Page 343 - Water Loss Control

312 Cha pte r Ei g h tee n

Pressure
45

20
Reservoir Pressure controlled at peak flow
h
6 12 15 Time

100

Critical point
PRV
Flow Outlet pressure
125 45

25 20
h h
6 12 15 Time 6 12 15 Time

FIGURE 18.9 Effects of fi xed outlet control.

Fixed outlet control in areas other than previously quoted may be inefficient, as outlet
pressures have to be set high enough to meet minimum pressures during peak demand.
As system demands reduce usually at night the head losses in the system reduce and
system pressure returns toward the static pressure, which in many cases is far in excess of
that required to meet nighttime demand plus fire demand. See Fig. 18.9.

18.4 Leakage Control—Pressure Leakage Theories
As discussed in Chap. 10 it is now proven that the relationship between leakage and
pressure is not merely related to the square root of the pressures in question, but rather
an expanding power law. As well as PVC leaks, many other types of leaks, in particu-
larly joints, are subject to a change in area as pressure changes. This means that poten-
tial benefit in pressure reduction, on the volume of these leaks has a much greater
impact, as not only the velocity of leak flow changes, but also the leak area.

18.4.1 Traditional Calculations
Traditional calculations for reduction in leakage through reduction in pressure assumed
a fixed area leak. The calculation for this type of situation was
.
L ⎛ P ⎞ N1 = 05
1 varies with ⎜ 1 ⎟
L 0 ⎝ P ⎠
0
and still is as follows:
When the pressure is changed from P to P , the leakage rate changes from L to L .
0 1 0 1
Therefore: L = L (P /P ) 0.5
1 0 1 0