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16/01/2012
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Four Honeywell 620 PLCs, a bespoke batch computer and a PCIM SCADA (supervisory control and data acquisition) system at the Alloa glassworks in Scotland have been replaced by four Siemens S7 PLCs and a the Siemens WinCC SCADA system.
 Boulting Electrical Systems was commissioned for the batch control system upgrade at this Owens of Illinois-owned plant, which was necessitated by difficulties with system maintenance and support due to lack of spare parts.
Tom Wilson, systems development engineer at O-I, explains that the system controls the mix of ingredients – sand, soda ash, limestone, cullet (a form of recycled glass) and additives – that are fed into a furnace and melted to make glass, under recipe control.
"The existing batch computer was effectively a black box and we had to recreate its functionality in the new system where it was split between the Siemens PLCs and the WinCC SCADA [system]," states Andy Marshall, Boulting lead engineer.
Marshall says that his organisation had to work closely with the O-I team to ensure full understanding of the current operation, so that engineers could accurately reproduce process control in the new system.
For example, one key requirement during the changeover was that the conveyors must run every four hours in order to charge the furnace with cullet.
Once the survey was completed, he says, a detailed functional design specification was created, covering all aspects of the existing system, as well as operation of the new system, and the processes required for testing and installation.
Prior to installation, the new systems were bench tested in Boulting's officees, starting with the basic functions and finishing with a complete system test.
Marshall explains that changeover then involved removing the existing PLCs, I/O and wiring before pre-wired harnesses from each of the new racks – connected to the panel side of the field termination rail – could be switched on, one system at a time.
"We are very happy with the results and the batch system is running well," comments Wilson.
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Author
Brian Tinham
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