Figure 3.8 DD simulation (left: z J3 = 25 msl, right: z J3 = 55 msl) - pipes: Q (l/s), nodes: p/ρg (mwc)
Figure 3.8 shows the results of demand-driven simulations where the elevation of node J3 has
been increased first at 25 msl (left) and then at 55 msl (right). Consequently, only the
pressure in that node changed, while all other nodes and pipes kept the same pressures and
flows, which is the well-known anomaly of the demand-driven mode of calculation and
obviously inaccurate representation of the reality.