Achieving pilot-scale process control flexibility and agility
By Chris Marinucci
Pilot-scale process control has posed some of the biggest automation challenges I have faced working in an advanced manufacturing environment. Extreme turndown ratios, process modularity, and rapid and frequent data acquisition, combined with the need for high accuracy and repeatability are hallmarks of any research and development process. The request from our client was straightforward: Make the pilot lab more flexible, accurate, and productive while maintaining the Class 1, Division 2 hazardous area classification.
The existing pilot lab was a combination of rigidly constructed and permanently affixed pumps, valves, heat exchangers, tanks, and instruments. Over the years, a spaghetti-like arrangement of bypasses and spool pieces were added to suit the process testing needs. Our solution was to break down each unit process operation into single systems and make them portable. Existing pumps, Coriolis flowmeters, and heat exchangers were mounted on wheeled carts, making it easy to mix and match the correct equipment for the pilot run. Cam-lock hoses replaced rigid stainless-steel piping, and 250-gallon totes or 55-gallon drums became the vessels of choice. The programmable logic controller (PLC) control panel itself was mounted in a console-style cabinet with a graphical interface and placed on casters.
Another challenge was how to deal with all the different kinds of instruments, flow pressure, turbidity, color, and temperature, that could not be permanently affixed to the process equipment. Each trial posed unique challenges for process control and data acquisition and its need to mimic real constraints at a variety of manufacturing plants in North America. The instruments had to be just as modular as the process equipment and allow the technician to place them anywhere in the process. Further complicating matters were the specialty instruments like turbidity and color analyzers that were large, heavy, and expensive.
With dozens of instruments and process elements that could be combined in seemingly infinite combinations, a wireless networking solution could tie all our pieces and parts together, but which wireless solution? Splitting our connectivity needs into real-time and periodic ones, we focused on the wireless Ethernet for real-time applications and wireless HART for periodic applications.
Wireless Ethernet provided near real-time control and data feedback for our process equipment. The centrifugal and positive displacement pump carts used variable frequency drives networked to a wireless Ethernet radio. The Coriolis flowmeter had to provide near real-time feedback to operate either pump in a closed-loop mode, so it, too, was fitted with a wireless Ethernet radio. Lastly, our control panel was fitted with a wireless Ethernet radio. To coordinate all the wireless Ethernet devices, a wireless Ethernet access point was mounted in the center of the 2,000-square-foot lab space on a beam 20 feet above the process equipment.
For those devices that required only periodic monitoring, a wireless HART system was used. Various battery-operated pressure and temperature instruments came with HART wireless thumbs, allowing them to broadcast their data back to the HART gateway approximately every 5 seconds. Being battery powered, the periodic pressure and temperature-sensing devices could be placed anywhere in the process by simply selecting the correct hose and pipe fitting.
This left our specialty analytical instruments, as well as our real-time pressure and temperature devices, to be placed. All of these instruments found a home mounted to the back of our PLC/human-machine interface console. This allowed us to use a combination of hardwired network cables and traditional 4-20 mA signals directly to the PLC.
The HART wireless gateway and the wireless access point were hardwired together through a managed switch with CAT 6 Ethernet cable. A Modbus TCP card allowed the PLC to read the HART wireless device data through the HART gateway for the purposed of alarming and graphical display. The hardwired Ethernet network linked the supervisory control and data acquisition (SCADA) workstation to the HART wireless gateway and the wireless Ethernet gateway, giving the SCADA system access to the PLC.
To maintain the area classification, the pump variable frequency drive panels and PLC panels used panel purge units. Hardwired devices used intrinsically safe I/O, while hardwired network devices were explosion-proof and used rigid conduit. To date, the wireless network system has operated without failure of service. It has proven to be every bit as reliable as a wired solution, while providing much-needed simplification and flexibility to the pilot lab process.