In the process industry, Coriolis flowmeters are known as reliable and robust field instruments. Process dependability and efficiency have become increasingly vital, and the result is an increasingly wide field of application for these versatile all-round devices.
Today's flowmeters have to fulfil functions more diverse and more demanding than ever before. Meeting tight budgets, while optimising process efficiency and maintaining or improving product quality, are all putting pressure on flowmeters to meet growing measurement and control requirements. The installation problems associated with the formative years of Coriolis mass flowmeters have been overcome and they are now widely used in industrial flow measurement.
Large span accuracy
Coriolis mass flowmeters are one of the most accurate (over large spans) on the market. Large spans are required, because liquids with high viscosity do not reach high fluid velocities - yet an accurate measurement must be achieved here as well. The accuracy for smaller flow rates is limited, with zero point errors. The achievable zero point stabilities range within a magnitude of 0,005% from the end value.
In the case of Coriolis flowmeters, the key to good zero point stability lies in the de-coupling of the measuring element from the process environment. The flow signal is derived from the phase shift of the sensors at an oscillating measuring tube. If there is a poor de-coupling of this oscillating measuring tube from the process environment, the connected pipelines will also start to vibrate. This, in turn, causes the flowmeter to be excited by these undefined vibrations - it cannot differentiate between signals coming from the flow and signals caused by external vibrations.
Vibrations coming from external forces, such as pumps and valves, are not critical, because they are not correlated with the measuring tube vibrations. Critical, however, are disturbances with exactly the same frequency as that of the measuring tube vibrations - the disturbances caused by the self-oscillation of the mass flowmeter. This causes zero point errors.
Oscillation compensation brings immunity to pipeline vibrations
Excellent accuracy of mass flow measurement is one of the outstanding characteristics of a Coriolis meter. The multivariable Promass I sensor consists of a single, straight measuring tube. The patented TMB system (Torsion-mode-balance) system keeps the system perfectly balanced, making it immune to external influences such as pipeline vibrations.
A counterweight mounted on the measuring tube (Figure 3) imposes a torsional movement (oscillation) of the tube that is utilised for measuring the viscosity of the fluid. The torsional movement creates a velocity profile in the fluid across the tube's cross-section. This velocity profile is an expression of the fluid viscosity.
The viscosity of the fluid dampens the torsional oscillation of the measuring tube, so if viscosity is high, more excitation current (power, in other words) is needed to sustain the torsional oscillation. Thus, dynamic viscosity is determined by measuring the required excitation power.
Fluid density is measured independently and simultaneously, so the kinematic viscosity can be determined as well.
The compensation operates through a wider density range, than with the previous counter transducers. For example, for a nominal diameter DN 50 the output force on the flange over a density range of 500 to 1500 kg/m3, a sensor with a pendulum emits far less force than the same sensor without a pendulum. This makes the installation of the device far less problematic and the ambient influences on the zero point are practically eliminated.
Instrument construction
The entire structure has been mounted in a sealed housing. Special attention has been given to the connectors, which are made from titanium - as is the measuring tube. The titanium welding process provides even transitions from the hydraulic connectors into the measuring tube. The device can be cleaned easily and has the EHEDG certification. Two versions are available for almost all connectors. The so-called 'full-bore' versions with straight-through section provide the lowest loss in pressure and the most careful handling of the medium.
Four primary variables with a single sensor
The ability to concentrate a number of process parameters into a single measuring point is one of the major strengths of Coriolis meters. Mass flow, density, temperature and, now, viscosity can all be measured by one single device. These measured variables can be combined, which means that a single process-metering device is all that is needed to calculate derivative variables - and no additional measuring points are necessary.
Multivariable metering - improved quality and safety
In their ongoing efforts to increase process efficiency, meet higher quality requirements and improve protection of plant machinery, companies find that they need to track an increasing number of process variables. As a result, the demand for true multivariable meters capable of registering multiple process variables simultaneously - without the need for extra sensors - is increasing.
Working in-line, Promass 83 I can now register four primary process variables/fluid properties simultaneously and independent of each other:
* Mass flow.
* Density.
* Temperature.
* Viscosity.
Since the introduction of the first Coriolis meters in the '70s, a continuous process of product development has ensued. Hence, other parameters of importance in process monitoring can be derived from the primary measured variables of these meters - the Promass 83 I is no exception, providing the following additional outputs:
* Fluid concentration calculated on the basis of measured density.
* Kinematic viscosity (ratio of dynamic viscosity to density).
* Temperature-compensated viscosity on the basis of models stored in the meter software.
Advantages
The meter has features that offer users numerous benefits - not just for measuring viscosity but also for all purposes in which it is used:
* Straight measuring tube, hygienic design, easy cleaning.
* Easy installation (fit and forget).
* No inlet/outlet runs required.
* Balanced system, immune to pipeline vibrations.
* Robust design for harsh environment.
* High accuracy and long term stability.
* No wear and tear, low maintenance costs.
* Space-saving, compact design.
With these features, Promass 83 I has proven to be a suitable choice for a wide range of applications - not least on account of the lower costs of installation and maintenance when compared with conventional installations typically requiring several different sensors.
The measuring system is EHEDG certified, has 3 A approval and is available with a wide range of process connections. PROline Promass 83 I has several approvals for use in hazardous areas, including ATEX, FM, CSA, etc. Certification in accordance with the Pressure Equipment Directive, often required for the chemical industries, is also available.
Measuring flow of liquids with entrained solids
Measuring silicone rubber containing particles of quartz (Figure 4) requires highly accurate mass flowmetering - the entrained solids must not affect this accuracy. In this particular case, due to the high viscosity of the medium, only a very small pressure loss can be tolerated. The single straight tube meter, Promass I, from Endress+Hauser is suitable for this application. Hygienic design makes the meter easy to clean and inhibits the formation of solids deposits. In this application, space was at a premium, making the compact Promass I even more attractive - it dispenses with inlet and outlet piping. Additional mounts are not necessary, due to the unique balancing for the single-tube meter achieved by the Torsion Mode Balancing (TMB) system. With the TMB system, a new single straight tube Coriolis mass flowmeter was realised, offering balance similar to that of a double tube meter.
Manufacturing meters with optimum application stability is one thing, but Endress+Hauser also realises that manufacturers require more and more accurate information about their processes. It is precisely for this reason that Promass I has the ability to measure viscosity, in addition to mass, density and temperature - Its makers report that the Promass I is the first Coriolis meter in the world with direct-in-line viscosity measurement. This meets the increasing requirements for multivariable measurement technology.
Viscosity (the ratio of shear stress to shear rate) is a crucial variable that can provide an indication about product and process changes. For example, non-Newtonian fluids either exhibit reduced or increased viscosity when the shear rate is high. For viscosity measurement, Promass I utilises the patented TMB system. A pendulum mounted on the measuring tube causes a torsional movement, in addition to the lateral oscillation movement of the measuring tube. (Figure 3). These movements cause a velocity profile. Based on this profile, shear forces and corresponding shear rates develop in the fluid. As with rotational viscometers, these shear rates are related to the viscosity of the medium. The shear forces dampen the tube oscillation, resulting in a larger drive force being required in order to maintain the tube oscillation. Hence, the viscosity can be determined by measuring the required power (excitation current) for tube oscillation and torsional movement. It is important to realise that the shear rate in Promass differs from a laboratory viscometer, making comparison between the two measurement technologies complex.
Why measure viscosity?
Viscosity is an important fluid property and a crucial parameter in process control. As a variable it can be used in monitoring product quality and changes in production processes in various industries:
* Food and beverages.
* Chemicals.
* Pharmaceutical.
* Petroleum.
* Mining.
Numerous applications underline the importance of measuring viscosity. The wide range of fluids includes shampoo, body lotion, honey, vitamins, paints, coatings and oils, to mention a few.
Measurement results
Figure 4 demonstrates how easily the new single tube systems can be installed. In this case, one instrument was calibrated, then installed on different plant units and tested. No precautionary measures for the installation were taken. The test cycles comprised various media with different densities, viscosities and temperatures. The instrument measured glycerol with a viscosity of 200 cP, glycerol with a viscosity of 500 cP, ethanol with a density of 810 kg/m3, water and polywolframate solution with a density of 1230 kg/m3. The measurement took place at a maximum flow rate of 1 kg/s, which corresponds to 2 m/s fluid velocity. The zero point and the sensitivity of the instrument remained stable during each measurement, despite different process conditions and plant units. This level of stability can only be achieved if the oscillating measuring tube has an optimum detachment from the environment.
Like most of the Coriolis Promass range of flowmeters, Promass I is also capable of measuring density. The density measurement also requires a very good detachment from the environment, because the density signal is derived from the oscillation frequency of the measuring tube. A connected pipeline, which vibrates as well, naturally changes the resonance frequency of the oscillating system. The density measurement, therefore, becomes dependent on the ambient conditions, if the instruments are not properly decoupled.
Multiple applications
Among the applications that have already been successfully used are loading of paint into tankers at loading stations; determination of the quality ratios in mixers for dispersions; determination of the density of lime milk (only a single tube can resist the high abrasion forces); measuring the fuel feed in oil burners (the single tube is also capable of measuring very dirty oils); and quantity determination in the rubber industry.
For more information contact Grant Joyce, Endress + Hauser, 011 262 8000, [email protected], www.za.endress.com
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