Fieldbus in biopharma applications, Part 2

Likewise, Micromotion mass flowmeters are used extensively in our facility, for example, on chromatography skids. From one fieldbus connection to the device, we can realize all four process variables measured or calculated by the mass flowmeter. These include mass flow, volumetric flow, fluid temperature, and fluid density. Typically, volumetric flow is the primary value of interest, and is a calculated value from the device (mass flow/density). Volumetric flow is used as the process variable for the flow control loop on the chromatography skid. Initial running of the skid requires priming the inlet buffer lines.

While executing the prime phase, we’ve noticed that when two-phase flow is introduced through the meter, the density will drop in proportion to the two-phase mixture, causing the volumetric flow to read high. Since we prime the various buffer lines based on passing a certain liquid volume, it’s important to only totalize liquid flow and not two-phase flow. To prevent totalizing of two-phase flow, we use the density input, detect when it’s no longer that of a liquid, and then turn off the totalizer. In this case, the host controller is actually writing to a parameter in memory in the fieldbus device. This ensures that all buffer lines have had the predetermined liquid volume transferred, and shows the two-way communication available with fieldbus devices.

In addition, Gemu manufactures valve actuators for on/off diaphragm valves. These valve actuators communicate to the host by way of AS-i bus . The valve actuators have a solenoid valve and open/close limit switches in the actuator’s topworks. It’s not unusual to find diaphragm valves open for SIP, and then indicate valve not closed when commanded closed at the end of SIP. This is due to the diaphragm being more malleable when heated, which allows the actuator stem to drive the diaphragm slightly deeper into the weir of the valve body past the previously set or cold diaphragm closed position. This device includes a programmable hysteresis for the open and closed limit switch that allows for process condition. Typically, it’s the closed limit switch we’re interested in confirming in the case of sterile boundary valves.

Genzyme reports that fieldbus helped it achieved its technology objectives in five areas:

1. Expanded view for operators, engineers, and technicians. Successfully realized because, from any operator workstation, it’s possible to drill down to the instrument configuration and quickly assess current instrument operating status.
2. Reduce wiring and installation costs. Though we didn’t track and compare costs with a conventionally instrumented system, our qualitative sense tells us that, because of the reduced number of home run cables and associated conduit size reductions and quantities, our installation cost must be less.
3. Reduce I/O equipment and control cabinet size. Successfully achieved. Though our controller cabinet sizes are reduced, we may, in fact, have more cabinets compared to a conventionally wired system due to the added cabinet count for field termination boxes.
4. Reduction in man-hours for commissioning and start-up. Not realized. We believe the effort is essentially identical for fieldbus and conventionally instrumented system, at least, that’s been our experience. Perhaps we haven’t optimized this activity because there are many proponents that advocate this point. We likely have more to learn on this topic.
5. Fieldbus reduces total cost of ownership. We believe this will be answered in the affirmative, when we realize a predictive maintenance model, and eliminate run to failure or scheduled maintenance. Given the fact that cell culture production can run for months at a time, it’s imperative that the instrumentation platform be extremely robust, so bioreactor runs aren’t terminated early due to instrument problems. To this end, we have evidence to date that the fieldbus platform has enabled us to remedy imminent device failure prior to the start of a production run.