Chapter Five: Pipelines Design

The quantity of water to be supplied to a particular community has to be conveyed from source to consumers. Normally, the medium through which water has to be conveyed in pipelines. This chapter describes the pipelines design. It presents the calculation of pipelines hydraulics and associated design approaches.

5.1 Design Requirements of pipelines

There are five principle operational requirements for a pipeline. The requirements are;

• It must convey the quantity of water required,
• It must be capable of resisting all external and internal forces,
• It must be durable and meet the design working life,
• It must be properly laid and embedded,
• The material from which it is made should not adversely affect the quality of the water being conveyed.

5.2 Types of pipelines

Broadly, there are two types of pipelines which should be considered for design. They are transmission and distribution systems. Transmission and distribution systems vary in size and complexity but they all have the same basic purpose, which is to deliver water from the source(s) to the consumer.

5.3 Security considerations for pipelines

When designing a water supply project, pipeline route should be located. It should be accomplished by ensuring to obtain pipeline way-leaves. For security reasons we propose marker posts to be provided for the boundaries of the way-leave. For all pipelines it is important to obtain and secure a way-leave so as to avoid problems later on. Even in road reserves the alignment should be agreed with the road authority in advance and officially recorded so that even many years later there can be no argument when it comes to any dispute or compensation claim.

5.3.1 Methods of water transmission and distribution

There are three (3) methods which should be considered when transporting water from the source to the treatment plant, if any, and the distribution system, and eventually reach consumers. The methods are;

• Through gravity flow
• Through pumping with storage
• Through direct pumping to the distribution system

5.3.2 Gravity flow

This is the ideal set-up when the location of the water source is at a considerably higher elevation than the area to be served. The operation cost of a gravity system is very low, as it does not require energy cost.

5.3.3 Pumping with storage

Water is either (a) pumped to a distribution pipe network, then to consumers, with excess water going to a storage tank, or (b) pumped to a storage tank first, then water is distributed by gravity from the tank to the consumers. The maintenance and operation cost of this system is higher than a gravity system.

5.3.4 Direct pumping to the distribution system

In this system, water is pumped directly from the source to the distribution system to the consumers. Where capital cost for a reservoir is not affordable at the initial stage of the water system, direct pumping to the distribution is usually resorted to. Variable speed or variable frequency drive pumps are most ideal for direct pumping operations, but the capital costs for such equipment are higher than for conventional water pumps.

5.4 Pipeline Hydraulics Assessment

5.4.1 Pressure

Pressure is generally expressed in N/m², also called Pascal. Because of the level or amount of pressure in a water supply system, pressure is commonly expressed in kilopascals (kPa) or simply in meters (m).

Pressure increases linearly with the depth of water. For water at rest, the variation of pressure over depth is linear. The pressure exerted by a column of water is called h = pressure head, and can be calculated using the formula below:

h=P/ɤ

(5.1) Where,P= pressure; ɤ= specific weight of water

5.4.2 Determination of Head Losses

The commonly used formulae for computation of head loss due to friction (also called friction loss) are:

• Darcy-Weisbach formula
• Hazen-Williams formula
• Manning's formula
• Combined Darcy-Weisbach and Colebrook-White equation

This Manual recommends the use of Hazen Williams among the above formula. The formula, which is the most widely used, relates the velocity of the flow, hydraulic mean radius and hydraulic gradient. In terms of head loss due to friction, the formula is:

h_L=(10.7LQ^1.852)/(C^1.852D^4.87)

(5.2)

Where, h_L = head loss due to friction; L = distance between sections or length of pipeline (m); C = Hazen – Williams C-Value; D = internal diameter (m); Q = pipeline flow rate (m³/s)

The C-value is a carrying capacity factor that is sometimes referred to as the roughness coefficient, which varies depending on the pipe material being considered. Higher C-values represent smoother pipes and lower C-values are for rougher pipes. Higher C-values indicate higher carrying capacities. C-values increase with pipe size but decrease with pipe age. Although C-values are affected by changes in flow rates, the effect is negligible. Thus, network designers usually assume uniform C-value for different flow rates. Table 5.1 presents the recommended C-values for various pipe materials.