OTA utilizes the most accurate and accepted method to determine emission rates from storage vessels, separators, and other storage and process equipment. We offer Direct Measuring, Gas-To-Oil Ratio (GOR), Gas-To-Water Ratio (GWR), and Simulation testing methods for determining flow rate and [gas or liquid] stream composition. Results are used for properly sizing emission control equipment and for determining applicability to Federal and State regulations.

OTA provides its clients with accurate and comprehensive tank battery emissions testing to meet the requirements of the federal, state, local, and/or tribal issued air permits and provide information to properly size a VRU or control device. All sampling, analysis, and calculation methods are preapproved by the appropriate governing body and/or authorized agencies (i.e. ASTM, API, GPA, federal or state agencies, etc.) to ensure that the information we provide is valid and compliant. We offer our clients state DEQ and industry preferred emission methods for the purpose of determining a facility’s (i.e. produced water and production tanks) potential to emit or PTE: Simulation software (E&P Tank v3.0), Direct Measurements, and Gas-to-Water Ratio (GWR). E&P Tank v3.0 has the capability to estimate working/breathing separate from flash emissions, whereas the Direct Measurements method will only provide the WBF emissions all together. The GWR method only accounts for flash emission, therefore, the WB losses will need to be factored in, to determine total PTE for produced water facilities. Both Simulation and Direct Measurement methods provide our clients with total PTE (total WBF losses) for purposes of determining applicability to NSPS Quad O rules and state issued air permits.


It should be noted that OTA carefully selected these flash estimation methods based on accuracy foremost, industry standards, percentage of error, and regulatory acceptance. All of the following outlined flash estimation methods are highly preferred by the Texas Commission of Environmental Quality (Air Permit Reference Guide 5942, 2012).

Pre-Test Inspection

Before testing can occur, a client is asked to complete a questionnaire designed to assess each testing location’s characteristics. This questionnaire enables both OTA and the client to prepare before testing occurs in order to ensure the most accurate and reliable results. The questionnaire requests pertinent site-specific information that is essential to conduct a field test and implement reports, such as production rates (BOPD/BWPD), process setup, liquid sample ports, and equipment integrity. It is imperative that all equipment is in good working order and that there are no leak points in the system. Fugitive losses are separate from a facility’s (tank vessel) PTE, therefore OTA does not account for those losses.

Safety is of the upmost importance. All of OTA’s technicians are properly trained to manage any and all associated risks with the testing procedure. A job safety analysis (JSA) is performed for each and every test to ensure all hazards are properly identified and addressed. Safety equipment and best practice procedures are utilized at all times.


Direct Measurements

This sampling technique is utilized to determine actual emission rates from various sources (i.e. separator vent, tank vessel manifold, wellhead vent etc.), and can be quantified to determine a facility’s PTE. OTA utilizes an easily transportable metering tube with size varying orifice plates designed based on the American Gas Association, Report No. 3 - Third Edition, Part 3, “Orifice Metering of Natural Gas” standards (AGA, 1992). Our proprietary calculation program utilizes the corresponding equations from Part 3 to determine flow rates. Our highly trained field technicians choose an orifice based on site-specific characteristics to ensure the vapor is measured accurately. A data logger is installed inline, downstream of the orifice at a precisely located tap hole to measure pressure and temperature differentials. Readings are measured in increments contiguously over a twenty-four hour testing period. This measuring technique yields detailed vapor flow results that show total, incremental, and peak flows.

A natural gas sample is extracted directly from the tank battery by securely fastening a vacuum pumped sample cylinder to the single point of emission, impeding atmosphere contamination.

This test measures the storage vessels’ working, breathing, and flash emissions.


OTA utilizes the latest version of API’s (American Petroleum Industry) emissions estimation software (E&P Tank 3.0v) to estimate working/breathing and flash losses (WBF) from tank batteries. The program data input is obtained from the laboratory analysis of a pressurized liquid hydrocarbon sample and site-specific information. The sample is collected from the outlet of the final separator and upstream from the final (low) pressure drop (i.e. storage tank). The client has the option to have an additional sample extracted from the water line output from the separator to accurately identify emissions from the produced water tank (produced H20 tank emissions must be accounted for, when determining applicability to NSPS Quad O regulations). This software does not simulate WBF from produced water tanks. E&P Tank v3.0 interprets the information provided from the compositional lab analysis, field observational data (i.e. bulk temperature, separator pressure/temperature, tank characteristics, etc.), and client provided data (i.e. production rates) using complex equations of state and modeling to simulate WBF losses from storage vessels at an oil and gas site (OGS).

This test measures the storage vessels’ flash emissions.

Peng-Robinson (EOS) is best suited for modeling upstream operation emissions (i.e. tank batteries).

Gas-to-Water Ratio (GWR)

The GWR is an analysis conducted on a pressurized liquid (water) sample directly in a certified lab. The sample is collected in a spun cylinder straight from the separator water outlet, therefore pressurized to the conditions of the separator. The lab utilizes sophisticated gas chromatography equipment to flash the sample, capture the vapors, and identify the flash constituents. This method only accounts for flash losses; therefore working/breathing losses must be combined with flash losses when determining a facility’s total PTE. It is important to realize that not all hydrocarbons are removed from produced water in the stream separation process, and depending on site-specific conditions the VOC content released in a produced water tank can be significant. This method is considered a bona fide representation of flash emissions from produced water vessels (Emissions Representations for Produced Water, 2012). Produced water VOC emissions must be accounted for and reported to the appropriate state DEQ.

This test measures the storage vessels’ flash emissions.

Sampling Methods


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