Using microwaves to detect hopper blockage

VEGA Australia Pty Ltd
Monday, 14 February, 2011

Schwenk Zement of Ulm has been a pioneer of the cement industry for more than 160 years and is recognised as one of the most innovative companies in the sector. The cement works in Allmendingen, Germany, was founded in 1889 and is the oldest manufacturing location of Schwenk Zement KG today. Large deposits of high-quality limestone were - and are still today - the basis for the site.

Cement is an important raw material for the construction industry. But production involves many complex processes and must be constantly monitored. Schwenk Zement uses the microwave barrier VEGAMIP from VEGA as a robust and reliable solution for reporting material blockages.

In the cement plant, a furnace burns the raw meal (usually composed of the raw materials limestone, clay, sand and iron ore) into clinker at approximately 1350°C. The hot clinker powder is discharged from the furnace into the clinker cooler, which cools the material down to between 150 and 350°C. At the end of the cooler, the clinker falls through a hopper, onto a plate conveyor belt that transports it to the clinker silo. In this hopper, the VEGAMIP microwave barrier is used to report any blockage that might occur, so that the clinker cooling can be stopped in time. If production was stopped too late, the clinker cooler would become completely blocked and require expensive and time-consuming cleaning. This would lead to a shutdown of the entire plant with a corresponding loss of production.

The volume flow at the clinker discharge amounts to approximately 250 t/h during normal operation. Falling material should not be detected by the blockage sensor. The sensors should only respond and stop the material flow when blockage occurs in the hopper. Transmitter and receiver are mounted with a high-temperature adapter directly on the hopper at a distance of approximately 1.5 m apart. The mounting adapter has a ceramic window cover that protects VEGAMIP from abrasion and temperatures up to 450°C, yet allows the microwaves to pass through.

The empty adjustment of VEGAMIP is carried out during normal volume flow, when material is falling through at a normal rate. When blockage occurs, the material density in the funnel increases, causing the VEGAMIP instrument to output a full signal. The switching signal of VEGAMIP is passed on to a PLC and immediately evaluated. In-contact detection techniques would be physically damaged in this environment. The radar waves of VEGAMIP penetrate the falling material and the electronics reliably trigger the switch output when a blockage occurs.