GUIDANCE NOTE Performance of Electromagnetic Meters www.tuvnel.com
Performance of Electromagnetic Meters Introduction Electromagnetic flowmeters designed for use in part-filled pipes may be considered as a replacement for existing open channel flow metering systems (eg weirs and flumes) used in monitoring effluent discharges, storm water and sewer flows, and for industrial process control measurements. Such measurements are common in water and waste treatment, chemical and food industries. This Guidance Note summarises the results obtained from a DTI Flow Programme project in which the performances of a number of electromagnetic flowmeters designed for use in part-filled pipes were measured. These measurements were preceded by a literature search to identify the following: Manufacturers and suppliers of such meters The principles of operation of electromagnetic meters for use in part-filled pipes Technical details and installation requirements Previous experience and potential problems with the calibration of such meters. Manufacturers and Suppliers Four companies were identified as manufacturing electromagnetic flowmeters designed specifically for part-filled pipes. It was established in discussion with those companies that the most common meter sizes in use and consequently the most readily available were 200 mm and 400 mm nominal bore. Three of the companies agreed to supply one meter of each size to NEL for testing: Bailey-Fischer and Porter Ltd (Workington, Cumbria). Meters type PARTIMAG II range, supplied by J W Fairbairn Ltd, Glasgow. Krohne Altometer Ltd (Dordrecht, The Netherlands). Meters type TIDALFLUX IFM 4100 PF, supplied by Krohne Ltd, Wellingborough, Northamptonshire Marsh-McBirnie Europe (Belgium). Meter model 253 Flo-System TM, supplied by Flowline Manufacturing Ltd, Borehamwood. Flowmeter Operation General The flowmeters from the three manufacturers all operate on the principle of Faraday s law of magnetic induction. By that principle, a voltage will be generated across a conductor if that conductor moves at right angles through a magnetic field. Mains powered coils mounted in the meter body provide the magnetic field and the flowing liquid acts as the conductor. The voltage generated as the liquid moves through the magnetic field is sensed by electrodes mounted flush with the inner wall of the flowmeter. Alternatively, capacitive coupling may be used and this method avoids direct contact between the electrodes and the liquid. Bailey-Fischer & Porter flowmeters The meters are fitted with electromagnetic coils on a vertical axis to provide the required magnetic field across the meter section. Four pairs of electrodes are provided, one on the normal horizontal diameter location and the other three at lower positions. There is also a single electrode positioned near to the top of the meter bore. This single electrode, on sensing the presence of liquid, is used to switch the meter between part-filled and fully-filled operation. At each flow cross-section, full or part-full, the appropriate, weighting factor corrected, electrode pair is used for the voltage (flow signal) measurement. In addition to measuring the average flow velocity, the four pairs of electrodes detect a superimposed alternating current field for determination of the fill level. The signal voltage is corrected and converted to an output signal proportional to flowrate by using the fill level information and characteristic curves stored in the converter. The maximum measurement uncertainty claimed for the Bailey-Fischer & Porter meter in part-filled conditions (with flow velocity greater than 0.2 m/s and flow level above 0.15 x nominal meter diameter) is 3% of flowrate reading for flowrates above 0.02 of the range maximum. Below this rate and down to the minimum flowrate of 0.001 range maximum, the quoted uncertainty is extended to 5% of reading. These values are for pulse output. In fully-filled conditions, the corresponding claimed uncertainty values are 0.04% of range maximum down to 0.04 of that maximum and 1% of rate below this value and down to the minimum flowrate. Krohne flowmeters One pair of electrodes is employed and these are positioned on a level of approximately 0.1 of the meter bore diameter on the lower part of that bore. Flow velocity is determined on the Faraday principle and the wetted cross-sectional area is computed from a capacitive level measuring system built into the meter tube liner. An additional feature of the Krohne flowmeter is that the ratio of flow level and flow rate is continuously measured and stored during normal operation. This data is evaluated for flow measurement at times when the electrodes are temporarily not in contact with the liquid, such as during the passage of wave motion. It is claimed that accurate measurements are therefore possible down to a flow level of 10% of meter tube bore diameter. The best measurement uncertainty claimed for the Krohne meter in part-filled conditions is 1% of the full scale flowrate for flow velocities 1 m/s. For fully-filled flowrates the uncertainty is quoted as 1% of the measured value for flow velocities 1 m/s. For fully filled flowrates less than 1 m/s the uncertainty is quoted as 0.5% of the measured value plus 5 mm/s.
June 1999 Marsh-McBirney Flo-System flowmeter This flowmeter (see Fig 1) is entirely different from conventional electromagnetic flowmeters (see Fig 1). It consists of a small velocity transducer which is installed in the bottom of a pipe bore. The transducer is held in position by a circular steel band sprung against the pipe bore wall. Calibration Methods Krohne Altometer at Dordrecht uses a calibration system involving two reservoirs which are connected by a low level pipe which always runs full and a high level pipe in which the liquid level can be adjusted by appropriate filling of the reservoirs. Flow is generated by a pump installed in the low level pipe and that flow is measured by a calibrated flowmeter also installed in that pipe. The flowmeter under test is installed in the high level pipe. Bailey-Fischer & Porter has a calibration facility at Göttingen in which full charged reference flowmeters are used at a low level in the facility while the meter under test is installed in a pipe at a higher level. The reference meters are calibrated against a gravimetric flow measurement system incorporated in the facility. The method used for the Marsh-McBirney meter is one of in situ calibration where the calibration coefficient is derived from a profiling (velocity traversing) technique using either the Flo-System sensor attached to the end of a rod or by some other means. Figure 1 - Installation of Marsh-McBirney meter in 200 mm pipe The meter signal is transmitted by a cable attached to the steel band and that cable emerges through an opening on the top of the pipe. Liquid level is measured by a piezoelectric pressure sensor located inside the velocity transducer. The velocity transducer generates a local electromagnetic field and the flowmeter instrumentation combines the transducer signals with level signals and uses this together with pre-established data concerning velocity profiles in a part-filled pipe to give the flowrate value. The claimed velocity measurement uncertainty of the Marsh- McBirney meter is 2% of reading over a defined velocity range. Liquid level measurement uncertainty is quoted as 6.4 mm and the conversion uncertainty (basic measurements converted to flowrate) is quoted as 2% of reading. NEL Calibration Facility and Programme The NEL large water calibration facility operates using flying-startand-stop diversion with static weighing. It was designed to operate with pipes fully charged. The 200 mm and 450 mm test lines of the facility were adapted to enable the flowmeter tests to be conducted under part-filled and fully-filled conditions. Conductivity of the facility water is between 3000 and 5000 µs/cm, this high value being caused by the sodium nitrite used as an anti-corrosion additive at NEL. The facility is NAMAS accredited with a best measurement uncertainty of 0.1% of flowrate with a coverage factor k = 2 giving a confidence level of 95% over a flowrate range of 3 l/s to 1.5 m 3 /s. Flowmeter Installation The Krohne and Bailey-Fischer & Porter meters have to be installed with the electrodes accurately positioned on a horizontal axis. There must be at least 10D of straight pipe upstream of each meter. Sudden changes in pipe section upstream of the meter are to be avoided and any flow control should be downstream of the meter; by at least 3D if a gate valve is used. The flow axis of the Krohne meter must be within 1% of the horizontal while the maximum pipe inclination with the Bailey- Fischer & Porter meter is 5%. The Marsh-McBirney meter can be installed in an inclined pipe (no inclination limits are quoted). The meter is programmed with a correction coefficient appropriate to the particular installation and determined in accordance with the manufacturer s instructions. Figure 2 - Installation of Krohne 200mm TIDALFLUX meter at NEL An example of one of the installations is on Fig 2 which shows the 200 mm Krohne meter installed in the 200 mm test line. Under part-filled flow conditions the test lines were vented to allow the contained air to remain at atmospheric pressure.
Performance of Electromagnetic Meters Control of flowrate and meter fill level were achieved by use of gate valves on the facility. Transparent Perspex viewing sections were installed to enable the fill level to be measured by a Vernier height gauge. Full details are given in Reference 5. Tests were conducted initially without controlling the water level at any particular value. The water flowed through the test meter and discharged freely into the end-of-line closed manifold. Under those conditions the maximum flowrate was limited by the resistance of the pipeline. The fill levels were then controlled using a downstream valve so that the test meter performance could be measured over a range of flowrates at each fill level. Three fill levels were selected and in the case of the Bailey-Fischer & Porter meter this allowed a different pair of electrodes to be brought into operation at each level. Krohne meters No satisfactory calibration could be obtained on the first 200 mm meter which was submitted. This was despite reprogramming of the meter by the manufacturer to suit the high conductivity of the water. Krohne offered to submit a new meter. This offer was accepted and the replacement meter performed normally. Results for the 200 mm, meter and the 400 mm one which performed normally, are shown on Fig 4 and 5 respectively. Measurements were also taken with the test meters operating under fully-filled pipe conditions. Results Bailey-Fischer & Porter meters The 200 mm meter did not operate correctly on initial installation nor after replacement of an electronic circuit. The manufacturer suspected the cause to be the high conductivity of the water. The meter was repaired and recalibrated in Germany. On its return to NEL it performed well at high flowrates but badly at low flows. The manufacturer again suspected that high water conductivity was the cause. The 400 mm meter initially gave erroneous results but after adjustment for high conductivity it operated normally. The results are shown on Fig 3. Figure 4 - Results from Krohne 200 mm TIDALFLUX meter Figure 3 - Results from 400 mm Bailey-Fischer & Porter meter Figure 5 - Results from Krohne 400 mm TIDALFLUX meter
June 1999 Marsh-McBirney meter Normal performance was obtained from this meter when it was installed in 200 mm and 400 mm pipes using the appropriate supporting band (see 3.4). Results are shown on Fig 6 and 7. Conclusions The NEL large water flow facility was adapted satisfactorily to allow the calibration of flowmeters operating under part-filled conditions. Of the five flowmeters tested in the project, two performed normally without adjustment or repair. The high conductivity of the NEL facility water was suspected by the manufacturers to be a cause of meter malfunctions and while this may have been the case it was not considered to be the only cause. Bailey-Fischer & Porter quotes a required minimum conductivity of 0.05 µs/cm but no maximum value. Krohne literature quotes a minimum value of greater than 50 µs/cm but, again, no maximum value. The Marsh McBirney literature does not appear to specify liquid conductivity requirements. As a result of the project, Bailey-Fischer & Porter and Krohne have both modified their meters to cope with higher conductivities. It would always be important, however, that a flowmeter manufacturer or supplier is fully informed regarding the specific application for a particular meter including the range of conductivities expected. Figure 6 - Results from Marsh-McBirney meter in 200 mm pipe The percentage deviations (see Fig 3-7) computed from the calibration data show that electromagnetic flowmeters for partfilled pipes are a viable alternative to the normal open channel flow structures. The electromagnetic meters also have the distinct advantage of being able to deal with fully-filled pipes and to measure the flow of liquids which would not be suitably transported in an open channel due to their hazardous nature. It is recognised that part-filled pipe flows may also be measured by ultrasonic means but this method was not included within the scope of the project. Figure 7 - Results from Marsh-McBirney meter in 400 mm pipe
Performance of Electromagnetic Meters References 1 Literature Survey on Electromagnetic Flowmeters for Use in Partially Filled Pipes (Manufacturers, General Principles of Operation and Methods of Calibration), Project WSDC54, Report No 027/97, National Engineering Laboratory, East Kilbride, November 1997 The work described in this Guidance Note was carried out as part of the Flow Programme under the sponsorship of the DTI s National Measurement System Policy Unit. NEL is grateful to the three companies who agreed to supply flowmeters to NEL for the project and for the assistance and information which they gave during it. 2 Performance Evaluation of a 200 mm Krohne Altometer Electromagnetic Flowmeter Installed in 200 mm Partially Filled Pipes, Project WSDC54, Report No 186/99, National Engineering Laboratory, East Kilbride, April 1999 3 Performance Evaluations of a Flowline (Marsh-McBirney) Electromagnetic Flowmeter Installed in 200 mm and 400 mm Partially Filled Pipelines, Project WSDC54, Report No 190/99, National Engineering Laboratory, East Kilbride, April 1999 4 Performance Evaluation of 200 mm and 400 mm PARTI-MAG II Bailey-Fischer & Porter Electromagnetic Flowmeters Installed in Partially Filled Pipes, Project WSDC54, Report No 195/99, National Engineering Laboratory, East Kilbride, April 1999 5 Evaluation of Electromagnetic Flowmeters Designed for Measuring Liquid Flow in Part-Filled and Fully-Charged Pipes, Project WSDC54, Report No 207/99, National Engineering Laboratory, East Kilbride, April 1999 The purpose of this Guidance Note is to provide, in condensed form, information on measurement methods and technologies. It was produced as part of the UK Government s National Measurement System. For further information, contact: TUV NEL, East Kilbride, GLASGOW, G75 0QF, UK Tel: + 44 (0) 1355 220222 www.tuvnel.com Email: info@tuvnel.com TUV NEL Ltd 2010 Re-issued 2010 This publication is to provide outline information only which (unless agreed by TUV NEL in writing) may not be reproduced for any purpose or form part of any order or contract or be regarded as representation relating to products or services concerned.