Page 349 - Water Loss Control

318 Cha pte r Ei g h tee n

cost can be a small reduction in revenue from the direct pressure user component. This
cost should only be used if there are no ideas for water conservation within the utility.
If the utility is trying to get their consumers to reduce consumption then pressure con-
trol will form a very efficient part of this program and reduction of consumption will
become a benefit not a cost.
The benefit is calculated from the reduction in leakage volume, reduction in main-
tenance costs, deferral of costs to build a new source of water if water is scarce, and
reduction of supply to nonpaying customers and increased storage management.
The cost to benefit is calculated as a function of the cost divided by the benefit and
is usually displayed as a ratio and also as the number of months required to payback
the initial investment. In most utility cases a good payback is somewhere inside of
24 months. In many cases advanced pressure control is giving paybacks of less than
12 months due to the huge impact on leakage and the simplicity of installation.

18.13 How Do ACVs Work?
There are various types of ACVs available on the market. Some use a diaphragm, some
a piston and some have a collapsible sleeve arrangement.
However most of the valve bodies for each
manufacturer are interchangeable for type of con-
trol. For example:
Most valve bodies can be A valve which was designed originally as a
piloted for a number of dif- pressure-reducing valve, can easily be changed into
ferent pressure management a level-control valve, a pump-control valve, a flow-
activities. control valve, or any number of other functions, by
changing the way that it is tubed and the type of
pilot control fitted.
Hydraulic ACVs basically work by using the upstream force of the pressure to
either open or close the valve by water entering or leaving the head of the valve, in
function to the pilot setting. This can be seen in the previous Fig. 18.8. A good rule
of thumb is to size the ACV to work within a range which is 20 to 80% open.
When considering a pressure management scheme for the first time it is a good idea
to talk to various valve manufacturers to ensure that their valves can be altered into
various functions, by altering the tubing and piloting. This will ensure the optimization
of your investment as systems change character.
For example a utility may install a 6-in valve for pressure control in a zone this year,
correctly sizing all parts, however in 2 years a construction company may construct a
large condominium, therefore changing the demand conditions. The 6-in valve may
now be undersized, however it is a simple job to replace this valve for a more represen-
tative one and reuse the 6-in valve in another location. Care should be taken when siz-
ing the bypass assembly to allow for flexibility.
Another important point is maintenance. For ease of maintenance most utili-
ties only use valves from one or maybe two different manufacturers. This saves on
having to stock the same size parts for various different makes of valve. Obvi-
ously price is a concern for most projects but local support and product and spares
stocking should also be taken into consideration. After all in any ongoing project
the initial investment is often only a small part of the overall investment on the
life of the project.