Environmental Engineering Reference
In-Depth Information
The following is the list of specific conclusions:
-
There has been little difference in the results for normal range of elevations, i.e. as long
the negative pressures are not occurring in the 'pdd1' variant. Consequently, the results in
Table 3.1 showing the situation of flat terrain are almost identical. The results in Table
3.2 showing the situation of moderately elevated node
J3
, too.
-
The negative pressure occurs in the case of more radical elevation of
J3
, which is shown
in Table 3.3. The 'pdd2' reacts to it by disconnecting the pipes connected to that node. As
a result, the EPANET calculation produces the result zero, both for the nodal demand and
the pressure in
J3
. The 'pdd3' variant will have the same nodal demands as the 'pdd2'
except that the pressure in
J3
is kept negative. Furthermore, the results of 'pdd4' are the
same as those of 'pdd3' for the negative pressure is above the lower limit of -10 mwc.
Finally, the results of 'pdd5' will comply with those of 'pdd3' and 'pdd4'. The implications
on the demand reduction are that the pipe flow and friction loss distribution are the same
in all cases.
-
The difference in the results starts in the case of more radical change of the elevation of
node
J5
, which is shown in Table 3.4. Due to very low negative pressure in
J5
, the results
of 'pdd4' are here identical to those of 'pdd2', showing more radical loss of demand than
in the case of 'pdd3' and 'pdd5'. The basic option of 'pdd1' also complies with 'pdd3' and
'pdd5' because the negative pressure has been registered in non-demand node.
Accordingly, the pipe flow and friction loss distribution will be the same for 'pdd1', 'pdd3'
and 'pdd5', while the results for 'pdd2' and 'pdd4' will be different (but mutually, the
same). In the first group, the total demand drops from 90 to 83.76 l/s, while in the second
group, the total demand shall be 73.92 l/s.
-
The results in Table 3.5 also show the compliance between 'pdd2' and 'pdd4', and 'pdd3'
and 'pdd5' but, unlike is the case in Table 3.4, the pipe flow and friction loss patterns will
be the same in all four cases because the total flow has been uniformly reduced to 60 l/s,
as a result of extreme elevation of node
J3
.
-
In case of a 'high hill' along the diagonal
J4-J5-J2
, the flow pattern shown in Table 3.6
will be uninterrupted in 'pdd3' and 'pdd5'. The demands in
J2
and
J4
will be lost due to
high elevation but the one in
J3
will be delivered 100% as the sufficient pressure (above
20 mwc) has been generated in that node resulting from the demand reduction in the other
two nodes. This is not the picture as created by 'pdd2' and 'pdd4' where the demand in
J3
is also zero, because of the disconnected pipes. Also, the node pressures calculated in
both cases do not look quite realistic.
-
For the elevation of 55 msl in
J3
, all four variants in Table 3.7 show logical no-flow
condition and the pressures based on the source head, except for 'pdd2'.
-
The results in Tables 3.8 to 3.10 show that the lowering of the lower pressure limit
actually makes the results of 'ppd4' equal to 'pdd3' instead of 'pdd2'.
-
Finally, a general observation is that in all the calculations the 'pdd3' and 'pdd5' results
complied suggesting the same numerical approach used in both algorithms.
3.6
CALCULATION OF AVAILABLE DEMAND
In the next round of testing and comparisons, the same network has been calculated for the
same set of node elevations by failing each pipe in order to calculate the total available
demand. These results are presented in Tables 3.11 to 3.17.
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