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Water Distribution Systems: Modeling and Computer Applications
Chapter 7
These results indicate that pipes P-2 and P-8, and the grouped pipes P-3 and P-7 have a roughness value that is about what
would be expected for the installed ductile-iron pipe. However, the results for pipe P-1 show that the roughness value is much lower.
This could indicate that a valve is partially closed, the pipe is blocked, or that the pipe diameter may be smaller than expected. In this
case, pipe P-1 should be investigated to determine the cause of this low roughness value. If there is a problem and that problem was
fixed, new field measurements should be taken.
These roughness values can be entered into the model and further simulations can be conducted. With enough field data, a model
that closely simulates the actual system can be created. Keep in mind that many times the person doing the modeling must decide
what values to put into the model. The software can only calculate values based on what is entered. The person doing the modeling
must judge how accurate the model is and whether the model can be used to make decisions.
7.8 PRACTICAL APPLICATIONS OF
COMPUTER-AIDED WATER SUPPLY Whenever a water flow in a pipe changes its direction,
SYSTEM ANALYSIS the direction of the slope of the HGL will also change
accordingly. A major fire, leak, or other unexpected high
A complete water supply system typically consists of raw water demand may draw the HGL down to a point that vac-
water reservoir, raw water intake, pump stations, valve sta- uum (i.e., negative pressure) is created at consumers’ service
tions, raw water transmission pipelines, water treatment connections. This negative pressure could potentially cause
plant, finished water storage reservoir, finished water trans- backflow from a non-potable contaminated source of water.
mission pipelines, finished water storage towers, water distri- The variables for a series of computer-aided water system
bution system, fire hydrants, meters, and other appurtenances analysis may include the number, location and magnitude of
(Figs. 1.1 and 4.4). The purpose of the water supply system is fire, leak or unexpected high water demand (such as emer-
to deliver adequate quantities of water at sufficient pressures gency water demand request from another water system),
at all times under continually changing conditions while at and so on.
the same time protecting water quality. A SCADA (supervisory control and data acquisition)
Section 6.9 discusses how a new water system should system is an industrial control system (ICS) commonly used
be designed and how an existing water system should be by all modern water systems and most of improved old water
operated and maintained. A properly designed water system systems. With a SCADA system, a water manager is able to
will be able to withstand the physical stresses imposed upon it observe and document the real-time, live information of water
and to deliver the water supply in sufficient quantity within a flow, direction, velocity, pressure, water quality data, major
proper pressure range. A well-operated and maintained water facility, elevation, and exact location. Since the sampling,
system will preserve its integrity and ensures high water monitoring, and remote sensing instruments are expensive
quality at all times. Hydraulic analysis of a complete water and limited, a SCADA system alone cannot provide the tech-
supply system using the Hardy Cross Method is very complex nical information of all pipes, predict the future changes, or
and extremely time consuming, due to the fact that there are determine the causes of past failures. It is possible to feed the
too many variables. Application of a computer-aided analysis real-time SCADA data into the computer-aided Hardy Cross
will allow an engineer to repeat same analytical procedure analytical system for generating an approximate, real-time
many times using various parameters for optimization of the hydraulic and water quality data of all pipes and facilities, if
water system within a short period of time. The following are there is sufficient number of SCADA data points available.
a few typical examples for application of computer analyses. This could be an interesting research or engineering project
The major hydraulic concerns of a water supply system to the students if a modern municipal water treatment plant
are water flow, flow direction, velocity, and available water can be available for collaboration.
pressure. Of special importance is the maintenance of a con- Another factor in the computer-aided water system anal-
tinuous positive water pressure, 20 psi or 140 kPa minimum, ysis is the roughness of all water pipes which resists water
in the water system under all conditions, so as to protect the flow and causes a drop in water pressure under dynamic
water system from the entrance of pathogenic or toxic sub- (flowing) conditions. Equations (5.32)–(5.37) show how the
stances. Excessive pressures (greater than 100 psi or 694 kPa, Hazen–Williams roughness coefficient C affects velocity (v),
2
or 6.94 kg/cm ) will cause pipe breaks in old water distribu- head loss (h ), and flow (Q). The smoother the inside of the
f
tion systems and will damage the customers’ facilities and pipe, the higher the C value, the higher the velocity and flow,
fixtures. Any water system modifications, such as pipe addi- and the lower the head loss. The pipe roughness increases
tion/deletion, pipe enlargement, pipe interconnection, water with pipe corrosion and deposition of suspended matters
storage tank relocation, new subdivision development, new along the pipeline over a long period of time. Some exam-
raw water source, new pump station with higher head, and ples in Chapter 5 illustrate how the Hazen–Williams rough-
fires in commercial district, will all affect the water flow, flow ness coefficient C of an existing pipe can be experimentally
direction, velocity, and water pressure. determined.