JUSTIFICACIÓN Y OBJETIVOS

2.7.1 Introduction

In this section, components of bulk water distribution systems are discussed. A good understanding of the entire system and its components is absolutely critical in order to develop a strategy of how best to implement condition assessment interventions, such as the one to be carried out in this study.

Bulk water distribution systems generally require large capital investments. Table 2-8 presents data, from an annual report (Rand Water, 2007) published by a South African bulk water supplier - Rand Water, of the capital requirements of their various asset categories. Rand water supplies potable water to the Gauteng province and other areas of South Africa and is the largest water utility in Africa.

Table 2-8: Value assets of the Rand Water Board (Rand Water, 2007)

Asset Categories Value (R’000) % of total value

Land and buildings 246 042 5.6

Plants and reservoirs 1 720 065 39.3

Bulk water distribution pipelines 2 292 794 52.3

Vehicles 121 961 2.8

Total 4 380 863 100

From the data in Table 2-8 it can be seen that the pipeline assets constitute the largest percentage of the total capital requirements, at 52.3%. Furthermore, the annual report also explains that bulk pipelines are critical from an operational point of view because they are used to distribute water to the end users, and any pipeline failure will impact on the level of service experienced by the affected end users. This re-emphasises why it is critical for bulk pipelines to be well maintained, and that leaks are detected and repaired when they occur.

In this study, the following components were identified as critical when it comes to understanding bulk water distribution systems: the pipelines used to convey the water, the pump stations, the valves isolating the pipeline, and the storage tanks.

2.7.2 Pipelines

Bulk water distribution pipelines are critical from an operational point of view, as they are used to convey large amounts of water from one point to another. The range of pipe diameters can

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vary from system to system. In this study bulk pipelines are not classified based on their diameter, but rather the pipe function; in other words pipelines that, for example, transfer water from a reservoir to another reservoir or from a treatment plant to a reservoir; such a pipe will be considered a bulk pipeline. However, any pipeline directly servicing a consumer is not considered a bulk pipeline.

The American Lifeline Alliance (2005) report on guidelines for water pipelines states that general pipeline design approach entails designing a system to safely accommodate internal pressures, vertical earth load, surface live load, pipe deformation, fatigue and fluid transients. Pipelines are made of a range of different materials, each material with its own unique characteristics. These pipe materials are mainly classified as either metallic or non-metallic. Pipe materials are discussed in detail in Section 2.8. The failure of a pipeline can have a wide- ranging impact on an economic, environmental and social level (Agrawal & Sinha, 2015). Pipelines may fail due to age-related material failure, considering the fact that pipelines have a limited design life. Table 2-9 presents the design lifespan associated with commonly encountered assets of Rand Water. The table shows that the design lifespan of pipelines can vary significantly, with some pipelines lasting as long as 75 years whereas others lasting only up to 20 years.

Age, however, is not the only reason why pipelines can fail. External factors such as environmental conditions around the pipe can also play a role. Knowledge and information about these two factors are critical to predict the pipeline failure (Nel, 2009).

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Table 2-9: The design life of bulk water distribution assets (Rand Water, 2006)

Asset Design lifespan (Years)

Storage facilities

Reservoir and storage tank 80

Pipelines

Pipeline – Steel shell 75

Pipeline – Steel with cement mortar lining 50

Pipeline – Steel with epoxy lining 50

Pipeline – Steel with HDPE lining 50

Pipeline – Steel with Bitumen lining 25

Pipeline – Steel with Bitumen coating 50

Pipeline – Steel with Bituguard coating 75

Pipeline – Steel with Sintercoat coating 50

Pipeline – Steel with cathodic protection 20

Pipeline – Pre-stressed concrete 30

Pipeline - HDPE 25

Pipeline - GRP 30

2.7.3 Pump stations

Pump stations consist of a number of sub-systems that form the pump system. These sub- systems consist of pumps, motors, controls, power transmission and valves. The reliability of a pump and its electrical power supply is important within the bulk water distribution environment, when pumping is required (Cullinane, 1985).

The reliability of the whole pump system is a critical aspect of bulk water distribution. Cullinane (1985) indicated that the reliability of a particular pump system, consisting of a series of systems, can be calculated by using equation 2-29:

𝑅_𝑆 = 𝑅_𝑃× 𝑅_𝑀 × 𝑅_𝐶 × 𝑅_𝑃𝑇× (𝑅_𝑉)2

Equation 2-32

Where Rs is the reliability of the pump system, Rp is the reliability of the pump, Rm is the

reliability of the motor, Rc is the reliability of the control unit, Rpt is the reliability of the power

station, and Rv is the reliability of the valves (1 intake and 1 delivery valve).

This equation suggests that sub-system failures are all independent of each another. In other words, the failure of one sub-system component is neither influenced by, nor does it influence, the failure of another component of the sub-system. Reliable electrical power supply is important within the bulk water distribution, especially when pumping is required.

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2.7.4 Pipeline valves

Valves in bulk water pipeline systems control the flow of water and are used to isolate a section of the distribution pipeline to facilitate repair. Large valves are usually housed in valve chambers providing easy access for maintenance or future replacement.

The following types of pipeline valves are used (Burstall, 1997):

• Line valves: their function is to section the pipeline into manageable lengths. This is useful when work is carried out on a section of the pipe.

• Bypass valves: these are typically fitted on main valves. Their function is to provide a slow charging rate across the main shut valve. This avoids surging and allows the air in the section of pipeline being charged to escape by way of the air valves.

• Branch valves: their function is to isolate a branch line from the main pipeline. They are fitted as close as possible to the main pipeline.

• Air valves: their function is threefold: first, to allow air to escape when the pipeline is charged; second, to allow air to enter the pipeline when water is being discharged from the pipeline; and third, to allow dissolved air to escape at minor high points.

• Scour valves: these need to work when they are needed. They are vital to the emergency operation of a pipeline, e.g. shutdowns.

• Non-return valves: these valves allow flow in one direction only. Typically, they are found on scour valve outlet pipes, fitted to prevent backflow into the main pipe.

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