Page 357 - Water Loss Control

326 Cha pte r Ei g h tee n

in conjunction with pressure-reducing valves will be used. An alternative if possible is
to install a small tank, which will pick up lost pressure only during the peak hours
when the system is most stressed and pressures uncontrollable.

18.18 Valve Selection and Sizing 2
Valve selection and sizing is often done using average values of flow and pressure and
as valves are fairly forgiving, in most cases the valves work. This however is not recom-
mended practice!
In the case of pressure reduction for leakage control, it is strongly recommended
that flows and pressures be measured in the field. As previously discussed the impact
of reduced pressure on leakage is often critical to the operation of the valve and without
accurate data, valves may be installed incorrectly
and operate erratically.
Field measurements are also beneficial when
Select the flow range that
seasonal corrections need to be made and ensure
meets your system require-
that the valves can cope with the top end flows,
ments and correct valve(s)
without creating too much head loss. This is also
size(s). Note: Maximum flow
true when calculating the effects of emergency
rates in this table allow for water use such as fire fighting.
continuous flows at velocity of All valve manufacturers provide valve-sizing
20 to 22 feet per second charts, an example is shown in the chart in
Table 18.2.

18.18.1 ACV Types—Diaphragm, Piston, Rolling Diaphragm, Sleeve
All pilot operated hydraulic ACVs operate use similar principles, however the mode of
control changes significantly. Each manufacturer will quote his/her type of benefit and
will attempt to justify why his/her valve is better. Full bore valves such as the rolling
diaphragm and sleeve will quote low head losses at high flows, while globe style dia-
phragm manufacturers will quote stable modulation and control. At the end of the day
it is important for the utility engineer to understand what it is that he or she wishes to
achieve and then to select the best valve for the job.
As well as technical benefits the engineer should also consider two other very
important points, local support and ongoing maintenance costs. One of the biggest
problems facing a utility when they have many valves installed is the cost and avail-
ability of quick and fast maintenance. A utility should try to avoid installing many dif-
ferent make of valve as the cost of stocking parts increases significantly and the
availability of local support drops drastically.

18.18.2 Valve Sizing and Limits—Qpmax and Qpmin, Cavitation, Head Loss
In many cases control limits are set as a function of the maximum pressure controllable
at the valve, while providing a constant minimum pressure at the critical node(s). If a
substantial amount of pressure is to be controlled, the manufacturer’s cavitation chart
should be consulted to ensure that the valve is operating within its limits. See example
in Fig. 18.22.