Fieldbus in biopharma applications, Part 1

The fieldbus architecture lends itself to maintaining an orderly appearance to controller cabinets. Figure 3 below represents one of our typical conventionally wired control systems, and Figure 4 (below) is typical of fieldbus. This cabinet uses five bus I/O cards that communicate with approximately 200 I/O points.

Fieldbus Segment Design
The buses selected for our facility also fall into two distinct categories—a powered bus and an unpowered bus. The powered buses include Foundation fieldbus and AS-i bus. The unpowered buses include Profibus-DP and DeviceNet. The powered bus includes the communications and device power on the same wires. 

FIGURE 4: FIELDBUS VERSION
Fieldbus reduces the amount of space needed.

The overall topology used for the powered bus is known as chicken foot (see Figure 5 below). One design element we thought was important to carry over from the conventional world is short-circuit protection for each device. Because one trunk provides power and communication to all devices, it would be undesirable to a have one device potentially take down the entire segment if it encounters a short. Moreover, it would be difficult to identify the offending device without disconnecting devices one by one until the shorted device or cable was found. Short-circuit protection is normally provided for the segment trunk with the segment power supply. 

The FTBs would house this short-circuit protection equipment for field devices of both powered buses, and we hoped to identify a common short-circuit device type that could be used for both Foundation fieldbus and AS-i. At the time of our detail design effort, Relcom offered its Spurguard for Foundation fieldbus. We’d also hoped to find something in a similar form factor for AS-i. At our request, Relcom designed and manufacturing a Spurguard for AS-i, which enabled our powered buses to have the same look and feel. Figure 6 below illustrates where he Relcom’s Spurguard devices are located.

FIGURE 5: CHICKENFOOT!

A five-spur fieldbus drop. (Click image to view an enlarged PDF.)

Our project’s ultimate goal was to have the stick-built process equipment, which included vessels, transfer panels, and skid equipment using an identical segment design approach. This allowed us to develop segment design standards that were followed by our engineering design contractor for the stick-built process equipment and by the various skid vendors.

FIGURE 6: SPURGUARD TO THE RESCUE!

Fieldbus, AS-i, anybus…it all works. (Click image to view an enlarged PDF.)

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Are Fieldbuses Right for Biopharma and You?

CRITERIA used for assessing the viability of fieldbuses include the following:

1. Instrumentation availability: are instruments typically found in a biopharm facility available with fieldbus?
2. Bus selection and bus quantity: would it require two, three, or four fieldbuses to address the various I/O types?
3. Electrical classification: could bus instrumentation be deployed in electrically hazardous environments?
4. Cost: is bus implementation more expensive? If so, is there payback over time based on a predictive maintenance model (inherent in bus systems) compared to our present run-to-failure?

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About the Author

William T. Dolan, PE, is principal instrumentation and control engineer at Genzyme Corp. in Allston, Mass. He can be reached at william.dolan@genzyme.com.