The IWA Approach to Calculating Unavoidable Annual Real Losses

The IWA approach is described in detail in the December 1999 issue of the IWA AQUA Magazine,8 and can be seen as a natural development of previous North American attempts to take key local factors into account. The component-based approach is based on auditable assumptions for break frequencies, flow rates, durations; background and breaks estimates concepts9 to calculate the components of unavoidable real losses for a system with well-maintained infrastructure; speedy good-quality repairs of all detectable leaks and breaks; and efficient active leakage control to locate unreported leaks and breaks.

Parameters used in the calculation, taken from "Water Loss Management in North America"7 and converted to North American units, are shown in Table 7.1. Table 7.2 shows these parameters in a more user-friendly format for calculation purposes.

Infrastructure Component

Background

(Undetectable)

Losses

Reported Breaks

Unreported Breaks

Mains

8.5 gal/mi/hr

0.20 breaks/mi/year at 50 gpm for 3 days duration

0.01 breaks/mi/year at 25 gpm for 50 days duration

Service lines, main to curb stop

0.33 gals/service line/hr

2.25/1000 service line/year at 7 gpm for 8 days duration

0.75/1000 service line/year at 7 gpm for 100 days duration

Underground pipes, curb stop to meter (for 50 ft ave. length)

0.13 gal/service line/hr

1.5/1000 service line/year at 7 gpm for 9 days duration

0.50/100 service line/year at 7 gpm for 101 days duration

gal = U.S. gallon; all flow rates are at a reference pressure of 70 psi Source: Ref. 7.

gal = U.S. gallon; all flow rates are at a reference pressure of 70 psi Source: Ref. 7.

Table 7.1 Parameters Values Used for Calculation of Unavoidable Annual Real Losses (UARL)

Infrastructure Component

Background Losses

Reported Bursts

Unreported Bursts

UARL Total

Units

Mains

2.87

1.75

0.77

5.4

gal/mi mains/ d/psi of pressure

Service lines, mains to curb stop

0.112

0.007

0.030

0.15

gal/mi/d/psi of pressure

Underground pipes between curb stop and customer meters

4.78

0.57

2.12

7.5

gal/mi u.g. pipe/d/psi of pressure

Table 7.2 Components of Unavoidable Annual Real Losses

Table 7.2 Components of Unavoidable Annual Real Losses

"UARL Total" values, in the units shown in Table 7.2, provide a rational yet flexible basis for predicting UARL values for a wide range of distribution systems. The calculation takes into account length of mains, number of service lines, location of customer meters relative to property line (curb stop), and average operating pressure (leakage rate varies approximately linearly with pressure for most large systems). An important aspect of Table 7.2 is the value assigned to unavoidable "Background (undetectable real) Losses," shown in Col. 2. These figures are based on international data, from analysis of night flows in sectors just after all detectable leaks and breaks have been located and repaired. This component of unavoidable real losses does not appear to have been quantified previously in North American practice, yet it accounts for at least 50 percent of the unavoidable real losses components in Table 7.2. Estimates of background (undetectable) leakage following intensive leak-detection surveys in small U.S. systems have been compared with IWA unavoidable background loss predictions based on the Col. 2 of Table 7.2. Initial comparisons are encouraging, and more comparisons are being actively sought.

There are many different ways to present the UARL equation. Figure 7.2 shows UARL in gal/mi/d/psi of pressure (Y axis) plotted against density of service lines. The large variation of unavoidable losses per mile of mains for different densities of service lines shows why it is not recommended to use "per mile" for comparisons of real losses. However, Fig. 7.2 can be used to estimate unavoidable annual real losses for any system, as the following example shows.

Example A water supply system has 60,000 service connections and 600 mi of mains (a connection density of 100 service lines per mile of mains), and the average operating pressure is 70 psi. Calculate the unavoidable annual real losses from Fig. 7.2 if the average distance of customer meters from the curb stop is (a) 100 ft or (b) 20 ft.

Answer At a connection density of 100 per mile of mains (X axis), from Fig. 7.2 the UARL is

(a) 34 gal/mi/d/psi of pressure x 70 psi = 2380 gal/mi/d x 600 mi = 1.43 mgd (for customer meters 100 ft from the curb stop); or

(b) 23 gal/mi/d/psi of pressure x 70 psi = 1610 gal/mi/d x 600 mi = 0.97 mgd (for customer meters 20 ft from the curb stop).

Blood Pressure During Exercise Chart

Figure 7.2 Unavoidable annual real losses (Gal/mile of mains/day/psi ) vs. density of service connections. (Source: Ref. 7.)

Density of service lines (service lines/mi of mains)

Figure 7.2 Unavoidable annual real losses (Gal/mile of mains/day/psi ) vs. density of service connections. (Source: Ref. 7.)

Comparison of IWA system-specific values of unavoidable annual real losses in gallons per mile of mains per day compare well with the range of 1000 to 3000 gal/mi/d usually quoted for North American systems. However, the IWA prediction method has the considerable advantage that it allows estimates to be made on a system-specific basis, taking account of density of connections, average operating pressure, and locations of customer meters (relative to the curb stop). The last of these factors is particularly important in a region of diverse climates such as North America, where some customer meters are close to the curb stop and others are in buildings more distant from the curb stop.

Density of service lines (service lines/mi of mains)

Figure 7.3 Unavoidable annual real losses (gal/Service Line/d/psi ) vs. density of service lines. (Source: Ref. 7.)

Density of service lines (service lines/mi of mains)

Figure 7.3 Unavoidable annual real losses (gal/Service Line/d/psi ) vs. density of service lines. (Source: Ref. 7.)

The UARL values in Table 7.2 can just as easily be plotted as a graph of gallons per service line per day per psi of pressure versus density of service lines, as shown in Fig. 7.3.

In well-run systems worldwide, the greatest annual volume of real losses occurs from long-running, small- to medium-sized leaks on service connections, except at low densities of service connections. This is why the IWA Task Forces recommend using "per service connection" instead of "per mile of mains" as the basic performance indicator for real losses, for connection densities exceeding 32 per mile. Using the previous calculation example, for the system with 60,000 service connections and 600 mi of mains, the UARL derived from Fig. 7.3 would be

(a) 0.34 gal/service/d/psi of pressure x 70 psi = 23.8 gal/service/d x 60,000 services = 1.43 mgd (for customer meters 100 ft from the curb stop); or

(b) 0.23 gal/service/d/psi of pressure x 70 psi = 16.1 gal/service/d x 60,000 services = 0.97 mgd (for customer meters 20 ft from the curb stop).

The curved lines in Fig. 7.3 are relatively flat for a wide range of connection densities. In calculating unavoidable annual real losses, for example, systems with customer meters 50 ft from the curb stop, and connection densities in the range 80 to 200 per mile, an acceptable simplification from Fig. 7.3 would be to say that the UARL is 0.25 gal/mi/d/ psi of pressure (=±10%).

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