Keeping air under control

11 April 2008

Tessenderlo Chemie produces dicalciumphosphate at its production facility in Ham, Belgium. Recently, a flat-operated flow meter was replaced with a thermal dispersion flow sensor, a move claimed to simplify the acquisition of preventative indications. Alfons Calders, of Industrie Technisch Management, explains how.

Problem solved: dicalciumphosphate production facility
Problem solved: dicalciumphosphate production facility

The chemical conversion takes place in high silos, in which an air stream keeps a mixture of sand and hydrochloric acid moving. To make sure the blowers, that manage the inlet, were supplying sufficient flow a mechanical meter was used to test the flow on the pipe.

It appeared that, in spite of the almost constant supply to the blowers, the airflow was not constant because of resistance as the air passed through the sand and acid. As a consequence the float was often smashed against the upper and lower float catchers, frequently knocking the upper one loose before being sucked into the silo and sticking in the first duct. The air pipe became blocked, the swirling effect interrupted and the mixture settled down. The production had to be interrupted and the silo had to be emptied.

Magnetrol’s TD2 was tested. For the production of dicalciumphosphate, the sensor was coupled to a writer with a delay to suppress alarms that would only indicate large flow fluctuations. Besides a direct indication of insufficient pressure, the flow trend can be controlled. After proper calibration airflow between 0 and 2.575 Nm³/u can be tracked via the 4 to 20 mA signal.

This measurement principle seemed to offer a solution but there was a problem with oil in the airflow. There are two versions of the Thermatel TD2: a twin-tip type and a spherical type, which contains the 2 sensors. Using the spherical type avoids the pollution of the sensors. To test this, four production units were equipped with TD2s. After six months, the measurement did not appear to be sensitive to oil settlement.

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