Level Measurement in Recycling Bulk Material

As always, level measurement technology plays an important role in bulk materials. The challenge to the corresponding devices, including the system requirements, can be illustrated well using recycling processes. The medium-sized company Arbeitsgemeinschaft PVC-Bodenbelag Recycling (AgPR) requires smoothly working bulk level measurements as a basis for safety and reliability in the entire recycling procedure. The company achieved this with the vibrating single-rod technology of the MBA700 as a limit switch, a measuring instrument from MBA Instruments GmbH (Germany).

Reprocessing at AgPR focuses on used PVC floorings that are processed in different work steps to produce recycled material. "They then serve as a basis for the production of PVC floorings," Heinz-Ahlerich Lübben, works as the manager of AgPR, describes the entire recycling process, adding: "The challenge is in filtering out the foreign substances from the chopped material."

AgPR needs a reliable instrument when recycling old PVC-floorings. "After shredding old floorings, a dosage container fills the resulting bulk goods into a cooling screw," Lübben reports. "This process is discontinuous, which is why we depend on sensitive fill level sensors." Before the change to the vibration limit switch MBA 700, the fill level was monitored with a rotor indicator. This measuring procedure is characterised by a motor continually turning a blade via a shaft. Once the charge touches the measuring blade and impairs its rotation, the measuring device emits a signal that interrupts filling. The disadvantage of this concept: In particularly fine and light-weight charges, the blade must be sized accordingly so that the switch will be triggered fast enough. When use of a large wing is not possible due to the system setup, there may be incorrect reports because the fine substances will stop blade rotation too late. AgPR were facing this problem as well. The choice thus fell on the single-rod vibration switch MBA 700.

As compared to other vibration rods, the patented limit switch MBA700 is a true, vibrating single rod. While vibration fork and false vibration rods have been competitors for a longer periods of time, the sword-shaped sensor, also known as a vibration blade or vibrating paddle, is unique to date.

It vibrates with a low energy at 290 Hz, so that it will not "dig itself free" on its own; incorrect measurements by cavity formation is thus avoided. Instead, the MBA700 uses the vibration blade procedure to find even extremely light-weight materials with a bulk weight of up to 10 grams per litre.

The vibration fork was used as a basis for the development of the single rod vibration fork. It was modified and improved time and time again until a single solid, vibrating stainless steel rod was developed. For best measuring-technical safety, the rod was given a cross-section shaped like a flattened diamond that led to a unique sword shape. This prevents adherence of powder when the device is installed in the direction of the material flow. The desired properties are achieved: High sensitivity to detect very light-weight powders and at the same time best mechanical stability to be used in heavy or viscous bulk goods. If the vibrating blade is bent by mechanical effects, it still continues to vibrate.

The measuring procedure is based on making a rod of steel swing with a piezo element. Vibration is a better term here, since swinging happens at a high frequency. If the vibrating element comes into contact with a liquid or solid substance, the vibration is dampened. The electronics recognise this, switch off the vibration and sends a signal.

Since the measuring procedure was integrated into the recycling process of AgPR subsequently, the challenge was in integrating the device signal into the programmable logic controller (PLC) of the unit. "This serves to inspect the process control via our digital system," Lübbel explains. "There was no problem here regarding implementation of the MBA 700. The unit now has two devices that react reliably and quickly and warrant secure switching."