By Robert Sherman and Jim Montague
Process analyzer sample conditioning systems (SCSs) do not always get the attention they deserve. This is in part because analyzers were not able to do low-parts-per-million (PPM) analysis 40 years ago. So in past decades, process users could get away with crude, less highly developed measurements of what their applications were doing, and many of these traditional habits still persist today.
However, today's analyzers and sample conditioning systems can routinely do low-PPM analysis. Also, since 1990, new rules and regulations are requiring analyses of lower and lower PPM levels of H2S and total reduced sulfur (TRS) in continuous emission monitoring system (CEMS) applications regulated by the U.S. Environmental Protection Agency (EPA), so users must now achieve defined levels of precision and accuracy in their processes. In fact, the first rigorous sample conditioning and reporting requirements were part of the EPA's original Clean Air Act circa 1969-1974. It was revised in 1990 and had its third-generation revision released last fall, including a staged application with compliance deadlines in December 2011 and December 2012.
Lack Coordination? Be Meticulous
Unfortunately, in recent years, there have been an increasing number of sample conditioning systems built by integrators that were non-functional on shipment. This can happen when a user buys an analyzer from a vendor and then has a system integrator (SI) design the SCS. Another common problem is when the user keeps the existing SCS, but it doesn't match the requirements of the new analyzer. This means the overall system may not work, so the user may have to pay daily fines if the application is a CEMS.
This problem often happens because of the lack of cooperation between analyzer suppliers, unaligned SIs and end users. The SI may not have the right field application experience. The vendor might have the field application experience, but the vendor does not feel it should have to fix a competitor's sample conditioning product. Users can be caught in between, sometimes incurring fines of $100,000 per day per erroneous CEMS value.
The solution is to be meticulous! Spend the funds wisely to get your SCS properly designed for the specific application, especially in northern climates. If a user and an SI were to expend the same attention to all details to design their process analyzer sample conditioning systems as they do when designing an operating process unit, then most sample conditioning systems would work reasonably well.
Most users do not complete application data sheets (ADS), especially for their CEMS. It is extremely important that the analyzer manufacturers have these sheets completed with all sample-plus-matrix components and MIN/NORM/MAX values for each identified component. A user will often indicate they require a 40 CFR 60 subpart Ja system for a flare, and know roughly what that requires, but then tell the analyzer vendor to just sell him something designed for this general application. There will be time made to fix it later, but no time taken to properly specify it prior to order placement. This means the user is also asking his SI to have enough knowledge to give him something that "just works."
Unfortunately, the process details for a given process application are often very specific: Products at one company are different than at others; different types of crude oil come in; and different towers create different sample conditions, including many different temperatures, pressures and other parameters.
As a result, users often believe they do not have the time to do sample conditioning systems right the first time. They perceive other higher priorities, so they hope that the equipment they specify will operate properly. If not, they will be able to deal with any problems later. This adage comes to mind: "Seldom time to do the job right; always time to do the job over!"
Diagnose, Fix—and Gimme Shelter
For example, in past years, a Gulf Coast refinery had a flare system with a 150-ft to 175-ft long sample line. This line was operating at nominal saturation due to water washes of the sample, but the sample system was not working properly. When asked about the dew point of the sample, no one knew what it was! This application had a new, insulated, heat-traced, sample-transfer line that was not specified or routed properly. The vendor checked the order for this sample line. It was specified for a low temperature of 300 °F and a high of 360 °F. However, it was still cool to the touch on a day when the ambient temperature was 65 °F. Do you recognize a problem here?
You need to consider heat-traced enclosures for your calibration gases in low-PPM level CEMS applications. This is important! While ambient temperatures in northern Texas can be -15°F in late winter, they can reach 115 °F in summer. Add incident sunlight in southern Texas and you can have 160 °F wall temperature for your calibration gas cylinder stored outside on the south side of an analyzer shelter. This ambient temperature difference can cause stratification inside the cylinder containing the calibration gas. The analyzer depends on the uniform composition of this gas to develop its results. Previously, bottles of calibration gas were put inside of the analyzer shelter, but today's they're installed outside for safety (Figure 1). They should have a separate, climate-controlled shelter for low-PPM CEMS and process analyzer applications. Some refineries are installing 2-ft x 8-ft footprint, climate-controlled calibration gas enclosure adjacent to their analyzer shelter, and installing their calibration gas cylinders inside with heat-traced-and-insulated delivery tubing to carry the calibration gas from the cylinder to the analyzer.