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- spot, composite and on-line BTU analysis for single-phase gas streams (there is no standard for sampling two-phase or supercritical streams)
\n- the elimination of liquid compounds and contaminants at the temperature and pressure of the gas stream to avoid changes in sample composition.
\n- the importance of measuring the dew point of hydrocarbons and maintaining the sample above the dew point (need for reheating).
\n- recommended packaging materials and guidelines for materials of construction.
\n- probes
\n- pressure regulators
\n- sample transfer tubes
\n- calibration gas considerations
* GPA 2166 Obtaining samples of natural gas for chromatography analysis, API 14.1 Measurement of natural gas (harvesting and handling of samples of natural gas for custody transfer) and ISO 10715 Natural Gas sampling guidelines.
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The sole reason for extracting samples is to determine the composition of the source gas. It is therefore imperative to extract a representative sample.
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GPA 2166- \"The objective of the sampling procedures listed is to obtain a representative sample of the gas phase portion of the stream being analyzed. Any subsequent analysis of the sample, regardless of the accuracy of the analytical method, will only reflect the true composition of the stream if a representative sample is obtained.\"
\nAPI 14.1-\" Composition identical or as close as possible to the sample source stream.\"
\nISO 10715-\"A sample having the same composition as the sampled material, when the latter is considered as a homogeneous whole.\"
A representative sample can therefore be defined as follows: A representative sample of natural gas is the vapor phase of the sampled stream as it exists at the pressure and temperature in the pipeline.
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This diagram shows a typical sample conditioning system.
\nThe standards recommend a sampling probe for sample extraction, to ensure that the liquid film along the pipe wall is not extracted with the sample. Wherever possible, the probe should be fitted with a membrane to keep solids and liquids out of the sampling system and reject liquids at source conditions. This is the most important task of the sample conditioning system. If the sample is modified at this stage, it is no longer valid and there is no way to recreate it.
\nOnce the sample has been extracted, it is usually regulated to a lower pressure acceptable to the analyzer. The regulator may be incorporated in the probe, or it may be external. Single-stage and multi-stage regulators are available. It is sometimes necessary to place the sampling tube and regulator in a heated enclosure.
\nThe sample must then be transported to the analyzer in a way that does not alter its composition. In the example above, a tracer tube has been added for sample transport, and the analyzer cabinet is heated to prevent condensation.
\nA \"safety net\" separator should be placed as close as possible to the analyzer to protect it from any liquid that might condense during sample transport in the event of a tracing malfunction (it's a bit like car insurance... you hope it never comes in handy, but you're glad to have it when you need it). Placing the diaphragm separator immediately after the regulator near the sampling point does not protect the analyzer from liquids condensing after this point. If you see liquid after the sampling point, the composition of the sample has changed, and the purpose at this point is to protect the analyzer.
\nBypass flow and sample flow to the analyzer must be controlled. Pressure and flow must be limited to provide the analyzer with only what is required for reliable results over time.
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The sample conditioning system comprises all the components used to extract a sample from a source, condition it so that it is compatible with the analyzer, then transport it from the extraction point to the analyzer (from sampling probes, regulators and filters to the tubing).
\nThe sampling system must meet two objectives:
\n1) Preservation of sample composition
\n2) Protection of the analyzer.
\n- Remove components from the sample stream that may harm the analyzer or influence its readings without affecting the components of interest.
\n- Adjust sample flow conditions, such as pressure, temperature and flow rate, to meet analyzer specifications.
It's important to keep both objectives in mind. Usually, the emphasis is on protecting the analyzer, but preserving the sample is just as important. The analyzer will analyze what we send, regardless of whether the sample composition has been altered or not.
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A properly designed sample conditioning system is the key to analyzer performance and accurate analysis.
\nThe design and operation of the sample conditioning system represents the greatest challenge for those involved, and almost always requires a bespoke design.
\nIf the sample conditioning system is not designed correctly, it will cause numerous problems for the analyzer and compromise sample integrity.
\nAround 80% of analysis errors are attributable to the sample conditioning system.
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- Type of analysis (BTU, H20, H2S, etc.)
\n- Gas composition (rich or lean).
\n- The quantity of contaminants in the source gas.
\n- Condensates, amines from gas sweetening, glycol from H2O removal and compressor oils are all contaminants that need to be treated with care. If they are not removed, they can affect sample analysis.
\n- Source gas pressure and temperature conditions.
\n- Flow rate required by the analyzer.
\n- Ambient temperature
Natural gas industry standards for sampling* provide guidelines on: - spot, composite and on-line BTU analysis for single-phase gas streams (there is no standard for sampling two-phase or supercritical streams) - removal of liquid compounds and contaminants at the temperature and [...].
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