Proper Filtration of Samples to On-Line Process Analyzers

Contaminants in samples are reported to be a major cause of problems with on-line analyzers used in the downstream sector of the oil and gas industry. On-line process analyzers such as gas and liquid chromatography, ion chromatography, laser optic instruments, atomic absorption instruments, and specific ion analyzers are widely used. Plant operation factors, such as continuous sample feed and test frequency, can magnify the challenges of delivering acceptable samples to analyzers and cause instrument failure. These issues can be minimized by installing an appropriate filter upstream of the analyzer.

Analyzer filter recommendations

While the need for an effective filter in the sample line is generally recognized, it is also important to understand the necessity of using a filter specifically designed for sampling applications, rather than trying to make do with a general-purpose filter.

Characteristics of a filter appropriate for sampling applications:

1. The filter must not change the composition of the sample, other than to remove unwanted impurities. Therefore, the choice of filter media is limited to a few chemically inert materials: glass, stainless steel, and polytetrafluoroethylene (PTFE).
2. Since the sample filter is often in a remote or inconvenient location, it must be capable of operating for a reasonable period of time between scheduled maintenance checks. More importantly, it should not be susceptible to unscheduled problems, such as filter element plugging or rupturing, between regular maintenance checks.
3. Sample filter maintenance in the field is usually performed under adverse conditions by personnel who are not trained chemists; therefore, the filter should be designed for easy and uncomplicated maintenance. Filter elements should be rugged and not susceptible to handling damage; the unassembled housing should have a minimum number of loose parts, and the housing should be designed so that it is virtually impossible to install a filter element incorrectly.
4. A filter should introduce minimum lag time into the system. Lag time can be handled via sample system design (slipstream sampling, for example), but sizable dead volume in the filter housing should be avoided. Since large reservoir volume is desirable in many filter applications – such as compressed air or water filters – filters not specifically designed for sampling usually are not suited for analyzers.
5. The filter must be able to efficiently separate a non-continuous phase contaminant from the continuous sample stream phase. Specifically, the filter must be able to make the following separations, in addition to removing solid particles:
   • Gas samples - remove liquid droplets
   • Liquid samples - remove immiscible liquid droplets and gas bubbles.

Most filter media will do an adequate job of removing solid particles from liquids or gases, but the only practical commercial media that will separate liquids from gases, gas bubbles from liquids, and two immiscible liquids is resin-bonded glass microfiber media.

Parker Balston sample analyzer filters are specifically designed to protect analyzers from sample impurities by removing solids and liquids from gases with 99.99999+% efficiency at 0.01 micron.

Read this white paper for guidelines for proper filtration and delivery of samples to on-line process analyzers.

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This post was contributed by Compressed Air and Gas Treatment Technology Team, Parker Hannifin.

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