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EPA Allows Refineries to Measure Flare Vent Gas NHV with Mass Spectrometers

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EPA has published a broadly applicable approved alternative test method (ALT-124) allowing refineries subject to 40 CFR 63, Subpart CC to use mass spectrometers to measure the net heating value of flare vent gas. The original language of the refinery sector rule (RSR) only referenced gas chromatographs (GCs) and direct measurement of net heating value (calorimeters). Essentially, EPA will treat mass spectrometers just like gas chromatographs.

In ALT-124, EPA approves the following measurement approach:

  1. Perform a pre-survey to determine the list and concentrations of components that are present in a flare vent gas feed and an appropriate analysis method.
  2. Calibrate the mass spectrometer using calibration gas standards consisting of the mix of compounds identified in the pre-survey.
  3. Compounds that are not detected during the pre-survey, but have mass fragments identical to compounds detected during the pre-survey will be included in the NHVvg calculation if they are present during flare gas analysis. This is basically stating that compounds found in very low concentrations in vent gas may “show up as” compounds included in the analysis method.
  4. The average instrument calibration error for each calibration compound must not differ by more than 10% from the cylinder gas value or tag value.
  5. For triplicate injections at each calibration concentration, no calibration measurement shall deviate more than 5% from the average concentration measured at that level.

EPA approved this measurement approach while also issuing requirements that mirror those established for continuous GC monitoring systems in Performance Specification 9. Also, the alternative method states that all other requirements of §63.670 and §63.671 (which includes daily calibration requirements) must be followed.

If you have any questions about flare gas measurements, give the CleanAir Flare Team a call or send an email. We love to talk about flares.


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