A benchmark for flow measurement accuracy

Anglian Water has commissioned one of the world's largest site flow audits

in strategic move to ensure accurate discharge flow measurement. The project is described here by David Tyler of Anglian Water, Paul Cherry of Bestobell Service and Alasdair Ward of Meggitt Mobrey.


It is increasingly becoming clear that a high proportion of open channel flow measurement structures in use by water companies throughout the UK do not conform to BS3680. Problems in design, installation and maintenance, and deterioration over time, are all contributing factors, but the net result appears to be that much of the water industry's open channel flow measurement is significantly inaccurate.

In a strategic decision to address this problem, Anglian Water has taken the lead and enlisted the help of some of the UK's leading flow experts from Bestobell Service and Meggitt Mobrey in a major project to survey and calibrate its open channel flow measurement equipment. By, and insisting on high standards of accuracy for flow discharge data, Anglian Water has not only highlighted the issue but has set a benchmark for flow measurement accuracy in the water industry.

BS3680 Methods of measurement of Liquid flow in open channels Part 4 is the British Standard which covers design and operation of primary flow structures including flumes and weirs.

In 1996, Anglian Water took the decision to begin a complete flow audit of its main waste water treatment sites, with aim of achieving a measurement accuracy of (8% or better over a flow range of 25-100% on discharge flows. All the treatment works serving more than 1000 population equivalent, a total of 450 sites altogether, were included in the survey.

Anglian Water engineer David Tyler explains: "The aim of the project was to achieve a high level of confidence in all our measured sewage flows, using a rigorous, accountable methodology, based on accurate flow measurement. This would enable us to ensure that we were complying with the EA regulations on issues of both quantity and quality of discharge. In addition, the accurate flow data is essential for optimum design and management of the entire treatment process."

A specialist team made up of engineers from Bestobell Service and Meggitt Mobrey was contracted to carry out a large part of the work. Set-up specifically to address this need, the team brought together flow analysis expertise, civil engineers and project managers who between them developed a structured approach which facilitated systematic appraisal of each site and efficient execution of remedial work. A formal reporting procedure helped Anglian Water to control the project easily, despite the volume and complexity of the work involved.

The team followed a prescribed procedure at each site which included visual inspection of primary and secondary flow devices, followed by measurement and verification to BS3680, plus a full review of the upstream and downstream conditions. All the site flow data is recorded in standard format, along with location details and physical dimensions. It is then analysed using custom developed software, which produces a full professional engineering report including flow calculations, uncertainty and discharge curves, instrumentation parameter settings, as well as description, comments and recommendations.

In addition, a single-page summary report produced automatically by the software provides management with essential details, helping them to track and control the project. In each case, the flow measurement system is considered in the context of the treatment process and recommendations made in light of any capital schemes which may impact on it.

In cases where the primary flow structure was found to meet BS3680 significantly, instrumentation was checked and re-calibrated, and a revised report and Certificate of Conformity issued, including the new instrumentation parameters and flow data.

If the flume or weir does not meet BS3680, then the site data is studied, maximum flow rates are calculated, and designs and costings for the necessary remedial work are produced for approval by Anglian Water. Only when the plans have been approved is the work carried out, then flow data is checked and a Certificate of Conformity issued.

Paul Cherry, team project manager explains: "By bringing together our experts from different fields into a dedicated team, we were able to provide Anglian Water with a complete turnkey service that achieved their objective of accurate flow measurement on all sites. We didn't just give them the figures and walk away. We re-calibrated and re-sited instruments, cleaned up flow structures and carried out remedial work. And by issuing formal Certificates of Conformity, we actually take responsibility for the accuracy of the flow measurement."

RESULTS

Primary flow measurement structures - Although many of the flumes and some of the weirs did not comply with the geometries and ratios stated in the British Standard, the primary devices were found to be of good design and construction, with only small defects such as the absence of a chamfer on a thin plate weir, or a badly made notch. Only 4 per cent had problems arising from incorrect installation, but 30 per cent of flumes and 10 per cent of weirs had suffered from distortion over time, such as corrugation, bowing or separation of the flume cheeks.

The study also highlighted a tendency to over-design such structures, to make sure they would be able to handle future capacities. The effect of this is that they are operating with lower flows and therefore higher uncertainties. Conversely, 18 per cent of the flumes surveyed were operating with larger capacities than they were designed for, and were being drowned (Fig 1).

In each case, the Bestobell Service / Meggitt Mobrey team made recommendations for refurbishment, or in some cases, replacement and this work is near completed now.

Upstream and downstream conditions - BS3680 specifies a minimum straight upstream channel of five times the channel width, to ensure steady flow through the structure and prevent variations in the velocity profile. A significant proportion of the flow measurement systems surveyed for Anglian Water were directly influenced by upstream or downstream conditions, or both. In many cases the problems resulted from the way the system had been designed.

A side weir upstream of the flow measurement flume, for example, causing significant turbulence (fig 2), or bends or irregularities in the channel. In other cases, the problem is caused by the way the process is set up - re-circulating flows, installation of pen-stocks and screens. On some sites, for instance, the grit removal system down stream was causing grit to build up in the approach channel and flume, affecting the accuracy of the measurement by creating a false liquid level.

Some of the upstream / downstream effects on flow measurement can be neutralised by installation of baffles which smooth the approach flow and improve the accuracy of measurement. Where the effects are pronounced, the audit team has recommended design and installation of complete new structures.

INSTRUMENTATION

A secondary device - usually an ultrasonic level meter - is used to measure the liquid level and compute flow, based on an established head to flow relationship.

The main problem here highlighted by the audit was that 90 per cent of the level sensors installed were positioned such that the floor level at the measurement point differed from the hydraulic datum of the flume, by amounts varying between 1 and 25mm. The consequent errors are significant - up to 15 per cent for a difference of 8mm.

Also, 23 per cent were located either too near or too far from the primary flow measurement structure. Too far away, and the flow velocities in the approach channel may not fully develop, possibly resulting in measurements that are too high. Too close to the flume or weir, and the instrument suffers from the effects of draw-down and will read low.

All of these problems were solved by either relocating the sensor or introducing an offset to compensate for differences from the datum.

In other instances, instruments were found to have been set incorrectly, with thresholds too low so that peaks of data were being missed, or too high so that signal output was of poor resolution. In response to the results from this part of the survey, Anglian Water has stepped-up its routine checking of instrumentation, and is fitting datum brackets to every sensor to facilitate quick and easy calibration of the instrument and confirmation of the head measurement.

CONCLUSIONS

The work carried out by the specialist team for Anglian Water demonstrates the many opportunities for error in the measurement and calculation of open channel flows, and shows that, with positive action and close maintenance, these errors can be minimised or avoided altogether. The project has not only given Anglian Water a high degree of confidence in the measured discharge flows, but it has also provided an invaluable bank of data for optimisation of existing processes and design of future sites.

David Tyler again: "It may be the case that the British Standard - based as it is on laboratory calculations - is just too prescriptive for field flow measurement. Some of the structures that it is now being applied to were built before even the earliest flow measurement standards were published. Yet without standards there would be no consistency. In the absence of any other guidelines for open channel flow measurement, there is no choice but to aim to satisfy the criteria of the British Standard as they stand. However, there is an urgent need for further studies to expand the range and application of the standard, to present criteria which designers, manufacturers and installers will find more readily useable."

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