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Accurate Flow Measurement
in Water Supply Management


Dr. Richard Furness, CEng, ISA Fellow
Chief Flow Technologist
JDF and Associates
Gloucester, UK

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Introduction
Flowmeter usage and applications.

Flow measurement applications in the water industry range from small dosing and treatment flows in pipes of a few millimetres in diameter, to the flow of treated or wastewater in trunk mains and aqueducts of 2m diameter and above.

Often in the larger cities, large interceptor sewers are used to collect vast volumes of effluent and spent water, to convey them to waste water treatment plants for processing. Some interceptors are several metres in diameter, maybe of special shape and usually running partially full.  

Flowmeter usage is therefore wide and diverse and is at the centre of the entire water cycle within the industry.  Metering influences resource management, process control, new works planning, distribution management, leakage detection, financial control and environmental issues either directly or indirectly.
 

Managing Water Distribution Systems
Increase Reliability and Quality of Supply

Water imbalances are becoming a key focus to increase the reliability and quality of supply.  The complete supply system consists of a number of elements.  Each needs to be effectively managed and controlled if the overall supply cycle is to remain within tight control. A typical system usually consists of the following main elements:  

  • Raw water piping system between abstraction and primary treatment plant

  • Piping and components within the water treatment works

  • Transmissions mains and supply storage reservoirs

  • Local distribution supply mains

  • Connection pipes to the consumers premises

  • Piping in the consumers premises after the point of final metering

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Losses or imbalances can occur in any or all of the above. In the first area, large volumes of water are involved through a few key flowmeters.

Venturi Flow Element (Photo Courtesy of Primary Flow Signal, Inc.)

Consider a large Venturi meter for example in a 1600mm pipe measuring a flow of 250MLD. 

Over a period of time the inside surface of the pipes and meter will change (due to deposits) and general experience shows the meter may begin to under-read.
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Water Deposits (Photo Courtesy of Delta Cascade)
The intrinsic ‘accuracy’ is therefore fundamental to managing the water balance.  If the same type of meter were used through the supply system, then all could drift in the same way and at almost the same rate.  We could have the situation were the meters may balance, but the quantitative assessment of the flow is in error. 

This has been the case in a good proportion of the older networks. Invariably these meters have not been properly installed, calibrated or maintained, but are still used to measure the amount of water put into supply. From our knowledge and experience of installing flowmeters, it may be argued that this total is never precisely known. Estimating leakage therefore becomes extremely difficult.

 

Moncenisio Dam on the Italy and France border
Sources of Water Imbalance and Leakage
Assessing Water Balance

Leakage rates within small areas (or small towns) can be estimated by direct measurement of the flows at times when demand is low. 

Water Companies make regular ‘soundings’ during night-time levels to try to estimate system flows.

However most water balances are made directly from totalized values over longer periods, by taking the difference between a measured input volume and the accumulated volume of many estimated or measured outputs over the same period. 

The uncertainty of metering directly affects the balance result.  Thus accurate flowmetering is fundamental in water conservation projects.
 

The total water balance is made up of a number of components. These are:  

  • Metered consumption

  • Unmetered consumption, estimated from the population statistics

  • Real losses or leaks

  • Apparent or inferred losses

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Domestic Water Meter Reader
Metered consumption is very clear and has an estimated uncertainty.

Unmetered consumption is less clear and has a much larger uncertainty associated with it. This larger uncertainty is partly due to the fact that the exact number of consumers is never precisely known and the rate of consumption may vary within an area depending on the balance between domestic and/or industrial users.

Real losses consist of leaks and losses prior to the point of final measurement and may be measured through the use of system balance meters, district meters or zone flowmeters. 

More measurements may mean a better estimation of the flow totals, but regular verification is essential to error analysis. Apparent losses are largely due to the errors in the assumptions made on consumer demand, population etc.
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