Avoiding the Culture of Double Fieldbus

Keeping things simpler with process instrumentation and discrete control on one bus.

Claude Hestroffer -- Control Engineering, 9/1/2006

Schenectady Europe Ltd., located at Four Ashes, U.K. (near Wolverhampton), has produced intermediate industrial chemicals for over 50 years. A wide-ranging clientele, covering automotive to health care and pharmaceutical sectors, uses its products. The facility is part of the U.S.-owned Schenectady International Group's 15 affiliated companies worldwide.

The five production-unit plant has been using Profibus for nearly five years. Martin Jahn, instrument engineer, says, “I'd like to say we did not initially choose Profibus, but we drifted into it. When we truly started to look at fieldbuses as a means for instrumentation and control onsite, we investigated available options, coming to the conclusion that Profibus was the best fit, 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 component manufacturers and suppliers.”

Jahn continues, “One of the principal 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. 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 that is always desirable when purchasing high-tech components. Our local suppliers proved very willing to help, so the decision was painless and straightforward.

Profibus can support both process instrumentation and discrete devices using Profibus-PA and DP technologies. He adds, “This single solution allows many cost savings. For example it means less installation time and training, configuration, and commissioning, together with easier maintenance. Basically it helps avoid the need for a 'double fieldbus culture,' which can lead to time and cost savings throughout the plant's life. We have also started to use Profisafe, which is based on standard Profibus technology, as part of our SIL (safety integrity level) system and therefore adds to these benefits.”

A total of five processes are now controlled with Profibus networks, ranging from small, recipe-controlled, specialty-chemical batch manufacturing, high temperature continuous-vapor-phase production to large continuous liquid-phase processes. With experience, Profibus' penetration in the Four Ashes site has grown. The last plant commissioned is the most Profibus-dependent — with over 300 Profibus PA instruments and many digital I/O devices. Inverter drives and even the UPS utilize 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 where Profibus was applied — a multiple reactor batch plant — was originally operated by local loop controllers. The upgrade allowed decentralized system hardware (that is now connected to the control system) with 'conventional I/O' running over Profibus DP; while existing analog and digital field-devices are connected using Profibus DP via remote I/O modules. The specialty chemical batch-plant came next. Having learned some lessons from the first installation, the second still closely followed that of the original installation. It is also based entirely on 'conventional I/O,' and there is dual redundancy for increased security and reliability.

Originally 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 — were used, but during the design it was decided that the soon-to-be-built production line would use Profibus PA throughout. A handful of Profibus PA process instruments were introduced to gain vital experience.

Commissioned in February 2004, the latest plant was the facility's first complete Profibus solution, featuring Profibus PA and Profibus DP. The plant was designed and built almost totally in-house, a step, which 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 readily accessible positions and with their cable entry glands exactly at hand. That really counts when you've got a breakdown.”

Jahn and a colleague did the design and engineering, so the work is very much a personal success story. At the conceptual stage, there was initial pressure to use other, more conventional technology. Says Jahn, “This was understandable, but we felt that the well-proven Profibus world, with its well-integrated DP and PA solution, plus our four years' experience, clinched the decision. We didn't need closed-loop control in the field so we were unconvinced that the additional cost was justified.

“It helped, I guess, that local Siemens staff were extremely helpful with technical support. They are always keen to let us try their latest products. So we feel we see the latest technology — keeping us in front (of competitors). This is important operationally because we can maintain the highest possible productivity and (product) 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 employs Siemens' Simatic-PCS7 process-control and is the biggest Profibus-based line yet at Four Ashes with 300 field devices. Dual redundant Profibus DP trunks are connected via segment couplers (link devices), each connected to 10 Profibus-PA process-instruments such as pressure, temperature, level, and flow plus 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.

Profibus PA segment-couplers are mounted in Zone-2-rated areas to allow local connection to the Profibus PA field instruments. Fisco (fieldbus intrinsically safe concept) modules are used to connect field devices. The control room is over 200 m from the plant, so process variables are transmitted over the Profibus DP network — acting as a backbone fieldbus — simultaneously integrating all instrumentation since the Profibus protocol is the same for both Profibus PA and DP devices. Profibus' tightly integrated hybrid nature offers unique value because both process and discrete automation systems function on one protocol. It's also noteworthy that a 20 kVA UPS — installed to mitigate power failures — has a Profibus DP interface for monitoring the UPS' state over the network.

“Cost savings in engineering, installation, and commissioning have been consistently high,” says Jahn, “but the real benefits came during operations with better diagnostics, easier maintenance, and improved accuracy which lead to 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 applications with a common solution. Other process fieldbuses require a second fieldbus in these situations and often that will be Profibus too.

Profibus' dual — or hybrid — capability 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. With the central configuration point and additional diagnostics, fault finding is faster and more precise than with traditional instruments and I/O. I can diagnose an instrument fault from home. In the event of an instrument failure, a new instrument can be installed, addressed, and set up in minutes with no meters or test equipment.

“Most instruments have extra information available that can be used to confirm other instruments' functionality. Finally, one standard user interface for configuration gives confidence and familiarity no matter what instrument is being used. 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.”