Monitoring the flowmonitors
Common guidelines for auditing flow monitoring systems should allow standardised reporting and improved assessment of performance. NEL explains how the guidelines were drawn-up.
In the UK the drive for improved flow metering comes principally from regulatory bodies such as Ofwat and the Environment Agency. As a response to this many water companies have substantially improved their flow metering, data acquisition and transmission technologies. This has necessitated the investment of considerable time and money. To take full advantage of the investment it is extremely important for the companies to use a single, recognised method of flowmeter auditing. The main advantages to the companies will be:
- the development of common reporting practices across the industry for auditing flow metering systems,
- easier assessment of flowmeter performance through the use of standard auditing requirements.
The guidance document outlines the components and principles involved in flowmeter auditing in such a way that companies can easily incorporate them into their own internal procedures. This is done by compiling check lists for each of the main audit areas. These are:
- meter specification,
- installation procedures,
- pipe specification,
- flow and verification details.
The second project, undertaken as part of the overall study, examined the issue of uncertainty in flow metering. The project looked at the issues that can affect the quality of flow data and assessed the different analysis techniques used to indicate flowmeter performance. The resulting report runs to three volumes and contains a comprehensive and objective account of current flow metering practices in the UK water industry.
In this project, flowmeter data quality and data analysis techniques were investigated principally through a series of case studies conducted with four of the participating companies. The uncertainty analysis methods used in each were illustrated by a worked example. Project reviews were held once a quarter and were timed to coincide with the Water Industry Flow Advisory Liaison Group (WINDFAL) meetings. WINDFAL is a focus group which provides a forum for all flow measurement and flow related issues within the industry. Its home page may be found at www.windfal.co.uk.
The first case study involved looking at ultrasonic clamp-on flowmeters. The popularity of these meters is increasing because they provide the advantage of being able to be installed without the need to interfere with the operation of the network. Their application tends to be as a means of flow verification and are therefore only used at a particular location for a short period of time. The study identified, described and quantified the sources of uncertainty for this type of meter using theoretical techniques. Where possible, reference was also made to experimental results to provide back up to these findings.
The study found that a critical issue in the assessment of flowrate using this type of meter is knowledge of and proper use of the pipe material and dimensional information. Pipe and liner material properties and thickness must also be known accurately. The study also highlighted that care must be taken when attaching the transducers to the pipe.
In the second case study the uncertainties associated with insertion flowmeters were covered. These meters are widely used in industry but have a major disadvantage over full bore meters in that they can only provide a flowrate based on a single-point velocity measurement. For this reason they are being increasingly utilised in the water industry as a means of verifying the performance of permanently installed full-bore meters. This ability to verify the performance of a meter in-situ, without the need to remove it and have it tested elsewhere is extremely attractive. In some applications, such as reservoir and outlet installations, there may simply be no potential for removal of the meter without causing major disruption and inconvenience to customers. Insertion probes provide a cost-effective means for verifying such meters.
The principal sources of uncertainty for insertion flowmeters were found to be:
It was concluded that errors are introduced into the signal in the data path. Although they are reasonably small it is considered important that procedures tracing the progress of the signal should be regularly performed. This will allow any problems in the data path to be highlighted at an early stage and, if performed regularly, will provide ongoing confidence that the data path is reliable.
The last case study assessed a number of sets of flow data provided by water companies. The data was examined in detail and a number of different analysis techniques applied. One technique standing out as showing particular promise is the application of cumulative sums (CUSUMS), a statistical method which allows changes in the mean level in a noisy signal to be highlighted and quantified. Overall, the findings indicate that a great deal of information can be extracted from flowmeter data. It is also emphasised that such techniques show particular promise if data assessment and analysis were to be combined in real-time as the data is being acquired.
Both of these projects will serve to standardise and improve uncertainty analysis and flowmeter auditing techniques in the UK water industry. The resulting improved accuracy of water metering systems will enable companies to prioritise their flow metering activities and make informed cost-benefit decisions