It is a magnificent achievement for a dairy to process about 14 million liters of raw milk every week. The Müller dairy in Leppersdorf which is near Dresden in the German state of Saxony achieves this capacity by combining five different dairy operations under one roof. The logistical, product-related and control engineering requirements of processing at this scale can be seen in some impressive data: about 650 employees manufacture a good 35.000 tonnes of milk or whey powder every year, as well as producing about 18.000 tonnes of butter, 30.000 tonnes of semi-hard cheese (Gouda and Edam), 10.000 tonnes of mozzarella and 10.000 tonnes of curd cheese and filling 140.000.000 units of UHT milk and five million units of fresh milk per annum.
All system states can be
visualized
at any time in the control room
Curd and yogurt products (750 million units every year) round off the product range. These enormous output figures can be achieved thanks to modern state-of-the art technology at all levels. The entire plant (comprising a built-on area of 59.000 sq. meters) is controlled using the Proleit OS-NT process control system with a total of three control rooms. The almost uncountable number of measuring, checking or movement procedures in this plant are controlled using digital or analog input and output modules from the INTERBUS-ST range.
System Availability is the Top Priority
In 1994, the Müller dairy acted through its subsidiary, Sachsenmilch Anlagen Holding AG, to take over the former Sachsenmilch plant, the planning and foundation of which date back a further two years. As Matthias Baum, a technician and experienced electrical specialist responsible for planning new projects at Leppersdorf explained, the high level of availability of the machines and systems is largely due to the use of modern INTERBUS technology. Thanks to the excellent diagnostics capabilities of the system, it is possible to detect and rectify malfunctions in the operating sequence or in individual systems extremely quickly. Steps can then be taken to prevent them reoccurring in the future. The control system planning for the plant was entirely conducted by Proleit of Herzogenaurach and, from the very beginning, was based on the high level of automation and rationalization potential afforded by the INTERBUS system. The use of 100 INTERBUS cabinets and 150 pilot valve cabinets with an INTERBUS interface considerably minimized the complexity of cabling between the units and the control desks in the dairy. The INTERBUS modules perform diverse tasks in the extensive plant: the automation equipment in all production areas is connected to the peripherals using the INTERBUS system. Specifically, this involves opening and closing valves, activating pumps, measuring temperatures and monitoring pressures and flow volumes.
The very great number of sensor
and actuator systems found in
the Leppersdorf plant cover an
extremely broad spectrum
Appropriate Provision for Cleaning and Maintenance
The Müller dairy operates nine filling systems for curd milk and yogurt products at Leppersdorf. These can be used to fill 267.000 units every hour. Consequently, the existing systems have to alternate between products in order to deal with the large number of yogurt or other milk products made by Müller and Sachsenmilch. It is easy to imagine that this involves a high level of control complexity. The raw milk is stored in tanks (total capacity of the tank farm is 12 million liters) and when milk is pumped from one of these tanks, it may be supplied to an extremely varied range of processing systems.
Obviously, a milk processing company has to comply with very exacting hygiene standards, so the various processing systems have to be cleaned regularly. For this reason, certain machine downtimes cannot be avoided. However, the ingenious process control system Proleit OS-NT puts the Leppersdorf plant in an excellent position to achieve high output levels nonetheless. The opportunity to shut down individual INTERBUS segments in a targeted manner represents an important prerequisite for making the time to perform necessary repairs on the individual machines.
Susanna Stock, Düsseldorf