In a steel production plant, time is money. Any unplanned shutdown of essential equipment leads to major losses for operators such as OneSteel Whyalla, and the customers who rely on their products.
OneSteel Whyalla is a large-scale operation built around a key South Australian port. Its diverse operations encompass mining and export of iron ore, as well as manufacture and structural milling of steel for the Australian and overseas markets.
The plant’s blast furnace is the power house of its steel manufacturing operations.
The furnace operates around the clock, producing molten iron for conversion to steel.
Two massive skips alternately provide the raw materials required by the furnace: pellets, coke, lump ore, limestone and dolomite. If the skip hoist shuts down, furnace operations are disrupted, which could potentially cost tens of thousands of dollars per hour of lost production.
In 2008, the existing skip hoist process control system was nearing the end of its operational life, but opportunities to upgrade it were few and far between.
According to Craig Foulkes, Iron Making Electrical Engineer with OneSteel Whyalla, the plant’s operations allowed for two 24 hour shutdowns per year.
“That doesn't mean that the skip system needs to shut down for this amount of time, but rather that these are the only available windows for scheduled maintenance,” he said.
A rare chance to overcome this hurdle was to timetable installation and commissioning of a new skip hoist control system during a planned blast furnace repair scheduled in 2011, which would involve approximately 60 days’ break in furnace operations. On average, major furnace shutdowns occur every 15 to 20 years.
OneSteel Whyalla contracted Rockwell Automation to locally design, manufacture and commission an upgrade solution within this tight time frame. The company was selected because of its expert knowledge and experience in maintaining the existing control system and its history of excellent service and support.
History and customer requirements
The original skip hoist system was installed in 1965 and an updated power module was added in 1990. In 1998, Rockwell Automation commissioned an upgraded process control system, which consisted a pair of Automax controllers interfaced to the third party power module.
The Automax units provided control and monitoring of a pair of drive motors, either of which could be switched in to operate the skip hoist drum.
“Effectively there were two independent switched systems,” said Andrew Simpson, Senior Design Engineer, Global Solutions Group, Rockwell Automation. “If a motor failed, it could be switched out and the system would run at a reduced rate.”
Even though Rockwell Automation still offered after-sales support for the Automax controllers, the scarcity of spare parts and limited maintenance availability were both issues affecting the viability of the system.
Additionally, on-site technicians and engineers did not have the knowledge to work with the skip hoist controller, as it was a specialised system and not in use in other parts of the plant.
For these and other operational reasons, OneSteel Whyalla decided to upgrade the whole control system while retaining existing system functionality.
Additional complications were that the power module interface hardware was also obsolete; maintenance and diagnostics were limited to a stand-alone ‘black box’ system, and the control system did not comply with current safety standards in South Australia.
OneSteel Whyalla required a solution which replaced the obsolete system components with products that were familiar to in-house maintainers. The upgraded skip hoist system also needed to be in line with standards that protected the well-being of workers and machinery.
Combining safety and availability
The high operational cost of skip hoist shutdowns made availability of equipment a design priority for the upgraded system.
There are also many operational hazards when working with large machinery, so safety features were paramount. Achieving a system design with an acceptable balance between safety and availability was a challenge.
Simpson was responsible for preliminary design and risk assessment of the new skip hoist control system.
“The main thing is to make sure that the hoist doesn’t go too fast, or too far. So you need to test for overspeed and overwind to keep the system safe,” he said.
“Having two individual controllers, one system online and one as backup, each with its own overspeed and overwind detection devices, means that if one fails the other can take over to maintain availability. Having the built-in redundancy minimises disruption to operations,” Simpson explained.
Cost effective solution
Rockwell Automation designed a control solution that interfaced to the existing third party power module. According to Foulkes, this was a unique service which was highly beneficial to OneSteel Whyalla. “The current power module was reliable, and we had both an online system and spare to back it up. Being able to retain this part of the system reduced project costs significantly,” said Foulkes.
The Automax controllers were replaced with Allen-Bradley GuardLogix systems, and the outdated power module interface hardware was also upgraded, with SD3000+ interface hardware bringing it up to current technological standards. Each of the GuardLogix cabinets was equipped with a PanelView Plus operator interface which provided two fully redundant diagnostic displays.
The first step in improving the safety of the new skip hoist control system was selecting an appropriate standard for compliance. According to Simpson, there was no definitive standard because the skip hoist incorporated aspects of mine winders, cranes and conveyors.
Simpson chose the New South Wales Government guideline for mine winders, MDG 2005 Electrical Technical Reference for the Approval of Power Winding Systems. Mine winders are subject to more rigorous safety regulations because they may be required to carry people as well as equipment. Although the skip hoist does not take human cargo, the importance of the system demanded a high level of reliability, and Rockwell Automation had extensive experience in implementing control systems for mine winder operations.
To determine the appropriate level of safety for the new system, OneSteel Whyalla and Rockwell Automation undertook a system risk assessment.
This stage of the project was facilitated by an accredited third party company.
The outcome of the risk assessment was that critical functions including the overspeed, overwind and emergency stops needed to meet Safety Integrity Level (SIL) 2.
To achieve this, Rockwell Automation incorporated extra safety functions into the design. Two overspeed and overwind devices were included in the new system, one attached to each of the GuardLogix controllers by an absolute encoder module.
The existing system had only one device, which was decommissioned during the upgrade.
These features meant that if the new online system detected an overspeed or overwind, it would trigger a system shutdown. The machine operators could then investigate the problem and manually switch to the backup system if required. During maintenance periods, operators would be able run test modes to safely determine whether the overspeed and overwind devices were functioning correctly.
The new system also provided test modes for each of the brakes on the skip hoist’s dual motors. “Operators can lift one brake, and try and drive through that brake to a certain level. If it doesn't move it passes, if it moves they know they need to do maintenance,” said Simpson.
The New South Wales design guidelines exceeded the current safety requirements in South Australia. According to Foulkes, this prepared OneSteel Whyalla for the anticipated nationwide standardisation of machine safety regulations.
The most critical milestone was to ensure that the new skip hoist control system was installed and commissioned within the 60 day shutdown period allocated for the blast furnace repair. To achieve this, work had to be scheduled around other demolition and construction tasks that took place during that time.
Foulkes explained that access to the skip hoist system was therefore limited to only 10 days, spread over the 60 day shutdown period. “Rockwell Automation was very flexible and came whenever they were needed,” he said.
To ensure that the project did not run over time, Rockwell Automation undertook considerable pre-installation work and testing. A new control desk was also supplied as part of the solution, so that the new system could be pre-wired and factory tested before it was delivered. Installation of the new system was carried out by OneSteel Whyalla and a local contractor so that it could be scheduled around other furnace reline activities.
Since commissioning, the upgraded skip hoist system has been operating well, with only minor teething problems. Many of these issues can be rectified by on-site maintainers, with the knowledge that Rockwell Automation field engineers are available if needed.