The right quantity in the right place

Ulrich Nabholz, Markus Gericke

All the liquid raw materials used in the manufacture of emulsion paints were already supplied automatically at Bosshard. The goal was therefore to implement a new, fully automatic silo, conveying and feeding system for powdery raw materials that would simplify handling and storage of more than 2500 tons of powdery products while helping to improve the quality of the end products. The majority of these powders, especially the very fine-grained titanium dioxide, exhibit extremely poor flow characteristics. Particular attention was consequently focused on the angle of discharge and the conveying speed. The system evolved by Gericke complies with the targets specified by Bosshard in every respect. The conveying concept is based on a combination of longstanding experience in the design of pneumatic conveying systems and extensive laboratory tests.

Six aluminium silos, each with a usable volume of 50 m3, are individually supplied with various titanium dioxides and fillers from bulk powder tankers. The walk-in silos are installed in the open. They were not required to be insulated despite the considerable seasonal variations in temperature. The very fine, very dusty prod-ucts (which often have a particle size in the region of a few hundred nanometres) necessitate special filling level sensors that are capable of measuring continuously as well as silo discharge aids based on fluidisation nozzles. These sensors warn the driver of the bulk powder tanker if there is a danger of the silos being overfilled. The precise filling level information is also used by the process controller for raw material management. The vibrating discharge hopper and the fluidisation nozzles supply the product to the pneumatic conveyor system via screw feeders and rotary dischargers. Each product is then conveyed individually, and in the quantities required by the formulation, from the six silos via pipe sidings to one of two weigh hoppers installed approximately 25 m away (full capacity: 7 m3). In order to guarantee the specified component accuracies, the feeding devices for the delivery lines are designed with coarse/fine feeding functions and filters with automatic cleaning. The components can also be conveyed to the weigh hoppers from three Big Bag discharge and metering stations as well as from two bag filling hoppers for small and medium-sized product. These components are pre-weighed and filled in the correct quantity. A central aspiration system reduces dust emissions from the Big Bag discharge stations and the bag filling hoppers to a minimum. As soon as a new batch is requested by one of the two mixing vessels, the appropriate agitator discharge apparatus discharges the supplied components from one of the two weigh hoppers and transports them to the pneumatic conveyor system. A medium-pressure conveyor transfers the complete batch to the mixing vessels via pipe sidings and filter separators. All blower groups and fans are fitted with acoustic hoods to minimise noise emissions.

It was not possible to arrange the two weigh hoppers directly above the mixing vessel owing to the restricted space available in the existing plant. The lines were therefore routed with the aim of keeping the conveying sections between these units as short as possible. The required conveying capacities and times for coarse and fine feed were calculated with the aid of material balances and timing diagrams, in order to ensure the necessary formulation precision in the specified batch times. The result was a maximum coarse feed capacity of approximately 4 t/h and a maximum conveying line diameter of 100 mm. Optimally positioned flexible connections all but eliminate the influence of the connected conveying lines on the load cells. The formulation components must be discharged as completely as possible, as a precondition of reproducible mixtures and hence formulation-compliant emulsion paints. Caking in the weigh hoppers, conveying lines and filter separators is avoided through the use of suitable materials (stainless steel and rubber) as well as a high-quality surface finish.

The entire silo, conveying and feeding plant can be controlled either automatically by a PLC or manually on the operator panel on the front of the control cabinet. The operator panel is equipped with an LCD graphics screen that displays process images and fault messages in plain text. The weighing and feeding system communicates with the PLC according to the master-slave principle. It is responsible for managing formulations, generating production and batch logs, and archiving production data and consumption statistics for components and formulations. The commands that cause the various products to be conveyed in coarse or fine mode are transferred to the PLC via the Interbus interface. The existing mixing vessel controller was adapted to the new weigh hopper control system. In-Touch software was chosen to programme the visualisation functionality. Remote hardware and software diagnostics and expert support for operating personnel are now a reality, because remote maintenance of the weighing and feeding system is carried out by modem.

cpp 406

Products from Gericke

Powder/Bulk Portal

Powtech – International Trade Fair for Mechanical Processing Technologies and Instrumentation