This article was printed in CONTROL's May 2009 edition.
By Greg McMillan
I can envision a wireless world, but is wireless control ready for prime time in industrial processes? Is wireless reliable, secure and fast enough for process control? Will battery life be long enough? What is the motivation other than to be part of a neat new technology?
The savings in installation costs is the most obvious benefit for using wireless devices. My six years in E&I construction makes me more appreciative of the potential savings. I can remember sorting through rolls of schematic diagrams, installation and termination details, and the drawings for the fabrication and layout of the cable tray, conduit, junction boxes and interface panels. It took the best electrician supervision to get the whole picture right and correct the inevitable design and drafting mistakes. Projects routinely used a factor of three to four times the instrument cost to estimate the total commissioned cost of an instrument, most of which was associated with design, implementation and checkout of the wiring system. The other biggest part was the making the process connection. One leading chemical company and some plants have standardized on the elimination of sensing lines and sample lines by using of inline meters for flow and liquid analysis and connecting differential pressure or pressure transmitters directly to the process pipeline or vessel. While the original goal was to improve reliability and maintainability, since most of the performance problems originate in the sensing and sample lines, it turned out the side benefit of savings in installation cost was impressive as well. The combination of direct process connections and wireless communication can drop the total commissioned cost for a transmitter to less than twice the hardware cost for large projects where there is an economy of scale.
But for me, cost takes a backseat to performance. So what are the functional benefits of wireless? One possibility is that more instruments can be used to provide a better view and understanding of the process because of portability.
In the old days, we added extensive standardized pressure and temperature gauge pipe connections and bought an assortment of gauges. The field operator would pick a gauge with the right range and connect it to the process to better track down problems. The gauges provided information on the source and profile of changes in plant performance.
For distillation column control, we sought the tray that had the largest and most symmetrical change in temperature for a change in the reflux-to-feed ratio and, in some cases, the steam-to-feed ratio. While simulation programs could give you some insight, that best tray was not found until after startup. Additionally, the best tray would change with process improvement and debottlenecking projects. More tray temperature sensor connections were provided than used, with the expectation that the best points for monitoring and control needed to be found online.
We started to use packaged equipment on skids significantly in the 1990s to service multiple-unit operations and meet multiple needs with reduced installation and operating cost and time. There is an increasing variety of skid operations. A significant example is the single-use bioreactor (SUB) with a disposable plastic liner that eliminates the bioreactor sterilization step time and cost. Originally used in pilot plants, the SUB has greatly increased in size to the degree where the SUB can be used for the commercial production of high-value-added biopharmaceuticals. Figure 1 shows a 100-liter SUB skid with WirelessHART pH, temperature and pressure transmitters and a lab-optimized DCS for research and development.
Today, we have the advantage of wireless transmitters to provide the flexibility and portability needed for process analysis and diagnostics, process control improvement and packaged equipment.
For the E&I plant people in the trenches, the time spent troubleshooting instruments is a primary concern. Even if the wiring is not the source of the problem in the instrument signal, it needs to be checked out. When grounding and noise problems occurred, they were difficult to track down. Signal shielding and grounding standards were developed in the 1970s. However, even systems designed per the standards developed rampant problems with electromagnetic interference (EMI) when inexpensive inverters for variable-frequency drives (VFD) were installed. It took years for some plants to sort out their VFD-induced EMI problems.
The instrument suffering the most from ground problems for the last 50 years is the pH transmitter. I wish I had a dollar for every spike I have seen or users reported in a pH trend chart. The spike is usually caused by ground noise. In some cases, the spikes disappear when the electrode is removed and inserted in a beaker, indicating one side of the ground loop is through the process. In most cases, the ground noise is gone when the electrode is left in place, and a lab meter is connected to the electrode cable instead of the transmitter wiring. This indicates the other side of the ground loop is somehow established in the transmitter wiring. Figure 2 is a trend chart of a WirelessHART transmitter and a wired conventional pH transmitter on the SUB in Figure 1. A spike from ground noise appears in the wired pH, but not the wireless pH.
Single-use bioreactor with wireless transmitters and a lab-optimized DCS.
Most of the new measurement diagnostic research and development is going into wireless transmitters. For example, wireless pH transmitters have improved glass impedance measurement, enabling better glass diagnostics, finer measurement resolution and the setup for smart electrodes that have calibration history and electrode health embedded. If you want the latest advances in measurement technology, you need to consider wireless.