Users expect flowmeter readings to be accurate, independent of process parameters and media properties such as viscosity, temperature or pressure. An installation at BYK-Chemie in Wesel demonstrates how a mass flowmeter’s design impacts the accuracy of the measurement for products with different viscosities.
BYK-Chemie is a member of the Altana Group and a leading supplier of coating and plastics additives worldwide. These are used in minute quantities and improve the qualitative properties of finished products. BYK-Chemie generates revenues of about €348-million with 935 employees. This specialty chemical company is one of the most innovative in the sector and highly values research and development and customer orientation. This is why BYK-Chemie operates application laboratories around the world, enabling the company to test new processes and products.
Fig. 1: Dosing Xylene, viscosity 0,8 mPas – both units in the green zone.
Originally, BYK-Chemie used straight-tube mass flowmeters from another supplier in dosing various chemicals with differing viscosities. Up to eight chemicals needed be added sequentially to a container during the production process for coating additives. After several faulty batches, BYK-Chemie did some troubleshooting to determine what was causing the problems, particularly since the high price tag for chemicals was quickly driving the total cost to an unacceptable level. During the investigation, measurements taken using precision scales revealed deviations of various magnitudes for different media. The manufacturer stated that these were clearly viscosity related. The supplier then recommended that BYK-Chemie replace the straight tube product with twin-tube meters. If this recommendation had been followed, the dramatic increase in dosing times would have meant additional shifts for the company. Furthermore, one of the stated objectives was that there should be no appreciable drop in pressure when dosing. This is not possible with twin-tube meters. BYK-Chemie therefore rejected the recommendation. As an alternative, the supplier recommended a zero-point adjustment for every product change. This also would have led to longer dosing times. Zero-point comparison takes several seconds and is very complicated to do automatically. BYK-Chemie therefore decided to test and directly compare Krohne’s Optimass with the original supplier’s mass flowmeter.
Fine dosing – the comparison test
At Krohne it is standard practice to thoroughly test every Optimass against all specifications before shipment. This characterisation of the Optimass goes far beyond normal calibration. All of the meter’s calibration constants are set in the meter, and its metering behaviour, including zero-point stability, linearity, mass and density measurement accuracy and repeatability, is tested at a minimum of three different product temperatures and at various flow rates. All test results are documented and are available at any time. Every established standard recommends adjusting the zero-point on a Coriolis mass flowmeter on site. When it comes to metering highly viscous media, the readings usually fall within the low end of the measurement range. For these types of applications, excellent zero-point stability plays an extremely important role in achieving very precise measurements. BYK-Chemie decided to use Krohne’s mass flowmeter straight from the factory and not to do the zero-point adjustment on site when it was tested against the Coriolis meter they were presently using. An Optimass with a nominal diameter of 40 was installed vertically in series and flange-to-flange with the competitor’s unit during the test run. BYK-Chemie then measured the mass flow rate for three products with different viscosities on both units. The company did several test runs for each product. During the test, the initial flow rate of up to 25,000 kg/h was reduced to a maximum of 1000 kg/h for fine dosing.
Both units met the customer’s accuracy specifications and had a stable zero point when comparative measurements were made on Xylene, a low viscosity product (see Fig. 1). When the product was changed to Bupol, which has a viscosity of 200 mPas, major shifts in the zero-point occurred with the first meter. Even after stopping the flow, the counter readings continued to change. These were corrected when the measured values were recorded (see Fig. 2). Every measurement taken during fine dosing had over ten percent error (see Fig. 3). The last product tested was Polyglycol, which has a high viscosity of 400 mPas and at a low flow rate. The measurement error increased even more, reaching almost 15%, while the Optimass readings remained reliable and accurate (see Fig. 4).
Fig. 2: Dosing Bupol, viscosity 200 mPas – significant zero-point displacement on a competitor’s meter corrected.
After completing the comparison test, BYK-Chemie decided in favour of the Optimass and has since tested T08, T10, T25 and T40 units in various applications. Exact and reliable measurement is essential to BYK-Chemie’s process, particularly for mass flow metering at the end of the distribution heads from which various products are delivered to the mixing vessel. The results of the mass measurement are fed to a process control system (DCS) that controls the dosing valves for about 35 production units. Up to 20 different substances required to produce BYK products can be dosed from each distribution head.
Fig. 3: Fine-dosing Bupol; only Optimass gets the green light.
At this point the question “Why is the Optimass straight tube meter so much better than the unit used up until now?” deserves to be answered. Part of the answer lies in the differences in the design. For example, the first meter installed is also supposed to be able to detect the viscosity of a particular product. The application we described here confirms this statement. However, the actual mass flow readings become inaccurate with changing viscosities. In other words, the meter readings are dependent on viscosity. The second part of the answer is related to the Optimass design and the electronics used. By employing adaptive sensor technology (see box) in the design, Krohne has succeeded in making its Coriolis straight tube meter independent of process conditions. In addition, compared to conventional mass flowmeters, the new electronics used in the Optimass have significantly improved the measuring behaviour at the low end of the metering range. This enables users to very precisely measure even the low flow rates that occur with highly viscous media, without having to reduce the pipe diameter, which in turn would lead to a higher pressure loss. Since the units are able to measure low flow rates over a wide range of nominal diameters, overdosing does not occur and meter readings remain accurate.
The straight tube design
Krohne has always focused on the straight tube design, since the advantages over mass flowmeters with bent or double tubes are clear. Curved or o-shaped tubes and twin-tube designs with flow dividers have proved unreliable in many applications. For example, it is very difficult to meet food sector or pharmaceutical industry requirements when using meters with flow dividers and whose flow paths are not straight because the specifications call for no dead space or crevices. Some media, such as viscous, non-Newtonian, shear-sensitive fluids or solids-bearing fluids cause high pressure drops in twin-tube meters. Abrasive media can wear away flow dividers and tube bends, and metered media containing fibres, such as palm oil or cellulose, can lodge in the flow dividers and cause blockages.
Fig. 4: Fine-dosing Polyglycol, viscosity 400 mPas; only Optimass gets the green light.
The most important benefits of straight-tube flowmeters include:
low pressure drop
less wear due to abrasion
also suitable for high flow rates
wide measuring range
no risk of blockage
In addition, straight tube flowmeters are easy to clean and self-draining. The favourable relationship between the short measuring tube and large inner diameter makes the unit suitable for measuring even highly viscous media.
Each individual application will determine whether users will benefit from each and every one of the advantages of Coriolis straight tube flowmeters as described above. One of the top priorities in the development of Optimass was to design a meter with high accuracy over a wide measuring range while maintaining good zero-point stability. Various factors can influence the measuring accuracy and zero-point stability of Coriolis flowmeters, including the gas content of fluids, the homogeneity of solid- bearing fluids and also viscosity. The following example demonstrates how large an impact viscosity can have on a mass flowmeter’s readings if it is not designed properly.
Contact John Alexander, Krohne, Tel 011 314 1391, jalexander@krohnesa.com
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