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FEATURE PRESSURE MEASUREMENT


THREE INTO ONE DOES GO A


multivariable single device measures and calculates mass


flow as a function of differential pressure as well as absolute pressure and temperature.


WHAT DOES THAT MEAN? A multivariable transmitter incorporates three separate transmitters into one device which can measure and calculate static and differential pressures and, via an external sensor, process temperature, with the added option of a flow computer. A multivariable transmitter can identify changes (compensations) and dynamically adapt to calculate these variations in the process condition to produce a highly accurate mass flow reading.


HOW DO MULTIVARIABLE TRANSMITTERS WORK? DP-flow measurement applications are made up of two constituent parts: a primary element and a secondary element, such as a differential pressure transmitter or multivariable transmitter with temperature measurement and integrated flow computer. The primary element is used to constrict the flow of a fluid in order to create a pressure drop, which in turn allows differential pressure to be measured to ascertain the flow rate. There are many variants of primary


elements that use different geometric shapes to measure differential pressure including orifice plates, flow nozzles, wedge meters, Pitot tubes and Venturi tubes. Depending on the application, certain primary elements are more suitable due to the range of fluid properties. For instance, wedge meters are appropriate for fluids with high solid content, erosive or abrasive fluids and high viscosity fluids. Once the correct primary element


is chosen it is placed in the pipe, creating an area of differential pressure and is connected directly to the secondary element. The secondary element or transmitter then takes a measurement and converts the reading into a useful quantitative value by using calculation methods within


24 MAY 2018 | INSTRUMENTATION


the multivariable transmitter such as ISO 5167 / AGA3, Natural Gas compensation acc. AGA8.


HOW DO THEY HELP TO SIMPLIFY MEASUREMENT? The ability to use a single device to measure multiple variables offers advantages during installation and commissioning. In a conventional installation, the same functions handled by a single multivariable device are instead split between separate instruments for each type of measurement. For users, this means having to purchase multiple instruments, which then have to be individually set up and wired in. The need to install multiple


devices means that extra installation space is required, something which can be difficult in a cramped or compact location where space may be at a premium. Where devices are purchased from


more than one manufacturer, there is also the added scope for error. For example, different manufacturers are known to calibrate their devices under different conditions, such that a measurement from one device may not necessarily be directly comparable with that from another.


WHAT BENEFITS DO THEY OFFER? Multivariable transmitters offer a variety of benefits, not least of which is cost. By replacing separate dedicated instruments with a single device that can measure multiple values and calculate flow, multivariable transmitters can typically cut the cost of a mass flow installation by around 40 per cent. In addition to reducing the amount


of devices, users can continue to save money due to minimising process connections, less cabling and fewer I/O modules used for recording and transferring measurement data to a centralised system. All these factors help reduce installation time, cutting costs further, without compromising on reliability or measurement accuracy. Taken together,


As a means of providing an insight into what is happening within a process, instrumentation provides the front line in any drive to find ways to maximise productivity and profitability. This Q&A-style article from ABB explains how multivariable transmitters are providing a cost-effective and highly accurate alternative to conventional technologies for measuring the flow of gases, vapours and liquids in industrial applications


the reduction of ongoing maintenance and enhanced functionality means that potential savings from opting for a multivariable device can reach up to 30 to 40 per cent. Another advantage of choosing


multivariable transmitters is the ability to limit the number of pipe penetrations. Installing just one instrument reduces the impact on flow and any negative effects on measurement accuracy. Furthermore, with the inclusion of integrated calculation functionality, the transmitter can measure any compensated levels of gas, liquid and steam, ensuring that any variations in temperature, pressure, density or viscosity are accounted for to maintain an accurate reading.


WHAT FACTORS SHOULD BE CONSIDERED FOR SELECTION? The main factor to consider is the choice of primary element, which will then determine the differential pressure range. Due to different fluid properties certain primary elements are better suited to different applications. Some examples include: Orifice plates - suitable for clean liquids


at low velocity and / or high viscosity, and low density gases. Venturi tubes - suitable for clean liquids and gases. Flow nozzles - suitable for high


velocity fluids and flow testing for steam-raising plant. Pitot tubes - suitable for liquids, gases and saturated and super-heated steam. Wedge meters - suitable for fluids with


high solid content, erosive and abrasive fluids and high viscosity fluids. Another factor to consider is the need for temperature and pressure compensation. The density of a fluid can change relative to temperature and pressure changes. This density variation can then have an impact on the measurement accuracy if the flow rate is uncompensated. This adjustment occurs when flow rate is affected by temperature, pressure, density or viscosity.


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