Several filtration techniques are used in the downstream sector of the oil and gas industry to ensure that samples are free of impurities for on-line process analyzers. These techniques include slipstream or bypass filtration, slipstream with coalescing filtration, membrane separation of sample streams, and stack gas filtration. Contaminants in samples can cause a host of problems such as skewed data analysis, clogged orifices, instrument failure, and frequent maintenance.
Since analyzers are often located a distance from the sampling point, samples usually travel to the analyzer at relatively high flow rates to minimize lag time. The sample is divided at the analyzer, with the analyzer using the portion it requires, normally a small fraction of the total sample, and the balance being recycled to the process or vented.
Ideally, a filter specifically designed for sampling applications should be located at the point where the low-flow stream is withdrawn to the analyzer. This arrangement permits the main volume of the filter to be swept continuously by the high flow rate stream, thus minimizing lag time. At the same time, only the low flow stream to the analyzer is filtered, thus extending the life of the filter.
A slipstream filter requires inlet and outlet ports at opposite ends of the filter element to allow the high flow rate of the by-passed material to sweep the surface of the filter element and the filter reservoir. A third port, connected to the low flow rate line to the analyzer, is also required. This allows filtered samples to be withdrawn from the filter reservoir.
If bubble removal from a liquid is required, this function may be combined with slipstream filtration, since the recommended flow direction for bubble removal is outside-to-inside, and the separated bubbles will be swept out of the housing by the bypass stream. In this case, liquid feed should enter at the bottom of the housing and the bypass liquid will exit at the top of the housing.
If coalescing is needed to continuously drain suspended liquid droplets from a gas or liquid stream, a second stage of filtration is required. The coalescing filter should be located in the sample line to the analyzer and should be as small as possible to minimize lag time.
A coalescing membrane combination filter is recommended to remove entrained liquid and particulate in gas samples and to prevent contamination or damage to the analyzer and system components. The membrane contains microscopic pores that permit molecules or gas vapor to flow through easily, keeping the composition of the sample gas unchanged. The high surface tension of the membrane effectively traps even the smallest liquid molecules. The membrane is extremely inert and is recommended for most process liquid applications. Because of the low absorption characteristics, a membrane combination filter is also recommended for use in systems designed for parts per billion (PPB), parts per million (PPM), and percent level component concentrations. A combination filter is typically located upstream from the analyzer.
Stack gas sampling presents the challenge of high temperatures, condensed liquids, and high solids contamination. In many applications there is a requirement to trap and analyze the solids in the gas. This is accomplished by filtering the gas sample at the draw point in the stack. This requires filter media capable of withstanding extremely high temperatures, upwards of 900°F. In addition to filtering the gas stack sample stream at the stack, it is also necessary to filter the sample gas just prior to entering the analyzer. The gas sample cools as it is pulled to the analyzer and consequently condenses liquids along the way. Considerable volumes of liquid may be present which can be highly corrosive in nature. Therefore, it is important to install a high efficiency coalescing filter just prior to the inlet of the sample analyzer to remove this corrosive condensate and protect the analyzer from damage and skewed analysis. It may be necessary to heat the sample line between the coalescer and the analyzer to prevent additional condensation. Stack gas sampling is widely practiced in the oil and gas industry and can be very challenging to accomplish, but proper filtration specifically designed for the sampling application will ensure trouble free and accurate analysis.
Related content: Proper Filtration of Samples to On-Line Process Analyzers
This post was contributed by Compressed Air and Gas Treatment Technology Team, Parker Hannifin.