How should one automate a tank farm for polyester resins? Correct - with INTERBUS and Process Interface. This, at least, is how Lorenz Kunststofftechnik GmbH (plastics company) in Wallenhorst near Osnabrück, Germany, did it. Lorenz Kunststofftechnik GmbH is seen as an expert in the field of developing, producing and processing semifinished polyester materials. These materials are supplied in two different forms: BMC (Bulk Moulding Compound) semifinished materials which have a paste-like consistency, and SMC (Sheet Moulding Compound) semifinished materials or sheet moulding compounds which are rolled up. Both types are glass fiber reinforced composites with a low weight and good mechanical and thermal properties, and are therefore a real alternative to conventional materials. Typical consumer products are, for example, automobile shock absorbers, reflectors and motor pans for automobiles, outdoor housings, fuse boxes or conductor retainers along railway lines.

New tank farm increases capacities

At the end of 1997, Lorenz Kunststofftechnik GmbH built two tank farms positioned wall-to-wall, each comprising six stainless steel tanks, in order to increase their existing production capacities. Today, tanks with volumes between 17,000 and 40,000 liters are used to store different polyester resins. The polyester resins flow as basic components into the SMC and BMC production in progress. When the system was automated, the main task was to use the existing control and IT platforms. In order to allow it to be integrated into the central recipe management and production control, the new tank farm had to be connected to the control room, which is located 130 m away, in any case. Production control is based on three Siemens S5 155 U PLCs which are already used to operate three corresponding INTERBUS host controller boards. For the tank farm, the requirement was to control eight pneumatic flap valves for each tank with each valve returning two position messages (open/closed) via NAMUR initiators als Ex-i signals. In addition, filling and conveying pumps had to be switched, and numerous signals from measuring transducers for pressure, mass flow rate and temperature also had to be processed. Another requirement was to integrate an operating console on the tank truck filling system in the automation concept. Since both tank farms are located in closed spaces, it also had to be taken into consideration that these are declared zone 2 potentially explosive environments.

Mastering distances reliably with INTERBUS and FO

Due to the distance between the control room and the tank farm, it was clear that only a remote I/O system could be used. Since experience with the simplicity of starting up and adding to INTERBUS had in the past been positive, it was decided to operate a fourth Interbus host controller board on the existing PLC structure. In order to rule out the harmful impact of ground potential differences and to effectively exclude strong EMC influences from the production environment, the distance was bridged with OPTOSUB-PLUS plug connectors and HCS optic fibers. Initially, the central switch cabinets for the tank farm were installed directly in the zone 2 areas where the tanks are located. But the scenarios of pressure-proof encapsulated distributor boxes or components, low flexibility with the I/O modules and limited servicing during production acted as a deterrent. Instead, the switch cabinets were installed in a neighboring room in a safe area and wired in parallel with the sensors, actuators and measuring transducers of the tank farm. In the cabinet, the signals are routed via Process Interface modules from where they are interconnected with different INTERBUS-ST modules. For each tank, eight of the pneumatic flap valves need to be controlled, in order to enable trouble-free change-over between the different operational statuses of a tank system.

Process Interface for the Ex-i signals

Each of the flap valves returns two digital position signals via NAMUR initiators so that for each tank there are 16 digital Ex-i input signals. It is this structure that is also present in the switch cabinet. The floating PDT outputs of the Process Interface NAMUR isolating amplifiers are also wired to 32-channel INTERBUS-ST modules. Connection to the control room is via this interface. In addition to the Process Interface NAMUR isolating amplifiers, six transmitter supply units with HART (PI/Ex-RPSS-I/I) are also used. These supply other two-conductor pressure measuring transducers which monitor the particle filters with energy and provide information about the pressure by means of 4..20 mA analog signals.

The advantages

The main arguments which made Lorenz Kunststofftechnik GmbH decide on Process Interface were summarized as follows by the project manager, Olaf Hemmersmeier: "The slim design of all Process Interface modules of only 12.4 mm enables a clear and structured switch cabinet design with extremely low space requirements. Furthermore, the Process Interface concept offers high flexibility during start-up as well as for system expansions. If service is necessary, no conductors need to be disconnected; all you need to do is replace the electronic plugs. This procedure is considerably easier and, above all, simpler than with common DIN rail housing solutions which have fixed screw terminal blocks or screw plug connectors". A large number of other PHOENIX products were used for the tank farm project at Lorenz Kunststofftechnik GmbH. The CM 125 5 A primary switched-mode power supply units, for example, reliably supply the switch cabinets with 24 V. Different PLC-Rel and ST-Rel types amplify control signals for pump management.

Summary

The combination of the Process Interface range of products with INTERBUS and other PHOENIX CONTACT products has provided a universal solution for the automation of process engineering systems at Lorenz Kunststofftechnik GmbH from almost one supplier.