IO solutions for hazardous areas
19 February 2013
An alternative approach to bringing IO signals from hazardous areas to a DCS is said to deliver cost and efficiency benefits. Steve Leech, product manager for the Process Automation business at Siemens Industry Automation in the UK, explains.
Overcoming the challenges presented by hazardous operating areas can be an issue for many industries, the process sector in particular, where gases, vapours or mist can be released into the environment. For this reason careful consideration should be given to the safe use of equipment in such areas, including that of distributed automation solutions.
Some necessary operations can create the potential for explosion, due to gas or dust being present in the air, including bringing IO signals from hazardous areas back to a distributed control system.
With ATEX legislation governing the rating for components in hazardous areas, a traditional approach to the problem has been commonly adopted, which involves the installation of IO housed in a safe zone in conjunction with hazardous area barriers that are hardwired to allow signals to be passed into the affected zone. This is a costly solution, with IO, barriers, components and hard-wiring over long distances having an impact on both installation cost and longer-term operational efficiency.
An alternative approach lies in the use of technology that can be installed directly into the hazardous area, negating the need for barriers and hard-wiring at installation phases. Such a solution can also offer the flexibility for onward operation, having an ability to be configured during normal operation as well as offering hot swapping – where module replacements can be undertaken in a ‘live’ environment – and diagnostic and redundancy capabilities.
At the centre of Siemens offering for such an approach is a modular remote I/O station which has been designed for use under hazardous ambient conditions. The ET 200iSP is designed to be used in different hazardous specified zones, as described in directives EN 60079-10 and EN 61241-10, either in Zones 2 and 1 in the case of gaseous atmospheres, or in Zones 22 and 21 in the case of dusty atmospheres and the connected sensors and actuators located in Zones 0 and 20.
The station utilises the PROFIBUS DP network to link communication on a local level between the field devices and the process control system. Use of an isolating transformer makes PROFIBUS DP intrinsically safe by isolating the bus and limiting the energy in the safe area.
Running a single PROFIBUS cable connection to locally mounted IO reduces installation hard wiring costs and time, with instances of commonly used terminal blocks as well as distribution boards and IS barriers for the IO signals able to be omitted.
The ability to use a standardised control cabinet-mounted solution that is already fully certified to comply with legislative requirements, allows a more flexible installation solution in the field in applications where space can often be at a premium, reducing hardware installation levels, and also reduce overall system risk.
Along with the infrastructure benefits that works with space restraint, complies with hazardous zones needs and significantly reduces wiring levels, the intelligent system also provides operational benefits once up and running.
Downtime can also be reduced as diagnostic information is generated as internal or external faults occur allowing informed decisions to be taken or ongoing proactive maintenance programmes adopted. The independent wiring also supports easy replacement of modules during normal operation and the ET 200iSP can accommodate hot-swapping of the power supply without arcing, as well as redundancy. The flexibility extends to an ability to add stations or expand stations with modules or even change module parameters during normal operation – again, maximizing operational efficiencies and production control.
Recent examples of the solution in action include a UK drinks manufacturer, which has used the solution to drive production efficiencies and flexibility and set it on course for a standardised approach for the future specification of industrial automation technology across its plant. Estimates say using this approach has contributed to significant time and cost savings through reduced engineering complexity, but with the added benefit of increased functionality.
Siemens worked alongside its Solution Partner Kigtek on a process system upgrade for the company. Gordon Fleming, from Kigtek, explains more: “The customer wanted to increase production output by adding new lines and generating more flexibility to the manufacturing streams. This would include automating certain areas of the plant to remove existing manual processes. The underlying objective was to enable the manufacturer to add new products easily to its production schedule, but be mindful of capital and operational expenditure.”
In order to undertake these improvements required installation of a process control system that would oversee better throughput and process efficiencies across the plant. For example, automatically adding water to the whisky line was achieved through reconfiguration of the control system the water would in future be added to the whisky within the pipelines as opposed to being added as a separate batch process.
Dealing with hazardous conditions
“An important element of the control enhancement was the need to deal effectively with the potentially hazardous conditions that form the backdrop to the process, “ said Fleming. “An essential component of the manufacturing process in question, ethanol is considered an explosive substance and as such housed in hazardous zones 1 and 2.” The ET 200iSP zoned I/O offered a solution.
In another application, a manufacturer of fine chemicals has included the solution as part of a PCS7 distributed control system installation. With geographically spread chemical process vessels on its production site, the company has been able to locate the iSP close to the vessels in question and link them via PROFIBUS back to the main control system – without worrying about performance and suitability for such potentially hazardous areas and raw materials. The company was faced with a one off installation cost, but then just subsequent short cable runs from the vessel to the safety box in situ. Together with the enhanced diagnostic capability offered by the system, the cost benefits of faster installation has offered the company ongoing capital expenditure and operational benefits.
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