The Gernsheim site of Merck KGaA produces, amongst other things, pearl lustre pigments, that are used as admixtures for automotive and industrial paints and cosmetic products. In recent years, completely new plants have been constructed in order to expand production capacity. One of the objectives was to achieve automation using cutting-edge technologies, whereby fieldbus technology, long since accepted in the field of process engineering, was incorporated in the concept from the outset.
A smaller plant with similar process engineering was set up in 1998/99, whereby the idea of a fieldbus installation was shelved in favor of remote I/O technology because, at the time, none of the fieldbus systems under consideration offered a sufficiently wide range of bus-capable field device or manufacturers. However, on erection of a second plant in 2000/2001 PROFIBUS as the only fieldbus system was able to completely meet the conditions of the user. The plant was therefore universally automated with PROFIBUS (and AS-I) technology, whereby construction "on a greenfield site" presented particularly favorable conditions for the consistent utilization of all technological options.
The plant is equipped with a redundant PCS 7 process control system from Siemens. At higher levels of the plant automation are the process bus (with controls, central servers and engineering station as bus devices) and the terminal bus (with operator stations, engineering station and batch server). Process and terminal bus are designed with Fast Ethernet as an optical ring with redundancy management.
The logical and consistent use of PROFIBUS (and AS-I) and the standardization of bus segments and control cabinets are key features of the plant structure at field level.
The plant is divided into approx. 130 plant sections. Each plant section contains at least one AS-I-, PROFIBUS DP and PROFIBUS PA bus line each, to which the field devices are connected in recurring topology (tree for AS-I and PA, line for DP). The bus lines are connected to the corresponding links in the allocated standardized control cabinet. Two plant sections share a plant control cabinet. Each plant control cabinet is redundantly linked over PROFIBUS (optical fiber, coupling over Optical Link Modules, OLM) to the controls in the two instrumentation and control rooms.
The use of fieldbus technology with its extremely simple and flexible topology goes hand in hand with an enormous reduction of the devices and cable lengths normally required for conventional connection technology. This has a direct knock-on effect on the volume of documentation: for example, some measuring-circuit drawings and terminal diagrams may be dropped completely or, as was the case with this particular plant, be considerably reduced in volume. For future plants with fieldbus technology there are even plans to reduce the documentation to a simple list form.
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