When faced with increasing amounts of downtime due to an ageing automated operations management system, Harmony Gold Mine’s Virginia and Central plants, situated in the Welkom area, chose to upgrade to a technologically advanced 32-bit Adroit scada cluster and a state-of-the-art Modicon system on a separated Ethernet backbone.
"Harmony chose Adroit because it is a local firm and has good supervisory capacity for plant control," says Frans Swart, instrumentation technician for Harmony Central, which utilises the system in processing approximately 180 000 tons of ore per month.
The original Proscon system was installed in 1986, and the degradation of its outdoor substation coaxial cables was responsible for much of the incumbent systems downtime. The contract to upgrade the system fell to CS Holdings, who subcontracted the scada and PLC engineering to ITSI.
Quentin Slabbert, a representative of Adroit working on the project comments, "Due to the live nature of the plant, the contractors took approximately eight hours to rewire the plant, working between shifts. All total wiring was completed in 24 hours and the plant was switched over to the new system in a record fortnight."
The Proscon systems at both the Virginia and Central plants were upgraded to standalone Quantum systems, as these were the latest Modicon on the market. Additionally, the new Modicon could also use existing modules from the plant's drop sites. "The modules were easily transferable which represented a significant cost saving to Harmony," comments Swart. Each independent plant has two PLCs, one to run each mill where the ore is milled, has thickener added, and is leached with carbon before it is sent to the elution plant. The elution plant is controlled by an additional PLC that oversees the extraction of gold from the carbon.
The plant has a main control for all starts and stops and utilises the Adroit system to visualise the plant operation in its entirety. "Visualisation of the plant and accuracy of its representation has been dramatically improved by the Adroit system," adds Swart, who used Adroit internal graphics to create plant mimics, which he complemented with flat 3-D graphics created by using colour scaling. "The graphics created in Adroit are visually pleasing and user friendly," says Swart.
The system at each mill comprises three computers for purposes of redundancy. Two servers with 2500 scan points act as master and slave, with one server actively scanning, while the other acts as a hot standby. A third computer works in the same line and serves as a workstation ensuring redundancy in the system. The elution plants have an additional 750 tags and also relying on dual server, hot standby configuration with an additional workstation for redundancy.
The Ethernet backbone utilises switching technology to communicate with the scada and PLCs. Each plant can be controlled from five different user stations via the Ethernet: three central control units are located at each mill and two each at the elution plants.
To optimise speed and to avoid hang-ups in a system comprising a 16-bit Modicon and a 64-bit PC, each PC is equipped with two separate network cards running independent subnets. The result is two discrete networks running on the same software thereby optimising resources and network efficiency.
The circuit also comprises two printers (one each for alarming and reporting) and includes an additional workstation in the management office for remote visualisation and monitoring of the plant. Alarms are filtered and only visible per plant section depending on which section the operator is logged into.
The report system was enabled using VB scripting to automatically generate shift report (three daily and a weekly), monthly activity reports as well as yield reports (monthly and weekly) in Excel to proactively assist plant management in scheduling, planning and general operations management of the plants.
While viewing the plant operations mimic and control system, the trending and historical data runs on a continual on-screen side bar for realtime trending. "Trending is easier to read in Adroit and easier for technical officials operating the system," comments Swart. Historical data is stored in the agent servers and can be recalled and viewed as a major screen.
Swart concludes, "Adroit was very helpful in assisting us with the implementation of the system, which has improved production, operations and ultimately the plants bottom-line by minimising downtime at the Central and Virginia plants."
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