Techniques ebb and flow meter
July 1st 2005 Mark Dutton, Emerson Process Management’s measurement sales director for the UK and Ireland, reviews current flowmetering techniques In reviewing current flowmetering techniques, it might have been expected that modern microelectronics would finally have assured that all future industrial flow measurement systems would be based on one allconquering technology – and many would have suggested that this would be Coriolis technology – but the facts are different.
It is true that the general industrial use of Coriolis mass flow measurement systems continues to increase. Much of this is because the manufacturers have steadily improved accuracy and performance, reduced the meter size, and reduced the cost, so that Coriolis meters are far more effective and economical than a few years ago. This is a result of micro-electronic developments, but also the sensors themselves have developed, extending flow rates down to the lowest levels of grams per hour, and upwards to units for use in 12 inch lines.
Straight tube versions are available, particularly for use with slurries or for hygienic duties, and direct Modbus connection from the electronics processor allows simple interfacing in packaged plant and systems.
Coriolis meters are proved and accepted in custody transfer measurement for oil products, and also for fiscal gas metering. The Coriolis unit is widely used in those countries developing the use of compressed natural gas (CNG) for vehicles, where it is installed on the delivery pumps at the dispensing stations.
In industry, the use of Coriolis meters continues to grow, taking market share from other technologies as engineers realise the simplicity of the units and the benefits available from a higher accuracy meter, particularly where the liquid viscosity or concentration might vary.
Further benefits arise when the on-line liquid property measurement information available from the Coriolis sensor is used.
Most meters provide a density output, which can be a powerful process analytical tool, useful as an input to quality assurance and process control systems. This information might previously only have been available as the result of an entire off-line procedure in an analytical laboratory: having the information available on-line, provided at no extra cost, at all flow measurement points, is still a new concept.
Differential Pressure Flow Measurement Modern electronics has produced a real revival of interest in some of the oldest flow measurement techniques known: these have then also benefited from some innovative mechanical engineering developments. Orifice plates and averaging Pitot tubes represent some of the longest established styles of flowmeter, and these use differential pressure (DP) measurement to derive the flow. Modern technology provides DP transmitters with much higher levels of stability and long term accuracy. Special attention to the factory calibration points of DP transmitters intended for flow measurement duty enables configuration to enable a full 14:1 flow measurement turndown with an uncertainty of +/-1% of reading. This places the old established orifice plate and Pitot tube performance up in the same class as many modern flowmeter technologies that were previously considered superior in accuracy and rangeability.
Attention has also refocused on the mechanical design and limitations of the final orifice plate assemblies. Modern production and machining approaches have eliminated many of the complicated ‘impulse lines’, and the associated concerns over potential process fluid leaks. Impulse lines are now built into the orifice carrier itself, an integral part of the fully machined and welded assembly.
Another common problem in orifice plate installations has been the need for extensive straight lengths of pipe, both upstream and downstream of the orifice plate. An approach is now available that reduces this distance requirement to only two pipe diameters, simply by making the orifice a ‘Conditioning Orifice Plate’, with typically four separate smaller orifices.
This allows such units to be specified for the smaller skids and more compact plant designs now needed. The Compact Orifice Plate can offer a retrofit solution to existing orifice plate installations where flow profile induced errors are significant.
The averaging Pitot tube continues to expand in application, being one of the few true full pipe averaging techniques available when considering an insertion type flow sensor for retrofit onto an existing plant. Frequently applied to HVAC or duct flow measurement, flue gases in chimneys, or continuous emissions monitoring systems, Pitot tubes also find wide application in steam metering. Again the modern electronic capability of the DP transmitter has been adapted, to allow one ‘Multivariable’ transmitter unit to monitor pressure, temperature and DP, for example allowing one transmitter to derive steam mass flow. More articles from Emerson Process Management: | 
