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| Schenectady Europe Ltd; Four Ashes (England) | | Profibus avoids double fieldbus culture at UK chemicals plant | "Process instrumentation and discrete control all on one bus keeps things simpler"
Schenectady Europe Ltd, located at Four Ashes, near Wolverhampton in England, produces intermediate chemicals for industry. The materials manufactured are used in a wide variety of commercial and industrial applications ranging from automotive products to health care and pharmaceuticals. The site is part of the US owned Schenectady International Group which has 15 affiliated companies world-wide.
The Four Ashes site in England has been producing chemicals for over fifty years. It now has five production units and has been using Profibus for nearly 5 years. "I'd like to say we did not chose the initial route of Profibus, but we somewhat drifted into it". "When we truly started to look at fieldbuses as a means of developing instrumentation and control onsite we investigated the available options", says Martin Jahn, the Instrument Engineer now responsible for control and instrumentation, "and came to the conclusion that Profibus was the best fit for our needs, which reinforced our original installations. With some support from our suppliers and the Profibus website we were quickly able to increase confidence in our evaluation and make educated decisions on equipment manufacturers and suppliers. One of the principle reasons for choosing Profibus was that it can be used with a massive range of "conventional I/O" via remote I/O modules on Profibus DP". Said Jahn: "Profibus DP products are available from many sources. IEC 61158-compliant Profibus PA process instruments are widely available too, again from multiple vendors. This gives us freedom of choice which is always a comforting factor when purchasing high-tech components. Our local suppliers proved very willing to help, so the decision was made painless and straightforward". He added: "Profibus is also the only fieldbus which can support both process instrumentation and discrete devices on the same cable, using Profibus PA and Profibus DP technologies. This single solution promotes many costs savings, for example because it means less installation time and training, less configuration, less commissioning, and easier maintenance. Basically it helps avoid the need for a double fieldbus 'culture', which can lead to time and cost savings throughout the life of a plant. We have also started to use Profisafe technology as part of our SIL system (SIL: Safety Integrity Level) which is based on standard Profibus and therefore adds to these benefits".A total of five processes are now being controlled using Profibus networks. These range from small scale, recipe controlled speciality chemical batch manufacturing, high temperature continuous vapour phase production to large scale continuous liquid phase processes. As experience has grown, the extent to which Profibus has penetrated the plant has grown too. The last plant commissioned is currently the most Profibus dependent, with over 300 Profibus PA instruments and many digital I/O devices. The inverter drives and even the UPS have Profibus connectivity. Profisafe networks have also been installed in a number of critical locations using S7 300F failsafe PLCs and failsafe remote I/O modules. The first area to use Profibus was a batch plant running multiple reactors. This plant was originally operated via local loop controllers. The upgrade allowed us to decentralise system hardware which are now connected to the control system with 'conventional I/O' running over Profibus DP - in other words the existing analogue and digital field devices are connected using Profibus DP via remote I/O modules. The speciality chemical batch plant came next. We learnt some lessons from the first installation but the installation closely followed that of the original installation. This is also based entirely on 'conventional I/O' and there is dual redundancy for increased security and reliability.As new products and processes came along, the existing plants were upgraded. On one upgrade, the existing predominantly pneumatics based field instrumentation was to remain but was to appear as all-electronic as possible.Originally we used just Profibus DP solutions (mainly remote I/O, both IS and non-IS and starter equipment for an inverter driving a 90 kW fan heater) but during the design it was decided that the soon-to-be-built production line would use Profibus PA throughout. So a handful of Profibus PA process instruments were introduced to gain vital experience. The latest plant was commissioned in February 2004 and it was the site's first complete Profibus solution. This installation features Profibus PA and Profibus DP extensively. The plant was designed and built almost totally in-house, a step which Martin Jahn says involved a lot of hard work but meant that everything could be put in the right places. "Because you know you will eventually have to use and maintain the equipment, instruments are located in really accessible positions and with their cable entry glands exactly to hand. That really counts when you've got a breakdown".Design and engineering was carried out by Jahn and a colleague so the line is very much a personal success story. In the conceptual stages, there was initial pressure to use other, more conventional technology. Says Jahn: "This was entirely understandable, but we felt that the well-proven Profibus world, with its well-integrated Profibus DP and Profibus PA solution, plus our four years' experience, clinched the decision. We don't need closed loop control in the field so we were not convinced that the additional cost was justified. It helped - I guess - that our local Siemens guys were extremely helpful with technical support. (I am not on commission for this article) They are always keen to let us try their latest products so we feel we get to see the latest what technology can bring and thus keep ourselves in front. This is important from an operational standpoint because it lets us maintain the highest possible productivity and end quality. We are fortunate that our company supports this culture because it is operating in a highly competitive field and needs to stay ahead".The plant is controlled using Simatic PCS7 process control system from Siemens. In networking terms, the plant is the biggest Profibus based line yet at Four Ashes, having a total of 300 field devices. The dual redundant Profibus DP trunks are connected via segment couplers (link devices), each connected to ten Profibus PA process instruments such as pressure, temperature, level, and flow devices and control valves. Conventional (non IS) I/O were used when Profibus PA instruments were not available or impractical. Profibus DP is used for ON/OFF devices and the control gear and inverters driving the pump motors. Profibus DP I/O is also used to switch the trace heating system, which comprises 50 Profibus PA temperature transmitters coupled with 'conventional I/O' to switch the heater contactors on and off as required. The Profibus PA segment couplers are mounted in Zone 2 rated areas to allow local connection to the Profibus PA field instruments. Fisco (=FISCO= Fieldbus Intrinsically Safe Concept!) modules are used to connect field devices. The control room is over 200 metres away from the plant so process variables are transmitted over the Profibus DP network. This means the Profibus DP system acts as backbone fieldbus, simultaneously integrating all instrumentation because the Profibus protocol is the same for both Profibus PA and Profibus DP devices. The tightly integrated hybrid nature of Profibus offers a unique value proposition because both process and discrete automation systems exist on one protocol. Of interest is that a 20 kVA UPS is installed to protect against power failure and this has a Profibus DP interface which is used for monitoring the state of the UPS over the network. Cost savings in engineering, installation, and commissioning have been consistently high, say Jahn, but the real benefits come during operations because of better diagnostics, easier maintenance, and improved measurement accuracy which lead to a better quality, more consistent product.Modern process applications cover not only continuous or batch control, but also discrete applications such as filling, bottling, labeling, and packaging, where drives, fast digital I/O, and pneumatic valves are needed. Profibus covers both types of application with a common solution. Other process fieldbuses have to use a second fieldbus in these situations and often that will be Profibus too. Currently there are 1.6 million Profibus devices installed in process plants - around 5 times more than any other fieldbus.This 'dual' or 'hybrid' capability of Profibus is important for Schenectady because the mix of process instrumentation and discrete control is about 60/40 across the site. Says Jahn: "Using fieldbus saves hours per instrument during commissioning. Due to the central configuration point and with the added diagnostics, fault finding is faster and more precise than with traditional instruments and I/O. Effectively I could diagnose an instrument fault whilst sat home. In the event of an instrument failure, a new instrument can be installed, addressed, and setup in minutes with no meters or test equipment. Most instruments have extra information available which can be used to confirm the functionality of other instruments. Finally, the use of one standard user interface for configuration gives confidence and familiarity no matter what instrument you are using. Profibus keeps everything simpler because we don't need to train people on two different fieldbuses or keep two different types of equipment. This means we can maintain a single fieldbus culture plant wide".
published in: Control Engineering 9/06 |
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