Thesis_MSc_GSchutgens

Discharge capacity reduction in pressurised
pipelines
A study on capacity reduction in the pressurised system of Rotterdam due to gas pockets
Gieljam Schutgens
November 2011
Delft University of Technology
Faculty of Civil Engineering and Geosciences
Department of Water Management
Section of Sanitary Engineering
Stevinweg 1
2628 CN Delft
www.sanitaryengineering.tudelft.nl
DischargecapacityreductioninpressurisedpipelinesGieljamSchutgens

DISCHARGE CAPACITY REDUCTION IN
PRESSURISED PIPELINES
A study on capacity reduction in the pressurised pipeline system
of Rotterdam due to gas pockets
Gieljam Schutgens
for the degree of:
Master of Science in Civil Engineering
November 2011
Committee:
Prof.dr.ir F.H.L.R. Clemens Delft University of Technology
Sanitary Engineering Section
Dr.ir. I.W.M. Pothof Delft University of Technology
Dr.ir. J.A.E. Ten Veldhuis Delft University of Technology
Prof.dr.ir. W.S.J. Uijttewaal Delft University of Technology
Fluid Mechanics Section
Ir. Koos de Voogt Gemeentewerken Rotterdam
Sanitary Engineering Section, Department of Water Management
Faculty of Civil Engineering and Geosciences
Delft University of Technology, Delft

2 DISCHARGE CAPACITY REDUCTION IN PRESSURISED PIPELINES
2

PREFACE
This thesis marks the completion of the Master of Sanitary Engineering at the Delft University of Technology
(DUT). The motivation for doing this thesis goes back to my previous experiences at DUT. Two internships, one
of them at a Drinking Water Treatment plant in Panama gave insights into the conventional treatment of water
for drinking purposes. A second internship concerning the monitoring of an anaerobic treatment plant for
coffee wastewater in Nicaragua provided me with insights into the purification of agricultural wastewater.
Most of the water in those systems was transported by pipes. It then became interesting to me to find out
more about the transport of liquids through pipes as this is one of the most applied methods in urban drainage.
The work presented in this thesis would not have been possible without the gentleness of many people: among
others the head of the Strategy, Policy and Advice department (Dutch acronym: SBA), Daphne de Koeijer,
within the Water Management department at the Municipality of Rotterdam, who together with Koos de
Voogt and Jorg Pieneman helped in securing a working place for me in the offices of the Municipality. Thank
you for your supervision and guidance. Furthermore, thanks to the people of ‘Onderhoud en Procesvoering’
(both within the towers of GWR as well as outside in the field) who answered my many questions in relation to
the working of pumps. From the Ingenieursbureau of Rotterdam I am grateful to Alex Duinmeijer who helped
me with the use of Wanda and gave me advice at the beginning and end of the process of investigating the
hydraulics of a number of selected study cases.
From the DUT I would especially like to thank Prof. Clemens who, with his inspiring lectures, awakened in me
the interest for the world of urban drainage. His enthusiasm and accessibility towards students also gave me
the confidence to explore this field. Furthermore, I would like to thank Ivo Pothof for his guidance throughout
the process of the entire thesis and both Ivo and Marie-Claire ten Veldhuis who patiently answered my
questions at each scheduled session we had.
I am grateful to my friends, who were often motivating me to proceed with my thesis; helping me to relax at
the right moments and for their fellowship in daily life. Also I am grateful to Esther Kok and Ekamma Inanga
who helped correcting the drafts of this thesis.
To my parents, Wim Schutgens and Maria Luisa Quiej de Schutgens, I am grateful for the comfort, guidance and
support at all aspects of life, as well as to my sisters and niece for reminding me that joy in life can be found in
the smallest and most innocent moments.
Finally, I would like to thank God for the life and blessings I receive every day.
Gieljam Schutgens
The Hague, November 2011

4 DISCHARGE CAPACITY REDUCTION IN PRESSURIZED PIPELINES

SUMMARY
This thesis can be seen as a spin-off of the CAPWAT research project. This research project was a joint effort of
Deltares and TU Delft, supported by Water Boards, Municipalities and consultancy firms in order to discover
the major reasons for discharge capacity reduction in pressurised pipelines for wastewater transport. The
results of this research demonstrated that gas pockets can hamper the transport considerably in pressurised
pipelines.
Using the knowledge and insights developed in the CAPWAT research project, this thesis seeks to quantify the
effects of gas pockets using historical monitoring data from wastewater pressure mains in the city of
Rotterdam. Can capacity reduction be observed in the pressurised wastewater system of Rotterdam, and if this
is the case, what is the influence of gas/air pockets in pressurised systems? These are relevant questions that
are answered in this thesis.
In order to find answers to the aforementioned questions, an overview was first made of the different causes
of discharge capacity reduction. Simultaneously, the theory concerning liquid flow in pipelines was reviewed. In
addition, methods to identify the presence of gas pockets in pressurised pipelines were revised and a new
method was developed. This method uses historical monitoring data with low frequency (1 signal stored per
minute) to compare two steady state situations. The method compares the occurring energy losses with the
energy losses which should take place provided the pressurised sewer system works according to the design
objectives. In spite of the fact that gas pockets cannot be specified anymore as the unique reason for a
discharge capacity reduction, by using this new method, other advantages are obtained. One of these
advantages is that the performances of both the pipeline as well as the PS are separately addressed. In
addition, costs involved due to malfunctioning of either the PS or the pipeline can be identified with this
method. With the developed method an analysis was made of the major pumping stations (PS’s).
Subsequently, PS’s showing a discrepancy between their design flow capacity and the occurring flow capacity
were included in the selection procedure. Two pumping stations with their corresponding pipelines (4 in total)
were further analysed in accordance with the method developed in the thesis. The two systems chosen
represent one of the largest PS’s of the Rotterdam system and a medium-sized PS.
The results of the analysis demonstrated that for two systems (PS002 WWTP and PS010 WWTP) the
capacity reduction was only 6% in comparison with the design capacity. In both cases the capacity reduction
could not be traced back to gas pockets. The real reason for the capacity reduction is unknown. In a third
system (discharge to Meuse) the capacity loss diminished by 10%. In this system the influence of a high
streaming number indicated that gas pockets were being broken and transported during high flows. The fourth
system (PS010 Meuse) showed remarkable capacity loss due to the influence of air pockets in the system
which reduced the capacity to half (48%) of its designed capacity. In this case, a poor design of the system, in
which the hydraulic grade line of the system falls below the layout of the pipeline, proved to be disastrous for
the discharge capacity. This was exacerbated due to an incorrect way of positioning and choice of air-valve
design that was applied.
The method reveals a clear relationship between the size of the pumping station and the extra energy costs
due to excess energy losses in the pipe. Therefore, it is advised to first solve problems in those systems that
cause the most energy losses (and therefore, expenses) and the most practical problems, and then investigate
if it is worth investing money and time in solving problems which the smaller systems are coping with.
This thesis confirmed that air release valves are often poorly managed, and in some cases their position and
existence are unknown to people from Water Boards and Municipalities. The overall management and
maintenance of pressurised pipelines has been neglected for some time and there seems to be little
information about the condition of pressurised pipelines.

One of the major sources of inaccuracy in the models developed to simulate the system characteristic is related
to this lack of information. The build up of the roughness factor is difficult to estimate. The conditions of pipes
that have remained under the ground for 25 to 100 years are uncharted. The models developed in this thesis
have used the best case scenario, i.e. a high roughness factor has been applied, because this elevates the
system characteristic and thus creates a smaller difference between design working points and the historically
measured working points.
At the end of this report, general conclusions and recommendations are given for any individual or organisation
interested in investigating a discharge capacity reduction using historical monitoring data. Furthermore,
recommendations are presented for future research and for practical applications.

ABBREVIATION LIST
GRP Gemeentelijk Rioleringsplan (Municipal Sewage Plan)
WMb Wet Milieubeheer (Dutch Law on Environmental Management)
WVO Wet Verontreinigingen Oppervlaktewateren (Dutch Law on Contamination of Open
Water Courses)
CAPWAT Capacity Reduction in Pressurised Wastewater Pipelines
CSO Combined Sewer Overflow
DWF Dry Weather Flow
EV End of Variable flow setpoint
GFRP Glass Fiber Reinforced Plastic
mwc meter water column pressure
MIP Management Informatie en Procesgegevens (Management information and process
data)
NAP Normaal Amsterdams Peil (Amsterdam Ordnance Datum)
PE Polyethylene
PP Polypropylene
PS Pumping Station
PS002 Pumping Station Groene Wetering
PS010 Pumping Station Willem Schürmannstraat
PVC Poly Vinyl Chloride
rpm revolutions per minute
SWF Storm Weather Flow
SV Start of Variable flow setpoint
WWTP Waste Water Treatment Plant

LIST OF SYMBOLS
Symbols Unit Significance
A m
2
Area
b m Width
c m/s Wave propagation speed in conduit
cgas m/s Wave speed with gas pocket
c0 m/s Wave speed without gas pocket
d m Diameter gas pocket
dhf - Friction gradient in pipe
D m Diameter pipeline
e m Wall thickness
E N/m
2
Elasticity modulus of pipe material
f s
-1
Frequency pumps
F - Flow number
g m/s
2
Gravitational acceleration
hbasin m Head in basin (upstream)
hc,pump m Datum of pump centre
hdown m Head downstream
hf m Energy loss due to friction
hl m Total energy loss
hlocal m Local energy losses
hL,f,pres m Local & friction head loss between pump and pressure logger
hL,f,suction m Local & friction head loss between suction and pump
hstatic m Upstream and downstream level difference
hv m Velocity head
H m Energy head
Hn (Q,n) m Theoretical head at a given flow & speed pump
Hpump (Q,hbassin,Pdata) m Occurring head at a given flow, basin level and speed pump
Hpump m Head delivered by pump
Hpres m Pressure head relative to pump
Hstatic m Level difference between pressure logger and downstream condition
Hsuction m Suction head relative to pump
Ht m Total energy headloss
Ht (Q,hbassin,hdown) m Theoretical head at a given flow, basin level and upstream level
ΔHdyn m Dynamic energy loss
ΔHpipe m Energy loss due to gas pockets in pipeline or scaling
ΔHpump m Energy loss due to pump wearing/dirtiness
ΔHgas m Energy loss due to gas volume
k mm Nikuradse roughness
kL - Constant for particular type of fitting
K N/m
2
Bulk modulus or elasticity of the fluid
kpoly - Polytrophic constant
L m Pipe length
Ldown m Pipe length down sloping leg
Lgas m Horizontal length of gas pocket
n rpm Rotational speed of pump blades
Δp Pa Pressure difference
p N/m
2
Pressure

pi Pa.a Initial gas pressure
pdata bar Pressure measured at pressure transmitter
pf,L bar Pressure due to friction and local losses along the pipe
pstatic bar Static pressure between pressure logger & downstream water level
P kW Power
QG m
3
/s Gas flow
Q m
3
/s Fluid flow
Re - Reynolds number
t s Time
T °C Temperature
Tsteady s Time needed to achieve steady state situations
v m/s Velocity
V m
3
Volume
Vi m
3
Initial volume of gas pocket
z m Potential head
zp,P m Level difference between pressure transmitter and pump datum
ξloc - Friction coefficient for local losses
η - Efficiency of pump
θ ° Angle between pipeline and horizontal plane
λ - Darcy-Weisbach friction coefficient
λ0 m
2
Water hammer storage
vkin m
2
/s Kinematic viscosity
ρ kg/m
3
Density of fluid

INDEX
1. Introduction .................................................................................................................................................. 13
1.1 General information ............................................................................................................................... 13
1.2 Problem definition.................................................................................................................................. 13
1.3 Research objective.................................................................................................................................. 14
1.4 Research questions................................................................................................................................. 14
1.5 Study area............................................................................................................................................... 14
1.6 Structure of report.................................................................................................................................. 15
2. Methods and Techniques.............................................................................................................................. 17
2.1 Capacity reduction in pipelines............................................................................................................... 17
2.2 Origin of gas pockets in pressurised pipelines........................................................................................ 17
2.3 Fluid transport through pipelines........................................................................................................... 18
2.3.1 Energy level..................................................................................................................................... 18
2.3.2 Energy losses due to friction ........................................................................................................... 19
2.3.3 Energy losses due to local disturbances.......................................................................................... 20
2.3.4 Energy losses due to gas pockets.................................................................................................... 20
2.4 Interaction between pressurised pipelines and pumps.......................................................................... 21
2.5 Gas pocket transport through pressurised pipelines.............................................................................. 23
2.6 Methods to detect gas pockets in pressurised pipelines........................................................................ 25
2.6.1 Detection method 1: Working point ............................................................................................... 25
2.6.2 Detection method 2: Pressure comparison .................................................................................... 32
2.6.3 Discussion on detection methods................................................................................................... 34
3. Description of case studies ........................................................................................................................... 37
3.1 Criteria to select a case study................................................................................................................. 37
3.2 Wastewater system of Rotterdam.......................................................................................................... 37
3.2.1 Sewer system .................................................................................................................................. 37
3.2.2 Pressurised system.......................................................................................................................... 37
3.2.3 Pumping stations............................................................................................................................. 38
3.2.4 Monitoring of wastewater system.................................................................................................. 38
3.3 Choice for pressurised system for gas pockets detection ...................................................................... 38
3.4 Pumping station Groene Wetering......................................................................................................... 39
3.4.1 Overview ......................................................................................................................................... 39
3.4.2 Pumps.............................................................................................................................................. 40
3.4.3 Pressurised pipelines....................................................................................................................... 42
3.5 Pumping station Willem Schürmannstraat............................................................................................. 44
3.5.1 Overview ......................................................................................................................................... 44
3.5.2 Pumps.............................................................................................................................................. 45
3.5.3 Pressurised pipelines....................................................................................................................... 47
3.6 Pumping stations in context ................................................................................................................... 49
4. Hydraulic modelling of wastewater pressure mains..................................................................................... 51
4.1 Hydraulic modelling of sewage systems................................................................................................. 51
4.1.1 Modelling of system characteristics................................................................................................ 51
4.2 Uncertainties .......................................................................................................................................... 51
4.2.1 Uncertainty in the input.................................................................................................................. 51
4.2.2 Uncertainty in model structure and in parameter input................................................................. 52
4.2.3 Uncertainty in measurement values............................................................................................... 52
4.3 Model of Pumping Station Groene Wetering (PS002)............................................................................ 55
4.3.1 PS002 WWTP Kralingseveer ....................................................................................................... 55
4.3.2 PS002 River Meuse..................................................................................................................... 58
4.4 Model of Pumping Station Willem Schürmannstraat (PS010)................................................................ 60
4.4.1 PS010 WWTP Kralingseveer ....................................................................................................... 60
4.4.2 PS010 River Meuse..................................................................................................................... 64
5. Results and Analysis...................................................................................................................................... 67
5.1 PS002 Groene Wetering WWTP Kralingseveer.................................................................................. 68
5.1.1 Dry Weather Flow operation mode in PS002.................................................................................. 68

5.1.2 Rain Weather Flow operation mode in PS002, SWFmin ................................................................... 69
5.1.3 Rain Weather Flow operation mode in PS002, SWFmax................................................................... 70
5.1.4 Analysis............................................................................................................................................ 70
5.2 Groene Wetering River Meuse .......................................................................................................... 72
5.2.1 Discharge water to the river Meuse................................................................................................ 72
5.2.2 Analysis............................................................................................................................................ 73
5.3 Willem Schürmannstraat WWTP Kralingseveer................................................................................. 74
5.3.1 Dry Weather Flow operation mode in PS010.................................................................................. 74
5.3.2 Rain Weather Flow operation mode in PS010, SWFmin ................................................................... 75
5.3.3 Rain Weather Flow operation mode in PS010, SWFmax................................................................... 77
5.3.4 Analysis............................................................................................................................................ 78
5.4 Willem Schürmannstraat River Meuse .............................................................................................. 80
5.4.1 Discharge water to the river Meuse................................................................................................ 80
5.4.2 Analysis............................................................................................................................................ 81
5.5 Results from Flow meters....................................................................................................................... 85
5.5.1 Results for PS002 (Groene Wetering) ............................................................................................. 85
5.5.2 Results for PS010 (Willem Schürmannstraat) ................................................................................. 86
5.6 Pumping Station Groene Wetering (PS002) ........................................................................................... 87
5.6.1 Conclusions regarding the pipeline to WWTP................................................................................. 87
5.6.2 Recommendations .......................................................................................................................... 87
5.6.3 Conclusions regarding the pipeline to River Meuse........................................................................ 87
5.7 Pumping Station Willem Schürmannstraat (PS010) ............................................................................... 88
5.7.1 Conclusions regarding the pipeline to WWTP................................................................................. 88
5.7.3 Conclusions regarding the pipeline to River Meuse........................................................................ 89
6. Conclusions and Recommendations ............................................................................................................. 90
6.1 Conclusions............................................................................................................................................. 90
6.2 Recommendations for practical applications ......................................................................................... 92
6.3 Recommendations for further investigations......................................................................................... 93
Bibliography .......................................................................................................................................................... 95
Internet sources............................................................................................................................................ 95
Books, papers and conferences .................................................................................................................... 95
Personal communication .............................................................................................................................. 96

1. INTRODUCTION
This chapter forms an introduction to the thesis and provides at the same time the basis for it. The general
information gives the background outlining the seriousness of the problem presented in the problem
definition. After that, the problem description is outlined in the research objective followed by the research
questions. Then, the study area is delineated. Finally, the structure of the research is presented.
1.1 GENERAL INFORMATION
As a consequence of 19
th
century industrialisation and the associated changes seen in the formerly agricultural-
based economy, many people were obliged to move to the cities. Once in the cities, the lack of housing and the
agglomeration contributed to an unprecedented spread of diseases. This became especially apparent during
the cholera epidemics in Western Europe of the 19
th
century. Following these epidemics the use of pipelines
was introduced for the discharge of wastewater. In this manner, the first sewer systems were built towards the
end of the 19
th
century.
As a result the problem of contamination was relocated to areas where the risk of disease developing was
strongly reduced due to dilution (lakes and larger rivers). As knowledge about disease infection increased and
the effects of wastewater on open water courses became more evident, people began to understand that just
having sewer systems was not the ultimate solution. From 1950 onwards, wastewater treatment plants
(WWTP) have been built.
In 1970 the Surface Water Contamination Act (Wet Verontreiniging Oppervlaktewateren, Dutch acronym:
WVO) was introduced. As a consequence of this law the construction of WWTP’s accelerated. In Rotterdam the
sewer system dates back to the 1870s (GRP2, 2005). This system, however, discharged the wastewater in the
Meuse. It was not until the second half of the 20
th
century that the sewer system in the Netherlands was
connected to WWTP’s, through a pressurised pipeline system, to purify the wastewater.
1.2 PROBLEM DEFINITION
In the 80’s and 90’s of the last century, a wide spectrum of pressurised pipelines was examined due to a
reduction in discharge transport capacity (Lemmens, 2007). It was then recognized that pipelines form a black
box for operating personnel for whom the management of such pipelines often seems unpredictable. From
experience, it is known that a slow decrease in flow capacity transport through pipelines can have its origin in
scaling or sedimentation. However, fast fluctuations in discharge capacity are unpredictable and undesirable
(Lemmens, 2007). This resulted, in the year 2002, in a workshop in which different parties collaborated on a
single research topic. From this workshop the CAPWAT (CAPacity reduction in pressurised wasteWATer
pipelines) research was developed. The major discovery of this research was that the greatest cause of
discharge capacity reduction, with the exclusion of pump failing, can be found in air accumulation or gas pocket
presence. The consequences of gas pockets in pressurised pipelines are a reduction in discharge capacity, thus
leading to higher energy consumption, larger investments on infrastructure, higher operational costs and an
increase in combined sewer overflows (CSO’s).
In the Netherlands, there are more than 13 thousand kilometers of pressurised pipelines laid under the ground
(Riool in Cijfers, 2009), 60% of these are owned by Water Boards and the remaining 40% are owned by
municipalities. The Municipality of Rotterdam owns and manages 312 km of pressurised pipelines (GRP3,
2011). The researchers of the CAPWAT program have calculated that 19 million kWh extra is consumed on an
annual basis (STOWA, 2010) due to the presence of gas pockets. This means that with an average energy price
of € 0.23/kWh (Groeneveld, D.A. 2010) potential savings could be obtained of € 4.4 million in the Netherlands.
Discharge capacity reductions other than those caused by pumping stations have also been observed in the
Rotterdam wastewater transport system.

Within Rotterdam Public Works, operators and managers can profit from the knowledge of the extent to which
capacity reduction takes place in their system. They will therefore be able to anticipate on problems and take
measures. In addition, advantages such as a decrease of CSO events (odors and contamination in the
neighborhood) and a possible reduction in energy and operational costs give enough reasons to analyze the
Rotterdam wastewater transport system on discharge capacity reduction.
1.3 RESEARCH OBJECTIVE
The CAPWAT research has identified gas pockets as one of the major causes leading to a reduction in discharge
capacity in pressurised pipelines. The question that this thesis seeks to answer is: Is there a capacity reduction
of discharge in the wastewater transport system of Rotterdam? If so, can it be ascribed to gas pockets?
Through a comparison between a hydraulic model and the actual occurring (measured) values of the transport
system the difference between both will give an indication of a possible discharge capacity reduction in the
pressurised pipelines. With the obtained results a study can be performed to the influence of gas pockets on
the system.
1.4 RESEARCH QUESTIONS
To achieve the research objective, the following questions are posed:
1. Which processes can cause a reduction in discharge capacity in pressurised pipelines?
2. Which methods and techniques are available to detect capacity reduction?
3. How large is the discharge capacity reduction in a few selected pressurised pipelines of the Rotterdam
wastewater system?
4. Which processes between wearing of wear rings, scaling, obstructions and gas pockets cause the
discharge capacity reduction, if any, in the pressurised system of Rotterdam?
a. What is the most important cause of capacity loss?
b. How can this cause be quantified?
c. How can the Water Management department recognize this cause and react in time?
5. What are the consequences of discharge capacity loss in Rotterdam?
a. Which of these consequences can be assessed quantitatively?
i. Which cannot, and why not?
b. Which pipeline segments are the most prone to develop gas pockets?
c. How can these consequences be reduced?
1.5 STUDY AREA
This research is conducted within Public Works of Rotterdam and is based on the pressurised wastewater
system of Rotterdam. Public Works of Rotterdam is the institution responsible for the infrastructure of the city
of Rotterdam and is a daughter-organization of the Rotterdam Municipality. Figure 1.1 shows the different
districts in which the pressurised system is divided, 40 districts in total.
Every district has a network of sewer lines which, to a great extent, discharge their wastewater through gravity
on the pumping station of their district. From here the pressurised system is responsible of transporting the
wastewater to a next district or to a WWTP.
To answer the questions posed in the previous section, a few pumping stations with corresponding pressure
mains will be studied.

1.6 STRUCTURE OF REPORT
The thesis “Discharge capacity reduction in pressurised pipelines” consists of 7 chapters. In the first chapter the
outline is presented from which the research will develop. Therein the reasons which led to the investigation
are presented. In the second chapter ‘Methods and Techniques’ the theory on which the thesis is based, is
addressed. Next, in chapter 3 the situation in which the research takes place is drawn together with the
selection procedure followed; in order to select a few case studies. Chapter 4 describes the modelling of the
pumping stations as well as their corresponding pressurised pipeline complying with all necessary conditions.
Chapter 5 presents the results of the different pipeline systems and analyzes them. In this chapter the
consequences of gas pockets in these specific pressurised pipelines are described. Finally, chapter 6
summarises the conclusions and recommendations for a broad spectrum of readers as well as for those
interested in the study cases described.
Figure 1.1, Districts of Rotterdam

2. METHODS AND TECHNIQUES
After the reasons for the research have been outlined in chapter 1, this chapter describes the theory on which
this investigation is based. First, a broad spectrum of possible causes for capacity reduction of flow in pipelines
is presented. The focus is then directed towards the presence of gas pockets in pipelines. What is their origin?
How can these gas pockets be transported? Which methods exist in order to identify their presence?
2.1 CAPACITY REDUCTION IN PIPELINES
Transport of goods through pipelines is indispensable for our world today. Many of the goods we consume (like
potable water or oil) or discharge (such as wastewater) are transported through pipelines. When this transport
takes place under pressure, which it often does, different causes can produce a reduction in flow capacity.
Figure 2.1 presents different causes which can lead to a reduction in flow capacity.
On the right-hand side of the diagram, capacity losses due to break of pipeline or leakage are presented. Note
that, depending on the good being transported, a leak can result in extra flow and costs or it can become
dangerous contaminating aquifers used for irrigation or human consumption. However, this thesis focuses on
energy losses due to gas pockets and their reducing effect on discharge.
2.2 ORIGIN OF GAS POCKETS IN PRESSURISED PIPELINES
Taking notice of research performed over the last 7 years in the CAPWAT program, it is expected that in 80% of
the cases in which capacity reduction takes place, the cause can be traced back, directly or indirectly, to
accumulation of gas pockets (Tukker, 2010).
The term gas pockets may at first impression indicate the presence of hazardous substances in pipelines.
However, this does not necessarily need to be the case. More often gas pockets simply contain air, which
means that mainly nitrogen and oxygen are present. In some other cases, methane, hydrogen sulphide or
carbon dioxide may be formed. In this thesis gas pockets refer to any gaseous substance that has accumulated
and hampers the transport of wastewater.
According to Schuit (2009), gas pockets can accumulate in pipelines due to different reasons:
1. During construction and filling of the pipeline, air can enter the system
Figure 2.1, Capacity losses due to different reasons

2. Poor design and operation of pumping station, i.e. plunging water jets close to the pump intake create
air bubbles in the water which are suctioned into the pressurised system (Smit and Lubbers, 2007).
3. Low switch off levels in the reservoir which can lead to vortex formation and subsequent entrainment
of air into the system. When a pump is stopped in a system, due to the inertia of the water and the
impeller a large amount of water is still sucked from the reservoir. If the switch off level is too low, or
if the reservoir is too small, air is introduced in the pipeline.
4. In a branched system, often, pumping stations are connected to the same pressure main. When some
pumping stations are stopped, the decelerating flow in the pressure main will lead to lower energy
levels in the connection points. This will result in pump tripping while the pump is not in operation.
5. Air valves which are meant to release gas pockets can, when malfunctioning, introduce air into the
pipeline, e.g. when a water hammer induces negative pressures.
6. Processes inherent to the wastewater can biologically or chemically produce gasses. In anaerobic
conditions: CH4, H2S and CO2 can be formed. The formation of gasses will depend on temperature,
composition of wastewater and retention time.
7. The highest locations in a pipeline may present sub atmospheric pressure. When this is the case,
degassing of wastewater can occur or air entrainment from outside (pipelines in poor condition).
2.3 FLUID TRANSPORT THROUGH PIPELINES
Fluid transport through pipelines takes place under pressure, either due to natural force “gravitation” or due to
pressure exerted on the water through pumps. Often there is a reservoir that feeds the pipeline with fluid, in
this case with wastewater. The water is then transported to another reservoir or basin for further transport or
treatment. The basic principle is that water moves from places with a high energy level to places with a lower
energy level.
2.3.1 ENERGY LEVEL
The energy level is comprised of three types of energy: pressure head, velocity head and potential head. The
sum of these three is denominated as the total energy head (H), expressed in meters:
z
g
v
g
p
H ++=
2
2
ρ
[2.1]
In which,
H = total head [m]
p = pressure [N/m
2
]
ρ = fluid density [kg/m
3
]
v = velocity [m/s]
g = gravity acceleration constant [m/s
2
]
z = potential head [m]
When liquid is transported in a pipe, head is ‘lost’ from the liquid. This means that water flowing from point 1
to point 2 in a full pipe experiences an energy loss of hl, see Figure 2.2. The energy loss is caused by two
different mechanisms: friction losses (hf) and local losses (hlocal). Friction losses are caused by forces between
the liquid and the solid boundary (pipe wall). Local losses can comprise a variety of disruptions, i.e. bends,
changes in cross section, butterfly valves, t-sections and even gas pockets.
In case of Figure 2.2:
21 HhH l =− [2.2]
In which,
Hi = total head at point i [m]
hl = total head loss [m]

2.3.2 ENERGY LOSSES DUE TO FRICTION
In Figure 2.2 hl is only attributed to friction losses along the pipe. The friction losses along the pipe can be
expressed with the Darcy-Weisbach equation:
g
v
D
L
hf
2
2
⋅=
λ
[2.3]
In which,
hf = energy losses due to friction [m]
λ = friction factor [-]
L = length of pipeline [m]
D = diameter of pipeline [m]
Other parameters as previously defined.
From equation [2.3] only the friction factor lambda (λ) is not directly known. In pressurised pipelines the cross
section of the pipe is normally fully filled. A large part of the cross section presents then similar velocities and is
often of turbulent flow. But near to the wall the velocities are low and the flow tends to be more laminar.
Frictional losses are affected by the thickness of the laminar sub-layer relative to the size of the roughness of
the pipe wall (Butler and Davies, 2004). However, most urban drainage flows are rough or transitionally
turbulent flows (Butler and Davies, 2004). In 1937 White and Colebrook developed a formula to determine λ.
This formula is dependent on the Reynolds number and on the ratio between the Nikuradse roughness and the
diameter of the pipe:






+−=
λλ Re
51.2
7.3
log2
1
10
D
k
[2.4]
In which,
k = Nikuradse roughness [mm]
Re = Reynolds number [-]
kinv
vD
=Re (Only valid for turbulent conditions: Re > 4000) [2.5]
In which,
vkin = kinematic viscosity [m
2
/s]
Figure 2.2, Energy Grade Line (EGL) and Hydraulic Grade Line (HGL). Adapted from Butler and Davies (2004)

5.1
6
)5.42(
10497
+
⋅
=
−
T
vkin [2.6]
In which,
T = temperature [°C]
Table A.1 in Appendix A shows the influence of temperature on the kinematic viscosity. Appendix B provides an
example of temperature influence on dynamic losses for different velocities and diameter sizes.
2.3.3 ENERGY LOSSES DUE TO LOCAL DISTURBANCES
Local losses occur when the kinetic energy cannot be completely transformed into potential energy (pressure
head). Bends, valves, cross-sectional changes and other are examples of local losses. Local losses are expressed
as a function of the occurring velocities:
g
v
kh Llocal
2
2
= [2.7]
In which,
hlocal = local head loss [m]
kL = constant for particular fitting [-]
The constant kL for particular fittings can vary between 0.1 and 4. Smooth transitions of flow cause little loss (kL
= 0.1) whereas abrupt transitions or obstacles such as non return valves can cause large local losses (kL = 4). In
Table A.3 from Appendix A, a list of kL values for different fittings is given.
2.3.4 ENERGY LOSSES DUE TO GAS POCKETS
Gas pockets can also become a source for energy losses due to friction or local disturbances. These two
mechanisms are described in sections 2.3.2 and 2.3.3. Applying Bernoulli to Figure 2.3, it can be seen that the
following happens with gas pockets:
gasH
g
v
g
p
z
g
v
g
p
z ∆+++=++
22
2
22
2
2
11
1
ρρ
[2.8]
In which,
ΔHgas = energy loss due to gas pocket [m]
And thus,
Figure 2.3, Energy losses due to gas accumulation
Gas pocket

21 zzHgas −=∆ [2.9]
This approach is valid if the friction gradient (dhf) is small compared to the pipe gradient (dL*sinθ):
dLSindhf ⋅<< θ [2.10]
In which,
θ = angle between pipeline and horizontal plane [°]
Another mechanism that could take place is explained in Figure 2.4.
Figure 2.4 shows that an energy loss of ΔHgas will occur as a consequence of gas accumulation. When a gas
pocket encountered in the pipeline has a length Lgas which has extended over a long section, due to a reduced
flow profile the velocity will increase and, larger friction forces will cause additional losses. From experiments it
has been discovered that gas tends to accumulate only in downward sloping pipes and is strongly related to the
angle of declination (Tukker et al., 2010).
2.4 INTERACTION BETWEEN PRESSURISED PIPELINES AND PUMPS
In flat countries or areas, such as the case of the Netherlands, pumps are used to feed energy into the water. In
downward-sloping pipes that have to cross obstacles (such as roads, water courses, rail roads or dikes) gas
pockets may accumulate over time which will negatively affect the transport capacity.
The hydraulic performance of a pump can be determined from the ‘pump characteristic curve’ a graph of the
head added to the liquid, plotted against flow rate (Butler and Davies, 2004). A typical pump characteristic
curve is shown in Figure 2.5.
The pipe system on its own, on the other hand, has a system characteristic curve which expresses the static
head that needs to be delivered, plus the dynamic energy loss according to the flow rate that is transported
through the pipe. See Figure 2.6.
Figure 2.4, Friction loss due to gas pockets
Figure 2.5, Pump characteristic curve (Q – H curve)

The static head in Figure 2.6 represents the energy level difference between the suction side of the pipe
upstream and the overflow level downstream. In wastewater systems the water often flows from the basin of
the pumping station to an overflow level at a Wastewater Treatment plant (WWTP) or to another basin in
another district. See Figure 2.7.
When the system characteristic is combined with the pump characteristic curve, the working point or operating
point, which is used as the design parameter, is obtained. See Figure 2.8.
The working point of a pump serves as a design value when pumps are selected. In the first instance as a result
of the scheme provided in Figure 2.8 and equations [2.1], [2.3] and [2.7], the head that the pump has to deal
Figure 2.6, System Characteristic
Figure 2.7, Schematisation of transport system
Figure 2.8, Working point

with is determined by the network characteristic. The network characteristic is given by: static lift + friction
losses + local losses + velocity head, and is expressed as:
vlocalfstatict hhhhH +++= [2.11]
Or






++⋅+= 1
2
2
Lstatict k
D
L
g
v
hH
λ
[2.12]
In which,
Ht = total head loss [m]
hstatic = upstream and downstream energy level difference [m]
Nowadays many pumps work with speed pumps. This means that these pumps can vary their rotational speed
(revolutions per minute), in order to increase or decrease their flow, see Figure 2.9. However, they all have a
nominal speed according to which they have been designed. The nominal speed is often the most frequent
occurring speed according to the design.
2.5 GAS POCKET TRANSPORT THROUGH PRESSURISED PIPELINES
Due to the discontinuous operation of wastewater transport, gas might accumulate in elevated sections of the
pipeline, during dry weather flow (DWF) periods, when there are periods of no motion in the water (Pothof and
Clemens, 2008), due to an intermittent pump operation. When the pump starts and the water begins to flow
these gas pockets may turn into extended pockets (see Figure 2.4) which form a hydraulic jump at the tail.
From this hydraulic jump, gas bubbles will be ejected with the continuing flow depending on the velocities
occurring in the pipeline. Velocities which reduce this gas pocket size are denominated as clearing velocities.
After extensive research carried out in the CAPWAT program a new dimensionless parameter was proposed:
the flow number F (Tukker et al., 2010a).
gD
v
F = [2.13]
In which,
F = flow number [-]
Figure 2.9, Each operating point corresponds to a different rotational speed

In recent research, Pothof and Clemens (2010) presented a relation between critical values for the flow
number and the inclination angle θ of the downward sloping pipe, as shown in Figure 2.10.
Lubbers (2007) identified 4 regimes that take place in downward sloping pipes.
In flow regime a (Fig. 2.11) the velocities occurring in the pipeline are so low that no transport of air takes place
at all. With increased velocity, the flow regime b is achieved in which the hydraulic jump at the tail is more
pronounced and the transport of air to downstream reaches occurs. Velocities occurring in flow regime c (Fig.
2.12) have decomposed the elongated gas volume into several hydraulic jumps. Each of these hydraulic jumps
captures air bubbles from the upstream decomposing hydraulic jump. And each of these hydraulic jumps has
the tendency to move counter current (upward). After a certain velocity, the bubbles in the water are
completely transported; this is demonstrated in flow regime d.
Figure 2.10, Flow number depending on inclination angle
Figure 2.11, a) stratified flow with weak hydraulic jump; b) stratified flow with pronounced hydraulic jump

The research performed in the CAPWAT project focused on inclination angles between 5° and 30° and was
validated at diameters varying from 80 to 500 mm. Research was also performed for a 90° bend by Lubbers and
the conclusion was that with the total transport of gas an F-value of 0.4 then applies. This outcome can cause
one to think that from the 30° bend to the 90° bend a linear relationship holds.
Figure 2.10 distinguishes 3 sections. In the bottom section (Section 1) of the Figure, very low if any transport of
gas pockets at all takes place (flow regime a). The second section is a transitional section between gas volume
transport and gas pocket transport. In this section the gas volume is broken down into small gas pockets.
Energy loss due to the large volume of accumulated gas reduces in this section due to the occurring velocities
(flow regimes b and c). Finally, in the 3
rd
section all the gas volumes are transported (flow regime d). The
question then remaining is: how can we detect gas pockets in pipes?
2.6 METHODS TO DETECT GAS POCKETS IN PRESSURISED PIPELINES
There are different methods which can provide an indication of the presence of gas pockets in pressurised
pipelines. This thesis presents 2 methods which are developed on the basis of theory explained in Section 2.3:
The working point method in Section 2.6.1 and the Pressure comparison method in Section 2.6.2. Tukker et al.
(2010b) describe three other methods to detect gas pockets in pressurised pipelines, these are dealt with
brevity in Appendix C.
2.6.1 DETECTION METHOD 1: WORKING POINT
Gas pockets in pipelines can cause an increase in the total head that pumps should deliver in order to transport
the desired flow. If gas pockets accumulate to form a considerable gas volume, equation [2.11] changes into
equation [2.14]:
gasvstaticlocalft HhhhhH ∆++++= [2.14]
From which the last variable was defined in [2.8] and [2.9]. An increase in total head required will lead to a
reduced flow transport capacity due to a new working point. See Figure 2.13. The influence of the gas pockets
is demonstrated by means of the two-dotted-line. The total head, which should be delivered, increases and
therefore the capacity to transport flow reduces (Q’). However, due to the use of the frequency transformers a
new rotational speed (n) will be established to deliver the desired flow. In other words, a new Q-H curve is
Figure 2.12, c) Stationary bubbles along the pipe; d) bubble flow

created and a new flow Q’’ is achieved (Figure 2.14). In principle this Q’’ = Q. The disadvantage is then that due
to an increased head the power use increases as well:
( )HH
gQ
P ∆+=
η
ρ
[2.15]
In which,
P = power [W]
Q = flow [m
3
/s]
η = efficiency of pump [-]
THEORY
In order to detect gas pockets in a pressurised pipeline the design value for the working point (that is, the
desired flow given a certain pump speed) is compared to the actually occurring value (measured flow, pressure
and basin level). Design values for flow are calculated on the basis of the assumption that steady state
conditions hold; this means that the flow rate is constant for a certain time span. A discrepancy between the
two steady states (design & measured value) indicates that there is a problem either in the pumping station or
in the pressurised pipeline system.
By measuring and logging the pump or pumping station discharge, the pump speed or frequency, the pressure
at the upstream part of the pump and at the downstream part it can be discovered how the pump is working. If
Figure 2.13, Decrease in flow capacity due to gas pockets
Figure 2.14, New working point with increased rotational speed

the power consumption is logged additionally, the overall efficiency can be obtained as well. If no faults are
encountered in this section then it is known that something is occurring in the pipelines.
In order to observe whether a difference occurs between calculated (theory) and measured (practice) working
points it is necessary to model Q-H curves and system characteristics for specific cases. These modelled graphs
representing the designed conditions will be compared to the values that are generated by measured values.
The system characteristic of a proper working system has already been given by equation [2.12]. The Q-H curve
is given by the pump supplier. This Q-H curve represents the nominal speed at which the pump works.
According to the affinity laws, for flow a linear relationship holds for a change in pump speed and for head a
quadratic relationship.
nn n
n
Q
Q 11
= [2.16]
2
11






=
nn n
n
H
H
[2.17]
In which,
nn = rotational speed [rpm]
These equations are important when scaling the working point due to a changed pump speed in order to
maintain a desired flow.
GAS POCKETS OR SCALING
An increase in the network characteristics, resulting in reduced flow rate at a given pump speed indicates
problems that can either be caused by air pockets, scaling or obstructions in the pipeline. When the cause is
scaling, a gradual increase of friction in time will often have taken place. In order to unravel the real cause, the
same pipeline could be studied for an extended period, for instance every two months. If air pockets are the
cause of a reduction in flow capacity then the decrease in flow capacity variation can be unpredictable.
However, both previously-mentioned processes (scaling and gas accumulation) can remain out of sight if only
flow measurements are taken. Therefore it is also desirable to know at what frequencies and thus at what
pressures the pumps have delivered certain flows.
PRACTICE
Flow in pressurised pipelines is often unsteady during small storm events because pumps work at varying
frequencies. During heavy storm events pumps often work at maximum capacity and steady state situations
will develop. During dry weather flow (DWF) situations, steady state occurs only when the pumping time is long
enough.
Theoretically, steady state situations develop after a pump has been running at a constant speed for a certain
time. Generally, downstream of the pump there is a non-return valve which prevents the water bulk in the
pipeline from flowing back into the pumping station. Therefore the pipeline is assumed to be completely filled.
When the pump has been turned on, the time needed to develop steady state conditions will depend on the
downstream and upstream conditions. Pothof approximates this time T, by using equation [2.18].
c
L
Tsteady
20
= [2.18]

In which,
Tsteady = time to develop steady state [s]
L = length of pipe [m]
c = wave speed propagation [m/s]
eE
D
Kc
ρρ
+=2
1
[2.19]
In which,
ρ = density of liquid [kg/m
3
]
K = elasticity of the fluid [N/m
2
]
e = wall thickness pipeline [m]
E = elasticity modulus pipe material [N/m
2
]
If a pipeline is comprised of various materials equation [2.18] becomes:
∑=
i i
i
steady
c
L
T 20 [2.20]
With the subscript i representing the different materials from which the pipeline is made of. For example if the
material of the pipeline is concrete then the wave speed is approximately 1200 m/s. In urban areas pipelines
will often be shorter than 5 km. This means that Tsteady is reached relatively fast ≈ 80 seconds (concrete). An
example is given in Figure 2.15, where a steady flow is obtained after 97 seconds. The pipeline characteristics
can be found in chapter 3, Section 4.4.1 (pipeline Willem Schürmannstraat to WWTP Kralingseveer).
As is the case in Rotterdam, the monitoring takes place once every minute. This means that only one to two
values are recorded in between Tsteady. Therefore, in order to have more values supporting the aim that steady
state conditions have developed, it is necessary to study those events when at least 5 measurements in a row
show a smaller deviation in flow than 5%. Out of pump speed, pressure and basin level the most likely to vary
the fastest is the basin level. Due to the working of the frequency converter the pressure and the flow will not
vary much. However, in DWF conditions, the supply of wastewater is so small that the basin storage is emptied
Figure 2.15, Stabilization time after pump start
Time (seconds), each square 5 sec.
Discharge(m
3
/hr),eachline100m
3
/hr

in just a few minutes. Therefore, this thesis employs the criteria to study those measurements in which 5
measurements in a row have remained fairly constant: ± 10 cm change in basin level (in DWF the difference
between pump start and pump stop in Pumping Stations in Rotterdam is around half meter). In chapter 4 an
indication will be given about the influence of factors such as the local loss factors, the change in diameter and
the roughness (k). All these factors introduce an uncertainty in the system characteristic. Therefore these
uncertainties are taken into account in the calculations.
After looking at 20 events of DWF situations a mean value for ΔHgas is deduced. This will indicate how great the
gas pocket problem is in a specific pipeline. Once this has been established a calculation can be performed with
equation [2.15] to give a value for the extra amount of power consumed due to gas pockets. An average
number of hours per day in which a certain pump was working, during DWF, and an average number of days
per year in which DWF situations occur will give the yearly extra costs due to gas pockets in DWF situations.
This same approach can be followed for SWF situations.
The information required to model the pump in the model is: pump speed, head at different flow and efficiency
of the pump at the same flow values. If these parameters are known, the pump characteristics can be specified.
Regarding the pipe characteristics to be used for the model, the following information is required for each pipe
section: the presumed roughness (k in mm), length (L in m), inner diameter size (D in mm), thickness of the
pipe wall (e in mm) and the elasticity modulus of the pipe material (E in N/m
2
).
In addition, the upstream and downstream conditions have to be determined. Does the downstream level
vary? If the difference in the varying water level downstream is small, i.e. ± 0.05 m if the water level is
maintained between certain boundaries, then an average water level can be assumed. This will introduce an
uncertainty in the results and will be mentioned in chapter 4. If the difference is great, for instance due to the
tidal influence of the sea, then calculations can be performed for both the maximum and minimum water levels
downstream. These values can be obtained from the Ministry of Infrastructure and the Environment
(Rijkswaterstaat) in an online database on water levels at the Meuse. For upstream water levels, on the suction
side of the pump, measurements exist of water levels in the pump pit (basin level). Here an average of 5
measured values will suffice. The uncertainty introduced in this way is taken into account in the calculations.
The local energy losses in the pumping station are taken into account in the model system by modeling in
sufficient detail.
Furthermore, the influence of the water temperature and thus of the viscosity could be taken into account in
the modeling. However, due to the relatively small effect that this parameter has on the dynamic energy
losses, all the calculations can be performed on the basis of one predetermined temperature. The temperature
influence is much smaller than the influence of the roughness parameter. The influence of temperature has
been studied in connection with the Rotterdam situation and is described in Appendix B.
Figure 2.16, Detection method 1: research to operational points

In Figure 2.16 the green square represents the design duty point. The rhombus on the Q-H curve means that
the pump works fine, but that a problem is present in the pipeline. This problem can be the consequence of
either gas pockets or scaling. If scaling is the case, often a gradual increase (from right to left on the Q-H curve)
takes place during time (months to years). If air accumulation is the cause then by calculating the flow number,
equation [2.13], one can say whether the gas pocket can be broken down or not. Besides, a non predictable
behavior in time can be seen. The development of such gas pockets takes place rather quickly, typically within
weeks to a couple of months.
If the case studied represents one of the red dots, which is on the system characteristic curve, then the
problem is that the pump is not working properly either because the pump is dirty, wearing of the wear rings or
because the impeller has worn out.
In order to be able to draw the scheme presented in Figure 2.16 the following steps are performed.
First, the Q-H curve is drawn. This is done averaging the 5 measurements in a row that were studied on the
pump speed. This average pump speed will transform the nominal Q-H curve to the Q-H curve occurring during
those 5 measurements. This transformation is performed combining equations [2.16] and [2.17] (see equation
[4.1]), using these equations the head that should govern at a certain flow can be calculated; this head is based
on theoretical assumptions.
The second step is to draw the system characteristic curve. This curve can be drawn by applying equation
[2.12] with a certain flow. This equation uses the average flow of the 5 measurements studied. In addition, to
determine the static head it uses the basin level at the suction side of the pump and the basin level at the
WWTP or the highest delivery pump downstream. So, three of the averaged monitoring values are used for this
step.






++⋅+= 1
2
2
Lstatict k
D
L
g
v
hH
λ
[2.12]
updownstatic hhh −= [2.21]
In which,
hstatic = static head [m]
hup = head upstream (head at pump basin) [m NAP]
hdown = head downstream [m NAP]
Equation [2.21] uses average values for the varying basin level. This average has maximum outliers of ± 0.10 m.
Step one and two produce those conditions which should govern provided the pipeline and the pump are
working under ideal circumstances. The result of these ideal circumstances is depicted in the green dot, on
Figure 2.16.
The third step is to obtain the actual occurring dots. The circles or rhombus represent the actual occurring
heads provided by the pump. Therefore we need to use the measured pressures downstream the pump. These
pressures are combined with the basin level values at the suction side and thus provide the head that the
pumps are delivering. In order to understand the calculation for the actual occurring pressures, a scheme is
provided in Figure 2.17.

So the head provided by the pump is defined as the difference between the suction head and the pressure
head:
suctionprespump HHH −= [2.22]
In which,
Hpump = head delivered by pump [m]
Hsuction = suction head relative to pump [m]
Hpres = pressure head relative to pump [m]
The suction head is the head just before the centre of the pump impeller, whereas the pressure head
comprises the head just after the centre of the pump impeller.
pumpcsuctionfLbasuction hhhH ,,,sin −−= [2.23]
In which,
hbasin = drainage-basin water level [m NAP]
hL,f,suction = local and friction headloss between suction and pump [m]
hc,pump = datum of pump centre [m NAP]
presfL
data
Pppres h
g
p
zH ,,
5
,
10
+
⋅
+=
ρ
[2.24]
In which,
zp,P = level difference between pressure transmitter and pump datum [m]
pdata = pressure at pressure transmitter [bar]
hL,f,pres = local and friction headloss between pump and pressure logger [m]
Following steps one to three will provide for every average of 5 measurements a scheme like the one depicted
in Figure 2.16. From this scheme conclusions about the working of the pipe and pump can be made. In most of
the cases the working point will not lie on the system characteristics curve or the Q-H curve. On those
Figure 2.17, Scheme of a pump system

situations the more obvious to assume is that both are present, the pipeline has scaling problems or gas
pockets and the pump impeller has worn out or is dirty. By subtracting the head provided by the pump (step 1)
at a given flow with the pressure measured by the same flow (step 3) the influence of the pump
dirtiness/wearing is quantified in meters water column.
),,(),( sin databapumpnpump phQHnQHH −=∆ [2.25]
In which,
ΔHpump = Extra energy loss due to pump dirtiness/wearing [m]
Hn(Q,n) = theoretical head at a given flow and speed pump [m]
Hpump (Q, hbasin, pdata) = occurring head at a given flow, basin level and pressure [m]
This same procedure has to be followed to check the influence of scaling or gas pockets. The only difference
then is that Ht in equation [2.14] is equated with Hpump in equation [2.22]. From there it follows that:
),,(),,( sinsin downbatdatabapumpgas hhQHphQHH −=∆ [2.26]
In which,
ΔHgas = Extra energy loss due to gas in pipeline or scaling [m]
Hpump (Q, hbasin, pdata) = occurring head at a given flow, basin level and pressure [m]
Ht(Q, hbasin, hdown) = theoretical head at a given flow, up- & downstream level [m]
Note that in equation [2.26] ΔHgas could either imply gas or scaling in the pipeline. The results of the different
study cases will determine which one of these two mechanisms is responsible for the major consequences.
2.6.2 DETECTION METHOD 2: PRESSURE COMPARISON
This method is similar to method 1 in a sense that a model value is compared to a measured value; in this case,
the only value to be compared is the pressure downstream of the pump (see Figure 2.17).
Given a certain flow, supposing there are no gas pockets or scaling in the pipeline, a specific pressure should
govern at the pressure device site in order to overcome the static and the dynamic losses, recall eq. [2.12].






++⋅+= 1
2
2
Lstatict k
D
L
g
v
hH
λ
[2.12]
In this case, the static head is equal to the difference in height between the downstream water level and the
level of the pressure transmitter in the pipeline. This pressure is then compared to the measured pressure at
the pressure transmitter, second term in equation [2.24]. If these two measurements are not equal, the
difference is either the influence of scaling or air pockets. Again, the procedure to identify which cause
contributes more to the observed difference has to be sought in time analysis.
For this method only three monitoring values are needed: the pressure downstream of the pump, the flow and
the downstream water level. The following equations permit us to draw the measured line in Figure 2.18 and
2.19.
staticdataLf ppp −=, [2.27]
In which,
pf,L = Pressure due to friction and local losses along pipe [bar]
pstatic = Static pressure between pres. logger and downstream water level [bar]

5
2,
10
gH
p static
static
ρ⋅
= [2.28]
In which,
Hstatic,2 = Level difference between pressure logger and downstream condition [m]
One of the advantages of this detection method is that once a pipeline has been modeled correctly, the specific
pressure values at each flow are well known. This means that every time a new flow is generated, and steady
state situations occur, the measured pressure can be outset against this pre-defined modeled pressure. An
example is given in Figure 2.18. This could easily be implemented in a control system (SCADA or CAS) of pump
operators and serve as a tool to detect gas pockets formation in pipelines.
In Figure 2.18 the red line, with squares, represents the modeled value for the pressures (values have been
modeled with WANDA). As mentioned in Appendix B one of the major variables influencing the dynamic losses
is the presumed roughness. So, the red line resembles the dynamic and static losses with a specific k-value
which can vary along the pipe according to the different pipe materials which compose a pipeline.
The measured values are represented by the blue line (rhombus). A clear distinction is visible between the DWF
values (on the left) and the SWF values (on the right) of Figure 2.18.
In order to see more closely the effects of scaling or air pockets presence, the pressure which is responsible for
overcoming the static head pressure can be discounted, see equation [2.27]. Afterwards only the pressure
caused by friction is compared to the measured pressure, see Figure 2.19.
Figure 2.18, Example of pressure comparison
Figure 2.19, Frictional pressure compare to measured frictional pressure

2.6.3 DISCUSSION ON DETECTION METHODS
In the previous section two methods have been described which are likely to detect the presence of gas
pockets in pressurized pipelines. Some of them are more easily applicable than others, depending on the
available sources of information. Table 2.1 presents which parameters are needed in order to apply one of the
five methods and Table 2.2 is the corresponding legend. Methods 3 to 5 are described in the Appendix C. The
different methods are listed below:
1. Comparison working points
2. Pressure comparison
3. Flow progress during pump start
4. Pressure change after draining isolated pipe system
5. Dynamic measurement
Detection methods
1 2 3 4 5
Modelling
Properties
Pipe
L [m]
D [mm]
k [mm]
e [mm]
E [N/m2]
Fluid
T [°C]
K [N/m2]
ρ [kg/m3]
Pump
n [rpm]
It [kg/m2]
Q-H curve
η [-]
Monitoring
Parameters
Q [m3/hr]
p [mWc]
F [Hz]
Lbassin [m]
P [kWh]
Detection method 1 requires almost all parameters to be monitored except from the power use, which can be
calculated with equation [2.15]. This power use can be additionally monitored to develop an overall efficiency
Table 2.1, Necessary parameters in order to apply a detection method
Table 2.2, Legend for Table 2.1
Legend
Icon Explanation
Needed in high frequency
Parameter must be available
Optional parameter

(including energy use). However, this is not the study of this thesis and therefore it is denoted here as an
optional parameter. In order to distinguish the presence of gas pockets from scaling it is needed to study many
situations (DWF as well as SWF) and in a chronological way. This study will reveal if gas pockets are present or
not. The size and location of gas pockets is more difficult to assess. Nevertheless, a careful study of the high
and length profile of the pipeline will indicate which positions are susceptible for air accumulation.
Detection method 2 requires less monitoring parameters compared to the other methods. However, in order
to discriminate between gas pockets and scaling the same procedure must be followed as for method 1.
Additionally, it gives the possibility to program on an operating system such as SCADA or CAS the modeled
pressures which should go along each flow. From this an up to date result after every steady state situation can
be extracted. However, no information is given about the proper working of the pump.
The 3
rd
method requires the same parameters as the 2
nd
method; however, a high frequency for the discharge
recording device is needed. A frequency of 1 measurement per second should suffice. This method only gives
an impression if a gas pocket is present in the system. No information concerning volume or location can be
obtained.
The method number 4 requires the possibility to take a pipeline segment in isolation. This means that for a
certain time a pipeline will be out of operation and that a pipeline section can be isolated with valves. In
addition, there must be a place where pressure device transmitters can be placed. Furthermore, if the
possibility exists that more than one gas pocket is present in the pipeline; this method would require the
isolation of different pipe segments. This method can give an impression of the size of the gas pocket and is not
negatively influenced by scaling.
Finally, method 5 requires the most quantity of parameters in order to be able to take into account the water
hammer storage due to the compressibility of the gas pocket and the pipe material. This method can
differentiate within the presence of gas pockets and scaling. According to Pothof (2011) the volume of one gas
pocket can be estimated with accuracy of 50% (underestimation) and of 2 gas pockets with an accuracy of 80%.
Besides, this method contributes to find the location of a gas pockets with accuracy close to 85%. This method
requires the possibility of inducing a transient pressure wave without damaging the pipe and pipe fittings. The
most practical way of achieving this transient pressure wave is to generate the pressure wave in the pumping
station itself with the closure of a valve or with the non-return valve. The pressure transmitter should then be
placed after the valve and have a measuring frequency capacity of 20 to 50 or even 100 Hz, depending on the
pipe material and the precision of the fingerprints one would like to obtain from the pipeline.
Next, chapter 3 presents the cases which will be modeled and analysed to investigate the presence of gas
pockets.

3. DESCRIPTION OF CASE STUDIES
In order to answer the third research question, a number of study cases were selected. This section describes
the procedure that has been followed to select case studies and the characteristics of those case studies.
3.1 CRITERIA TO SELECT A CASE STUDY
The wastewater transport system of Rotterdam is divided into 40 major districts. Each of these districts has its
own pumping station to transport the wastewater. In order to select a pumping station, with its corresponding
pressurised pipeline system downstream, a number of conditions are required to be met:
• Depending on the detection method chosen, all parameters described in Section 2.6.3 must be
available or easily reproducible.
• Steady state conditions must be long enough in the measured values to guarantee Tsteady to occur, see
equation [2.20].
• There should be crossings with other infrastructural works or obstacles in order to have downward
sloping pipes in the length-height profile.
• Preference is taken to study pipelines which ‘stand alone’, that is, those which are not connected to
other pipeline system. This would require the hydraulic modelling of a different system as well.
• The pumping station and corresponding pipeline should be representative for the pressurised system
of Rotterdam.
• There must be indications that the pumping station does not deliver its designed discharge.
With the implications that these six criteria impose, 2 district pumping stations with their corresponding
pipeline will be chosen from the Rotterdam wastewater system. Therefore, the next section begins by focusing
on the wastewater system of Rotterdam.
3.2 WASTEWATER SYSTEM OF ROTTERDAM
The municipality of Rotterdam is in charge of collecting and transporting wastewater to WWTP’s. The Water
Boards responsible for the treatment of the water of Rotterdam are: Waterschap Hollandse Delta,
Hoogheemraadschap van Delfland and Hoogheemraadschap van Schieland en de Krimpenerwaard. They take
care that the effluent of their treatment plants is of such quality that it will not have major impact on the
receiving waters.
3.2.1 SEWER SYSTEM
The sewer system of Rotterdam comprises of 40 districts. The wastewater produced in a district is collected
and transported by gravity sewers to a central collector main, by either a combined system or a separate
system. In Rotterdam, the combined system accounts for 71% of the sewerage whereas the separate system
for 27% (GRP3, 2011). From the central collector storage place the water flows by gravity to the pumping
station basin.
3.2.2 PRESSURISED SYSTEM
From the basin of the pumping station the pressurised pipelines transport the water to WWTP’s. In several
pumping stations, when extreme rainfall supersedes the capacity of the pipelines to transport water to the
WWTP, wastewater is directed through other pipelines to the river Meuse.
The total length of pressurised pipelines in the municipality of Rotterdam now reaches 312 km (GRP3, 2011).
This length can be divided into two groups. The first group comprises the pipelines used for the transport of
wastewater between districts and pump stations (94.5 km ≈ 30.3% of force mains), whereas the second group
of pipelines connects houses to the main transport pipelines (217.5 km ≈ 69.7% of force mains). In this
research, however, only the first group is studied.

For the pressurised system, pipelines are needed which can hold heavy pressure loads, PE, PP and cast iron are
examples of materials used in force mains. Small force mains, the second group, (D < 315 mm) often use PVC,
PE and PP. Force mains for larger wastewater transport (D > 400 mm) are mostly built from cast iron or
concrete.
3.2.3 PUMPING STATIONS
The pumping stations are usually located in the deepest part of the districts to allow gravity flow to collect the
wastewater in the basin of the pumping station. In the municipality of Rotterdam all of the 40 districts have dry
well arrangement.
3.2.4 MONITORING OF WASTEWATER SYSTEM
In order to be able to select one of the detection methods outlined in Section 2.6, it is necessary that all
required information is available or easily reproducible. Recall tables 2.1 and 2.2 with the different monitoring
parameters needed. For the application of detection method 1 and 2 all parameters required are available.
Therefore, these methods can be applied. However, some processing of the information needs to be done
before it can be used for the purpose envisaged. Detection methods 3 to 5 need to record higher frequencies
for either pumped volumes or occurring pressures. Due to a limitation in the storage capacity of the Rotterdam
network, a higher recording frequency is as for this moment not viable. In addition, method 4 needs the
isolation of one system (pumping station). Method 2 generates only information about problems in the pipe.
Therefore, the conclusion is drawn that this thesis continues to study the detection of gas pockets by making
use of detection Method 1, because it provides a view on problems occurring in the PS as well as in the pipe.
3.3 CHOICE FOR PRESSURISED SYSTEM FOR GAS POCKETS DETECTION
In order to choose a wastewater transport system for the detection of gas pockets, general information about
the different districts is studied. First, a selection is made through the internal document MIP in which process
and management information is updated 4 times a year. From this list those district PS’s are selected which do
not reach their designed flow capacity and for which no plausible reason is given (see Table 3.1).
WWTP Code Pumping station % supplied Comment
Kralingseveer 1 B. v Kempensingel 63 due to relining
2 Groene Wetering 80
10 Willem Schürmannstr. 57 CSO pumps to Meuse
13 Molenplein 71 Old pumps
17 Fioringras 93
18 Alexanderlaan 95
Dokhaven 3 Heemraadsplein 81 Two models needed
9 Westersingel 91 Two models needed
16 Abtspolder 73 Old pumps
29 Merlijnpad 119
36 Kerkedijk 80 Old pumps
30 Pascalweg 97
23 Wolphaertsbocht 60 Old pumps
24 Everlo 55 Old pumps + relining
Hoogvliet 21 Toscalaan 85 Two models needed
Table 3.1 presents on the right column the main reasons why the design capacity is not met. From this list
those pumping stations are selected which have the potential to accumulate gas pockets. These pockets may
develop in crossings with waterways, with pipelines, roads, etc. Long downward sloping pipes also enhance air
accumulation. Table 3.2 presents those pipelines from which the length-height profiles were studied.
Table 3.1, Pumping stations which do not reach their design capacity

Culverts Sloping pipe Total
Pump
station
No.
Height
[m]
Declination
[m]
Longitude
[m]
Pot. ΔH
[m]
Diameter
I
[mm]
Pipe
length [m]
001 D 1 1.88 3.69 240 5.57 500 & 600 1201
002 R 4 7.21 3.21 30 10.42 400 1047
002 W 0 0.00 9.50 180 9.11 400 2377
003 W 2 25.30 3.20 45 28.50 1600 & 1400 2500
009 R 0 0.00 1.65 15 1.65 900 610
009 W 3 30.90 6.00 320 36.90 1000 & 1400 2650
010 R 3 10.45 1.35 160 11.80 1260 903
010 W 6 19.25 13.60 750 32.85 1200 4262
013 D 5 11.00 2.50 1188 13.50 900 & 1300 5000
017 D 2 4.45 1.40 135 5.85 800 & 860 2300
018 W 8 17.10 11.60 700 28.70 1000 5860
021 W 4 8.35 3.05 79 11.40 960 & 1200 2686
024 R 12 14.20 0.00 0 14.20 800 & 860 4150
024 W 11 10.80 0.00 0 10.80 800 & 860 3770
Table 3.2 presents in the first column the code of the pumping station. The letters (D, R, W) next to the number
indicate discharge to another District, to the River or to the WWTP respectively. The second column represents
the number of culverts with its corresponding height. The third column takes into account the downward
sloping sections, generally comprising angles between 0 and 2 degrees. Eventually, the fourth column presents
the total potential energy loss that can build up due to the presence of gas pocket. In Table 3.2 the ‘stand-
alone’ pipelines are 001, 002R, 002W, 009R, 010R, 010W, 013D and 017D.
Combining the selection criteria stated in Section 3.1 with the information gathered from pump operators
about the working of the selected pipelines in tables 3.1 and 3.2, two pumping stations are selected to be
investigated. These are pumping station 010 (Willem Schürmannstraat) and pumping station 002 (Groene
Wetering). These pumping stations (PS’s) are representative of the Rotterdam wastewater system in the sense
of their size. Pumping station 010 is one of the largest and pumping station 002 is small to middle-sized. The
following sections will systematically present these two systems.
3.4 PUMPING STATION GROENE WETERING
Pumping station Groene Wetering (PS002) is a small to middle-sized pumping station (PS).
3.4.1 OVERVIEW
An overview of the PS is given in Figure 3.1.
I
AVERAGE DIAMETERS
Table 3.2, Potential energy loss due to gas pockets (Total Pot. ΔH)
Figure 3.1, Overview scheme of PS002.

3.4.2 PUMPS
PS Groene Wetering has 3 pumps. Pump no. 1 and no. 2 are connected to the same basin and are responsible
for the transport of DWF and SWF to the WWTP. Both pumps work intermittently. Each pipeline connected to
the pump has its own non-return valve. Behind this the pressure transmitter is placed. At the time when the
data was acquired both pressure transmitters had a default value of (+1 bar). After the conjunction of both
pipelines, a flow meter is placed. Pump 3 is used to pump wastewater during SWF to the Meuse.
A summary of the characteristics of the three pumps is given in Table 3.3.
Pump Capacity (m
3
/hr) Pressure (mwc) Revolutions (min
-1
) Power (kW)
1 (GEHO RP200-480) 140 - 480 32.6 680 (35hz) – 980 (50hz) 55
2 (GEHO RP200-480) 140 - 480 32.6 680 (35hz) – 980 (50hz) 55
3 (GEHO RP200-480) 160 - 620 30.8 680 (35hz) – 980 (50hz) 45
The flow capacity (m
3
/hr) of each pump varies linearly with the frequency (Hz). The set points for the working
of pump no. 1 and no. 2 are given in Table 3.4.
Operation Capacity (m
3
/hr) Pump in (m NAP) Pump out (m NAP)
K1-in 240 -3.60 -4,00
SV 240 -3.59
EV 420 -3.40
K2-in 600 -2.50 -3,20
EV 600 -2.51
SV 600 -3.19
The K1 stands for characteristic (operation mode) 1. This is the normal DWF operation. When the water level at
the basin reaches -3.60 m NAP, one of the pumps begins to work (pump 1 or 2), see Figure 3.2. During SWF
conditions the water level at the basin may fill more rapidly and reach -3.59 m NAP. When this happens, the
frequency converter changes again and works linearly from the SV (start of variable flow) towards the EV (end
of variable flow) operation characteristic, meaning that between the water levels -3.59 to -3.40 a linearly
increase in flow takes place from 240 m
3
/hr to 420 m
3
/hr. This flow can be conveyed by one pump only.
At extreme SWF conditions, PS002 uses pump 3 together with pump 1 or 2. At that moment the second
characteristic of operation starts and 600 m
3
/hr is pumped towards the river through a different pipeline.
However, the latter situation is not desired due to the pollution contained in the wastewater-rainwater
mixture, but is preferred as a way of discharging wastewater in the inner open water courses: canals, ditches
and lakes.
Table 3.3, Characteristics of pumps for PS002
Table 3.4, Operation set points for PS002 (Pump 1 and 2)
Figure 3.2, Set points for PS002
WWTP
River

Q-H CURVE FOR PUMPS TO WWTP
The Q-H curve for pumps 1 and 2 of PS002 is identical and is presented in Figure 3.3. This data has been
obtained from the manufacturer and dates back to 1985.
The efficiency curve for the pumps is presented in Figure 3.4.
It can be seen in the efficiency curve that the maximum achievable efficiency for the pumps is 74%. Figure 3.4
portrays the Q-H curve at maximum speed, 980 rpm. This speed is commonly used to achieve the SWF
discharge (420 m
3
/hr), for the DWF discharge (240 m
3
/hr) a scaled speed is used. Formula [2.16] and [2.17] give
the governing relationships between pump speed, flow and head. In order to scale the Q-H curve to different
pump speeds Formula 3.1 can be applied.
2
1
2
1)(
Q
Q
HnH n
n ⋅= [3.1]
In which,
Hn = head with speed n, see formulae 2.16 and 2.17 [mwc]
H1 = head with a speed of 980 rpm [mwc]
Q
2
n = flow at a given speed n [m
3
/hr]
Q
2
1 = flow at a pump speed of 980 rpm [m
3
/hr]
In the municipality of Rotterdam the speed is linearly scaled to Hz. For PS002 this means that a speed of 980
rpm corresponds to 50 Hz and 685 rpm to 35 Hz. By regulating the speed between these two ends, the
efficiency of the pump remains above 70% for both DWF and SWF conditions. In order to obtain the theoretical
Q-H relationship, monitoring values flow (Q) and speed (n) are used. A minute value is recorded for each one of
these values.
Figure 3.3, Q-H curve for pumps 1 and 2
Figure 3.4, Pump Efficiency

Q-H CURVE FOR PUMP TO THE MEUSE
The Q-H curve is presented in Figure 3.5. The pump was installed in 1985. The efficiency curve is presented in
Figure 3.6. Figure 3.5 gives the Q-H curve at the maximum speed, 980 rpm. When pump 3 is in use, this is the
only speed the pump uses. The purpose is to achieve a discharge of 600 m
3
/hr. This flow, however, is rarely
achieved.
3.4.3 PRESSURISED PIPELINES
There are two pipelines exiting PS002. One discharges water to the WWTP, and the other to the river Meuse.
PIPELINE PS002 WWTP KRALINGSEVEER
The pipeline which transports water to the WWTP has a length of 2377 m. The inner diameter varies,
depending on the section, it can be 316, 388, 389, 399 or 419 mm. Approximately 95% of the pipeline has a
diameter of 399 mm, see Table 4.4 and Appendix E. The pipeline contains a total water volume of
approximately 300 m
3
. During DWF, a flow number of 0.27 is obtained, during SWF, 0.47. This indicates that in
both operation conditions gas pockets are prone to remain in the highest places of the pipeline (in case these
are present), unless good working air valves are present. However, the condition of air valves is unknown.
Figure 3.7 presents the height-length profile of the pipeline that connects the PS002 to the WWTP.
Figure 3.5, Q-H curve for pump 3 of PS002
Figure 3.6, Pump efficiency for PS002 to River

The upstream condition for this pipeline is dependent on the water level at the pumping basin. At the
downstream end of the pipeline the water enters into an elevated reservoir at the WWTP Kralingseveer. This
elevated reservoir receives water from other districts of Rotterdam as well as from districts of Capelle aan de
IJssel. In this reservoir the water level is maintained at +8.11 m NAP during DWF conditions, and during
extreme SWF conditions (> 20 mm/day), the built up of dirt at the bar screens can occasionally elevate the
downstream conditions to +8.25 m NAP. The pipeline consists of different materials these are listed in
Appendix E. Each material has its own roughness. For the modelling of the roughness see Chapter 4.
PIPELINE PS002 RIVER MEUSE
As can be observed in Figure 3.1, the OB (discharge to the Meuse) pipeline is also connected to pump 196. This
pump is managed by Public Works of Rotterdam. So, when the pump basin of PS002 threatens to overflow,
pump 3 is put into working and pump 196 is turned off. The pump basin of pump 3 can be isolated from the
pump basin of pump 1 and 2, but the gate between the two basins is generally open. The flow and pressure
transmitter for this pipeline therefore records SWF events from the PS002 as well as from PS196.
The pipeline described above is composed of two materials: steel (D = 389) and HDPE (D = 399); 95% of the
pipeline comprises HDPE. The length of the pipeline is 1047 m, see Table 4.9. The design flow for this pipeline is
600 m
3
/hr. With this flow a velocity of 1.33 m/s is obtained in the pipeline which implies a flow number of 0.67.
Recalling Section 2.5, this speed is sufficient to start the break-up of eventual gas pockets.
For the hydraulic upstream conditions of this pipeline the same remarks are applicable as for the previous
pipeline. At the downstream side of the pipeline two air valves exist. The first one is situated 50 m after the
pump but is out of order and the second one is situated 500 m after the pump and its working is unknown. The
2
nd
air valve is placed at a height of +2.50 m NAP. In the downstream reach, the outlet of the pipe is situated on
a height of +1.35 m NAP (centre line of pipe). The mean high water level is +1.27 m NAP. Therefore, the final
300 m of the pipeline remains empty for most of the time. In 2010, the outflow of the pipe was completely
covered only 1.5% of the time. Therefore it is assumed that the downstream condition is the bottom of the
pipeline at the highest point of the height-length profile: +3.84 m NAP. The times at which the water level
surpassed the +1.55 m NAP are presented in Figure E.1 (Appendix E).
Figure 3.7, Height length profile of PS002 WWTP Kralingseveer
Air valves

The water pressure at the downstream side of the air valve must be sufficient to overcome the dynamic energy
requirements over the final 300 m. With a flow of 600 m
3
/hr a dynamic force of 1.7 mwc is required to
transport the water in the last 300 m. Such a pressure is almost always present.
3.5 PUMPING STATION WILLEM SCHÜRMANNSTRAAT
Pumping station Willem Schürmannstraat (PS010) stands representative for the larger pumping stations of the
Rotterdam pressurised sewer system.
3.5.1 OVERVIEW
An overview of the PS is given in Figure 3.9.
Figure 3.8, Height length profile of PS002 River Meuse
Figure 3.9, Overview scheme of PS010
Air valve

Pump numbers 1, 2 and 3 are used intermittently for the DWF discharge and are connected to the same
pipeline which transports the water to the WWTP. During SWF discharge two of the pumps or all three are put
into work (as is the case in Fig. 3.1 and 3.9, the green colour indicates that the pumps are in operation).
Furthermore, each pump has its own non-return valve. The diameter size of the conveying pipeline increases
respectively after each pump from 700 to 900 to eventually 1000 mm (inside the PS). Hereafter the flow meter
is placed and then the pressure transmitter. Pumps 4 and 5 are used only when the SWF configuration of pump
1, 2 and 3 is insufficient to cope with the discharge of water. If this is the case, these pumps (4 and 5) discharge
their water into the Meuse through another pipeline.
3.5.2 PUMPS
Pumping station Willem Schürmannstraat has three pumps connected to the WWTP and 2 pumps connected to
the discharge to the Meuse. The characteristics of the pumps are presented below in Table 3.5.
Pump Capacity
(m
3
/hr)
Pressure
(bar)
Revolutions (min
-1
) Power (kW)
1 (Nijhuis RWV 1-500.735) 3.600 3.9 610 – 740 283
2 (Nijhuis RWV 1-500.735) 3.600 3.9 610 – 740 283
3 (Nijhuis RWV 1-500.735) 3.600 3.9 610 – 740 283
4 (Stork “Jaffa”) 4.200 1,8~2,0 515 – 585 155
5 (Stork “Jaffa”) 4.200 1,8~2,0 515 – 585 155
The setpoints for the working of the pumps are presented in Table 3.6.
Operation Capacity (m
3
/hr) Pump in (mNAP) Pump out (mNAP)
K1-in 2.160 -4,90 -5,40
SV 2.160 -4,15
EV 7.200 -3.75
K2-in 2500 -2,50 -2,90
SV 2500 -2,49
EV 7.200 -2,35
The characteristic K1 is the normal DWF operation. Between SV and EV, the SWF operation mode is used. The
K2-mode is used at higher rain intensities (discharge to Meuse) and when wastewater cannot be transported to
the WWTP anymore, either because the maximum flow of water that can be treated has been reached or
because an incident has endangered the transport capacity of the other pipeline. Extreme SWF may require the
use of K1 mode together with K2; however, the use of K2-mode seldom occurs (5 to 8 times a year). Figure 3.10
depicts the different operation modes for the pumps.
Table 3.5, Characteristics of pumps for PS010
Table 3.6, Operation set points for PS010
Figure 3.10, Set points for PS010
WWTP
River

Q-H CURVE FOR PUMPS TO WWTP
According to the flow dictated by the water level at the pump basin, one, two or three pumps are used to
convey the flow into the pipeline. These three pumps have equivalent Q-H curves. Therefore in this section
only one of these three curves is presented. This curve is shown along with the power use in Figure 3.11 and
represents the curve for pump number 1. The pumps were delivered by the Nijhuis Company in 1985. The
efficiency curve is given in Figure 3.12.
The maximum achievable efficiency in these pumps is approximately 88%. Figure 3.11 shows the Q-H curve at
the maximum speed of 744 rpm. This speed is only used in rainfall periods and in the transition from operation
of a single pump to operating more pumps. In addition, this high speed is used during extreme rainfall events
when the maximum capacity is needed. In order to reach the DWF (2160 m
3
/hr), the speed varies between 600
and 650 rpm. For this pumping station it holds that 50 Hz corresponds to 744 rpm.
Q-H CURVE FOR PUMPS DISCHARGING TO THE MEUSE
The pumps serving the pipeline for the sewer overflow into the river date back to 1955. Only an old photocopy
was found of the Q-H curve of these pumps and their performing values are shown in Figure 3.13. In addition,
Figure 3.14 presents the efficiency curve of the pumps. Figure 3.13 gives the Q-H curve at the maximum speed
of 585 rpm. When pump 4 and 5 are on, often, this is the only speed they are operating at. The purpose is to
achieve a discharge of 2500 m
3
/hr. This flow, however, is rarely met.
Figure 3.1, Q-H curve for pump 1, 2 and 3
Figure 3.2, Pump efficiency

3.5.3 PRESSURISED PIPELINES
Two pipelines are connected to PS010. One transports water to the WWTP and one to the Meuse.
PIPELINE PS010 WWTP KRALINGSEVEER
The pipeline which transports water to the WWTP has a length of 4262.5 m. This pipeline varies in diameter
and in material along the way. The mean diameter, however, is 1200 mm and the material used is either steel
or concrete. Table 4.11 and Appendix E provide details about the diameter size of the different segments of the
pipe. The total volume contained in the pipeline approximates 4820m
3
. During DWF conditions the flow
number (F) lies around 0.16 for almost 97% of the pipeline. When the flow has reached the 7200 m
3
/hr, it
corresponds to an average F-value of 0.52. This indicates that potential gas pockets are difficult to remove by
means of flow transport only. In the worst case scenario a potential gas accumulation could be reached of
32.85 meter water column (mwc), recall Table 3.2. Figure 3.15 presents the length-height profile of the
pipeline.
Figure 3.13, Q-H curve for pump 4 and 5 of PS010
Figure 3.14, Pump efficiency for PS010 to River

The upstream condition for this pipeline is dependent on the water level in the pumping basin, in the analysis,
a steady state situation is calculated every minute (based on 5 measurements).
At the downstream end of the pipeline the water enters into an elevated reservoir at the WWTP as has been
explained in Section 3.4.3.
PIPELINE PS010 RIVER MEUSE
In Figure 3.9 it can be seen that pumps 4 and 5 are connected to this pipeline. In case a calamity would nullify
the use of the other pipeline (PS010 WWTP) there is the possibility within the first 500 meters to divert the
water to this pipeline.
The pipeline is comprised of steel (21%) and concrete (79%). The length of the pipeline is 900 m, Table 4.15
gives details about the different diameter sizes. The design flow for this pipeline is 7200 m
3
/hr, but the
operation mode ranges between 2500 and 7200 m
3
/hr. With the lower limit, a flow number of 0.16 is achieved,
whereas with the upper limit a F-value of 0.46. This indicates that no gas pocket transport is possible. A layout
of the pipeline is presented in Figure 3.16.
Figure 3.15, Height-length profile of PS010 WWTP Kralingseveer
Figure 3.16, Height-length profile of PS010 --> river Meuse
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