Legacy system upgraded to support future manufacture
17 January 2012
A batch control system at the Alloa glassworks in Scotland, owned by Owens of Illinois has been upgraded by Boulting Electrical Systems.
The original system consisted of four Honeywell 620 PLCs, a bespoke batch computer and a PCIM SCADA, which controlled the mix of ingredients such as sand, soda ash, limestone, cullet and minor additives which are fed into the furnace and melted at high temperature to make glass. Different recipes vary the mix to enable different types and colours of glass to be manufactured.
However, the legacy control system became difficult to maintain and support due to a lack of availability of spare parts, and an upgrade was ultimately necessary. The original batching system had been used almost continuously throughout its life, stopping only for routine maintenance.
Boulting replaced the whole system with four new Siemens S7 PLCs and a Siemens WinCC SCADA. A key requirement during the changeover was that the conveyors would need to run every four hours in order to charge the furnace with Cullet –a form of recycled glass.
Starting with a detailed survey of the existing equipment, Boulting worked with the O-I team at the site in order to fully understand the current system. Andy Marshall, Boulting lead engineer, said: “We worked closely with Tom Wilson (Owens of Illinois systems engineer) and his colleagues. They had a great deal of experience of how the existing system worked and fitted together. The batch computer was, effectively, a black box and we had to recreate its functionality in the new system where it was split between the PLCs and the WinCC SCADA.
“We had to watch the system and ask lots of questions to make sure we understood the current operation so that we could accurately reproduce it in the new system.” Once the survey was completed a detailed Functional Design Specification was created, which covered all aspects of the existing system, operation of the new system, testing and installation.
The new systems were then bench tested, starting with the basic functions and finishing with a complete system test. Finally, installation commenced and changing one system at a time, existing PLC, IO and IO wiring were removed and pre-wired harnesses from each of the new IO racks were connected to the panel side of the field termination rail.
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