Active Leakage Control

The purpose of active leakage control (ALC) is to find leaks that do not surface or otherwise come to the attention of the operating company through customer contact, for example, poor supply, loss of water, and so on. These leaks are often referred to as reported leaks. The process of active leakage control involves teams of leakage detection staff sweeping an area to find leaks generally using sounding techniques or similar. This may be in response to an increase in a nightline if the area is sectorized, an increase in the output from a treatment works or service reservoir/tank or simply as a result of a regular sounding programme at an agreed interval.

This ALC activity will locate unreported leaks, which will then be repaired, and leakage levels will be maintained. If sweeping is carried out at more frequent intervals then leakage will be maintained at a lower level. Thus, there is a relationship between average leakage level and the time between surveys. This is shown as curve A-A in Fig. 9.2 and is referred to as the active leakage control curve. The vertical axis is usually expressed in cost terms and is simply the annual cost of the leakage detection resources. The horizontal axis is the average leakage level, over the same period (usually a year). On the assumption that some leaks would never come to the attention of the operating company if they did not come to the surface (e.g., if they break through to a sewer) and would therefore accumulate on the system, then the curve will asymptote to the horizontal axis. The curve will also asymptote to a line parallel to the vertical axis. This line B-B, will be equivalent to the level of leakage that would result if infinite resources were deployed on leakage control activity. This minimum level of leakage would equate to

Active Leakage Control Curve

Active Leakage Control Curve

Leakage (Ml/d)

Figure 9.2 Active leakage control (ALC) cost curve. (Source: Dave Pearson.)

Leakage (Ml/d)

Figure 9.2 Active leakage control (ALC) cost curve. (Source: Dave Pearson.)

background leakage, that is, leakage below the level of detection, plus the leakage from reported leaks plus the leakage from unreported leaks during the period they run between detection and repair, resulting from any given leakage control policy. This is sometimes referred to as the policy minimum level of leakage.

There has been much debate about the shape of the curve between these asymptotes. In the most simplistic model of regular sounding, the curve will be hyperbolic. This is based on the fact that the curve will be defined by the leakage during the period which unreported leaks run until they are detected. This will be directly related to the length of time they run before being detected and hence the intervention interval. As the intervention interval will be inversely related to the resources (doubling the resources will half the intervention interval) then leakage will be inversely proportional (i.e., a hyperbole) to the level of resources and hence the ALC cost. If the area is sectorized, or if other forms of flow measurement are used to direct resources more efficiently compared to simple regular sounding, the curve will be flatter than a pure hyperbole.

If the cost of the water lost at different levels of leakage is plotted on the same graph this would be represented by the line C-C. The cost will be the simple difference in cost in producing one more or less unit of water in terms of power, chemicals, and possibly labour. The slope of this line is referred to as the marginal cost of water. If the marginal cost of water is constant, line C-C will be a straight line. If the marginal cost of water production is not constant, then line C-C will be made up of a number of straight lines; usually increasing in slope with higher leakage as more expensive water is used. Curve D-D is the total cost of operation, that is, cost of leakage control plus cost of water production. As can be seen, the curve will be high initially due to the high cost of leakage detection required to achieve very low levels of leakage. The total cost then reduces before increasing again as the cost of water production increases with increasing levels of leakage. The point at which the total cost is lowest will be the short-run economic level of leakage. At this point, the marginal cost of leakage detection activity will be equal to the marginal cost of water. This point will also define the economic level of resources to be deployed on leakage detection and the economic period between interventions.

It can be shown that the minimum total cost of lost water and intervention costs occur when the accumulated value of lost water since the last intervention equals the cost of intervention. This simple relationship has been used by a number of people to develop methodologies to calculate the economic intervention period for a system.

The solution to the calculation of the economic intervention period in the case of regular sounding, that is, where all parts of the system are swept with the same frequency, is reasonably straightforward3 and this has been developed4 into methodologies that can be readily applied to distribution systems.

Where the system has been sectorized and information therefore exists for the rate at which leakage accumulates on different parts of the network then a more specific approach can be taken.5,6 In this approach, the actual volume of leakage is accumulated using night-line information since the last intervention and proactive detection is initiated when the value of this is equal to the cost of intervention on that sector. The advantage of this approach is that it can take into account sector-specific cost of water (say due to local boosting of water) and also sector-specific survey costs (say due to urbanisation or pipe materials).

An alternative approach has been to try and define the ALC curve itself. This can be carried out in a number of ways, which can be classified as either empirical or theoretical.

The former relies on the establishment of a number of points along the curve by analysing the results from actual ALC operations. When a number of points have been derived then a curve is fitted. This may assume a given shape to the curve.7 The difficulty with this approach is that the current position on the curve represents a static situation of the balance between average leakage over a number of years at a constant resource level. It may take a number of years to reach stability when detection resources are changed. It is therefore a long process to develop accurate estimates of a number of points on the curve.

Alternatively, a theoretical approach using component loss modelling methodolo-gies8 can be used to define the ALC curve, but this will require a number of assumptions, such as burst flow rates, although attempts can be made to calibrate these from actual data. A compromise is to establish the ALC curve by building a component loss model of the system and then to calibrate this such that it passes through the current operating position established by analysing the actual cost of operations. The economic intervention period can then be found by direct differentiation of this curve or by numerical methods.

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Responses

  • Gormadoc
    What is the level on the horizontal axis,on water loss due to leak to the quantity of water?
    3 years ago

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