Level Measurement & Control

Level measurement with radar

May 2010 Level Measurement & Control

The measuring principle

The active component is the antenna of the radar sensor; short microwave pulses in the GHz range are emitted and propagate down to the surface of the medium where a portion are reflected and received as echoes, again through the antenna. The elapsed time between sending and receiving is measured and translated into a level by the sensor electronics.

The main advantage of radar is the non-contact measuring procedure. Since the electromagnetic pulses do not need a propagation medium, process conditions in the vessel like pressure, temperature, vapour, dust and noise have less influence on the measurement. The method is equally well suited for liquids as it is for bulk solids.

Decor paper – level measurement in the broke pulper

Technocell G&$252;nzach in southern Germany is a leading manufacturer of special high quality types of edge papers and smooth one sided foil papers. These decor papers contain high quality fillers and pigments and are often produced in small batches according to customer specifications.

Colours and features are continuously checked, especially during the start up phase, via online and laboratory measurements. Until the required quality is reached, the paper is recycled. The paper web is first fed directly into the broke pulper under the reeling section and decomposed to a pulp density of approximately 4% by adding screen water at 30°C and agitating. A level sensor in the pulper controls the emptying and filling of the vessel and thus the material density.

The process conditions in the pulper result in agitated surface conditions due to water jets, falling paper web and a stirring unit, which make level measurement difficult. The previously installed hydrostatic measurement equipment did not work to the complete satisfaction of the user. Unfavourable factors were unstable measurements, high cleaning costs due to colour deposits and difficult access to the sensor which was located on the side wall of the pulper.

VEGAPULS 61, a radar sensor with plastic horn antenna for liquids measurement, was deployed as an alternative solution. The measurement is carried out in a vertical, approximately 2 m long DN 80 tube, which guides the radar signals and at the same time excludes the effects of the water jets and the falling paper web. Jets of fresh water periodically clean the inner surface of the tube. Due to the non-contact measuring technique and the configuration of the measurement setup, all maintenance work is eliminated and the sensor is directly accessible at any time.

Thermopaper – level measurement in the headbox

At one of the leading European suppliers of thermopapers, an effort was made to find a good solution for measurement in the headbox. The level and pressure measurement in the headbox serves to regulate the stock pump and therefore has direct influence on the cross-direction profile of the paper web.

The previous level measurement in the overflow outlet was monitored with using differential pressure transmitter. This differential pressure pipes installed for the measurement had to be continuously flushed with sealing water and completely cleaned at regular intervals. Furthermore, air bubbles in the sealing water had an adverse affect on measurement accuracy.

The level measurement setup in the overflow was then replaced by the VEGAPULS 63 radar sensor with front flush, PFTE-encapsulated antenna system. Its millimetre precise measurement guarantees a constant filling level in the overflow, with a filling level of only 450 mm at pressures up to 0,7 bar. The excellent signal focus avoids disturbing influences from the perforated roll.

Pulp production – level measurement in the thick lye tank

With a production capacity of 570 000 tons/year of bleached long fibre cellulose, Zellstoff Stendal in northern Germany is one of the largest manufacturers of market pulp in central Europe. The high-quality fibrous material is further processed into printing paper, amongst other things, and also used as reinforcement fibre in waste paper recycling.

The black liquor arising from cellulose digestion is thickened to a dry content of 75% in the evaporation system. A viscous liquor is thereby created, which is stored at 80°C in a 17 m high insulated tank. The set point level in this tank is evaluated via the control system to ensure continuous process.

Previously, the level measurement was monitored using a hydrostatic pressure transmitter, mounted laterally in the lower area of the thick liquor tank. Sedimentation of the liquor caused build up on the container walls and the sensor diaphragm, resulting in unreliable measurements and high maintenance costs for cleaning the transducer.

The radar sensor VEGAPULS 66 is optimally suited for level measurement under the conditions described above. A steady, light flushing of the antenna with compressed air prevents build up from vapour precipitation for maintenance free operation.

Book paper – level measurement in the starch silo

Mochenwangen Paper in southern Germany is a manufacturer of high quality papers for book production as well as for web and sheet fed offset printing. It delivers, among other products, the paper that was specially developed for the Harry Potter novel series.

Chlorine-free bleach, a slight wood content and a special surface and a feel, were among the requirements of the author and the publishing house. Important components to achieve this quality are peroxide bleach as well as a specially prepared starch used for gluing. The raw starch is stored in a silo 12 m high and 3 m in diameter. The level is monitored by the control system to ensure a sufficient supply of starch for production. The previous measurement system was via a mechanical plumb line, which worked only intermittently and required regular servicing.

During the course of modernisation, the setup was changed to one using a VEGAPULS 67 bulk solid radar sensor. This instrument is optimised for process conditions like dust and unfavourable reflective properties and operates completely maintenance free.

The rapid development of level radar has led away from large, costly instruments that require much time and effort to parameterise, to small, efficient, cost-effective and maintenance friendly sensors. Suitable instrument versions are now available for widely different measurement tasks. This development opens up still more diverse application possibilities for the radar measuring principle in the pulp and paper industry.

For more information contact John Groom, Vega Instruments SA, +27 (0)11 958 1901, john.groom@za.vega.com, www.vega.com


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