Water Supply
Advanced Zoning Methods (District Metered Areas)
Introduction
The conventional zoning methods are limited to manual identification of manageable command areas with similar attributes like pressure, nodal heads or sometimes the administrative boundaries are also used to identify suitable zones.
However these kind of approaches do not ensure the hydraulic reliability of water networks and the ease of operation and maintenance. As the complexity of water networks increased, the water utilities realized the need of partitioning the water networks into suitable zones.
These zones (sectors, clusters or district metered areas are used alternatively now a days) have become one of the efficient strategies of managing water networks. It was first introduced in the United Kingdom in 1980s and has been adopted across the globe.
The advanced zoning strategies not only ensure better operation in terms of leakage reduction but also promote the preventive maintenance and water quality and protection against contamination in the water networks. We will discuss all the modern methodologies of zones identification and will demonstrate how to successfully identify reliable zones in EPANET software in a programming environment.
There are many aspects which affect the identification of zones. (Here onwards we will use District Metered Areas or DMAs instead of zones). Basically it involves identification of boundaries of zones and closure of a few boundary pipes between the two zones and installing flow measuring devices in pipes which are kept open. The following figure highlights the DMAs and their boundaries in a given water network.
Every water network is unique in its topology, size and characteristics. Hence there is no standard procedure available for identification of DMAs but a series of guidelines provided by different water utilities. The most popularly used in the research community is IWA guidelines (Morrison 2007).
Factors affecting the DMA Design/ Design of Zones
As per the IWA guidelines, the following points should be considered while identifying the DMAs in a given water network.
· Infrastructure condition (leakage levels, availability of pressure at each node)
· Size of the network (number of customers served)
· Types of customers (residential, institutional, industrial, commercial)
· Ground elevation
· Water quality considerations
· Pressure requirements and firefighting capacity
Number of valves and flow meters required for implementing the strategy (Cost aspect)
In the next chapter we will discuss the general steps or work flow chart for identification of DMAs.
Activity: What is the recommended size of each DMA as per the IWA guidelines for DMA establishment? (Hint: refer to the IWA guidelines.)
References
[1] Morrison J, Tooms S, Rogers D (2007) DMA management guidance notes. IWA Publication
Hardy Cross method
The Hardy Cross method is an application of continuity of flow and continuity of potential to iteratively solve for flows in a pipe network.In the case of pipe flow, conservation of flow means that the flow in is equal to the flow out at each junction in the pipe. Conservation of potential means that the total directional head loss along any loop in the system is zero (assuming that a head loss counted against the flow is actually a head gain).
Hardy Cross developed two methods for solving flow networks. Each method starts by maintaining either continuity of flow or potential, and then iteratively solves for the other.
Assumptions
The Hardy Cross method assumes that the flow going in and out of the system is known and that the pipe length, diameter, roughness and other key characteristics are also known or can be assumed.The method also assumes that the relation between flow rate and head loss is known, but the method does not require any particular relation to be used.
In the case of water flow through pipes, a number of methods have been developed to determine the relationship between head loss and flow. The Hardy Cross method allows for any of these relationships to be used.
The general relationship between head loss and flow is:
hf=kQn
where k is the head loss per unit flow and n is the flow exponent. In most design situations the values that make up k, such as pipe length, diameter, and roughness, are taken to be known or assumed and the value of k can be determined for each pipe in the network. The values that make up k and the value of n change depending on the relation used to determine head loss. However, all relations are compatible with the Hardy Cross method
It is also worth noting that the Hardy Cross method can be used to solve simple circuits and other flow like situations. In the case of simple circuits,
V = K ⋅ I
is equivalent to
h f = k ⋅ Q n
By setting the coefficient k to K, the flow rate Q to I and the exponent n to 1, the Hardy Cross method can be used to solve a simple circuit. However, because the relation between the voltage drop and current is linear, the Hardy Cross method is not necessary and the circuit can be solved using non-iterative methods.
Method of balancing heads
The method of balancing heads uses an initial guess that satisfies continuity of flow at each junction and then balances the flows until continuity of potential is also achieved over each loop in the system.
Zoning for Water Distribution System
Zoning is the division of command area in to manageable small parts which have some similarity in property, requirement or function. Zoning helps in reducing the scope of problem to smaller modules, which can be tackled easily with existing tools and resources.
In town planning, Zones are used to demarcate areas reserved for specific proposes like the purpose of land use like Green Zone for agriculture, Industrial zone for industries etc. Generally a city or town is divided in number of wards for municipal management and for election of citizen repetitive. The city office has such maps and the authorities find it easy to organize the service work force accordingly. However zoning for design of water distribution requires more scientific approach and cannot adopt the same zones. There are obvious reasons. The requirement of water distribution system is equitable supply of water to all area with sufficient pressure and with cost optimization as regards both capital as well as maintenance cost.
Zoning is governed by many factors
Ø Location service areas- In case of rural water supply schemes, villages are clusters of population separated by long distances and village has to become one separate zone
Ø Population density - In order to achieve optimization in pipe sizes it necessary to divide the area in subparts which have equal water demand. Naturally, depending upon the density variations, the zones can be of various sizes. In densely populated area, zones will be smaller in size area wise and in the outskirts, where the area is not developed fully; the zone can cover large area.
Ø Type of construction. - From the point of fire fighting requirements, type of buildings also decide zone demarcation. Multistoried buildings can be provided with larger pipes to get required pressure.
Ø Type of activity -
Ø In case of industries, the requirement of water is not similar to that of residential area, Market place or garden also would demand different water supply. Zoning needs to be done taking this point in view.
Ø Physical barrier like express highway, canal, river or railway track also would force zone to be restricted.
Ø Topological characteristics example of Pophali
Type of water distribution system also would govern zoning.
1)Radial system
2)Tree or dead end system
3)Grid iron system
4)Ring system 5)Hybrid system
