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CALIBRATION BE CONFIDENT IN DENSITY MEASUREMENT


Norman Glen, Service Leader for densitometers and physical properties of fluids at TÜV SÜD National Engineering Laboratory, looks at mitigating the health & safety and fiscal risks of densitometer calibration


T


he accurate measurement of the density of fluids at the correct conditions of temperature and pressure is critical for many applications. These include where the fluid density is required for product quality assessment, process control (including safety aspects) and for inter-conversion between mass and volume flow rate. Fluid flow in pipelines is largely measured by volume, and conversions involving density are necessary for accurate financial reporting, fair trade and taxation when reporting by mass or by volume at standard conditions. Density measurement is therefore a key element of both mass and volume flow rate measurement in many industrial sectors and is fundamental to the commercial operation of facilities. Whilst the use of Coriolis flow meters to provide direct measurement of mass flow rates is becoming more common, the most widely implemented approach for mass flow measurement is to use a volumetric flow meter and a densitometer. Both of these require periodic calibration. Whilst the need for periodic calibration of


flow meters is widely recognised, the corresponding requirement for density measuring devices is not. One area where this has been recognised for some time is oil production in the UK. The regulatory body responsible for this, the UK North Sea Transition Authority (formerly known as the UK


22 OCTOBER 2022 | PROCESS & CONTROL


Oil and Gas Authority), requires the traceable calibration of densitometers where they form part of fiscal flow metering systems.


“ The performance of a suitably calibrated


densitometer or Coriolis meter is more than sufficient


The UK’s approach to densitometer calibration, developed by TÜV SÜD National Engineering Laboratory, is based on the off- line calibration of densitometers using traceable reference fluids and has been in operation for ten years. Based on the original work, further research and inter-comparison activities with device manufacturers and hundreds of calibrations of customer densitometers, the sector has strong confidence in this approach.


We believe that this approach could be more widely used. For example, a master densitometer calibrated in the laboratory could be installed on a bypass line onsite, either to provide real-time density data or to prove the installed density measurement system. In addition to providing real-time data, this approach offers a reduction in the health and safety risks associated with


”


sampling using pressure pycnometry. Pressure pycnometry is still widely used, particularly in the USA, when dealing with flammable liquids at high temperature and pressure.


Density is a derived quantity but pycnometry has the advantage of being as close as possible to its direct measurement. Density is the ratio of mass to volume, so in SI units it is reported in kg m-3 customary units in g cm-3


and in US . A pycnometer


provides a measurement of the mass of a known volume of fluid. Careful calibration of the volume of the pycnometer is required and appropriate corrections must be applied to account for changes in volume if it is used at temperatures or pressures different from its calibration conditions.


For liquid applications, pycnometers are usually calibrated with water, to determine their internal volume, since the density of water is known with very low uncertainty across a wide temperature and pressure range. Thus, in principle, pycnometry can achieve very low measurement uncertainty. However, in addition to the careful calibration of the pycnometer in the laboratory, considerable operator skill is required when using the device in the field, particularly for fluids at elevated temperatures and pressures. As a result, the measurement process is often time-consuming and does not provide continuous data output. The use of an oscillatory technique to measure density, either by a dedicated densitometer or via the secondary output of a Coriolis flow meter, has the advantage of providing a continuous output signal. Furthermore, once installed, no further operator intervention is required, eliminating the impact of operator skill and limiting their exposure to potentially hazardous conditions.


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