State Of The Art

University Of Maryland Baltimore County
Faculty Of Engineering Of The University Of Porto
A Sensor Network For An Early And Efficient
Leak Detection In Long Pipelines
Alexandre Santos
PROVISIONAL VERSION
Master in Electrical and Computer Engineering
Automation Major
Supervisor: Mohamed Younis (PhD)
Co-Supervisor: Paulo Portugal (PhD)
January 2011
i
© Alexandre Santos, 2011
Abstract
Our society relies on an extensive pipeline network to transfer and deliver water, gas,
oil, etc. In many cases the pipeline extends over hundreds of miles and run through
inhospitable environment. The pipes are often subject to erosion over time due to the
surrounding environment. In addition, pipes carrying valuable commodity may be subject to
theft, sabotage, etc. Leakage not only would waste resources, but also can be harm and
hazardous. Therefore detecting leakage and containing its negative effect is very important.
Current pipeline monitoring systems are inefficient and costly. They lack responsiveness
and often report the problem after significant fluid is spilled. Furthermore, current systems
involve mobile equipment and significant manpower. Sensor networks can be invaluable in
providing a comprehensive, economic and effective solution. This project opts to investigate
the design of a wireless sensor networks that continuously monitor a pipeline and provide an
early warning when leakage is starting.
iii
Resumo
A nossa sociedade está sustentada sobre uma extensa rede de “pipelines”, quer seja para
a transporte de água, gás ou petróleo.
Em muitos casos, estes “pipelines” estendem-se ao longo de vários quilómetros, passando
por ambientes inóspitos e estando sujeitos a processos de desgaste de forma prolongada.
Além disso, estes “pipelines” transportam muitas vezes matérias primas importantes,
podendo ser alvo de ataques como pilhagens ou sabotagens. As fugas não só desperdiçam
recursos como também podem trazer consequências graves. Nesse sentido, é importante
detectar e controlar os efeitos negativos que uma fuga pode acarretar.
Os sistemas de monitorização existentes actualmente são ineficientes e dispendiosos.
Apresentam falhas nos tempos de resposta aos acontecimentos provocando com frequência
fugas de fluidos em quantidades consideráveis. Além disso, os sistemas actualmente
existentes envolvem equipamentos portáteis o que implica a necessidade de mão-de-obra
para monitorizar os “pipelines”. Por estas razões, a implementação de uma rede de sensores
para a detecção de fugas trás elevados benefícios, fornecendo uma solução abrangente,
eficiente e de baixo custo.
Este projecto pretende assim investigar o desenvolvimento de uma rede de sensores semfio que possibilite uma monitorização de um “pipeline”, fornecendo informações sobre uma
fuga em tempo-real.
v
Contents
Chapter 1 ....................................................................................... 12 Introduction ................................................................................................... 12 1.1 Motivation ........................................................................................... 12 1.2 Objectives ........................................................................................... 13 1.3 Work Plan ............................................................................................ 14 Chapter 2 ....................................................................................... 15 Flow Measurement ........................................................................................... 15 2.1 Background .......................................................................................... 15 2.2 Basic Principles ..................................................................................... 16 2.3 Flow Meters ......................................................................................... 22 2.4 Detection Of Leaks ................................................................................. 29 Summary ................................................................................................... 30 Chapter 3 ....................................................................................... 31 Sensor Networks .............................................................................................. 31 3.1 Background .......................................................................................... 31 3.2 Application Areas For WSN ........................................................................ 32 3.3 Challenges & Requirements For WSNs ........................................................... 36 3.4. Energy Scavenging .................................................................................. 37 Summary ................................................................................................... 39 vii
List Of Figures
Chapter 1
Figure 1.1 - Property Damage 1990 - 2009 [2] ......................................................... 12 Figure 1.2 - Thesis work Plan ............................................................................. 14 Chapter 2
Figure 2.1 - Block diagram of a flow measurement system [7]. .................................... 16 Figure 2.2 - Laminar Flow versus Turbulent Flow [10] ............................................... 17 Figure 2.3 - Flow profile of a fluid [12] ................................................................. 18 Figure 2.4 - Hydraulic condition for pipe flow [9] .................................................... 19 Figure 2.5 - Electromagnetic flow meter [14] ......................................................... 23 Figure 2.6 - Ultrasonic flow meter - Doppler [9] ...................................................... 24 Figure 2.7 - Ultrasonic flow meter - Transit-time [9] ................................................ 24 Figure 2.8 - Working principle of transit-time flow meter [14] ..................................... 25 Figure 2.9 – Cross-correlation flow meter [9] .......................................................... 26 Chapter 3
Figure 3.1- Typical architecture of sensor network and sensor node. Based on [18] ............ 32 Figure 3.2 - A sensor network for military surveillance application. From [22]. ................. 33 Figure 3.3 - Enemy target localization and monitoring using WSNs. From [20]. ................. 34 Figure 3.4 - Forest-fire monitoring application. From [20]. ......................................... 35 ix
List Of Tables
Chapter 2
Table 2.1 - Comparison of flow meters technologies................................................. 27 Table 2.2 - Types of flow meters and its characteristics ............................................ 27 Chapter 3
Table 3.1 - Comparison of various potential power sources for WSNs. ............................ 38 Abbreviations and Symbols
Abbreviation list (alphabetical order)
EMF
Electromotive Force
LDS
Leak Detection Systems
MPPT
Maximum Power Point Tracker
PHMSA
U.S. Department of Transportation Pipeline and Hazardous Materials Safety
Administration
WSN
Wireless Sensor Network
WSNs
Wireless Sensor Networks
Symbols list
A
Area
Q
Volumetric Flow Rate
V
Volume
!
Velocity
!
Mass Density
xi
Chapter 1
Introduction
1.1
Motivation
Modern economy relies on an extensive network of pipelines for transferring billions of
dollars of crude oil or natural gas. The need to transport these commodities throughout the
world has led to sudden establishment of new pipelines that extends over hundreds of miles
and run through inhospitable environments.
Among the main issues associated with the transport of commodities are leakage
problems. According to the US Department of Transportation Pipeline and Hazardous
Materials Safety Administration (PHMSA), 25% of all pipeline failure incidents reported in the
US from 2002 to 2003 were caused by corrosion (leakage main causes)[1].
In addition, PHMSA’s reports reveal that pipeline incidents cause a significant damage to
property and product losses (Figure 1.1) [2].
Figure 1.1 - Property Damage 1990 - 2009 [2]
12
Moreover, leakage not only damage companies property and profits, it also damages its
reputation and most important, people’s lives and the world’s environment. This issue
becomes even more conspicuous in the oil industry case, where leakage can lead to
catastrophic disasters.
In the latest oil spill accident, in Gulf Mexico, “…scientists say, the impacts of oil spill are
likely to go far beyond the oiled birds and dead sea turtles, spoiled beaches and wetlands
that we think of when we think ‘oil spill.’ ”
In Newsweek [3]
For this reason, the development of leak detection systems (LDS) is critical in pipeline
networks.
Despite of its importance, current pipeline monitoring systems are inefficient, costly and
they lack responsiveness [4]. Hence, it is important to develop a system capable of detecting
leaks quickly, efficiently and with low operational costs. It is within this context that wireless
sensor networks (WSNs) emerge. The advantage of using this type of networks lies in its
features that provide. This comprehends low deployment cost, low energy consumption and
high communication reliability.
In this project, the literature review on LDS was done having in mind the design of a noninvasive monitoring system that continuously supervises a pipeline and provides an early
warning when leakage starts to occur.
As regarding to WSN, its literature review was done concerning some technical issues that
affect the network performance as well as the pipeline system requirements.
1.2
Objectives
This thesis aims to develop a pipeline monitoring system to detect leaks in real time. The
research project consists in using a large number of ultrasonic flow meters connected
wirelessly through an infrastructure based on like a wireless sensor network.
The assignment seeks to use these two technologies due to the fact of ultrasonic flow
meters being a non-invasive technique under employed in pipelines flow measurement. In
addition to this, ultrasonic flow meters installation tolerates the pipeline’s operation without
requiring major interventions.
Regarding the choice of wireless sensor network its deployment aims to cover an
application area that was been slightly covered yet.
13
14
1.3
Work Plan
The thesis will be developed over a period of approximately four months. During this
period, the performed tasks are intended to meet the objectives defined above. The work
plan undertaken during the research project is presented in Error! Reference source not
found..
Figure 1.2 - Thesis work Plan
During the course of the dissertation thesis it will be possible to check in [5] a full
description of the work carried out in each week as well some further information related
with the project.
14
Chapter 2
Flow Measurement
This chapter presents a literature review of the flow measurement techniques, giving
particular attention to use the flow meters. The chapter has been organized in four different
sections:
•
The first section gives an overview of the flow measurement systems (FMS), its
importance and components.
•
The subsequent section, highlights some basic principles involved in the process
of flow measurement.
•
The working principle of some types of non-invasive electronic flow meters,
within the context this thesis is covered in section 2.3.
•
Finally, the end of the chapter, discusses the correlation between flow
measurement techniques and leakage detection.
2.1
Background
As its name suggests, flow measurement is a technique used in fluids measurement for
both gases and liquids. The importance of quantifying this phenomenon is a consequence of
the need to transport the fluid from one point to another.
The application areas of flow measurement are wide and crucial. Either for charging some
supplied material or controlling industrial process, the task has to be performed accurately as
its value can affect many other process’ variables such as pressure or temperature. For
instance, in the operation and control of pipelines - LDS, flow measurement plays the most
important role [6, 7].
As stated previously, flow measurement is connected with the need to convey a fluid. This
transference occurs in a physical medium, which is frequently a pipe or pipeline. In order to
achieve the requirements of a flow measurement systems it is necessary to equip the physical
medium with sensors that have the ability to measure flow – flow meters. Other flow
15
16
measurement systems require additional devices to measure relevant data, such as pressure
sensors, temperature gauges or other pertinent instrumentation devices.
In addition to this, it is necessary to provide the system with a signal conditioner
component that correlates the measured value with a physical value, and data acquisition
module to empower the control and monitoring of the system.
Despite not being mandatory, the incorporation of a communication module to gather
data and remotely control the process can be considered.
The composition of a flow measurement system (FMS) is illustrated in Figure 2.1
Figure 2.1 - Block diagram of a flow measurement system [7].
The architecture and technology employed in each flow measurement system (FMS) can be
different accordingly to the requirements. Some of these issues are going to be examined into
further detail in the next sections.
2.2
Basic Principles
Before discussing the topic of flow meters in detail it is necessary to start with the basics
of flow measurement theory. These concepts constitute the basis to understand the
instrumentation systems based on flow.
17
2.2.1
Flow Definition
The volumetric flow rate in fluid dynamics, commonly known as flow rate, is the volume
of a fluid that passes through a given surface per unit time, i.e. the velocity of a certain fluid
times a certain surface. In equation 2.1 it is seen that given an area A at a designated point
and a fluid flowing through it with an average velocity V, the flow rate is [8]:
! = ! ∙ !
(2.1)
!"#$ !"#$ = !"#$%&'( × !"#$ = 2.2.2
!
!3
∙ ! ! =
!
! (2.2)
Reynolds Number
The properties of a fluid, such as velocity, density or viscosity, affect the behaviour of the
fluid itself. Accordingly to these physical properties, the flow pattern of a fluid can be
classified in laminar or turbulent.
The flow state of a fluid is considered laminar or streamlined when the paths of the fluid
particles are parallel to each other and to the tube walls. As the fluid velocity increases it
reaches a critical value and the arrangement of the fluid particles becomes chaotic. At this
point the fluid particles are no longer parallel to the tube walls and a transverse velocity is
evident This form of flow pattern is called turbulent [9].
Figure 2.2 illustrates the arrangement of the particles in laminar and turbulent flows,
respectively.
Figure 2.2 - Laminar Flow versus Turbulent Flow [10]
In order to be feasible to study the flow of fluid along a channel, especially a closed one
like a pipeline, it is helpful to use a criterion that allows comparing results of observations
made under different conditions. Osborne Reynolds carried out a work on this subject and
formulated his data into a dimensionless number – Reynolds Number [9, 11].
!" =
! ∙ ! ∙ !
!
(2.3)
18
Where:
Re is the Reynolds number;
D = the size of the channel (in circular channels like a pipe D is the diameter of the pipe);
! = velocity of the fluid;
ρ = density of the fluid;
µ = viscosity of the fluid;
Based on Reynolds number, fluids can be classified in laminar flow, for values lower than
2000, and turbulent flow, for Reynolds number above 4000. For values between 2000 and
4000 the flow is said to be in the transition region and classed as transition flow [8, 9, 11].
Another characteristic that can be inferred from Reynolds number is the distribution of the
velocity across the diameter of a pipe. This characteristic, designated by flow profile can be
different in agreement with the Reynolds number. For the laminar flow the profile is
parabolic. This means that the velocity at the center is greater than the mean velocity
(approximately twice the mean velocity). As for the turbulent flow, the profile is fairly [8].
The difference between these two flow profiles is evidenced in Figure 2.3.
Figure 2.3 - Flow profile of a fluid [12]
19
2.2.3
Energy Of A Fluid
The understanding of the law of the conservation of energy and types of energy associated
with the flow of a fluid are essential for the understanding of the Bernoulli’s principle
(Section 2.2.4) that will be covered in the next subsection.
The types of energy associated with the movement of a fluid comprise [9]:
•
Potential Energy – A fluid has potential energy as a result of its position with
respect to some fixed level.
•
Kinetic Energy – The fluid will have kinetic energy as a consequence of its velocity.
This form of energy is also known as energy of motion.
•
Pressure Energy – A fluid has pressure energy by virtue of its pressure.
•
Internal Energy – A fluid has internal energy due to its temperature and chemical
makeup.
2.2.4
Bernoulli’s Principle
Bernoulli's Principle, published in 1738 by Daniel Bernoulli, takes a considerable
importance when we are talking about fluids flow. As “all fluid flow formulas in a closed
pipe are based on Bernoulli’s Theorem” [9], and as this thesis concerns the leakage detection
of a fluid in a pipeline, it is important to study this topic meticulously.
Bernoulli’s theorem states that for an inviscid fluid within a closed pipeline the sum of all
the fluids’ types of energy is constant at any point along the pipeline – principle of the
conservation of energy [13].
This physical phenomenon can best be described by an example presented in [9].
Figure 2.4 - Hydraulic condition for pipe flow [9]
Considering 1 kg of fluid entering the pipe at section 1. The fluid’s energy at each section
of the pipe is the result of the sum of each type of energy (Described in Energy Of Fluid).
Therefore, the energy at section 1 and 2 is:
20
E! = Potencial Energy + Kinetic Energy + Pressure Energy + Internal Energy
!
= Z! ∙ g + V!! + p! ∙ v! + I!
(2.4)
!
E! = Potencial Energy + Kinetic Energy + Pressure Energy + Internal Energy
!
= Z! ∙ g + V!! + p! ∙ v! + I!
(2.5)
!
Recalling to the principle of conservation of energy, which states that energy cannot be
created nor destroyed, and since the energy cannot leave the pipe:
Energy at Section 1 (E! ) = Energy at Section 2 (E! )
!
!
!
!
Z! ∙ g + V!! + p! ∙ v! + I! = Z! ∙ g + V!! + p! ∙ v! + I! (2.6)
If the temperature of the fluid remains the same, the internal energy remains the same
(I! = I! ) which means that equation 2.6 reduces to
!
!
!
!
Z! ∙ g + V!! + p! ∙ v! = Z! ∙ g + V!! + p! ∙ v!
(2.7)
Considering the particular case of liquid fluids. Since they are considered to be
incompressible, its density does not change significantly and therefore its volume will remain
constant and as consequence equation 2.7 can be rewritten as:
!!
!!
!
!
Z! + !"! + !∙!! = Z! + !"! + !∙!!
(2.8)
Back to Figure 2.4 it is possible to detect a differential height between the two sections.
This difference is given by:
!
!
!
!
ℎ = !! + !∙!
− !! + !∙!
Replacing in equation 2.8 presented above:
!!
!!
!
!
ℎ = !!
− !!
(2.10)
(2.9)
21
Therefore,
!!! − !!! = 2!ℎ
(2.11)
Now the volume of liquid flowing along the channel per second will be given by Q m3
where
! = !! ∙ !! = !! ∙ !!
!! =
!! ∙!!
!!
(2.12)
(2.13)
Substituting in equation 2.11:
!!
!!! 1 − !!! = 2!ℎ
(2.14)
!
and dividing by
!!
1 − !!! and taking the square root of both sides
!
!! =
the factor
!
!
!!!
!!
!! !!
!!
(2.15)
is called velocity of approach factor and it is often represented by E.
!
!! !!
!!
Recalling to equation 2.12 the volume of liquid flowing in the pipe is:
! = !! ∙ !! = !! ∙ ! 2!ℎ
(2.16)
Note that the equations presented above only apply to laminar flow and not to turbulent.
To determine the actual flow, it is necessary to take into account other parameters. Also
note, that the effects of viscosity have been ignored in the above equation. In order to
determine the actual flow an additional factor would have to be considered - Discharge
coefficient. To contemplate these and other factors should be consulted the BS 1043 Part 1
1964 [9].
22
2.3
Flow Meters
After reviewing the basic principles involved in the process of flow measurement we can
proceed to the literature review on flow meters.
In this section will only convers the study of non-invasive electronic flow meters, as
previously highlighted. This section focuses on the analysis of three different kinds of flow
meters: electromagnetic, ultrasonic and cross-correlation. Further reference is also made
at the end of the section to other types of flow meters providing a brief summary of the
working principle, major application, advantages, disadvantages and variants of different
flow meters.
2.3.1
Electromagnetic Flow Meter
The working principle of this type of flow meters, also called magmeters, is based on
Faraday’s law of electromagnetic induction. According to Faraday’s law “if an electric
conductor moves in a magnetic field, an electromotive force (EMF) is induced whose
amplitude is dependent on the force of the magnetic field, the velocity of the movement,
and the length of the conductor.” See equation 2.17 [9]
! ∝!∙!∙!
(2.17)
Where:
E is the electromotive force (EMF);
B = magnetic field density;
! = rate at which the conductor is cutting the magnetic field;
l = length of the conductor;
In this type of meter, the fluid flowing through the magnetic field is used as conductor
and therefore has to be a conductive fluid. Unfortunately, this excludes most petroleumbased flows that are not conductive fluids [14].
To sum up, the working principle of electromagnetic flow meters, which is evidenced in
Figure 2.5, stated in [9] refers to BS 5792 1980 – The specifications for electromagnetic flow
meters.
“If the magnetic field is perpendicular to an electrically insulating tube through which a
conductive liquid is flowing, a maximum potential difference may be measured between two
electrodes positioned on the wall of the tube such that the diameter joining the electrodes
is orthogonal to the magnetic field.” [9]
23
Figure 2.5 - Electromagnetic flow meter [14]
2.3.2
Ultrasonic Flow Meter
In the literature were identified two possible techniques for the use of ultrasonic flow
meters: Doppler and Transit-time. Below is presented these two different techniques for
flow measurement.
Doppler Flow Meter
The origin of the name of this technique comes from the working principle in which this
meter is based – Doppler effect. According to it, “the frequency of sound changes if its
source or reflector moves relative to the listener or monitor” [9]
The Doppler flow meter use a single ultrasonic transducer, which enclosures both a
transmitter and a receiver, installed on the monitoring pipe wall. The flow meter sends an
ultrasonic beam of frequency !! into the fluid at an angle ! as it is illustrated in Figure 2.6.
The ultrasonic pulses send by the transmitter will reflect on some of the entrained particles
or bubbles of the fluid back to the ultrasonic receiver with a frequency !! (See Figure 2.6) [9,
14].
The equations correlated to the fluid movement are shown below [9]:
!
!! = !! ± 2! ∙ !"#$ ∙ !1
(2.18)
The equation 2.18 can be rewritten as:
!(! −! )
1
! = 2∙! 2∙!"#$
1
(2.19)
24
Where:
! is the velocity of the fluid;
!! = frequency of the ultrasonic beam sent – transmitter;
!! = frequency of the ultrasonic beam reflected – receiver;
! = angle of attack between the transducer and the fluid;
C = velocity of the sound in the fluid;
Figure 2.6 - Ultrasonic flow meter - Doppler [9]
Transit-Time Flow Meter
The difference between this ultrasonic technique and the one described previously
(Doppler) is that this technique is based on the transmission of an ultrasonic beam through
the flow stream. Contrarily to Doppler technique, this method does not depend neither on
fluid’s discontinuities nor entrained particles [9].
The operation principle of this method is based on the transmission of ultrasound signals
between two transducers separated by the fluid. In order to measure the fluid velocity, the
transducers placed in the pipe wall “transmit ultrasonic pulses with the flow and against the
flow to a corresponding receiver”. This means that each ultrasonic sensor works as both
transmitter and receiver like is evinced in Figure 2.7 [14].
Figure 2.7 - Ultrasonic flow meter - Transit-time [9]
25
The equations given below concern the operating principle of transit-time flow meter
taken from the example in [14] (See Figure 2.8)
Downstream equation
L
t !" = (C+v∙cosθ)
Upstream equation
t !" =
L
(C-­‐v∙cosθ)
(2.20)
(2.21)
Solving equation 2.20 and 2.21 results in equations 2.22 and 2.23:
L
t -­‐t
v = 2∙cosθ ∙ tdu ∙t ud (2.22)
du ud
L t +t
C = 2 ∙ tdu ∙t ud
du ud
(2.23)
Where:
v is the mean velocity of the fluid;
C is the velocity of the sound in the fluid;
t !" = transit time between transducer u to d;
t !" = transit time between transducer d to u;
L
= length between transducers;
θ
= angle of attack between the transducer and the fluid;
Figure 2.8 - Working principle of transit-time flow meter [14]
26
2.3.3
Cross-Correlation Flow Meter
Almost every fluid flowing in a pipe or tube presents random variations that can occur for
instance in fluid’s density or temperature. Appropriate sensors installed on some specific
locations can detect these variations. By placing transducers separately by a distance L, as is
shown in Figure 2.9, the upstream transducer will notice the variation (e.g. pressure or
temperature) t seconds before the downstream transducer. The flow velocity is calculated
dividing the distance L (distance between transducers) by transit time t. “In practice the
random fluctuations will not be stable and are compared in a cross-correlator which has a
peak response at transit time T, and correlation velocity V = L/T, meters per second.” [9]
!=
!
!
(2.24)
Where:
! is the flow velocity;
L = distance between transducer 1 and transducer 2;
t = time difference between fluctuation detection in transducer 1 and transducer 2;
Despite being possible in principle to measure almost every fluid, a narrow range of flow
measurement systems use this type of flow meter due to the slow response time of those
systems [9].
Figure 2.9 – Cross-correlation flow meter [9]
2.3.4
Comparison Of Flow Meters
The table provided below (Table 2.1) was taken from [15] and summarizes the main
characteristics of the various flow meters technologies reviewed above.
27
Table 2.1 - Comparison of flow meters technologies. Taken from [15].
Maximum
Temperature
Pressure
range
(psig)
(ºF)
0.15 to 60
5000
-40 to 350
Short
L, G, SL
0.5 to 200
6000
-40 to 250
Long
Down to zero
flow
L, G
1 to 540
6000
-40 to 650
Long
10:1
L, G, SL
1 to 60
Piping
limits
To 600
Long
Flow meter
Accuracy
Technology
(+/-)
Electromagnetic
0.2 to 2% R
10:1
L
1 to 30% R
50:1
0.5 to 5% R
0.5% R
Ultrasonic
Turndown
Fluids
Pipe Sizes
(in)
Pipe
run
Doppler
Ultrasonic
Transit Time
Correlation
Where:
R= Rate
G= Gas
L= Liquid
SL= Slurry
Table 2.2 presents a brief summary of the working principle, major application,
advantages, disadvantages and variants of different types of flow meters
Table 2.2 - Types of flow meters and its characteristics. Taken from [7].
Flow Meter
Type
Differential
Pressure
Turbine
Primary
Working
Principle
Pressure drop
caused by
pipe’s
restriction
Rotating
device
Quantity
Measured
Volume
(versatile –
almost all
gases and
low viscosity
liquids)
Volume
(lubricating
fluids)
Advantages
Disadvantages
Meter Name
Low-cost, versatile,
robustness, variety
of versions,
Square root
dependence, affected
by pressure and density
changes, high pressure
drop (exception for
nozzles)
Orifice,
nozzle
Venture,
elbow, vcone, Pitot
tube,
Annubar
No supply power
requirements,
extreme
temperatures and
pressures, certified
for gases
Only for low viscosity,
moving parts, sensitive
to contamination and
vibration
Single rotor,
dual rotor,
paddle
wheel,
propeller,
tangential
28
Flow Meter
Type
Variable Area
Magnetic
Oscillatory
Target
Positive
Displacement
Ultrasonic
Primary
Working
Principle
Quantity
Measured
Dynamic
balance
(impulsion,
weight and
dragging)
Volume
(liquid and
gas
applications
with enough
density)
Advantages
Disadvantages
Meter Name
Low-cost, no supply
power requirements,
simplicity, versatile
Vertical installation,
constant pressure loss
(inaccuracy low fluid’s
flow), affected by
density and
temperature changes
Rotametersdifferent
variants of
float designs
and sensing
systems
Electromagnet
ic induction
Volume
(almost all
measuremen
ts as long as
the
conductivity
is above a
minimal
value)
No moving parts, non
invasive, no pressure
loss, no dependence
of flow regime, not
affected by
temperature,
density, conductivity
and concentration
changes
Only liquids
applications, lower
conductivity limit (0.05
µS/cm)
Magnetic DCno
electrodes
two-wire
partially full
Coandă effect
Volume
(application
to a large
variety of
liquids as
long as RN is
above a
minimal
value)
No moving parts,
robustness, suitable
for different fluid
types (gas, liquid,
steam), linear
relation between
measurement and
fluid flow
Fluids that exceed a
minimum velocity RN
restriction, fluids with
viscosity above a
minimal value, complex
signal’s conditioning
Fluidic
vortex
Force of the
fluid in a fixed
body (target)
Volume
(clean fluids,
minimum
movement
quantity)
Low-cost, good
performance in large
pipes
Restrictions on RN,
fluid’s velocity,
material of
construction, low
accuracy, almost
impossible calibration
Target
Measurement
of fixed fluid’s
volume per
rotation
Volume
(clean and
non-abrasive
liquids)
High accuracy, no
supply power
requirements,
bidirectional
operation, no RN
restrictions
Only for liquids, high
pressure drop, moving
parts, sensitivity to
contamination and
overloading
Helical gear,
nutating
disc,
oscillating
piston, oval
gear, rotary
Volume
(clean
liquids and
some gases,
but
problematic
for the last)
No moving parts, it
could be noninvasive (transducers
are outside the
pipe), no pressure
drop, linear relation
between
measurement and
fluid’s flow, on line
calibration
Good accuracy only for
liquids, error due to
deposits, errors caused
by gas bubbles,
affected by sound,
velocity, temperature,
concentration and
density changes
Doppler
transit time
pulse
repetition
Acoustic
waves or
vibrations
29
Primary
Flow Meter
Working
Type
Principle
Quantity
Advantages
Measured
Disadvantages
Meter Name
Only large pipes,
requirement of data
processing for
acceptable values of
accuracy, RN restriction
(turbulent flow regime),
large pressure drop
One for each
type of
transducer
(metering
principle)
Insertion
Measurement
of fluid’s
velocity in
critical
positions
(e.g.: pipe’s
axis)
Volume
(liquids, gas
and steam in
large pipes)
Depends on
transducer types
because it can be
implemented using
almost all flow
meter’s working
principles
Correlation
Correlation of
measurement
data captured
in different
positions
Volume
(depend on
flow meter
type use for
correlation
purposes)
Typically noninvasive
measurement
method
(depend on flow meter
type use for correlation
purposes)
One for each
type of
transducer
(metering
principle)
Open Chanel
Sheet of fluid
above a
crested dam
or variable
height in a
restriction
(Parshall)
Volume
(liquid flow
measuremen
ts usually in
irrigation,
drains and
water works
systems)
Unique solution for
measurements in
open channels (e.g.:
irrigation systems,
river flows)
Pressure loss, not
versatile
Weirs
Parshall
flume
Conservation
of angular
momentum
Mass
(measureme
nt of:
liquids, gases
with
restrictions,
harsh
chemicals,
density)
True mass
measurement, no RN
restrictions, high
accuracy, unaffected
by pressure,
temperature and
density
Acceptable accuracy
only for liquids,
vibration sensitivity,
large size limitations
Coriollis
Hydraulic
Wheatstone
bridge
Mass
(measureme
nt of gases)
True mass
measurement,
robustness for
industrial and
vehicle applications,
large flow range
Only for gases, nonlinear inference
measurement of mass,
non-linear output
signal, bubbles
sensitivity
Hot Wire
Anemometer
Coriollis
Thermal
Thermal
properties of
materials
2.4
Detection Of Leaks
Till now we have seen some issues about flow measurement. These include both the basic
concepts involved in the measuring task and the instrumentation equipment to its
quantification. However, the most important topic to address within the context of this thesis
is: How can a leak be detected by measuring the flow of a fluid in a pipeline?
The idea behind the detection of leaks has already been addressed. The detection of leaks
is based on the conservation of mass that states that “, the amount of fluid moving through a
meter is neither added to nor taken from as it progresses from point 1 to point 2.” [8].
30
Therefore, if two consequent flow meters, one placed at point 1 and other placed at point 2,
diverge in the measured value would mean that there is a leak somewhere between these
two points.
The consequent product loss can be calculated by the difference between instantaneous
inlet and outlet flows and the average of the integrated inlet and outlet flows as it shown in
[6] and described by equation 2.25 [6].
∆! = !!" − !!"# − !! (2.25)
Where:
∆! is the leakage volume;
!!" = Metered inlet flow;
!!"# = Metered outlet flow;
!! = Calculated as the average of the integrated inlet and outlet flows;
This last term (!! ) is calculated as the average of the integrated inlet and outlet flow. In a
pipeline without leakage this term is zero as the integrated inlet and outlet have symmetrical
values.
To sum up, either for identifying a leak or determining its location, a cooperation of
different flow meters installed along the pipeline is required. It is within this context that
WSNs emerge. In the next chapter this topic will be discussed in detail.
Summary
The more important issues of flow measurement and leak detection systems were
reviewed in this chapter. The relevant information about the basic principles, formulas, and
theory involving the movement of fluids were analyzed in detail.
A great emphasis to flow meters and its measurement techniques was also given. Three
types of flow meters were discussed in the context of this thesis: electromagnetic, ultrasonic
(Doppler and Transit-time) and cross-correlation.
Finally at the end of the chapter, the correlation between flow measurement and leakage
detection was addressed.
Chapter 3
Sensor Networks
This chapter introduces the sensor networks’ state of the art. The chapter covers the
following topics:
•
Background on wireless sensor network.
•
Application areas for sensor networks;
•
Challenges and requirements;
•
Energy scavenging;
3.1
Background
A sensor network is a cluster of nodes that are connected using a communication
infrastructure for either monitoring or control purposes. The communication infrastructure of
a sensor network can be either wired or wireless. The present chapter gives a particular
emphasis to the study of wireless sensor networks (WSNs). However, most of the topics cover
can also be applied to wired sensor networks [16, 17]
Wireless sensor networks (WSNs), consists of a wireless network composed by large
number of sensor devices. These devices, called nodes, cooperate with each other to perform
a sensing task in a specific physical environment. Each sensor node is equipped with a [18]:
•
Transducer which generates electrical in accordance with the measured value;
•
Microcontroller which processes data. Is responsible for acting or sensing over
the environment;
•
Memory which store programs and intermediate data;
•
Transceiver which receives and transmits data over the communication
infrastructure i.e. antenna;
•
Power Source which supplies electrical power for node’s operating;
31
32
Figure 3.1 shows the typical architecture of a sensor network and sensor node.
Figure 3.1- Typical architecture of sensor network and sensor node. Based on [18]
3.2
Application Areas For WSN
In the latest years the interest in WSNs has increased significantly due to both research
and industrial players. This growing interest took this technology to be considered by
Technology Review at MIT as one of the “10 emerging technologies that will change the
world” [19].
The application areas for WSNs are immense. From military usage to environmental
monitoring, there are an unlimited number of possibilities for new application purposes for
WSNs. This section addresses some of the most relevant examples.
3.2.1.
Military Application
The usage of WSNs for military purposes can be a crucial part for military command
control, communication, computing, intelligence, surveillance, reconnaissance and targeting
(C4ISRT) systems. Some of the military application for WSNs includes: monitoring friendly
troops, Targeting and monitoring enemy forces or detecting biological and chemical attacks.
We will look in further detail which one of these applications [20, 21]
Monitoring Friendly Troops
Determining the best course of action in a battlefield is a very complex task that requires
knowing the position and status of every personnel, equipment and vehicle. This kind of
application aims to ease the coordination and monitoring among the friendly troops by using
WSNs.
33
Every troop, vehicle, equipment and critical ammunition has to be equipped with attached
sensors that monitors and reports the status to a sink node (basestation node illustrated in
Figure 3.2). The information gathered in the basestation is sent to the troops leaders or
command centers to determine the best course of action in accordance with that information
[20, 21].
An example of monitoring friendly troop application is illustrated in Figure 3.2.
Figure 3.2 - A sensor network for military surveillance application. From [22].
Targeting And Monitoring Enemy Forces
The purpose of this application is collecting valuable information of the enemy forces. The
sensor nodes, which are installed in the battlefield area, can detect a presence of an enemy
and disseminate this information throughout the network. This information not only would
help to determine the position of the enemy as also would empower the friendly troops to
strategically position in the battlefield and determine the best course of action, which
minimizes any possible casualties [20, 21].
An example of targeting and monitoring enemy forces is exhibited in Figure 3.3.
34
Figure 3.3 - Enemy target localization and monitoring using WSNs. From [20].
Detecting Biological And Chemical Attacks
In a situation of a biological or chemical attack it is essential to be close the ground zero
for timely and accurately detect the agents involved.
This application consists in deploying spatially distributed sensor nodes in the friendly
regions to detect the presence of toxic substances on the environment. This way, sensor
networks can provide an early warning in a situation of a biological or chemical attack [21].
3.2.2.
Environmental Monitoring
Environment monitoring is a natural candidate for the development of sensor networks
applications due to the characteristic of variables to monitor, which are usually spatially
distributed over large areas (e.g. temperature or pressure) [23].
Several applications can be considered in this context: Forest fire detection, animal
tracking and habitat monitoring or monitoring the environmental conditions [21].
Forest Fire Detection
As seen in the military application case, WSNs can densely cover a wide area. This type of
application uses sensor nodes to detect the exact location of a fire before it starts to spread
uncontrollably. As soon as a node detect a fire it disseminates a message over the network
till arrives the sink node. Afterwards a message would be sent to fire fighters with the exact
location of the fire [19-21].
Figure 3.4 illustrates the scenario of a fire detection using WSNs.
35
Figure 3.4 - Forest-fire monitoring application. From [20].
Animal Tracking And Habitat Monitoring
This kind of application permits researches to gather important data about a specifically
animal or habitat in an unobtrusive manner.
A well-succeeded example of animal tracking and habitat monitoring is identified in the
literature [19, 20].This example is one of the world's most advanced experiments in wireless
networking as it covers an area of approximately 90 hectares on Great Duck Island (USA). The
application focuses in the habitat monitoring of Leach’s Storm Petrel, a common seabird in
the western North Atlantic. Humans cannot perform the activity monitoring of this particular
specie as it can lead to nest abandonment or increased predation on chicks or eggs. For this
reason, the usage of WSNs is the only form to obtain information.
Monitoring The Environmental Conditions
WSNs also can be used for the measurement of environmental data. This comprises
temperature, pressure, humidity, solar radiation, chemical pollutants and wind speed among
others.
Some examples of this type of application comprehend the measurement of chemical
pollutants in garbage dump sites; surveillance the marine ground floor an understanding its
erosion process for the construction of offshore wind farms [18].
36
3.2.3.
Health Applications
Health is one of the application areas covered by WSNs. Some relevant applications in this
area are: drug administration, monitoring patient’s physiological data, tracking doctors and
patients, diagnosis or help the disable [21]. Monitoring physiological data and drug
administration is covered below.
Monitoring Physiological Data And Drug Administration
Patients can be attached with sensor nodes that would not only monitor the patient’s
physiological data but also include information about patient’s medical record, such as
allergies, prescribed medications, chronic diseases, among others.
This would help both doctors and medical personnel during the patients’ treatment as this
would permit: storing patients’ information for long periods of time and submit to doctors’
analysis, minimizing the adverse of drugs or preventing from erroneous drug prescription [21].
An example of a potential health monitoring application is shown in [24] for glucose-level
monitoring. Diabetes is a disease that requires constant observation of blood sugar level.
Using WSN for this purpose would not only guarantee a continuous sugar level monitoring but
also alert the individual to take corrective measures.
3.3
Challenges & Requirements For WSNs
Literature [17, 18, 20, 21, 25-28] identifies several challenges and requirements that are
shared among almost every WSN application. These comprehend both software and hardware
challenges to engineers. In this section we will look into further detail of some of the most
critical properties of a WSN system.
3.3.1.
Fault Tolerance
The fault tolerance of a system is the property of a system to continue to operate properly
in a situation of failure. This property is crucial in a WSN as nodes may run out of energy,
might be damaged or the communication with a certain node can be interrupted for a long
periods of time. The recognition and exhibition of these situations is desirable.
To support this property is usually required some kind of redundancy in the deployment,
which involves more nodes than the necessary.
3.3.2.
Security
WSNs are designed to operate in hostile environments (e.g. battlefield, f), sensor nodes
should support security at all levels of the network. The following security characteristics
must be guaranteed [21]:
•
Access control in order to prevent unauthorized access to sensor nodes;
37
•
Message integrity to guarantee non-unauthorized changes to the message;
•
Confidentiality, which consists in encrypting exchanged messages between nodes
and assures that only the nodes with the secret key can communicate;
•
Replay Protection that involves preventing a sensor node from gaining network
access by reusing an authentic packet;
3.3.3.
Scalability
WSNs are usually composed by large number of nodes that might be spatially distributed.
Therefore, the employment of the WSN’s architecture and protocols must be able to scale
the network’s nodes.
3.3.4.
Energy Efficiency
The biggest problem in WSNs applications is energy. Sensor nodes are usually powered on
batteries that have a limited amount of energy. Managing the energy source is therefore
mandatory. In addition to this, energy-efficient is the key to a long lasting life of the
application, as when a sensor node runs out of energy it can no longer fulfill its role.
In order to increase application’s lifetime, energy scavenging can be considered. Next
section covers some of the options that can be considered in energy scavenging.
3.4.
Energy Scavenging
An important issue to embrace within the context of WSNs application is energy
scavenging, as applications may require uninterrupted operation for long periods of time and
in inhospitable environments. Energy scavenging is the term used for meaning energy search
and collect from the environment. The review of this procedure, also referred as energy
harvesting, is important in application where replacing batteries or supplying with wired
energy is unfeasible. Note that, using energy scavenging to recharge battery does not exclude
the problem of nodes running out of energy. This technique just extends battery lifetime.
The scavenging sources analyzed in this section are the main sources of ambient energy
considered suitable for use with WSNs by the authors from [29].
3.4.1.
Solar
Solar power is the one of the most common forms of energy harvesting. Although,
photovoltaic panels require incident light to generate electricity and a low-power maximum
power point tracker (MPPT) to ensure that energy is not lost from the scavenging source to
the sensor node batteries [29].
Despite these disadvantages, solar power is still being a high viability for energy
harvesting.
38
3.4.2.
Mechanical
Man has been using this kind of energy for centuries. Take the example of a mill or
watermill where wind is used to transform corn into flour.
Vibrational, kinetic and mechanical energy can be used for harvesting energy from
movement objects. Vibration energy for instance can be harvested from the environment by
using piezoelectric capacitors. This kind of energy is highly potential in bridges or railways
were vibration is present all around [29].
3.4.3.
Thermal
Temperature variation can also provide a means by which energy can be scavenged from
the environment. A temperature difference between two junctions of a conducting material
generates electrical current. Thermal energy harvesting consists in generating electricity by
using this principle [29].
There are some commercial products, like citizen wristwatches, that use this technology
to recharge batteries [20]. In that application temperature difference between human body
and the surrounding environment is used to generate energy.
3.4.4.
Comparison Of Scavenging Sources
Table 3.1 summarizes the main characteristics of various potential scavenging sources
analyzed in this section.
Table 3.1 - Comparison of various potential power sources for WSNs. Taken from [20].
a
b
c
Power Source
P/cm3
(µW/cm3)
Solar (outside)
Secondary
Voltage
Commercial
Storage Needed
Regulation
Available
15000a
Usually
Maybe
Yes
Solar (inside)
10a
Usually
Maybe
Yes
Temperature
40a,b
Usually
Maybe
Limited
Air Flow
350c
Yes
Yes
No
Vibrations
200
Yes
Yes
Limited
Denotes sources whose fundamental metric is power per square centimenter per cubic
centimeter
Demonstrated from a 5ºC temperature differential.
Based on reported values at an air velocity of 5m/s and 11% conversion efficiency.
38
39
Summary
An effort has been made to give an overview of WSNs in this chapter. The concepts of
WSN, sensor node and node composition were reviewed.
A great part of the chapter was spent in analyzing the various areas of application for
WSN, its architectures and usage advantages. It has also been covered the main challenges
and requirements identified in the literature and shared among WSNs applications.
Finally at the end of the chapter, scavenging energy sources have been addressed. Three
types of energy sources were discussed within the context of WSNs usage.
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