Measuring in a cloud of dust

A large agitator with sharp knives ploughs its way through the mass in the storage silo for alternative fuels

Cement manufacturing is highly energy intensive because of the extreme heat required to produce it. However, by using alternative fuels and raw materials (AFR), the production process becomes more economical, primary raw materials and fuel resources are conserved, and landfill volumes are reduced.

AFRs

AFRs consist of whole and shredded tyres, waste timber or mixtures of plastics, paper, compound materials or textiles whereby their calorific value hardly differs from that of lignite (brown coal). The value of old tyres is even comparable with coal.

But the use of these alternative fuels has pitfalls. They must be processed prior to use in such a way that they do not influence the later production process and the quality of the cement.

Cementos Molins

The Barcelona subsidiary of construction materials company Cementos Molins has used a range of Vega sensors (pressure transmitters, capacitive measuring instruments and vibration limit switches) for over a decade.

Cementos Molins needed a solution for the storage silo that contained AFR. With a height of more than 20 m and a diameter of 9 m, the storage silo has an unusual agitator on the inside floor of the silo, which consists of very sharp, vibrating knives to grind the waste continuously.

About 35% of the fuel in the plant that is required for the later production of cement comes from alternative fuels. About one tonne of AFR material is unloaded into the silo per hour. The trucks loaded with waste must be completely emptied so the capacity in the silo must therefore be sufficiently great to hold a complete truck load.

Level measurement provides protection against the threat of overfilling and being empty.

Challenges with measurement when filling the silo

AFR materials stick very easily. Furthermore, when filling the silo from the trucks, a cloud of dust forms – creating heavy deposits on the sensor and making measurement extremely difficult. During simultaneous stirring and chopping, a 2 m high wave of material is pushed along in front of the agitator blades. The stirring itself also generates a cloud of dust in the container that never settles due to the vibrations. This dust influences the technology and measurements.

The previously installed 26 GHz radar sensor with horn antenna from another manufacturer created many issues. The dust stuck to the horn antenna and caused false measured values. The constant vibrations caused by the cloud of dust made reliable signals possible. Furthermore, the metal particles from the Tetrapaks in the silo also caused false signals in the sensor. Metal particles from Tetrapaks can create interference signals due to flying metal foils.

A radar level measuring instrument that performs in challenging conditions.

VegaPuls 69

Vega’s 80 Ghz radar level measuring instrument promises a much more reliable operation because of the higher focusing of the medium. At that time, a few test measurements had already been made in comparably difficult measuring situations. It

succeeded in also measuring media with poor reflection properties more accurately and easily. The high focusing also helps better differentiation of the actual measuring signal from the interference signal in containers with a lot of installations. With a measuring range of up to 120 m at an accuracy of ±5 mm, there are enough power reserves even for unusual applications.

These seemed to be ideal conditions for the difficult measuring environment. One also has to know that the opening angle of the radiated radar energy and the focusing therefore depends on two factors: the transmission frequency and the active antenna area. As a result, much better focusing is achieved with a higher frequency and identical antenna size.

The VegaPuls 69 operates with a transmission frequency of 80 GHz and an antenna size of around 75 mm. This achieves an opening angle of only 3 degrees. In a radar sensor with 26 GHz transmission frequency, the opening angle at the same antenna size is about 10 degrees. The 80 GHz beam also goes past installations or deposits on the container wall. This makes the measurement safer and more reliable. The measuring instrument itself is made from the robust PEEK material, which has a high temperature and chemical resistance. The lens antenna is also insensitive to deposits and dirt, and shows no signs of wear.

To simplify commissioning, an intelligent swivel holder was also developed with the aid of which the sensor can be aligned quickly, easily and optimally. To get the optimum angle of tilt for the installation of the measuring device, a smartphone is simply placed on the device so that the sensor can be aligned quickly and optimally with an app.

No loss of transmission signal

The Vegapuls 69 radar sensor has very good signal focusing and a high dynamic range. The 80 GHz penetrates the dense cloud of dust. Since the sensor has a purging air connection as standard, this can be cleaned quickly at any time so that any deposits ‘deal with themselves’. Despite the interference signals that could not be totally avoided due to the surrounding metal particles, an almost exact level measurement is achieved.

A remote display is located prominently at eye level on the silo to avoid the service team having to climb on to the silo during maintenance. It gives the service team an overview of all the data at all times.

The fuels must be processed before firing so that the processes run uniformly later and the same calorific value is consistently achieved

The trucks deliver the alternative fuels to the cement factory around the clock

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