The right tools for the job

As has been detailed in these pages previously, notably in the article that appeared by John Bentley, senior process engineer from Severn Trent Water (Ensuring Accuracy to meet Standards and Requirements, WWT, March 2002), the programme of work to make sure all WwTWs in Severn Trent meet Environment Agency (EA) requirements under the Urban Waste Water Treatment Directive has been in progress since the beginning of 2001.

To recap those standards, the EA has laid down two separate requirements. Firstly, at any WwTWs that discharge more than 50m3/d, the discharge should be measured with no worse than +/- 8% accuracy with 95% confidence. The EA requires self-monitoring of effluent flows by the water company. Each water company is therefore obliged to undertake an expert assessment of all sites falling within the ambit of the Urban Waste Water Treatment

Directive site to confirm it is capable of being measured ‘substantially’ to BS 3680 and identifying the programme of replacement or remedial work that will be undertaken as a result of this survey.

Back when these sites were first built, many did not have the benefit of good calibration work, so there has been work to do at the majority of sites. Part of self-monitoring is the certification of the site to prove the installations will meet the requirements.

As of January 1, 2004, a flow specialist accredited to the MCERTS standard must have provided that expert assessment. Severn Trent Water was one of the first of the major water companies to implement a programme to meet the standards, supported by an investment protocol that formed part of the firm’s investment strategy under AMP3. The Severn Trent Water flow measurement delivery team consists of Carl Bro Group of Bristol as consulting engineers together with two contractors, Droicon and Field Systems Designs (FSD).

Approximately 75% of the installations are being undertaken by Droicon of Droitwich, which, as a civil contractor, has been working with Gamma Automation for mechanical and electrical or instrumentation installations and Stirling Technical Engineering for telemetry
connections. Ultrasonic flow monitoring equipment has been supplied throughout by Pulsar Process Measurement.

The contract for the remaining 25% has been awarded to FSD regional office of Kidderminster, which has partnered with George Law for the civil works. In order to understand the issues in achieving the required accuracy within the outflow it is necessary to appreciate the situation within the system and the sources of uncertainty within the overall measurement. Approximately 95% of all WwTW outflows are open channel. Of these, perhaps three quarters use flumes as primary measuring devices (PMDs), the other quarter being v-notch weirs.

There are, therefore, three primary areas where inaccuracies can develop within the flow measurement – the flume or PMD structure itself, the measurement instrument and the transmission of the measured signal through the telemetry system.

flume inaccuracies

Taking these in turn – historically, the greatest inaccuracies in open channel flow measurement have probably derived from poor flume construction and geometry. There has been more than one incident where the flume has been installed backwards, but more typically, the walls have been poorly constructed, not parallel and made from rough or degraded concrete so there is both a restriction in flow due to the roughness of the material and a good surface for a bio-film to develop. Any restriction in flow may also cause turbulence in the flow, which makes the flow dynamics less predictable and can introduce further errors. With modern materials and a greater
understanding of the need for correct installation, these problems have diminished.

A flume manufactured from stainless steel or glass reinforced plastic (GRP) provides a smooth surface for water flow, and Droicon and FSD have proved themselves experts in providing civil installations that conform to the requirement of BS3680 (usually within 1mm), whether the solution is open channel or closed pipe. Equally important to get right is the measuring instrument itself. Pulsar’s Flow Oracle, as the majority of similar systems do, consists of a sensing head (transducer) connected to a controller (transceiver). The transducer produces a short pulse of sonic energy by the excitation of a piezo-electric crystal.
This pulse then reflects from the surface of the water in the channel, the returning echo re-energising the crystal. The controller analyses the echo signature, calculating the distance from the transducer face to the water from the time taken for the signal to return. The accuracy of this measurement is affected by a number of factors.

Although the instrument measures the distance from the face of the transducer to the surface of the water, the parameter we require for the calculation of flow measurement is the depth of water flowing in the channel, so the Flow Oracle needs to be programmed with the distance to the datum (bottom of the channel). Any inaccuracy in this measurement will have a direct effect on the total accuracy of the measured flow rate. A limit on the resolution of the system is the wavelength of the sound pulse produced. Pulsar has designed the dB3 transducer, with the open channel flow measurement application in mind, with a frequency of 125kHz.

This is a much higher frequency (which is inversely proportional to the wavelength) than other systems on the market, so the wavelength is as short as possible. Accuracy in ultrasonic systems is also determined as a percentage of the full range of the application, so it is important the transducer is mounted as close as possible to the water level. The Pulsar dB3 may be mounted as little as 125mm above the ‘high water mark’, further increasing the accuracy and resolution of the overall measurement. Air temperature will affect the measurement because it affects the speed of sound, so it is important the transducer (as in the Pulsar system) compensates for this. Once the depth of water is known, the Flow Oracle calculates the flow rate to BS3680. It has been the responsibility of Gamma Automation with Droicon and FSD with George Laws, as part of the overall M&E contract, to make sure the Pulsar Flow Oracle has been correctly installed and positioned.

With more than 300 units now in the field, Gamma and FSD have built up a great deal of experience in the application of this equipment. The Flow Oracle makes great use of digital technology for the signal processing, therefore avoiding the sort of long-term drift in accuracy that analogue equipment is prone to.

Once the Flow Oracle has calculated the flow rate, it provides a pulsed output (pulses per measured volume) to the telemetry system, together with a 4-20mA analogue output. To verify the installation, an engineer from Stirling simulates all the operational parameters on site and sets up a short-term archive at Severn Trent Water for verification of the signals.

Once the initial archive is verified as having been communicated accurately, the project is then finally verified by Aseriti, which is employed by Severn Trent Water. For final verification, the analogue and pulse signal outputs received at the Severn Trent Water data monitoring centre over the subsequent two weeks are compared and only when these readings are consistent with the project’s specified tolerance does the site get presented to the owner as verified. Maintenance will be important. Tests conducted showed a v-notch weir can show a drift in accuracy of about 4% over three months simply due to bio-film growth. The drift will probably be lower within a flume, but some drift will need to be taken into consideration nonetheless. Data is recorded at 15min intervals, as specified. There are approximately 600 WwTWs that fall under the scheme within the Severn Trent region, which gives an annual total of approximately 21M individual records.

The obvious next step will be to approve flow monitoring devices, including ultrasonics, to MCERTS – and a product approval scheme is being developed at the moment.

MCERTS already provides a standard for water
flowmeters and as soon
as test facilities are available open channel flow monitors will become part of the process. It is thought the first MCERTS approved instruments will be available in the middle of this year