There are a number of standard field and laboratory methods available for gas quality testing but there are many aspects to consider in choosing the best technique. What techniques are best for your requirements? Do you have to compromise speed with accuracy or vice versa? What is the best approach?

Gas composition and its contaminants vary significantly across Australia and it is important that multiple determinations are carried out by field and laboratory-based equipment to confirm results.

Composition

Field analysis of gas composition by portable gas chromatographs (GC) and online analysers, which provide accurate results within a few minutes and only require routine calibration checks and maintenance. However, the accuracy of these units can become an issue when faced with non-ideal situations such as dramatic changes in composition, moisture, carbon dioxide and nitrogen concentrations.

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In these conditions, it is important to collect samples for analysis in a laboratory. It is essential that experienced chemists go to the site and collect representative samples for return to the laboratory to determine gas composition by GC. Obtaining a representative sample can often be more difficult than the analysis itself.

Sample collection and analysis conducted by the same qualified person allows the analyst to account for unusual or unexpected lab results related to observed sampling or site issues such as liquid carryover from separators. The chemist can also provide valuable assistance and advice in areas such as gas treatment and sample point location.

Contaminants

Accurately determining gas contaminants, such as moisture, sulphur, hydrogen sulphide, chlorine, fluorine, radon or metals such as mercury, is different to determining composition.

While analysis of these contaminants can be performed in the laboratory, the most accurate tests are performed onsite or with a concentration stage onsite. Large gas samples are needed to reach the very low detection limits required by some gas specifications. This may require transporting large and numerous gas cylinders to the laboratory – especially if the sample pressure is low.

Testing onsite or concentrating the contaminant in a liquid or solid medium can eradicate the need for cylinders in most tests. Eliminating cylinders from the test removes the possibility of contamination between samples, and also minimises the losses of reactive components to the walls of the cylinder – which can be experienced even when inert coatings such as Teflon-lining are used.

Mix and match techniques

It is important to determine the right method of testing for specific situations. When basic gas composition is required and the conditions are known, the speed and accuracy of a portable/online instrument is satisfactory. Onsite sampling coupled with laboratory analysis is recommended to determine gas composition, contaminant testing or to resolve process issues.