Integrating Gas Chromatographs with Control and Device Management

Gas Chromatographs (GCs) are used in many process-manufacturing processes. Some GC applications include fuel & flare gas analysis, cracking furnace BTU firing rates, gas ratios, and a whole lot more.

Wikipedia defines the gas chromatography process:

…used in analytical chemistry for separating and analysing compounds that can be vaporized without decomposition. Typical uses of GC include testing the purity of a particular substance, or separating the different components of a mixture (the relative amounts of such components can also be determined). In some situations, GC may help in identifying a compound.

I share this for those not already well steeped in the knowledge of GCs as background for an article by Emerson’s Jonas Berge and Michael Gaura in Gas Today Australia. The article, GC made easy: integrating process gas chromatographs, describes how digital communications technologies and standards such as Foundation fieldbus (FF) and Electronic Device Description Language (EDDL) have made the information collected by GCs more readily accessible to improve the process and easier to maintain to provide continued accuracy over time.

I liked Jonas and Michael’s description of GC even better since it provides a picture of how the GC operates:

A process Gas Chromatograph (GC) determines the concentration of multiple constituents in a gas mixture. The analysis is cyclic. A gas sample is taken and travels through a separation column which selectively retains light and heavy components for a different duration such that each component exits in succession. The separated components then flow through a detector assembly. The time of exit identifies which component it is. The detector output is proportional to the concentration. The detector output is recorded in a chromatogram in which gas components appears as peaks. The concentration of each constituent is computed from the area of the corresponding peak and a calibrated response factor (RF). In some applications the heating value is also computed. Each cycle takes several minutes to complete depending on the components analysed. After purging, a new analysis cycle begins.

They describe prior ways of connecting GCs with the control systems and device management software. It involved either hardwiring signals from the GC to the control system or using protocols such as Modbus/RTU over RS-485 and Modbus/TCP over Ethernet and mapping the multitude of process variables, status, and diagnostic signals into registers. This process is often time consuming, expensive, and error prone. Beyond the engineering work required, the authors noted:

Special care had to be taken with documentation, configuration, verification at FAT [factory acceptance test], and change management – particularly when the GCs and DCS [control system] were not from the same vendor. Register changes late in the project often required major rework and caused delays.

With Foundation fieldbus-based GCs, these steps are eliminated including register mapping and integration into the control system. They describe inherent:

…alert mechanisms that are part of Foundation Fieldbus, analyser operations share the same process display, historian, audit trail, and alarm & event log etc. as the main DCS.

Where EDDL (IEC 61804-3) enters the picture is to provide a common user experience in calibration, configuration, and maintenance along with other devices including, “…drives, radar level transmitters, flowmeters, etc.” Before this common approach, technicians had to contend with many different software packages each with their own user interface. Jonas and Michael sum up the role of EDDL:

…the GC manufacturer’s expert defines how they want the GC to be displayed in the system in order to make it easy to use. The device management software renders each device display with the ‘content & structure’ defined by the manufacturer, but with a common ‘look & feel’ for all devices regardless of manufacturer, type, or protocol. The mix of devices around the plant is displayed consistently and is therefore easy to use. Thus EDDL interoperability makes it easier for technicians to manage the wide assortment of devices in the plant and complete their work faster.

Give the article a read for more on the task-based approach in the displays, wizard-based guided validation and calibration, and interpretation of GC diagnostic information.

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