Separately powered or unpowered buses such as Profibus-DP and DeviceNet have very straightforward wiring practices. They both follow a multi-drop wiring or daisy chain wiring schema. Wired in a similar fashion to each device is the required 24 VDC power supply. As mentioned earlier, DeviceNet is used to interface variable speed and single speed motors. Our motor-control center is scattered about the facility, and tends to serve more than one process unit. For example, two bioreactors and one seed reactor have all their respective motors for agitators and pumps in the same 480 VAC, UPS-fed, motor-control center (MCC). To maintain independent process units, three separate DeviceNet trunks feed this MCC, including one from each of the two bioreactor trains controllers and one from the seed reactor controller. Since each unit has its own separate controller, it follows that the MCC interface should be unique for each. Sometimes this results in some process units having very lightly loaded DeviceNet segments, and so the bus cost may be high per unit. The purification side of the facility exploits DeviceNet more fully from a device count perspective because of the abundance of buffer prep and buffer hold vessels, which are covered with only two DeviceNet segments.
Fieldbus in Hazardous Settings
The last item in segment design has to do with using fieldbus in an electrically hazardous environment. Most of our purification space is classified as Class 1, Division 2, Group C and D. Our first introduction to fieldbus was in the cell culture side of the facility, and this space is electrically rated as general purpose. Moving segment designs already in place for general purpose areas into an electrically classified environment required some changes which buses we could employ from those used in cell culture. Immediately, AS-i wasnt appropriate due to its high power levels, which dont facilitate an intrinsically-safe or an explosion-proof implementation. However, FF can be used in a Class 1, Div. 2 electrical environment and in a Class 1, Div. 1 space.
Added to the market at the time of our implementation was FFs Fieldbus Intrinsically Safe Concept (FISCO). It has many desirable attributes such as eliminating the requirement for entity parameter calculations, less documentation to develop and maintain, instruments that can be added later to the segment without the need for further segment analysis, higher power levels that allow more devices per segment. Before FISCO, segment device count usually couldnt exceed four devices for an Entity IS segment, but the count could be as high as 12 with FISCO. Because it was new, our design presented a concern regarding the availability of field devices certified for a FISCO implementation. We found all the devices we used on the cell culture side of the facility that would be used in purification were FISCO certified, and so we could continue to use the same suppliers and devices. Though the FISCO implementation is designed for a Class 1, Div. 1 electrical environment and our space classification is Class 1, Div. 2, we decided to pursue the FISCO design because it meant that all segment and device maintenance and troubleshooting could be accomplished with a powered segment or device.
Shortly after FISCO was introduced, another non-incendive design appeared. This Fieldbus Non-Incendive Concept (FNICO) is intended specifically for Class 1, Div. 2 electrical environments. Yes, we could have pursued this approach, but there was a comfort level in the facility about addressing electrically classified areas with intrinsically safe (IS) implementation, which ensures that facilities personnel never have to think about powering down instrumentation prior to performing maintenance or troubleshooting.
Also, FISCO requires that all elements in its segment be FISCO certified. This includes the fieldbus power supply/isolator, Relcom Spurguard, and field instrument.
Earlier, we mentioned that AS-i wasnt appropriate for IS applications. Therefore, another solution was sought for the discrete devices in the electrically classified space. Given that our continuous measurement and control devices followed, an IS implementation with FISCO and an IS solution were envisioned for the discrete devices. We tried to keep host controller cabinets in safe-area spaces, and only allow I/O subsystems in process spaces. IS remote I/O was installed in the process space to interface to low-power solenoid operators for valve actuators, valve position limit switches, rupture disk indicators, and transfer panel proximity switches. The remote I/O modules are intrinsically safe, but the communications, Profibus-DP, to each is not. Profibus-DP cant be implemented intrinsically-safe though its a non-incendive design.
Fieldbus Instrumentation Attributes
Our experience with fieldbus shows there are some fieldbus devices that lend themselves nicely to biopharma applications. A few instruments can illustrate this point.
The first is the Rosemount 848T temperature transmitter, which also allows for other input types besides RTD and thermocouple. Steam-in-place (SIP) is a common process in biopharmaceutical facilities for sterilizing process vessels and piping. SIP is usually automated with multiple temperature elements at various low point condensate drains, and usually has five to eight temperature elements per vessel. In the case of a bioreactor, the temperature element count can approach 30 or more. In the early design stages of our cell culture facility, there was some concern about how to handle the many RTD inputs with a bus system. Certainly, a Foundation fieldbus transmitter for each input would be unwieldy from a construction standpoint, not to mention the cost. Fortunately, 848T allows up to eight inputs per instrument, which reduces the device count. We also use this device to convert conventional 4-20 mA/DC devices to Foundation fieldbus.