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.

Table 5.13: Recommended pipe C-values (New pipes)

Pipe Material	Diameter	Recommended C-Values
Plastic	300 mm	150
	< 300 mm	140
Iron	300 mm	140
	< 300 mm	130

(Source: Philippines Rural Water Supply Design Manual, 2012)

Another contributing component of total head loss is the head loss from turbulence due to pipe fittings and appurtenances. This category of losses is sometimes called minor losses. The total minor losses in a distribution network is usually insignificant compared to the total head loss of the system, thus, the designer may ignore this component in network analysis computation.

5.5 Water supply transmission system

The transmission system’s function is to transport water from source to the reservoir, if any, and to the distribution point. Water conduits for the transmission system may be canals, aqueducts or tunnels, free-flow pipelines, or pressure pipelines. The transmission of water will either be under gravity or pumping. Pressure pipeline is generally the type of water conduit used for water supply transmission systems.

5.5.1 Determination of Transmission Pipe Size

Normally, the sizing of the transmission main is dependent on the total storage capacity and the way the supply is transmitted to the distribution system. The main should have at least the carrying capacity to supply water at a rate equivalent to the maximum day demand of the system for a given design year.

As a rule of thumb, for transmission by pumping, it is advisable to assume a preliminary head loss (hL) of about 5.0 m/km of pipeline. (As much as possible, head loss should be limited to 10.0 m/km of pipeline for transmission by pumping). For a gravity system with a considerably elevated source (e.g. highland springs), the transmission line could afford to have higher head losses as long as the remaining pressure head at the downstream end is sufficient for the distribution system’s needs. For a gravity system with source elevation that is not much higher than the distribution system, the head losses are lowered to attain just sufficient pressure head in the distribution system.

5.5.2 Maximum Pressure

The pipe material should be selected to withstand the highest possible pressure that can occur in the pipeline. For a gravity system, the worst-case scenario is for pressure to be at its maximum during shut-off at the downstream end when the static pressure is too high. For the transmission line design, a maximum computed HGL based on a minimum supply rate equivalent to 0.3 times the average day demand should be examined. However, practical experience from the ground suggests that there is no need to limit the maximum allowable pressure to 60m head. Instead, considerations should be given on the basis of the economic grounds. However, in any circumstances where the maximum allowable pressure be exceeded, ,break pressure tanks should be installed along the main. The break pressure tank will limit the static pressure by providing an open water surface at certain points of the transmission line. Design of transmission line should be undertaken with the use of hydraulic computer software, which is discussed in the succeeding sections. Figure 5.1 below shows the longitudinal section of pumping main showing hydraulic gradient and characteristic curve

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