Level measurement in a silica silo

A multinational company specialises in the mining, processing, and sale of fine materials such as silica and kaolin for the construction and other industries. Among its European plants, the company operates a quarry in the south of France.

The extracted solids are transported to conical silos with approximately 10 m / ~ 32.8 ft in height, which allow trucks to be filled from above. When the company decided to upgrade its infrastructure, it was also searching for a cost-effective, state-of-the-art instrumentation for continuous silo level measurement.

The previously used strain-gauge system was no longer up to the challenge and suffered from performance issues under changing temperature conditions. Even when the silos were neither being filled nor emptied and the actual level remained constant, the strain-gauge device reported weight variations of up to 20 tonnes. The customer required the new instrumentation to operate reliably regardless of temperature fluctuations and dust build-up, and it needed to integrate seamlessly with the existing PLC infrastructure.

The quarry operator selected the OPTIWAVE 1530 radar level transmitter to replace the outdated strain-gauge system. The KROHNE 80 GHz radar is a compact and cost-effective instrument for basic solids applications requiring reliable and accurate level measurement in dusty atmospheres with moderate process temperatures and pressures.

The radar level transmitter features a robust PVDF housing with fully potted electronics. Its flush-mounted DN70 lens antenna provides a narrow 4° beam angle, delivering a dynamic measurement signal with optimal strength and preventing interference from reflections within the silo. Its threaded connection (3 NPT), unique in its class, allowed easy installation in the customer-made opening.

Commissioning of the OPTIWAVE 1530 was carried out using the OPTICHECK Level Mobile app. Its user-friendly installation wizard includes a preconfigured silo template, allowing the customer to parameterise the 80 GHz radar via mobile phone and Bluetooth^®. Only a few inputs, such as the distance from the radar to the silo bottom and the desired measuring range were needed.

The 80 GHz radar is powered by the SCU 200 control and signal conditioning unit. This unit makes use of the existing silo infrastructure, including the cabling of the previous system, and transfers measurement data to the PLC while maintaining the PLC’s scaling of the measurement range. The SCU 200 also provides a local display for on-site indication. As it can display two values simultaneously, the customer can optionally have both weight and current silo level shown locally. In addition, the SCU 200 is equipped with three relays that can be used to monitor the filling process.

The quarry plant retrofit successfully resolved the performance problems of the old strain-gauge system. The customer received a stable, accurate, and low-maintenance radar level measurement. The 80 GHz radar performs reliably in the presence of dust and unaffected by environmental conditions. Unlike with strain-gauge systems, temperature fluctuations don’t cause drifts in radar level measurement and there is also no on-site calibration needed for the KROHNE device.

The combination of OPTIWAVE 1530 and SCU 200 allowed for a fast, very cost-effective replacement while maintaining the existing 4…20 mA signal range for the customer. By reusing the existing infrastructure, the customer received a low-maintenance level measurement solution with minimal installation effort. The measuring range was adjusted via the OPTICHECK Level Mobile App to make it perfectly fit the previous configuration. A simulation was quickly carried out via the app to check the main setting for the customer and to confirm that the process including the PLC scaling worked as before.