The chemical industry relies on tried and trusted measuring technology to measure the volume flow of electrically conductive media: well over three million electromagnetic flowmeters (EMF) are in use all over the world. The demands placed on devices when used in chemical plants are much more stringent than those in the water industry, for example. In addition to the electronics, the design and measuring tube material are crucial when it comes to determining whether the EMF is suitable for use in chemical processes.
The following properties of the measuring tube material are particularly relevant:
• Corrosion resistance to aggressive media.
• Form stability.
• High resistance to thermal shock.
• Vacuum resistance, form stability with under/over pressure.
• Resistance to abrasion.
• Diffusion tight.
• Leakage immunity.
Even if not normally the case, sometimes with certain tasks such as measuring nitric acid, several of these properties are required at the same time. The PFA and PTFE linings typically used for EMFs are not adequate in this case as they are not acceptably resistant to diffusion. For this reason, EMFs with measuring tubes and measuring tube liners made out of ceramic materials have been available for decades for this type of application.
Krohne was a pioneer in this area, introducing its first ceramic device as early as 1982. Since then, it has been constantly perfecting ceramic measuring tubes for various fields of application. Krohne currently offers a variety of different device types featuring ceramic measuring tubes: in addition to their use with acids and bases in chemistry, the flowmeters used in bottling machines in the beverage industry are almost exclusively equipped with ceramic measuring tubes. This is where another of the ceramic properties comes into play: extraordinary low surface roughness means good hygienic suitability.
The Optiflux process measuring devices used in the chemical industry feature measuring tubes made of oxide ceramics with a base of aluminium oxide or zirconium oxide. The electrodes are mainly platinum but depending on the application, other materials are also used. Variants without wetted metal electrodes, so-called capacitive EMFs, are also available.
Design and construction of the measuring tube
To understand the special properties of ceramic measuring tubes, users must first understand their design: a measuring tube is a single rotation-symmetric piece from face to face. It is manufactured as if it were a cast piece. This holds true for nominal sizes up to DN 300, sizes above that are lined with ceramic plates.
The starting material for the measuring tubes is an aluminium or zirconium oxide powder featuring special mineral quality which is pressed into a cylindrical shape under pressures >1000 bars. The cylinder is then machined into the shape of the measuring tube.
It is sintered above 1700°C in a process during which the ceramic shrinks to the pre-calculated size. The hardened measuring tube is now the desired basic size and any continued processing such as machining the outside diameter, end faces and inlet taper is only possible with a diamond-tipped tool. Following quality control, it is ready to be installed into the EMF.
This brief description does not come close to reflecting the extreme amount of effort and long years of experience and expertise necessary to manufacture high-performance ceramic. One example of the numerous innovations resulting is the fused-in-place cermet electrode: when inserting the electrode containing precious metal into the ceramic measuring tube, the goal is to unite two different materials with as little gap as possible and in such a way as to ensure that temperature shocks cannot cause any damage. A leak at this position could mean a safety concern and increased costs, depending on the measured product. To begin with, two holes are made in the ceramic raw form and the electrode is inserted like a massive pin, the ceramic then shrinks during sintering. The result is a leak-free compound of two materials.
Stability and reliability
This type of manufacturing justifies many of the features of the Optiflux ceramic EMF which are so valued in the chemical industry. The combination of accuracy, repeatability, robustness, long-term stability and process reliability is unsurpassed amongst process measuring devices.
The long-term stability (i.e. accuracy over many measuring cycles) of the measuring tube is achieved thanks to the extraordinary form stability of the ceramic. Plastic coatings, on the other hand, are malleable, exhibit vacuum sensitivity and are by far not as resistant to wear.
The extremely high long-term stability of the ceramic devices was scientifically investigated during cooperative research between Krohne and the PTB (National Metrology Institute). During the tests, the devices tested for the food and beverage industry were subjected to a total of 600 simulated clean-in-process cycles, one after the other, followed by another 60 sterilisations with hot steam (SIP) (extreme temperature change). That corresponds to a device life of approximately 4-5 years in average operation. The tests showed that the ceramic devices had an average deviation of only 0,05% compared to the tests in new condition and also featured stable repeatability at various flow rates.
A practical example
An application example illustrates the abrasion resistance of the virtually diamond hard ceramic: the Dutch company Nedmag Industries Mining and Manufacturing is Europe’s leading manufacturer of ultrapure synthetic magnesite DBM mainly used in fire-proof applications such as liners in blast furnaces. When producing DBM, magnesium hydroxide sludge with 53% solid content is also created. To process this extremely abrasive sludge, its flow rate must first be measured. Since a PFA or PTFE liner would fail in a short time, an Optiflux 5300 with ceramic measuring tube was used.
When it comes to corrosion resistance, the requirements of the chemical industry are extremely high: to equip standard processes and new production routes, ceramic is used because other materials are not sufficiently durable.
To increase process reliability further, EMFs with ceramic measuring tubes are available in both sandwich (DN25-100) and flange versions (DN15-300). The Optiflux 5300 is a ceramic EMF available with flanges, and thanks to the shorter screws (less expansion on heating), the flange version offers a high degree of fire resistance as dangerous products do not leak out at connection points if a fire occurs. The flange version of the Optiflux 5300 contains a ceramic measuring tube that features a Pressure Equipment Directive 97/23/EC with individual approval for zirconium oxide and on top of that has design approval for DN 15 to DN 300 from TÜV Rheinland. For the first time, this enables the chemical operator to use a ceramic EMF as per the Pressure Equipment Directive. This means the operator does not have to run his own test and can reduce costs.
Electromagnetic flowmeters with ceramic measuring tubes boast unrivalled high performance, particularly in the chemical industry. Thanks to the unique combination of material properties such as surface hardness and finish, mechanical strength, resistance to corrosion and temperature change, the operator benefits from the extreme accuracy and long-term stability as well as vacuum and diffusion resistance. The chemical industry’s own requirements concerning process reliability, longevity and durability are met perfectly.
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