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This article was originally published in our sister publication Chemical Processing.
By Robert N. Dubois, consulting analytical specialist
"When men got structural steel, they did not use it to build steel copies of wooden bridges," wrote Ayn Rand in her book "Atlas Shrugged." Today process sampling systems can benefit from advances due to the New Sampling/Sensor Initiative (NeSSI) — so, we should ponder whether we're really taking advantage of these innovations or just building steel copies of wooden bridges.
The Center for Process Analytical Chemistry (CPAC) at the University of Washington, Seattle, in 2000 launched NeSSI. This ambitious undertaking aimed to address reliability problems (and, yes, bad reputation) of process analytical systems. Many people associate NeSSI exclusively with the miniature mechanical footprint, adopted from an International Society of Automation (ISA) SP76 committee standard. That's Generation I, which already is well established. Today there's much more. Generation II, now under a full head of steam, automates the sample system — and sets the stage for Generation III, widespread adoption of microanalytical devices (Figure 1).
Automating a sample system always has been a struggle. The first continuous analyzers and their "evil" accessory, the sample system, appeared in pre-World War II Germany. Today the analyzers themselves have become modern marvels of automation. However little has changed with the sampling system. We still rely on spring and diaphragm regulators, on/off thermostats, manually adjustable needle valves and visual indicators for monitoring and control. We invariably need to do routine field checks and adjustments. Indeed, it's not unusual for analyzer technicians to make daily rounds. Process analytical has never caught up with the automation used by our instrumentation and distributed control system (DCS) associates. Sampling systems are one of the last bastions of manual operation left in a modern processing facility. Why does process analytical remain an anachronism in a sea of automation?
Figure 1. Development Roadmap: The ultimate
In one company where I worked, some process automation folks called analyzers "the technology of last resort." But these folks also were part of the problem because they didn't want to handle the multiple diagnostic inputs needed to adequately monitor performance of an extensive process analytical system. Typical analyzer-to-DCS connections include component concentration signals and an analyzer fault contact (and sometimes a flow switch in parallel) to give the ubiquitous "analyzer trouble" alarm. However the majority of diagnostic elements such as sample take-off pressures, sample disposal pressures, sample flow quantities, heat-tracing temperatures used to maintain sample dew points, filter performance, calibration system check flows, analyzer shelter environmental alarms and analyzer utilities that contribute to overall analytical system reliability typically aren't monitored. We generally remain analyzer-centric in predicting or reporting a failure to the process operator. This impacts reliability because an analyzer is only a small part (and in many cases maybe the most reliable one) of a multi-element system. To make things worse, the signals sent usually are discrete, don't predict the problem and only give an alarm when it's too late to do anything — by that time the plant may be down. When we attempt to become system-centric and send multiple signals to/from the control room, the cost of sensors, actuators, wiring and additional input/output (I/O) automation points (especially for conventional 4–20-mA signals) becomes very steep.
To make matters worse, doing closed-loop control and adding logic functions (e.g., stream switching routines) using the DCS for sample system control invokes another layer of automation that's perceived as overkill by the process automation folks. Although many sampling system automation tasks (or applets) could be standardized across the process analytical discipline, we as an industry have yet to come up with an open modular solution. So even if we get our input (and output) signals serially to the DCS, programming costs tend to ramp up because of the need for custom programming.
An Ugly Secret
Many sampling systems handle hazardous fluids (such as hydrocarbons and hydrogen) and are packaged enclosures. Yet the electrical engineer going out to map the plot plan for electrical hazardous area ratings doesn't classify the inside of an enclosure as a Division 1/Zone 1 environment — but that's what it is. We've learned to design and package our sample systems using a potpourri of protective methods and wiring techniques to allow at least some degree of automation — e.g., explosion-proof enclosures, air and inert purging, hydrocarbon gas detector interlocks, equipment encapsulation, intrinsic safety, filled conduit seals, rigid conduit and armored cables. Having to meet exacting requirements of various global electrical certification agencies intensifies the problem. It's difficult and expensive to automate a sample system to meet requirements of a Division 1/Zone 1 area.
There are instances where we have used 4–20-mA analog signals to send pressure, temperature and flow signals from our sample systems to the DCS. (In NeSSI-speak, we call this generation 1.5). However this requires extensive wiring in a confined space; significant cost and effort need to go into design of cabling, intrinsically safe barriers, wiring and conduit to meet the electrical classification. In some cases as many as 30 I/O may be required to adequately monitor and control a system.
Figure 2. NeSSI Architecture: Modular system includes
Having a suitable bus allows us to move on two other critical issues: How do we unburden the DCS and manage our own signals? And how do we do closed-loop control and execute simple control tasks for process analytical specific requirements? We can use a NeSSI-bus-enabled local controller rated Division 2/Zone 2 (since it can be located outside of a sampling handling enclosure). A physically large controller would defeat miniaturization efforts; we need a "hockey puck" sized programmable device that talks NeSSI-bus on one side and Ethernet or a fieldbus protocol on the other. We call this device a Sensor Actuator Manager (SAM). This SAM functionality to date has been typically embedded within smart analyzers such as gas chromatographs. Some SAMs employ a programmable logic controller (PLC) to control a sample system. Unfortunately the sample system applets developed for these SAMs have been platform-specific and proprietary. At one CPAC workshops, attendees came up with a list of 60 applets that could provide a standard set of functions to allow a technician to set up, monitor and control a sampling system (and microanalytical device) without custom programming.
Figure 2 shows the NeSSI architecture with mechanical and electrical bus rails along with a SAM. The SAM manages bus signals and controls the sample system via programmable applets. It also serves as an interface between a Zone 1/Division 1 NeSSI–bus handling the sample system sensors and actuators, and a higher-level communication bus. This arrangement allows plug-and-play capabilities of devices within a hazardous location. A wireless personal digital assistant (PDA) or personal computer (PC) enables interaction with the SAM and provides a graphical user interface to visualize flow paths. ("O&M" in the figure refers to an operator and maintenance station in a control room or even offsite.)
Breaking Old Habits
Making optimal use of a NeSSI-bus and distributed control embodied by the SAM will require us to rethink what we've been doing over the last 70+ years.
Sample system fabrication techniques. Size and weight do matter with NeSSI. Because our plan is to get the equipment by-line (i.e., next to the sampling point), it's important that a system is small and light so that service can be done on a replacement basis. Today we have the ability to fabricate modular miniature systems that should be able to be assembled Lego-style by an unskilled person. Yet in many cases we build modular systems that haven't been optimized for space or still require custom tubing work. We should aim to tightly integrate the modular system with its enclosure, to reduce size and weight, as well as eliminate custom tubing. Use of graphical indication of the flow paths certainly will overcome reluctance to use a densely populated sampling system.
Figure 3. Rotameter Replacement: NeSSI-bus-enabled
Use of visual indication devices such as rotameters and pressure gauges. We're addicted to their usage because we've felt a need to "see" the process fluid. However, as glass rotameters have given way to armored versions with magnetically coupled indicators, what we're getting now is an inferential view of the flow. The new NeSSI-bus-compliant flow devices can transmit flow or pressure signals (Figure 3). If you have a signal that's available on a graphical user interface you really don't need an indicator. Maybe it's time to remove the windows from our sample system enclosures and get a smaller transmitter in their place. Eliminating the rotameter also does away with an aggravating position constraint that dictates vertical positioning of the sample system.
Use of manual flow and pressure regulators. A rotameter generally comes with a needle valve, allowing manual flow adjustment by analyzer technicians. So, how can we adjust flow without a rotameter? Work is underway to supply a proportional valve coupled to a flow or pressure transmitter to give a real control loop on the sample system. This will allow us to monitor the flow using proportional-integral-derivative (PID) control and also input a set point. Consider the great strides gas chromatograph manufacturers have made with carrier-gas pressure controls. The days of matching flows using needle valves are history. Thankfully needle valves used in the majority of process gas chromatographs have been consigned to the obsolete parts bin.
Figure 4 shows a NeSSI-bus-enabled valve control module for actuating sample-system pneumatic valves as well as a NeSSI-bus-enabled pressure/temperature transmitter. The module is rated Division 1/Zone 1 and so can be mounted inside a sample system enclosure. When used with a gas chromatograph it can obviate separate pneumatic tubes between the chromatograph and the sample system. A single cable connection links the gas chromatograph to the valve control module.
Filter replacement routines. Do you know how effective your filter is or when to change it? Right now gauging system performance is a hit or miss activity. Use of a NeSSI-bus-enabled differential-pressure or moisture-breakthrough sensor (common in continuous emission monitoring systems) would give hard data. It would allow us to validate filter performance and move from preventive to predictive maintenance. Automation of the filter also will lead to adoption of more-intelligent filtration devices that predict life span and initiate self-cleaning routines.
Using the DCS to control analytical systems. The advent of low-cost miniature computing and control devices will enable sampling-system control functions to become distributed and local to the sample system. Simple programmable control applets will dominate, be interchangeable across platforms and available from third parties. Sensors and actuators associated with auxiliary systems such as carrier-gas generators, heat tracers, conditioners (vaporizing regulators, sample recovery systems, etc.) can be integrated on the NeSSI-bus. With these sensors we can monitor and apply set points to our auxiliary process-analytical support system. The process analytical SAM can significantly extend limited control functionality previously provided by the DCS and various controllers.
Figure 4. Valve Control Module and
Gas cylinders for calibration and validation. More-precise flow and temperature control in a sample system affords the opportunity to opt for more permeation generators to calibrate and validate analytical sensors. Today we use bulky gas cylinders to do this chore. It would be a tremendous advantage from an installation and operational point of view to eliminate calibration cylinders when the components needed are available as permeation sources.
Maintenance resources and routine rounds.NeSSI can eliminate the need for continual checks and adjustments. The new generation of smart analyzers such as gas chromatographs will have visualization built into the sample system as part of their local human machine interface (HMI) and remote workstations, helping analyzer technicians properly troubleshoot. Indeed, troubleshooting will become more of a science than an art. Portable zone-2-rated laptop computers or PDAs can effectively serve as the new "adjustable wrench" for the technician.
Block and vent valves for gas chromatograph sample introduction. Typically to ensure constant molecular volume we reference the sample pressure to atmosphere using a block-and-vent-valve arrangement prior to injecting a sample into a gas chromatograph. The ability to use an absolute pressure sensor will allow more-precise measurement and better control without the need for block-and-vent hardware. Of course, this would require the sample system to communicate with the gas chromatograph.
System-centric health monitoring. Sensors and networking will enable expansion of monitoring to all elements of an analytical system. It will permit overall analytical system performance to appear in the control room as a traffic light status signal that tells the operator whether the complete process analytical system is good, bad or is still good but will soon require maintenance. This will improve the operator's confidence in the performance of the analyzer system.
The Path Forward
Bus systems undoubtedly will mature fairly quickly; many components including miniature flow meters, pressure sensors, smart heaters and both proportional and on/off automated valves either are available or will be in the next couple of years. Ability to purchase functional applets that could work across multiple analyzer systems will be truly revolutionary — they even may be fun to use. Today SAM functionality is embedded in more-complex analyzers such as gas chromatographs. Extending it to other analyzers demands a compact NeSSI-bus-enabled SAM. Until that's available we'll struggle to bring standardization and simplicity to our discipline. Until then we'll continue to supply ad hoc and proprietary solutions that will work — but not support our general move to Generation III microanalytical and by-line installations. Our objective is to allow a microanalytical manufacturer to be able to plug into the mechanical and communication rails — and configure its devices sampling tasks using off-the-shelf applets. This architecture finally will enable the sampling and analytical measurement to go hand-in-hand as an integrated package.
The cost and technical effort to move to complete sample-system automation will be high. However end users will gain significant rewards including higher reliability and lower maintenance costs. It'll take a clear vision and concerted effort to change the game — but the horse is out of the barn and it's only a matter of time before we'll look back and wonder why we clung to our manual systems for so long. However, until that time comes we'll continue, out of sheer habit, to build steel copies of wooden bridges.
More Information on NeSSI
The Center for Process Analytical Chemistry at the University of Washington serves as the forum for NeSSI development activities. It conducts workshops and presentations twice a year in Seattle. For additional reading or contact information, refer to:
Robert N. Dubois is a consulting analytical specialist based in Sherwood Park, Alberta, and a member of the NeSSI steering team at the Center for Process Analytical Chemistry, Seattle. E-mail him at firstname.lastname@example.org.
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