Gas chromatographs (GCs) are widely used in the oil and gas industry for determining the component makeup of a gas sample. For high volume or custody transfer stations where an even greater level of precision is necessary, continuous "live" analysis of the flowing gas stream via online GCs is often preferred over spot and composite sampling. However, without a reliable routine for validating the results and preserving accuracy, online GCs can have a negative impact on measurement.
Validation of online GCs typically means verifying that the device is properly calibrated for measuring the molecular content of the gas stream. Generally, there are three steps to ensure that GCs are operating with acceptable accuracy:
- Monitor and track the GC response factors;
- Confirm that the calibration standard is representative; and
- Verify the GC by analyzing the calibration standard.
Tracking response factors over time aids in identifying GCs in need of recalibration. A change in response factor may require altering a GC’s mechanisms (e.g., valve event timing) to maintain accuracy. GPA-2198, an industry standard, recommends that fidelity plots be used for monitoring response factors.
Confirming that GC calibration standards are representative is also a critical step. Calibration (CAL) standards have a known component mixture that can be analyzed to determine the accuracy of GCs. Initially, a new CAL standard should be verified in accordance with GPA-2198. Afterwards, control charts should be used to ensure that CAL standards are not changing over time. Control charts, like fidelity plots, are covered in GPA-2198. Analyzing CAL standards also helps to affirm that the component concentration data produced by the GC is within the precision limits of GPA-2261, the industry standard for natural gas analysis by chromatography which includes acceptable repeatability and reproducibility criteria.
Best practices generally incorporate comparing spot samples pulled concurrently from the meter tube and at the online GC. For proper sampling, refer to API MPMS 14.1 (Manual of Petroleum Measurement Standards, Chapter 14 — Natural Gas Fluids Measurement, Section 1: Collecting and Handling of Natural Gas Samples for Custody Transfer). It’s also prudent to analyze the data path from the online GC to the flow computer, to verify that the flow computer has been receiving, processing, and logging accurate information from the online GC.
Following these guidelines will help ensure that online GCs are functioning properly and providing the measurement accuracy that’s appropriate for its deployment.
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