Distribution of Water

Table of Contents
1. Chapter 3: Distribution of Water
2. Key Features of a Ideal Water Distribution System
3. Types of Water Distribution Pipe Network Layouts
4. Distribution reservoirs
5. Storage capacity of distribution reservoirs
6. Pipe network analysis
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Chapter 3: Distribution of Water

The purpose of a water distribution system is to deliver water from the treatment works or source to the consumer with the required quality, quantity and pressure. The term distribution system collectively describes the pipes, valves, storage reservoirs, pumping stations and appurtenances arranged to supply water to points of use. The water supplied through this system may be drawn from surface sources such as ponds and lakes, streams and rivers, storage reservoirs, and in some cases oceans (after desalination).

Key Features of a Ideal Water Distribution System

• Water quality should not deteriorate in the distribution pipes; the system should avoid contamination and excessive residence time.
• The system must supply water at intended places with sufficient pressure head for normal use.
• It should be capable of supplying the requisite additional quantity during fire-fighting events.
• The layout should permit continued supply to consumers during repair or maintenance of any section, isolation by valves should be possible.
• Pipes should be laid so as to maintain safe separation from sewer lines - preferably at least one metre away or above sewer lines.
• The network should be fairly watertight to keep leakage losses to a minimum and to conserve treated water.

Types of Water Distribution Pipe Network Layouts

Distribution pipes are generally laid beneath road pavements and therefore often follow the road layout. Four principal types of pipe network layouts are commonly used singly or in combination:

(I) Dead-end (branch) system

In the dead-end system, mains branch out from a primary feeder and terminate at the extremities. It is simple and economical for small, low-density areas.

• Advantages: Low first cost, simple to construct and operate.
• Disadvantages: Poor circulation leads to stagnation and possible water quality deterioration; failure of a main may deprive many consumers of supply; difficult to provide adequate fire flow at extremities.

(II) Grid-iron system

A grid-iron system is formed by laying mains in a rectangular or square grid. Each block is supplied from two or more directions, improving circulation.

• Advantages: Better circulation and water quality, alternative flow paths reduce service interruptions, improved ability to meet fire demands.
• Disadvantages: Higher cost than a simple dead-end layout, more valves and fittings required.

(III) Ring  (circular) system

The ring system supplies each area from a circular main or multiple rings. Rings may surround high-demand zones.

• Advantages: Good reliability and uniform pressures; easy to isolate sections for maintenance.
• Disadvantages: Higher initial cost; may require larger mains to maintain velocities and pressures.

(IV) Radial system

A radial system has mains radiating from a central point (for example, a central reservoir). It suits towns where demand is concentrated around a central storage or pumping station.

• Advantages: Simplicity of flow control from a central location; clear hydraulic layout.
• Disadvantages: Failure at central source or main feeder can affect a large area; peripheral continuity may be poor unless rings or interconnections are provided.

Distribution reservoirs 

Distribution reservoirs also called service reservoirs, store treated water for meeting emergencies (fires, pump failures, repairs) and to absorb hourly fluctuations in normal demand. They form a vital link between supply capacity and consumer demand pattern.

Functions of distribution reservoirs

• To absorb hourly variations in demand (equalise supply and demand over short periods).
• To maintain reasonably constant pressure in the distribution mains.
• To provide stored water for use during emergencies such as pump or power failure and fire-fighting.

Location and elevation of distribution reservoirs

• Should be located as close as possible to the centre of demand to reduce main sizes and provide more uniform pressures.
• The water level in the reservoir must be at sufficient elevation to permit gravity flow and provide adequate pressure at the highest service point; the service level is set by hydraulic requirements of the system.

Types of reservoirs

• Underground (subsurface) reservoirs
• Small ground-level reservoirs
• Large ground-level reservoirs
• Overhead tanks

Storage capacity of distribution reservoirs

The total storage capacity of a distribution reservoir is the sum of the following components:

(I) Balancing storage

Balancing storage (also called equalizing or operating storage) is the quantity required to equalise fluctuating demand against a constant or varying supply. It is commonly determined by the mass-curve method:

Mass-curve method:

• Plot cumulative supply and cumulative demand versus time on the same graph.
• The vertical distance between the two curves at a given time represents the surplus or deficit of water relative to demand.
• The balancing storage required is the maximum deficit (maximum ordinate) of cumulative demand above cumulative supply over the period considered.

(II) Breakdown storage

Breakdown storage (emergency storage) is reserved to tide over emergencies caused by pump failures, power breakdowns, or other interruptions in supply. Provision for this storage varies by design practice; a value of about 25% of the total reservoir capacity, or 1.5 to 2 times the average hourly supply, is commonly used as a rule of thumb.

(III) Fire storage

Fire storage is the volume set aside for extinguishing fires. Provision varies with local regulations and fire risk; a provision of 1 to 4 per person per day is sometimes cited to meet fire requirements (check local standards and codes for the precise value to be used in design).

Total reservoir storage =  Balancing storage + Breakdown storage + Fire storage

Pipe network analysis

Analysis of a water distribution network requires determination of flows in the various pipes and the head losses so that residual pressures at junctions can be found. Two fundamental conditions must be satisfied in any steady-network analysis:

• The algebraic sum of head losses (pressure drops) around any closed loop must be zero - a statement of conservation of energy around the loop.
• The sum of flows entering a junction must equal the sum of flows leaving that junction - the continuity (mass conservation) condition.

Because head loss depends non-linearly on pipe flow, network equations are nonlinear and are normally solved by successive approximation (iterative) methods. The most widely used iterative technique in pipe-network analysis is the Hardy-Cross method.

(I) Hardy-Cross method (conceptual description)

• Assign an initial set of flows to all pipes such that continuity at nodes is satisfied.
• Identify independent closed loops in the network.
• For each loop, compute the head loss in each pipe using a chosen head-loss relation and take algebraic sum around the loop.
• Compute a correction to the assumed flows for the loop and apply the correction with appropriate sign to each pipe in the loop.
• Repeat the process for all loops until the corrections become negligibly small and both loop energy balances and node continuity are satisfied.

(II) Head-loss relations used in design

Head loss in a pipe is expressed in the general form hₙ = k Qⁿ, where hₙ is head loss, Q is discharge, k is a constant depending on pipe geometry and roughness, and n is an exponent determined by the chosen empirical or theoretical formula.

• Darcy-Weisbach equation: based on friction factor and is rigorous for all flow regimes; for turbulent flow the head loss varies approximately as Q² (n ≈ 2).
• Hazen-Williams formula: an empirical relation commonly used for water mains; the exponent n is about 1.85 in this formulation.