Transient Flow Analysis and Utilization in Urban Water Supply Systems- Juniper Publishers
Juniper Publishers- Journal of Civil Engineering
Abstract
This paper presents a brief review on the transient
flow analysis and utilization in urban water supply systems (UWSS). The
advancement and progress of transient flow theory and practice are
summarized and discussed with regard to the design and operation of
UWSS. The opinion and recommendation on the future work and
opportunities of transient flow analysis and utilization in UWSS are
also provided in the end of paper.
Keywords: Transient flow; Urban water supply system; Water pipeline; Defect detection
Introduction
The transient state of flows is variously termed
water hammer, fast transients, hydraulic transients, fluid transients,
pressure surges, or transient waves in the water hammer literature [1].
Transient waves are fast moving elastic shocks that travel at
relatively high velocities in pipeline systems (i.e., about 1000 m/s).
They are generally triggered by planned or accidental events in pipe
fluid systems that result in rapid changes in the pipe flow. Transient
events may be caused by operations of valves, starting and stopping of
pumps, variations in the supply or demand of the system fluid, and many
other situations. These sudden changes in system flow require the
imposition of large forces to accelerate or decelerate the fluid, and
consequently are capable of inducing severe or even catastrophic
pressures in the pipeline. For example, a water hammer accident caused
the hydro disaster in the Russia’s biggest hydroelectric plant in 2009 [2].
In engineering practice across a multitude of fluids
systems and applications, transient flows exert decisive influences on
practical aspects of engineering design and operation of pipeline
systems such as the structural and functional design of pipe strength
and material [3].
In recent years, transient waves, either passively received or actively
injected in urban water supply systems (UWSS), are widely utilized as
an efficient and economic tool for pipeline condition assessment such as
leakage and blockage detection [4]. The advancement and progress of these aspects of transient flows are briefly reviewed in the following sections respectively.
Transient Flow Analysis
In a practical pipeline, transient state of the flow
usually occurs from an original steady state and switches to another
steady state eventually due to the energy dissipation.
That is, the total energy of internal flows in pipes
is dissipated with time when the transient wave propagates back and
forth along the pipeline. For illustration, the pressure head signal at a
specific pipeline location in a typical water hammer event is
attenuated and reflected with time in the manner shown in Figure 1. In
addition, the shape of the pressure wave traces is also modified by the
damping factors (see enlarged picture from the circled part in Figure 1).
Predicting and evaluating pressure wave damping in determining the
maximum pressure envelop is important to transient system design of pipe
strength and operations of hydraulic devices in pipeline systems [1,5].
On the other hand, transient wave reflections may have great influence
on the flow profiles for water quality problems, such as reverse and
mixing flows [6].
Meanwhile, the decay of the transient intensity of pipe flows can
affect the discharge oscillation in pipes. Thus, it has impacts on the
optimal design of protection devices, such as air-chambers, surge tanks,
and air valves, which are used for protecting pipelines by adjusting
the flow storage during transient processes [7].
Many factors in pipeline systems can affect the
behaviors of transient wave damping and reflections, including unsteady
friction of wall shear, visco-elasticity of plastic pipes, uncertainty
of parameters, and wave-scattering of random in homogeneities. An
extensive literature review shows that the unsteady friction effect has
received the most attention in the past two decade, followed by the
visco-elastic effect, uncertainty effect, and wave- scattering effect.
However, the investigation in [8]
demonstrates that, in practical pipeline systems, an opposite trend
with respect to the order of importance of these four effects to
transient pressure wave damping. That is, among the four effects,
unsteady friction has the least contribution to the pressure wave
damping, while the other three can affect more significantly the
transient wave propagation in practical pipeline systems.
Figure 2
shows the ranking of importance of issues to be discussed in this work
on the basis of attention received from researchers and the order of
importance of these factors to practical systems as identified in
previous research. In fact, the transient wave damping and reflections
induced by these factors are especially critical to the utilization of
transient flows for defects detection in pipelines (e.g., leakage and
blockage), which is elaborated in the following section.
Transient Flow Utilization
While many practicing engineers think most often
about transients referred to their negative or damaging physical effects
on a pipe system, or deterioration of potable water quality, etc.,
there is a positive aspect to transients as an integrity management
tool. Transients have the ability to acquire and transmit a significant
range and variety of system information along the pipeline while
travelling through the system at high speeds.
This high speed transmission of information can be utilized in many
practical applications, such as pipeline leak detection and condition
monitoring. Leak detection continues to be more and more important to
the urban water-related systems. Specifically, as transient waves move
rapidly throughout a system, their waveforms are modified by their
propagation and reflection interactions with the pipe and its component
devices. Therefore, they can also be used as a potentially inexpensive
and diverse source of information in integrity management applications [4,8].
On this point, transient pressure monitoring and analysis appears to
hold considerable promise for estimating the state or condition of the
pipeline system as it changes over time.
Recently developed techniques for leakage detection
in water pipeline systems are utilizing the information associated with
the transient damping and reflections [4,9]. Utilization of transient data for leak detection could have great practical significance
Since pipe leakage is a common, costly and serious
water conservation and health issue worldwide. For example, estimates
made in 2002 in Hong Kong suggest that losses in the potable water pipe
system may exceed 30% of the total consumption, and a major contributor
to this water loss is leakage [10].
According to information from Water Supplies Department of Hong Kong, a
comprehensive pipe replacement scheme with price tag of more than US
$1.3 billion started in 2000. The scheme would replace around 3,000km of
pipeline within the Territory of Hong Kong mainly for reducing leakage
in the water piping system [10].
Since points of leakage also represent potential locations for
contaminant intrusion, identification and control of leak locations is
doubly important. Consequently, an improved understanding of transient
flows in pipes is important to advancing the practical utilization of
transients as a source of information and, at the same time, minimizing
their damaging impacts on the physical infrastructure.
Furthermore, the transient-based method has been developed and used for the detection of pipe blockages in UWSS [11]. Blockages in aging pipeline infrastructure can be caused by various reasons (see Figure 3),
including corrosion, failure of pipe lining and deposition of suspended
materials. Blockages in pipelines increase operational costs by
reducing the flow capacity as well as large energy dissipation
throughout the system [11].
Unlike leaks, the presence of blockages in a pipeline does not result
in clear external indicators and the problem often remains undetected
until the pipeline is close to fully constricted. Blockages are
classified based on their physical extent relative to the total length
of the system. Localized constrictions that can be considered as point
discontinuities are referred to as discrete blockages. Common examples
of this type are partially closed inline valves or orifice plates. In
comparison, blockages caused by pipe aging are more common and often
cover significant stretches of pipe relative to the total pipe length,
which are commonly termed extended blockages in the literature.
Finally but not least, transient-based method has
also been exploited for the identification and assessment of other
different types of pipe defects in UWSS. For example, transient
frequency response method was firstly developed and applied in [12]
for detecting ill-connected branches in water supply pipe systems.
Their results provided a promising alternative and tool for
characterizing and identifying this type of pipe defect, which could not
be well tackled by many traditional methods. Air-pocket is another
issue in water pipelines that cannot be well dealt with by current
methods. Based on previous studies, it is expected that the
transient-based method might be useful and helpful to solve this
problem, which is worthy of more investigations on this aspect in the
future work.
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