The effective monitoring of important process parameters, such as temperature and pressure, is vital in order to effectively optimise processes and bring about significant reductions in production costs. When dealing with complicated processes the problem is compounded and the engineer can often benefit considerably from the specialist knowledge to be had from a helpful sensor manufacturer.
The accurate determination of the conditions within a vessel is often of great importance for efficient processing. In many industries radar technology has recently been asserting itself as a measurement method somewhat worthy of being viewed as a reference. The introduction of the two-wire technique, together with continual improvements in signal processing, has been instrumental in the growing popularity of this measurement method. The result is that radar is now used in to determine the level in a large variety of vessels, from simple storage tanks and batch vessels with a whole spectrum of products to complex reactors with stirrers and accessories.
Despite all the progress in sensor signal processing, however, applications in which radar technology is at its limits crop up time and again. These are the cases where the know-how of the sensor manufacturer and cooperation with the customer are of essential importance. Only such joint efforts will result in the best performance from the sensor, permitting effective optimisation of the plant.
A typical application
At Wacker Chemie in Burghausen, ingredients with different physical characteristics are mixed in a reactor, where a chemical reaction is initiated by heat and applied pressure. The resultant polymerisation of the liquid changes the physical properties of the mix. These physical changes complicate the task of acquiring useful level measurements.
Differential pressure
When a differential pressure sensor is used, the different densities cause large measurement errors, making this method complicated and impractical.
Capacitive probe
The use of a capacitive measurement probe is made impossible by the changes in the dielectric constant of the product and, besides this, the probe would be subject to great mechanical stress because of the stirring and the strong product movement.
Ultrasonic level gauge
The use of ultrasound as measuring principle can be excluded as the high process temperature and the changing gas composition, which would cause a change in the propagation velocity of the ultrasound waves.
Radiometric level measurement
A much unliked, but generally reliable measuring principle, was often the only possibility left in the past. For the operator, this meant a high service and maintenance expenditure and an expensive sensor. Safety zones have to be created around the radiometric emitter, warning notices must be posted and a security officer for radiation protection must trained up and employed. When its working life is over, the emitter must be disposed of, a significant extra expense that should not be overlooked.
The problems in measuring levels - and the solution
The density of the product changes during polymerisation reactions. The basis of the radiometric measurements is the damping of radioactive rays, which are allowed to pass through the medium. Since the measurement is made through the walls of the vessel, these walls also contribute to the absorption by the medium. A scintillation counter determines the gamma radiation that arrives at the opposite side of the vessel. When the density of the medium changes, the damping will also change, resulting in measurement fluctuations easily confused with those caused by genuine level changes. This renders radiometric measurement unreliable for this application.
The problems were repeatedly discussed with manufacturers of level sensors, but even the new measuring principle 'radar' did not seem suitable for this application. That was before Vega set new standards for this type of measurement by introducing a new generation of radar sensors with their user-friendly two-wire principle of operation.
To establish the limits of these sensors, it was arranged to have a test measurement system installed at Wacker Chemie. Technicians installed a Vegapuls 54 radar sensor with a horn antenna set-up of 150 mm diameter and a transmission frequency of 5,8 GHz. The service technicians used mainly standard parameters in the parameterisation of the sensor, which worked on the pulse radar principle. No application-specific software was required. The display of the echo curve on the PC was particularly helpful during the start-up phase. It enabled the reflection signals to be monitored over the whole process and the reliability of the measurement principle to be fairly judged.
The measurement was found to be reliable over the whole measuring range; no measured value variations were found, not even during the polymerisation. The influence of the stirrer paddles was very small, and this was reduced by software false-echo storage. The only disadvantage was that the very poor reflection properties of the product meant that measurements very near to the antenna system were not possible. A minimum distance of 40 to 50 mm had to be maintained.
Although this measurement system was a vast improvement over radiometric measurement, it prohibited using the top 50 mm of the vessel - a solution that was not completely satisfactory.
Reclaiming the top 50 mm of the processing vessel
Sensors from a new radar generation, working at a frequency of 26 GHz, a far higher transmission frequency than that of previous sensors, appeared to be the optimal solution for applications requiring the shortest possible minimum distance. They allowed a considerably smaller antenna to be used, while maintaining a beam width just as narrow as that of the 5,8 GHz device, with its larger antenna. Another advantage was that the newer technology also brought with it improved accuracy.
While these properties seemed to predestine the new sensors for this application, there was still concern that the surface of the polymerised product might not adequately reflect the extremely high frequency radar signal. This could only be known for certain if the 26 GHz, Vegapuls 44 was tested in this scenario.
Conversion to the new device was simple. The new sensor had the same flange and the same electrical connections, so no mechanical work or rewiring was required. The sensors were quickly swapped.
Before starting up for the first time, the interfering reflections from the multistep stirrer were stored. An initial test was performed, using water. After the engineers were certain that the sensor functioned safely, the vessel was filled with product and the process started. During this first run, the system logged the echo curves for a later analysis of any faults that may have occurred. It was found that the reflective properties of the product were far better than had been expected. The measurements were reliable throughout the reaction process. The Vegapuls 44 could measure reliably almost right up to the end of its small antenna - the sensing problem was solved.
Process optimisation with exact level measurement
With accurate and reliable processing information, the polymerisation process could now be effectively optimised. This, together with the ability to make use of the entire volume of the process vessel, meant that output increased, enabling the cost-effective production of a high quality product.
The example demonstrates how experience - and readiness to cooperate - can lead to teamwork between the sensor manufacturer and the plant operator, in this case MSR technology, which permitted the parties to find ways of making full use of the benefits of radar sensors under unusual conditions.
Powerful sensors are the prerequisite for the certainty of measurements, but the working together of people is the decisive point for the solution of such problems.
For more information contact: Alan Wynn, Vega Instruments SA, tel: 011 958 1901, e-mail: alan.wynn@za.vega.com
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