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216 Chapter 6 Water Distribution Systems: Components, Design, and Operation
not controlled by a service reservoir until the circuit laws discussed in connection with
the method of relaxation are satisfied throughout the system. These laws can be summa-
rized as follows:
1. At each junction ∑Q inflow ∑Q outflow
2. In each circuit ∑H 0
n
3. In each pipe H kQ or Q (H k) 1 n .
To program the operation, number each pipe and junction and identify pipe ends by
junction numbers. Then tabulate pipe resistances, junction pressures (including assumed
values where pressures are unknown), and net inflows at each junction (zero at all but en-
trance and exit points of the system), and feed the tabulated information into the computer.
The computer instructions are then as follows: Calculate by circuit law 3 the total flow into
the first junction for which the water table elevation is unknown; adjust the assumed value
until the total inflow and outflow are balanced in accordance with circuit law 1; proceed in
sequence to the remaining junctions; and readjust the first water table elevation. Repeat the
cycle of operations until all circuit laws are satisfied.
Camp and Hazen (1934) built the first electric analyzer designed specifically for the
hydraulic analysis of water distribution systems. Electric analyzers use nonlinear resistors,
called fluistors in the McIlroy analyzer, to simulate pipe resistances. For each branch of the
system, the pipe equation, H kQ 1.85 , for example, is replaced by an electrical equation,
V K I 1.85 , where V is the voltage drop in the branch, I is the current, and K is the non-
e
e
linear-resistor coefficient suited to pipe coefficient k for the selected voltage drop (head
loss) and amperage (water flow) scale ratios. If the current inputs and takeoffs are made
proportional to the water flowing into and out of the system, the head losses will be propor-
tional to the measured voltage drops. Some large, rapidly developing communities have
found it economical to acquire electric analyzers suited to their own systems.
Of course at present, several handy commercial software programs are available for
modeling and design of water systems (see Chapter 7). Software is available as a stand-
alone interface for Windows or integrated into GIS or CAD systems, for example:

1. Haestad Methods Solutions (Bentley Systems Inc.): Water GEM and WaterCAD
2. MWH Soft: InfoWater
3. Wallingford Software: InfoWorks WS (for water supply).

6.7 INDUSTRIAL WATER SYSTEMS
Large industrial establishments, with a heavy investment in plant, equipment, raw materi-
als, and finished products, concentrated in a small area, are generally equipped with high-
pressure fire supplies and distribution networks of their own. Because such supplies may
be drawn from sources of questionable quality, some regulatory agencies require rigid sep-
aration of all private fire supplies and public distribution systems. Others permit the use of
protected cross-connections and require their regular inspection for tightness. How the two
sources of supply can be divorced without denying the protective benefit and general con-
venience of a dual supply to industry is illustrated in Fig. 6.16. Ground-level storage and
pumping are less advantageous.
A widely approved arrangement of double check valves in vaults accessible for inspection
and test by the provision of valves, gauges, and bleeders is shown in Fig. 6.17. No outbreak of
waterborne disease has been traced to approved and properly supervised cross-connections of
this kind. Automatic chlorination of the auxiliary supply can introduce a further safeguard.