No matter how sophisticated a fluid analyzer system may be, it will be ineffective if a sample flow fails to reach the analyzer sensor or if the sample is contaminated or stale. Failing to monitor fluid flow to the analyzer may result in unsaleable product or environmental harm. The costs of analyzer failure are potentially huge in terms of lost production, damage to equipment, regulatory fines, liability and more. This article gives guidelines for choosing a flow monitoring system.
Gas chromatographs (GCs), mass spectrometers, optical spectrometers and photometers are a few examples of analyzer technologies applied in process plants where assurance of sample flow is required. It is an accepted industry best practice that sampling systems have some type of flow monitor to assure valid samples.
While there are a number of fluid flow monitoring technologies on the market, immersible thermal dispersion technology combined with packaging and sensor designs optimized for sampling systems has emerged as the new best-in-class technology. Thermal dispersion mass flow sensors have proven themselves for decades as extremely reliable in other demanding process and plant applications.
The trend to mount the sample handling system at the process has greatly enhanced process efficiency. More and more systems now provide analysis of the process in real time, making the integrity of the readings that much more important to the process success. The ideal flow monitor should provide the form, fit and functions that will accommodate these new-generation analyzer systems as well as traditional legacy and hybrid designs.
Standards for Sampling Interfaces
Depending on the analyzer type, sampling fluid is often transported in tubing with a diameter of 1/8–1/2 inch. Most systems typically draw small samples in 1/4-inch tubes. Also growing in popularity are systems based on the industry standard SP76 manifold. SP76 is an ANSI/ISA standard approved in 2002 which has been adopted by major chemical and refining companies worldwide.
Leading the way for the SP76 standard is the New Sampling/Sensor Initiative (NeSSI) organization, which originated in 1999 at the Center for Process Analytical Chemistry (CPAC). Other organizations that have embraced NeSSI/SP76 include the International Forum for Process Analytical Chemistry (IFPAC) and the Federation of Analytical Chemistry and Spectroscopy Societies (FACSS).
Each block of a typical SP76 train measures 1.5×1.5 inch square and provides a convenient plug-and-play modular surface mount interface for sampling system components. Beyond the SP76 manifold itself, the NeSSI group has defined progressive levels of sophistication — known as Generations I, II, and III respectively — which provide for digital bus communications, hazardous location approval types and the Sensor Actuator Manager (SAM).
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