Rethink Sample System Automation

Sample system design methodology. Software configurator tools available for designing NeSSI systems ultimately will expand to include the heaters, the microclimate enclosure, the wiring interconnections and the applets specified for the SAM. These tools will allow an end user to do a rapid detail design of a sample system based on company best practice and generate a detailed bill of materials and estimate. Because the NeSSI bus is intrinsically safe, use of pre-certified components allows us to virtually self-certify our system as an entity regardless of where in the world the system is installed. The assembler or integrator now will be able to validate the automated performance and operation of the sample system as part of the check-out procedure by enabling self-checking routines available in the SAM. Contrast that to current designs where much of the detailed design is farmed out and systems are mainly checked only for mechanical operation.

Stream switching norms. For sample and calibration/validation fluid switching, we often use double-block-and-bleed stream-switching valves with bubbler systems to indicate a leak. This mandates maintenance rounds to regularly check if there's a leak at the bubbler. Use of miniature, modular close-coupled systems minimizes upswept voids; the need for double-block-and-bleed valves to reduce dead volume all of sudden becomes less important. Another reason for multiple valves has been to compensate for leaky valves that were standard fare in the "bad old days." Early stream-select valves were ball valves with poor seating — later followed by explosion-proof solenoid valves that probably were even worse. Today we have better valves (see "Streamline Your Sampling System," www.ChemicalProcessing.com/articles/2009/076.html). And if they leak smart flow and pressure sensors can monitor valve performance. Use of close-coupled systems and smart sensors allows us to simplify our systems and reduce size and costs by minimizing the need for double-block-and-bleed stream-switching hardware. This also frees us from the burden of providing visual indications of leakage.

 

Figure 3. Rotameter Replacement: NeSSI-bus-enabled unit can sense fluid flow, pressure and temperature, and serve in Division 1/Zone 1 areas. Photo courtesy of Circor.

 

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.