Emitter Based Algorithm for Pressure-Driven Demand Calculations of Water Distribution Networks - Pattern Recognition for Reliability Assessment of Water Distribution Networks - page 37

A is the surface area of the orifice/tap opening, and C is the shape factor of the orifice, which
corresponds to the minor loss factor ξ .

Low heads or high losses calculated in the DD mode can result in negative pressures, as
shown in Figure 3.2. This is a clear indication of irregular supply conditions that makes
proper interpretation of the results difficult, because the pipe flows are based on the nodal
demands that are false.

Figure 3.2 Negative pressures as a result of DD calculation (Trifunović, 2006)

Figure 3.3 Pressures as the result of PDD calculation (Trifunović, 2006)

In the PDD mode, the nodal discharges will be reduced, causing a slower drop of the
reservoir levels, as Figure 3.3 shows. This is the result of reduced pipe flows causing smaller
friction losses.

Tanyimboh et al. (2001) describe the PDD relationship as:

H = H

min

+ K Q

n

3.2

i i i i

where H i represents the actual head at demand node i , H i min is the minimum head below
which the service becomes terminated, K i is the resistance coefficient for node i , Q i is the
nodal discharge, and n is the exponent that theoretically (and usually in practice) takes value
of 2.0 (Gupta and Bhave, 1996). Equation 3.2 actually follows the similar format as the one
of Equation 3.1. To determine the unknown value of Q i for any given nodal head, Equation
3.2 should be rearranged as:

1 / n



min



H − H

Q =



i i



3.3

i

K

 

i