Environmental Engineering Reference
In-Depth Information
Chapter 5 come forward in the fact that the values of
I
n
(and
I
r
) are predominantly driven
by the heads, while those of
NBI
are better responsive to the nodal connectivity.
Consequently, the two (three) indices give the results that may suggest different economic
choices, although they will follow the similar general trends.
2.
The values of
I
n
(and
I
r
), look more consistent. They are mathematically 'neat' evaluating
networks as reliable, as long sufficient head is available in the network, which is
somewhat questionable. On the other hand, in case of supply under stress conditions, the
formulas used to calculate these indices, based on demand driven simulations, allow
negative values both in the numerator and/or denominator, leading to strange peaks or
sudden negative values, as shown in Figure 8.31. Such results may have occurred by
inappropriate combinations of diameter- and pump speed multipliers, but would be self-
corrected in the next increment.
3.
The NBI-values can reduce for the increased network volume, which may look weird but
is again a result of a strange combination of diameter- and pump speed multipliers: the
reducing pump speed causing more loss of head than it has been gained by the increase of
the pipe diameters. These are the situations mostly occurring under stress conditions, as it
can also be seen on Figure 8.31.
4.50
4.00
3.50
3.00
2.50
NBI
Ir
In
2.00
1.50
1.00
0.50
-
-
500
1,000
1,500
2,000
2,500
Network Volume m
3
Figure 8.31
Discrepancy in the values of
I
n
and
I
r
for supply under stress conditions
4.
Finally, the sudden drop of NBI-values, experienced during the analyses in several
situations with the balancing tanks, can also be attributed to the situation that is to be
avoided in reality. A substantial reduction of pump speed/capacity leading to supply
controlled mostly or exclusively by the balancing tank is a signal of serious stress; the
reduction in pressures around the pump may cause the loss of demand even without any
pipe failure. Neither of the two resilience indices,
I
n
and
I
r
, clearly alarm this kind of
situations, which is seen as serious deficiency, illustrated in Figure 8.32.
5.
The NBI-values will therefore describe the situations of supply under stress better than
the values of
I
n
and
I
r
, which is mostly the consequence of pressure driven demand nature
of
NBI
. This is then also what makes the shape of NBI curves shown in the diagrams
occasionally less 'neat' but more true representation of the network reliability than is the
case with
I
n
and
I
r
.
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