Paint & Coatings Industry - March 2009 - (Page 50) Mass Flow Measurement in Varnish Production M ixing, dosing and grinding are essential processes in paint and varnish production. In order to ensure a high level of automation, it is essential to be able to measure flows accurately and reliably. As some raw materials used in the manufacturing processes are very expensive, product wastage and equipment downtime can lead to considerable losses. This is doubly annoying because the measuring technology currently available on the market can effectively prevent or at least help prevent these losses. This is illustrated in the example of BASF Coatings AG (Figure 1), who uses KROHNE’s Coriolis flowmeters to detect leakages. and gaseous media, mass flowmeters are becoming increasingly popular. Coriolis measuring technology is already the second most common modern measuring method behind electromagnetic flowmeters and, together with ultrasonic flow measuring technology, boasts the fastest growth rates. OPTIMASS in Varnish Grinding The Coriolis Measuring Principle Coriolis flowmeters can be used to calculate the mass flow rate from the deformation of the measuring tube caused by the inertia. The media density can also be derived from the resonance frequency of the vibrating tube. The sensor in the OPTIMASS comprises a straight measuring tube, which is caused to vibrate by a field coil attached to the center of the tube. The excitation is controlled in such a way that the measuring tube always vibrates in its resonance frequency. Two sensor coils record the Coriolis effect, each of which is fitted equidistant from the field coil on each side. If there is no flow, both sensors record the same sinusoidal signal coming from the natural vibration of the measuring tube. As soon as a flow begins, the Coriolis force acts on the flowing mass particles of the medium and deforms the measuring tube, thus causing a phase shift between the sensor signals. The sensors measure the phase shift of the sinusoidal vibrations. This phase shift is directly proportional to the mass flow. As the Coriolis principle allows an exact measurement of fluid BASF Coatings AG uses KROHNE’s OPTIMASS mass flowmeters in the color pigment grinding process. In the past, however, mobile paddle mixers (Figure 2) with a volume of 12 m3 were guided and connected to the mill using a hose. The mobile mixer supplies the components before the medium goes into the mill and out again. This usually happens several times to ensure that each pigment particle is pumped through the mill at least once. FIGURE 1 | Water tower at BASF with the product logo. Due to the time taken to complete this process – between 36 and 48 hours in some cases – the system also operates nights and weekends. BASF Coatings wanted a solution to allow unsupervised operation of the mill by connecting a hose. This would have run the risk of potential leakages. As the whole process takes the form of a circuit, there was the option of integrating a flowmeter in the outlet, which would trigger a leak alarm in the event of a sudden deviation from the regular flow and automatically shut the system down. Although this type of leakage rate measurement only requires one meter, this meter must fulfill high specifications in terms of its accuracy. In the case of BASF Coatings, the properties of the media made things even more difficult. The raw materials for varnishes are, for the most part, high-viscosity media which, moreover, are processed at a low flow rate. The media have viscosities of 7000 mPas and over, and are generally also shear thinning; in other words, By Christoph Engelbert, Plant and Electrical Engineering | BASF Coatings AG, Münster, Germany and Reiner Scheulen | KROHNE Messtechnik GmbH & Co. KG, Duisburg, Germany 50 MARCH 2009 | W W W . P C I M A G . C O M http://WWW.PCIMAG.COM
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