Representative Sampling: Determining VOC Emissions

Is there an easier, less time consuming, and money saving way to determine a production or midstream site’s emissions for air quality permitting purposes? Why yes there is! The Texas Commission of Environmental Quality’s Air Quality Division developed the standard for determining the emissions from crude/condensate stock tanks without physically testing each and every site. This procedure is termed, “representative sampling” because one site’s stock tank emissions is used to represent that of another. This procedure was developed with the best interests of the end user (operator). When an operator is under a time crunch to file for air permits, the laboratory is behind, or sampling is physically impossible when filing for a pre-construction permit, then representative sampling is the only way to go.

What is exactly being represented? The laboratory analysis of a natural gas or liquid hydrocarbon sample that gives the flash gas composition is what can be used to represent that of sites that meet specific criteria. Therefore, the emission rate of each site still has to be determined. OTA’s simulation testing program is highly recommended in conjunction with representative sampling because individual sites’ emission rates do not have to be physically tested. However, if a site is already setup with a means to measure actual emission rates from stock tanks, then OTA’s direct measurement program may better suit the operator.
To make things even easier, OTA developed a step-by-step procedure for determining which sites can be deemed representative (Representative Sampling Protocol). If a site is in question whether it passes as a representative or not, please refer to TCEQ’s fact sheet, “Representative Analysis Criteria” or contact OTA’s environmentalist at

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