Winter 2009

A Quarterly Newsletter of the Instrumentation Testing Association

ITA Enews

Spring 2011

Arthur Arscott1, Richard Furness PhD2

1 Regional Director, 2 Measurement Consultant

RPS Water Services, 6 Manaton Court, Manaton Close, Matford Business Park, Exeter, Devon EX2 8PF, UK

1Tel: Office: +44 1392 677 333;

Direct Dial: +44 1392 680 712;

1Mobile: 07966 125 210; Fax: +44 1392 677 335.

1E-Mail: arscotta@rpsgroup.com

2E-Mail: rydalhall46@aol.com

Copyrighted by RPS Water, Reprinted with Permission.

Instrumentation Testing Association  (ITA)

 

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DO YOU REALLY KNOW YOUR FLOW RATES?

             his article looks at the methodologies used to determine water losses from source to end usage point and in particular leakage.   

Leakage in Australia is generally half that reported within the United Kingdom (UK) even though network characteristics and meters are very similar.  The UK Regulatory process has forced water Companies to provide audited evidence of system losses for a decade. 

The article reviews the current methods of loss assessments.  It provides important lessons for other companies worldwide who are facing similar accountability in their water balances.

There are other uncertainties associated with each element in this overall balance and these can be summarized as:

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Introduction

Balancing water supply and distribution is notoriously difficult.  This is because several of the components involved in the balance are estimated. 

 

In the UK the ‘Top down’ and ‘Bottom up’ approach is adopted.   This is shown in Figure 1.   Water balances across water works consists of 3 elements:

 

(1) water abstracted;

(2) wash water returned to source; and

(3) works output (termed distribution input or DI). 

 

It is essential that waterworks are correctly balanced for the top down approach to be meaningful. In the ‘Top down’ approach to leakage assessment, the overall balance is:

Figure 1: Methodology for leakage estimation in the UK

All components that affect water balances from source to distribution are examined. Only by ensuring each element is properly accounted for can a true picture of losses be established.  Accurate flow measurement is a cornerstone to understanding.  If works output figures are inaccurate, the whole balance will be inaccurate. 

Experience now shows accepted standards and suppliers’ advice for installation may not be correct in larger diameters.  Only rigorous and regular in-situ meter verification can establish the actual flow measurement uncertainty. 

Distribution input – water delivered (A) – Distribution operational usage (B)   =     Distribution loss (C)

DMA—District Meter Area

 

MLE—Maximum Likelihood Estimation 

 

OSU—Operations System Use

 

PCC— Per Capita Consumption

 

A = measured usage + unmeasured usage + unmeasured supply pipe losses + illegal usage

B  = water used in process (mains flushing etc)

C = Trunk main loss + service reservoir loss + distributions mains loss + communication pipe loss

This approach uses the amount of water leaving all the waterworks as the benchmark.

 

Meter accuracy here is paramount.

 

In the ‘bottom up’ approach, the night flow usage (estimated from many local measurements) is the benchmark and values shown in Figure 1 are built up from this. 

 

This approach usually results in a GAP (shown in green in the centre of Figure 1) because of the larger uncertainty in measuring lower night flows and to the complexity of the method as shown in Figure 2 (UK Managing Leakage).

Figure 2: Components of ‘Bottom up’ approach

In the UK, regulatory processes dictate these two figures should agree within 5%.  If they do not then the ‘Top down’ approach is used alone.  If the agreement is within 5% then a process called Maximum Likelihood Estimation (MLE) is used to give a single value for balance. This is the massaging of data based on uncertainty estimates agreed between Water Company and regulator.  The right hand column in Figure 1 shows such an approach applied to the data in question.  

 

Figure 3 shows a real example based on actual UK figures. The interesting point in Figure 3 is the estimated metering uncertainty (3%) and the overall estimate for the yearly value (also 3%).  Recent field work verifying meters in the UK suggests this value is low and that fluid dynamic effects alone can account for this amount.  It would therefore seem that such an approach may be incomplete. A more likely estimate for installed meters is thought to be 4-6% (or more) as discussed in the following sections.

 

If the ‘top down’ approach is used alone then clearly measurement uncertainty must be estimated correctly. The following section examines the components which must be included in each flow measurement uncertainty analysis. 

Figure 3: Example of the MLE approach to reported leakage. 

T

 

Initial Estimate  (Ml/d) for 2001/02

Confidence interval            + or - %

Range  (Ml/d)

% of total range

Adjustment (balance * %)

Revised MLE  Estimate  (Ml/d)

Unmeasured Household Consumption

1078.00

8

86.24

37.54

-52.67

1025.33

Unmeasured Non-Household Consumption

18.80

25

4.70

2.05

-2.87

15.93

Measured Water Consumed

861.30

3

25.84

11.25

-15.78

845.52

Water taken unbilled (legally + illegally)

28.60

50

14.30

6.22

-8.73

19.87

Distribution system operational use

14.50

50

7.25

3.16

-4.43

10.07

Trunk main and SR losses

0.00

 

0.00

0.00

0.00

0.00

DMA total losses

914.00

10

91.40

39.79

-55.82

858.18

Total Losses bottom up

914.00

 

 

 

 

858.18

Sum of components

2915.20

 

 

 

 

2774.90

 

 

 

 

 

 

 

Distribution input estimate for year

2774.9

3

83.25

36.24

-50.84

2825.74

Integrated Flow Leakage

773.70

 

 

 

 

909.02

Reconciliation balance

-140.30

 

 

 

 

50.84

% of distribution input

-5.06

 

 

 

 

1.80

 

 

 

 

 

 

 

 

 

Total Uncertainty

229.73

100.00

Percentage Total