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For temperatures up to 400 °C

High-temperature radar sensors for liquid level measurement
For temperatures up to 400 °C

For process pressures of 100 bar and temperatures over 400 °C, it is difficult to find a suitable liquid level measurement system. In cooperation with a major American oil and petrochemical concern, a high-temperature radar sensor has been developed for non-contact liquid level measurement.

Hanspeter Oswald

In distillation and stripper columns, up to now liquid levels (e.g. of sump, plate and head products) could generally only be measured by means of pressure sensors or differential pressure measurements. The installation required for such pressure measurement systems (pressure lines, pressure converters) is elaborate and expensive, often amounting to several times the value of the sensor itself. Because there were no realistic alternatives, instrumentation departments had to put up with not only this, but also high maintenance costs (for rinsing tubes, errors due to condensate, scale formation on diaphragms etc.) and often had to accept inadequate accuracy (due to temperature errors, density variations, installation faults…).
The requirements of the petrochemical industry for a non-contact liquid level sensor resulted in the following specification:
• independent of temperature and pressure,
• process temperature up to 400 °C,
• process pressure up to 150 bar,
• to be made of heat-resistant and universally applicable materials,
• accurate to 0.1%,
• robust metallic housing,
• flameproof (EEX d or EEX ia),
• loop-powered and compatible with digital networks.
In principle, radar sensors were the only way these demands could be met.
Mastering widely different coefficients of thermal expansion
Radar sensors for high temperatures have been in use for many years, but they all have to be cooled with compressed air, or water, or both. The reason for this lies in the technology employed for coupling the radar signal into the container so that it can sense the liquid level. In this coupling zone, the high frequency radar signal (electro-magnetic waves at 5.20 GHz) passes from the wiring into the coupling material. From the point of view of the high frequency signal, this material has characteristics similar to air or empty space. The radar frequency and the type of coupling material determine the exact geometric form that must be imparted to the latter. The coupling component behaves in the first instance like an precisely tuned waveguide. It operates, in the zone through which the radar signal passes from the coupling material into space, as a focusing lens which bundles the signal to a high-frequency beam. The dielectric constant of the coupling material governs its „refractive index“.
Up to now, a plastic (PPS, PVDF, PTFE…) was always required as the coupling medium for microwave signals and as a gastight barrier between the process and the electronic components. However these plastics neither retain their mechanical strength at temperatures above 150 °C to 200 °C, nor have they adequate resistance to such temperatures without cooling.
The VEGAPULS 56 is a sensor that owes its existence to the latest advances in materials science and production technology. A specially developed ceramic, which has high frequency properties similar to the plastics usually employed, is used as the coupling material. This ceramic is both chemically and thermally extremely stable.
All parts of the sensor that come into contact with the process are made of highly resistant materials. This refers not so much to the flange material, which is a highly alloyed stainless steel (1.4571 or superior), as to the specially developed ceramic (Al2O3) and its gastight, pressure and temperature resistant gland. An elaborate stepped sealing system is employed between the stainless steel flange and the ceramic body to create a joint that is gas-tight at a molecular level. The gland has a sealing ring of tantalum – a material that is so expensive to produce that the price is comparable to gold – and a special bonding process that makes it possible to join materials that have very different coefficients of thermal expansion.
At high temperatures, the thermal stresses between highly alloyed stainless steel and a normal ceramic would be extremely large. Stainless steel (a = ca. 18.5 x 10-6 K-1) expands more than twice as much as the ceramic (a = ca. 8 x 10-6 K-1). With this unique, ultra-gas-tight bonding process – similar to brazing – the ceramic is joined to a metallic sleeve that has the same coefficient of thermal expansion as the ceramic itself.
The joint may be compared to a slice of hot toast into which butter is allowed to melt, and then deep-frozen. Like the toast and butter, the ceramic is then inseparably bound to the metal sleeve (brazed sleeve) at a molecular level. This first brazed sleeve is then electron beam welded to a second mounting sleeve. As described above, the ceramic body and brazed sleeve have identical coefficients of thermal expansion, but the mounting sleeve into which the ceramic body and brazed sleeve are welded expands on heating more than twice as much as the latter. The brazed sleeve is therefore designed so that it can absorb the thermal expansion of the highly alloyed stainless connection flange without fatigue, like a spring.
Finally, the whole assembly, ceramic, brazed sleeve and metal mounting, is laser beam welded into the flange/process connection. The result is a system which is thermally absolutely stable, and, thanks to the tantalum seal, and welded and brazed joints, one that is completely gastight.
A tubular construction separates this hot zone at the process flange thermally from the micro-wave module and evaluation electronics. To protect against radiant heat, the rear face of the flange up to the first tube segment must be fitted with the normal container insulation. Plants of this type are anyway always insulated as standard, so that the process side of the sensor can simply be included in the process insulation. The temperature of 400 °C on the process side of the flange is thus reduced to 40 °C at the electronics.
Pulsed radar or FMCW radar?
A metallic waveguide in the tube system and a small hole in the rear face of the ceramic body couple the radar signal from the high frequency module into the ceramic. The ceramic body, at its conical end (a focusing lens) works alternately as sender and receiver. Fast and intelligent fuzzy logic evaluation electronics process the received echo images in 0.1 s cycles. A pulsed radar measurement process is used in which radar signals are sent as a short pulse of 1 ns duration. The liquid in the container reflects the pulse, and, in the pauses between pulses which last 278 ns, the sensor, now operating as a highly sensitive directional micro-phone, receives the radar echo. In this time, the travel time of the signal (less than a millionth of a second) must be precisely processed, and the echo image evaluated in a fraction of a second. This is achieved by a special time transformation process. The time transforma-stion, without which the more than three million echo images per second would be difficult to evaluate, are stretched and frozen in a slow-motion process. In this condition, the electronic system can evaluate the radar echo images very precisely. Without the time-consuming frequency analysis needed with FMCW radar, the VEGAPULS 56 sensor described here is able to re-calculate the liquid level continually in 0.1 s cycles, and present the results to the nearest millimetre (0.1% accuracy).
Vega
Fax: ++49/7836/50-190
Further information cpp-201
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