Hydraulic design and management of wastewater transport systems
154
Hydraulic design and management of wastewater transport systems
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Overview
Product Details
| ISBN-13: | 9781780407821 |
|---|---|
| Publisher: | IWA Publishing |
| Publication date: | 04/08/2016 |
| Pages: | 154 |
| Product dimensions: | 6.12(w) x 9.25(h) x 0.75(d) |
Read an Excerpt
CHAPTER 1
Foreword
1.1 NECESSITY OF THE MANUAL
The creation of the manual 'Hydraulic design and management of wastewater transport systems' arose from the research project CAPWAT (CAPacity loss in wasteWATer pressure pipelines), which researched the mechanisms for the creation, stagnation and discharge of gas bubbles in wastewater pressure pipelines. During this six-year research programme, it was recognised that there is no hydraulic manual/guideline that focuses on the entire wastewater pressure pipeline system, the processes it includes, and the interaction between the pressure pipeline and the pumping station.
Processes that hardly or never occur in clean water transport systems (such as cooling water, drinking water) must be taken into account when designing a wastewater transport system. In case of wastewater, we have to deal with discontinuous supply. The type of sewer system (combined, separated, improved separated) determines the distribution of supply flow during both dry and wet weather.
The characteristics of the wastewater (such as surface tension and turbidity) vary in time and per location, as well as the waste load (floating and non-floating parts).
Usually, the designers and managers are used to thinking in terms of stationary processes. A wastewater pressure pipeline does not operate according to a stationary process, certainly not in dry weather conditions. Knowledge about the dynamic processes (variation in time) that occur in a wastewater transport system is necessary in order to determine the design and management guidelines.
Two important processes that the designer/manager deals with are:
• The process of the creation, stagnation and transportation of gas bubbles, and
• The water hammer phenomenon.
Another aspect is that the wastewater transport system is becoming more complex. Due to building larger sewage water treatment plants, wastewater is being transported over greater distances and increasingly more (and smaller) pipelines connect to the main sewers. The operation of the pumping stations is largely determined by how the entire system behaves. Insight into this operation is, therefore, crucial for proper design and management.
The manual 'Hydraulic design and management of wastewater transport systems' provides an overview of all the aspects and interrelatedness that are crucial for the hydraulic design and management of a wastewater transport system. A wastewater transportation system is understood to mean the pressure pipeline and/or the pressure pipeline system, including the pumping station and the receiving basin.
The central point of the design is to create an independent and safe system with the necessary transport capacity at minimum societal costs. Predominantly, the management aspect focuses on guidelines to maintain the design principles regarding capacity and required energy.
1.2 SCOPE OF THE MANUAL
The purpose of this manual is to create a compilation of all the hydraulic knowledge that is necessary for designing a wastewater transport system and to manage it operationally. The wastewater transport system is the link between the collection and treatment of the wastewater. The collection system includes, among others, the gravity flow sewage system from the house (or consumer) and service connection through street and main sewers up to the suction basins. The transport system, for which this manual was written, includes the suction basin, the sewage pumping station and the pressure pipelines.
In the Netherlands, municipalities and district water boards are the organisations responsible for the wastewater transport systems, as shown in Table 1.1.
This is a supplement to the existing and generally accessible information such as the Dutch Sewage Guideline. Modules B2000, B2100 and B2200 focus on the hydraulic design of the gravity flow system and provide designed flow rates for sewage pumping stations. Module C6000, pumping station management, mostly focuses on the design of the pumping station, as well as on management and maintenance organisation. This manual is also a supplement to existing standards, especially the Dutch standards NEN-EN 752:2008 Drain and sewer systems outside buildings – mostly about gravity flow sewerage – and NEN-EN 1671: Pressurized sewerage systems outside buildings – about pressure sewerage. This manual completely focuses on the hydraulic aspects of the design and management of wastewater transport systems.
In addition, many organisations have manuals about the design and management of pumping stations. These manuals mostly describe civil engineering, mechanical and electro-technical issues.
For now, this manual 'Hydraulic design and management of wastewater transport systems' should not be viewed as a replacement of such manuals, but as a supplement.
The following stages are recognised in the life cycle of a pipeline system (see also the Dutch standard NEN-EN 3650 'Requirements for Pipeline Systems'):
- Design
- Construction and testing
- Usage stage (operational management)
Before the design, the development stage takes place, also known as the preliminary design. The preliminary design is mostly determined by the usage requirements (functional requirements) and planning aspects. The design stage can be divided into the basic design stage and the detailed design stage. In the basic design stage, the definite points of departure (schedule of requirements) for the design are determined. In the detailed design stage, the calculations, drawings and specifications are established for the realisation and operational management stage. There is no fine distinction between the two design stages and, in this manual, it is summarised as 'design'.
The flow chart in Figure 1.1 describes the scope and interconnectivity of this manual. The starting point is that the preliminary design is available, although some points of attention are still mentioned. Therefore, this flow-chart emphasises the design of the transport system, followed by a chapter about the delivery of the installation, which describes how to test whether the built installation complies with the hydraulic design criteria. The construction of the installation is a stage that takes place between the design stage and delivery. In this stage, there are no specific hydraulic focal points and, therefore, this manual does not include a separate chapter about the building stage of the transport system.
During the utilisation stage, the purpose of this manual is to maintain the desired capacity at minimal societal costs.
1.3 AUTHORS AND EDITORIAL STAFF
In 2010, the first version of this manual was drafted by Michiel Tukker (B.Eng), Kees Kooij (B.Eng) and Ivo Pothof (PhD) of Deltares. The editorial staff included:
François Clemens Deltares/TU Delft (prof.)
In 2012, a second Dutch version was published in which the experiences and comments of users were included. This manual was translated into English and published in 2016 (this edition).
1.4 READER'S GUIDE
The connection between the various components of the piping systems and the mutual interaction means that the reader cannot read this manual sequentially in one go, but will often have to return to previous sections. The theoretical background information has been compiled in a separate annex. The emphasis of this manual is on describing the design and management processes, and the interconnectivity.
Chapters 2–6 focus on the design process, which is tested again in Chapter 7. Chapter 8 discusses the delivery and acceptance stage. Maintaining the hydraulic capacity is discussed in Chapter 9.
CHAPTER 2
Designing wastewater transportation systems
The flow chart of the design process (Figure 2.1) shows that the design of a wastewater transport system is an iterative process. Every choice influences other components, which is why choices made might have to be repeated.
This does not only apply for the design process; Renovations and adjustments are also included in the same iterative process, because every change made to the system may have a far-reaching impact on the other components. This is why a system adjustment cannot be seen as a separate process, but as a design process with additional preconditions.
If the design process concerns an existing system, it is desirable to include the current situation of the system in the design process and not to just assume the starting points of the previous design. The design process must, therefore, be undertaken integrally and not divided into separate objects.
In many cases, it will not be possible to fully comply with all the main functions. In such cases, assessments will have to be made and priorities will have to be set. An assessment can be made by providing the designer with thorough theoretical knowledge. This manual contains the necessary hydraulic knowledge for the design and management assessments regarding wastewater pressure pipelines.
The transport system includes the entire system, starting from the receiving basin (suction basin), subsequently the pumping installation, the pressure pipeline and the endpoint (treatment plant or another receiving basin). The receiving basin and pumping installation together form the pumping station. This manual discusses both components separately.
Additionally, there are other arrangements that are necessary to guarantee the integrity of the system (water hammer surge protections) and to carry out management activities. First, the boundary conditions must be clear before discussing the design. The primary boundary condition is the discharge capacity in dry and wet weather conditions. In addition to transporting wastewater, the system must also be tested for the following requirements:
• Transportation of gasses (mixed-in air or chemically or bio-chemically formed gases)
• Transportation of solid elements (sediment, floating waste)
• Reliable and safe operation, also in extreme situations (water hammer)
• Low maintenance
It is important to monitor the energy-saving and efficient operation of the system. Furthermore, the design must take into consideration the management stage, so that maintenance to the system can be carried out efficiently with minimum use of staff, as well as without delays and extra costs that can be avoided with a good design.
The following chapters will further elaborate on the design aspects of each component of the wastewater transport system.
CHAPTER 3
Pipeline design
We refer to a pressure pipeline when it is a pipe entirely filled with liquid (with the exception of local gas bubbles) at a positive pressure. The pressure is created in flat areas by a pumping installation. However, a main that connects a high reservoir with a low reservoir can also be considered a pressure main under the condition that no free surface flow occurs. Therefore, this is a pipeline flow driven by gravity.
In its simplest form, a pressure pipeline consists of a single pipe that connects a reservoir (suction basin) with another reservoir (basin, receiving construction, etc.).
A pressure pipeline can also connect to another pressure pipeline. Initially, this chapter will discuss the simple pipeline. In principle, the design aspects for a connecting pipeline do not deviate from this. The extra requirements can be converted into additional boundary conditions for the design process.
The pressure pipeline dominates the design and management process. The distance that the wastewater needs to bridge is often an established fact. In addition, carrying out a crossing (this includes all crossings such as culverts, horizontal directional drilling and dike crossings) with other objects (roads, railways, watercourses, dikes, etc.) is an important element in the question of capacity.
Most design choices regarding the pressure pipeline influence the design of the pumping station (pump and other appendages).
The design of a pressure pipeline is primarily determined by economic aspects, constructive aspects (choice of material, strength), and liquid-mechanical aspects. This manual only describes the liquid-mechanical aspects.
Figure 3.1 presents the preconditions and design activities for the design process of the pipeline(s).
3.1 BOUNDARY CONDITIONS
3.1.1 Flow rates
The flow rate is not a design choice, but is a fixed boundary condition. The pipeline system must be dimensioned to carry the maximum designed flow rate Qmax, usually the maximum expected flow rate in wet weather conditions. Most of the time (approximately 80%) we deal with dry weather supply, which means discontinuous pumping operation.
Wastewater transport systems are driven by supply. The distribution between dry weather supply and wet weather supply, the installed pumping capacity and the type of control determine the distribution in the discharge. A pumping station with one on/off pump (while disregarding the variations in water levels in the basin) will always pump at the same flow rate regardless of the dry or wet weather supply. Only the duration that this flow rate flows through the pipeline depends on the supply.
3.1.2 Choice of route
The length of the pipeline is determined by the beginning location and the end location. Usually, the shortest distance possible is chosen while taking into consideration the existing infrastructure and zoning of the area, which is why the length of the pipeline is fixed.
During the preliminary design stage, it may be useful to investigate possible problematic points caused by the profile of the pipeline. When designing a wastewater pressure pipeline, the starting point is that the wastewater is in a situation of positive pressure in normal operation conditions, but also during standstill of the pump. At high points in the route, it may be that there is permanent negative pressure, which makes degasification possible. The released gas can cause extra loss of energy. Another source of energy loss can be the downward inclined pipe components of (drilled) pipes if gas bubbles accumulate here. That is why it is recommended to minimise the number of culverts and drilled pipes when choosing the route.
3.2 DETERMINING THE DIMENSIONS OF THE MAIN
The dimensions of the pipeline and the flow rate determine the loss of energy during operational circumstances. There are various equations for calculating this frictional loss (Darcy-Weisbach, Chézy, Manning). In the Netherlands, but also internationally, DarcyWeisbach is used for entirely filled pressure pipes:
ΔH = λL v2/D 2g (3.1)
In which,
ΔH= Friction loss [m]
λ = Friction coefficient [-]
L = Pipe length [m]
D = Diameter [m]
v = Flow velocity [m/s]
g = Gravitational acceleration [m/s2]
The friction coefficient λ is determined, among others, by the wall roughness of the pipeline, expressed in k (mm) – see Annex A.4.
Due to the planning boundary conditions, the pipe length is a parameter that can hardly be influenced. That is why the designer makes a great impact in determining the behaviour of the system when he chooses the diameter and, thus, the velocity in the pipeline. The choice regarding the material of the pipeline hardly has any influence on the capacity calculation, but does determine, to a large extent, the dynamic behaviour of the system (water hammer).
3.2.1 Profile of the pipeline
The height of the pressure pipeline is primarily determined by the local surface level. To limit earth moving, the pipeline will not be laid lower than what the local applicable regulations prescribe (frost proof, landowner, safe protection against damage due to digging, laying method, crossing objects, etc.).
An assumption is also that during all operation conditions, thus also during standstill of the pump, there is positive pressure in the pipeline. During negative pressure situations, as shown in Figure 3.2, there is a risk of creating gas bubbles in the system as a result of air entry from outside (leaking connections) on the one hand, and degasification of the wastewater (see Annex A.8.5) on the other hand. Primarily, the pipeline and connections must be resistant to all loads: external loads, such as ground and traffic loads, as well as internal loads, such as stationary and dynamic pressures.
(Continues…)
Excerpted from "Hydraulic Design and Management of Wastewater Transport Systems"
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Copyright © 2016 Deltares.
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