Drinking Water Distribution Systems Assessing and Reducing Risks (2006) / Chapter Skim
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5 Hydraulic Integrity
5 Hydraulic Integrity
Pages 192-220
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From page 192... ...
Variations in demand have an important influence on water distribution system operation and in the determination of water age which in turn influences water quality, as discussed later in the chapter. From an infrastructure perspective, a water distribution system is an elaborate conveyance structure in which pumps move water through the system, control valves allow water pressure and flow direction to be regulated, and reservoirs smooth out the effects of fluctuating demands (flow equalization)
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From page 193... ...
Many water quality parameters change with length of time in the distribution system, a factor directly related to the hydraulic design of the system. For example, chlorine residuals decrease with the increasing age of water and may be completely lost, and trihalomethanes concentrations may increase with time.
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From page 194... ...
FACTORS CAUSING LOSS OF HYDRAULIC INTEGRITY There are many different ways that a water distribution system can lose its hydraulic integrity, such that water quality becomes impaired. A loss of hydraulic integrity implies a loss of positive line pressures, flow reversals, rapid changes in velocity, a reduction in hydraulic capacity, a detrimental increase in water residence time, or a combination of these events.
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From page 195... ...
They can cause the collapse of thin-walled pipes or reinforced concrete sections, particularly if these sections were not designed to withstand such strains. In less drastic cases, strong pressure surges may cause cracks in internal lining, damage connections between pipe sections, and destroy or cause deformation to equipment such as pipeline valves, air valves, or other surge protection devices.
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From page 196... ...
The system is pumping drinking water to an elevated storage tank while serving the intermediate customers with adequate pressures. Due to an unexpected power failure, the pump quickly runs down (loses speed)
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From page 197... ...
Retention time or water age is strongly related to the characteristics of the system and its operation. For example pipe roughness, which affects water flow and residence time, may be modified by repair or rehabilitation.
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From page 198... ...
pump scheduling and planned maintenance, or unplanned effects, such as unexpected changes in demand, will all have an effect on water age. A particularly important issue that demonstrates the interaction of system operation and water quality is the ability or inability of utilities to ensure adequate mixing intensity and time in storage tanks to minimize short circuiting and to limit residence times to be within acceptable limits.
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From page 199... ...
In the event of a large intrusion of pathogens, the disinfectant residual normally sustained in drinking water distribution systems may be insufficient to neutralize contaminated water (see Chapter 6 discussion on Adequate Disinfectant Residual)
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From page 200... ...
Other deposits such as microbial slimes can also result in a significant decrease in the hydraulic capacity of water mains. The reduction in the hydraulic capacity can lead to a subsequent unwanted reduction in system pressure due to the higher head loss.
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From page 201... ...
within the water distribution system as a result of hydrant flushing and valve and pump operation can scour sediments, tubercles, and scales from the interior pipe walls and degrade water quality. For example, when the water velocity is increased or flow direction is reversed, sediment deposited on the pipe walls during periods of low flow may be re-suspended and scales may detach.
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From page 202... ...
The report uses the term "water age" as a surrogate for water quality. However, it should be noted that while water quality may depend on the age of the water, it may also depend on the specific residence time distribution at that point in the network or on one of its statistics (such as its variance)
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From page 203... ...
at a specific point in a water distribution system but cannot provide an overall understanding of the spatial and temporal changes, complex flow pathways, and interactions among the various water system characteristics. Thus, water distribution system network models are attractive supplements to monitoring for evaluating hydraulic and water quality changes throughout the distribution system.
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From page 204... ...
Temperature in storage tanks and reservoirs could also be monitored via SCADA to detect thermal stratification that results from poor mixing characteristics. Temperature differential between the inflow and the bulk water in the reservoir can result in density gradients inside the storage facility and cause stratification and poor hydraulic mixing and, thus, the greatest potential for water quality deterioration (Mahmood et al., 2005)
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From page 205... ...
. They can calculate the spatial and temporal variations of flow, pressure, velocity, reservoir level, water age, source contribution, disinfectant concentration, and other hydraulic and water quality parameters throughout the distribution system.
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From page 206... ...
To maintain the hydraulic integrity of water distribution systems and ensure the highest possible water quality, travel times in the system should be kept as short as possible and large fluctuations in the hydraulic regime and low flow and pressure conditions should be avoided. This can be accomplished by implementing best design, management, operational, and maintenance practices, as discussed below.
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From page 207... ...
The pressure differential range, which specifies the operating values for maximum and minimum pressure to be maintained, is based on local engineering standards and conditions. Many states have established requirements for the design, construction, operation, and maintenance of drinking water distribution systems that relate to hydraulic parameters.
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From page 208... ...
Figure 5-3 illustrates typical locations for the various surge protection devices in a water distribution system. Because no two distribution systems are hydraulically the same, there are no general rules or universally applicable guidelines for eliminating objectionable pressures in distribution systems.
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From page 209... ...
It was discussed briefly in Chapter 4 in association with the cleaning and disinfection of water mains following pipe installation, repair, and replacement. It is a topic of Chapter 6, which focuses on water quality integrity, because flushing is routine in areas with repeat customer complaints about color, taste, or odor; in dead ends mains; and in storage facilities.
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From page 210... ...
Operation and Design for Water Age Minimization As discussed in Chapter 1, a primary reason for water quality problems within distribution systems is the advanced water age necessitated by the provision of adequate standby fire flow and redundant capacity. This requires that utilities use standpipes, elevated tanks, and large storage reservoirs, as well as Hydrant flushing.
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From page 211... ...
. An important aspect of hydraulic integrity maintenance is to ensure sufficient mixing and to minimize water age in storage facilities -- issues which if not addressed can generate pockets of stagnant water with depleted disinfectant residual and associated water quality problems.
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From page 212... ...
RECOVERING HYDRAULIC INTEGRITY When a distribution system experiences high head losses, inadequate pressures or flows, high turbidity from scale loosening or resuspension of sediment, or low disinfectant residual and high bacterial counts from advanced water, there are several steps the utility should take. One of the first steps is to consider one or more of the standard techniques available to remove any loose sediment, biofilm, and tubercles that may be the cause of the problem.
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From page 213... ...
Unidirectional Flushing Unidirectional flushing involves the closure of valves and opening of hydrants to create a one-way flow in the water mains (see Figure 5-5)
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From page 214... ...
. There are often long-term water quality benefits because deposits and water of questionable quality are actually removed rather than being re-routed to other parts of the distribution system.
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From page 215... ...
This is due to the fact that cleaning potentially exposes unprotected metal pipe which would result in additional water quality problems. Scouring, swabbing, and abrasive pigging are progressively more aggressive cleanup techniques that involve more specialized equipment and specialized skills.
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From page 216... ...
In addition, slow moving water or changes in the flow regime (including flow reversals) and advanced water age can negatively impact finished water quality.
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From page 217... ...
Furthermore, limited understanding of the stochastic nature of water demand and water age makes it difficult to assess the water quality benefits of reduced residence time. Research is needed to investigate such questions, as well as how to achieve minimization of water residence time while maintaining other facets of hydraulic integrity (such as adequate pressure and reliability of supply)
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From page 218... ...
2004. Managing Distribution Retention Time to Improve Water Quality -- Phase I
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From page 219... ...
1998. Modeling water quality in drinking water distribution systems.
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From page 220... ...
2005. Distribution system water qual ity report: a guide to the assessment and management of drinking water quality in distribution systems.
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