System Integration & Control Systems Design

Coal handling plant replaces scada system

May 2000 System Integration & Control Systems Design

Industrial System Integrators (ISI) was commissioned by the Project and Technology Services (P&TS) department of Sasol Coal to handle the replacement of the scada software and related activities as part of a control system upgrading project. ISI's scope included installation of the scada software, the setting up and configuration of the software, the drafting of mimics and finally the commissioning of the software. The work was required to ensure optimum operation of the Sasol Coal Supply (SCS) plant.

Sasol Coal operates a comprehensive coal handling plant in Secunda, which consists of six stockpiles, each being served by a stacker and a reclaimer. A number of conveyers interconnect the stockpiles with each other. The stockpiles are supplied with coal from seven underground mines, one opencast mine and a coal beneficiation plant. The coal from these sources is transported with long overland conveyors to the stockpiles. The coal on the stockpiles is used as feedstock for the Sasol Synthetic Fuels plant in Secunda.

The SCS plant is controlled with Mitsubishi PLCs. Previously, they were connected to two control rooms via Conet networks. The final interface to the operating personnel was achieved by means of a number of Mul-T-Link scada packages running on industrial type 486 PCs.

SCS decided to upgrade the control systems - ISI was awarded the contract to complete commissioning of the scada part by the end of 1999. ISI handed the project over to P&TS on 15 December 1999 and this is how:

ISI draughted and got approved a comprehensive functional design specification (FDS). They also recommended a hardware platform that would suit the FDS. ISI replaced the Mul-T-Link packages with Citect Software, which was sized to include for possible future expansions to the system without major alterations to the proposed system and still provide full redundancy. Citect was also set-up so that it could easily interface to a management information system and a G2 simulation package.

The scada hardware platform is in a client/server configuration, which includes hot standby servers and makes bump-less transfer possible. The plant was divided into four geographical separate parts with each part being controlled from its own input/output (I/O) server. Each of the I/O servers also acts as hot standby for one of the other I/O servers. Alarms, trends and reports (ATR) on the system, are handled by a separate ATR server with its own hot standby. The hardware is completed by four separate servers for file handling, management information system, G2 and a network monitoring facility. ISI configured the system at the design stage to allow future expansion, should it be required.

Interface with operators is via a minimum of 10 workstations, a supervisor's workstation and a separate engineering workstation with a full access to all facilities on the system. Every workstation is able to address every server and hot standby server; all interfacing between servers, the workstations and the PLCs is via Ethernet. The system can also be remote accessed from any workstation on the office network at SCS.

A separate Citect package running on a freestanding server acts as back-up system to the main system. This 'secondary network' system connects via a Mitsubishi proprietary network to all the mission critical PLCs on the plant. The 'secondary network' server can be used to control the vital parts of the plant should the main networks fail. The system uses the same mimic screens as the main system.

The scada configuration meant that Citect was installed on the machines for the first time. All configuration of Citect was according to the FDS. The draughting of the mimic screens, workstation server connectivity, set up of the database for alarms, events, trending, logs, security and reporting needed to be completed in a minimum time frame with no downtime and full redundancy.

Allowance was made for the development of at least 200 screens with a maximum of one overall screen for the total plant. The screens for each section of the plant were developed with a minimum quantity of screens applicable to the information in that area of the plant.

Screens were developed for geographical areas, area start/stop, trajectory, mine bunker, factory bunker and the 10 000 t bunker. Various geographical screens that prevented control from that screen and allowed control from other screens, machine screens, stockpile screens, scale screens, ash and moisture analysis, electrical reticulation, weather stations, control room monitoring, utility, reports, daily downtime and production, alarms, exceptions and events, trending, scada configuration.

ISI had to assist the project team of Sasol Coal to write a proper driver for the PLC programs that would be capable of communicating with Citect and that would recover after fault conditions.

The project had to be undertaken as a brownfield one, which implied that the work had to be done on a producing plant and not on a new plant, and commissioning had to be done with minimum disruption to production. This was achieved successfully. Sasol were adamant that there would be zero production loss tolerated on the factory feeding system. Using Citect software it was possible and no production time was lost.

Roger Girdler of ISI stated that the project was one that took dedication from the ISI engineers and designers as they had to be available on site at all times during commissioning and for a minimum of two days after commissioning. The engineers worked hard to complete the project two weeks ahead of schedule and successfully handed it over to SCS on 15 December 1999.

ISI was, in the meantime, also commissioned to investigate the whole matter of the management information system and the G2 package. As part of that investigation they have to again draft an FDS. The work was scheduled to be completed by May 2000.

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