Enhanced Fuel Tank Venting: A Comprehensive Guide

Enhanced fuel tank venting is a critical aspect of managing fuel storage systems, particularly for facilities that handle flammable liquids. The primary objectives of enhanced fuel tank venting are to ensure safety, prevent overfilling, and accurately quantify venting and fugitive emissions.

Venting and Fugitive Emissions Estimation

Guideline PNG035 v.1 provides a comprehensive approach to estimating venting and fugitive emissions from storage tanks. To ensure accuracy, site- and equipment-specific data should be used when quantifying vent gas. This includes:

• Detailed tank specifications, such as diameter, height, and volume
• Fuel properties, including vapor pressure, density, and molecular weight
• Environmental conditions, such as ambient temperature and wind speed
• Operating parameters, including fill rates, withdrawal rates, and tank turnover

The guideline outlines various calculation methods, including the use of emission factors, mass balance equations, and computer models. These techniques allow for the estimation of both working losses (during filling and emptying) and standing losses (due to temperature changes and other factors).

Tank Gauging

Tank gauging is the measurement of liquid levels in large storage tanks, with the primary purpose of quantifying the volume and mass of the product. Accurate tank gauging is essential for managing fuel inventories and preventing overfilling.

One advanced technology used in tank gauging is radar level gauges, which are commonly employed in full containment tanks for cryogenic or refrigerated liquefied gas. These gauges can measure liquid levels accurately even under surface boiling conditions, ensuring reliable data for enhanced fuel tank venting applications.

Key specifications for tank gauging systems include:

• Measurement range: Typically 0 to 80 meters, with an accuracy of ±2 mm
• Response time: Less than 1 second for level changes
• Operating temperature: -196°C to +450°C, depending on the application
• Explosion protection: Suitable for use in hazardous areas (e.g., ATEX, IECEx, or NEC certifications)

Tank Pressure Measurement

Measuring the gas pressure in the ullage (headspace) of the storage tank is crucial for assessing vent emission patterns. Releases can occur when the pressure exceeds the cracking pressure of the pressure/vacuum (P/V) valve in the vent pipe.

To capture these pressure dynamics, the tank pressure is measured every 4 seconds, and 2-minute average values are stored. This high-resolution data allows for a detailed analysis of the pressure fluctuations and their correlation with vent emissions.

Typical tank pressure measurement systems include:

• Pressure transmitters with a range of 0 to 100 kPa (0 to 14.5 psi)
• Accuracy of ±0.1% of the full-scale range
• Response time of less than 100 milliseconds
• Explosion-proof housing for use in hazardous areas

Vent Emission Measurements

To quantify the evaporative fuel releases through the vent pipe of a storage tank, the volumetric flow of the mixture of gasoline vapor and air is measured directly in the vent pipe. This is typically done using a dry gas diaphragm flow meter, which measures the gas flow rate.

The time-dependent volumetric flow rate of the gasoline vapor/air mixture through the vent pipe is then calculated by integrating the time series of measured flow volumes over an averaging period and dividing by the duration of that period. This provides a detailed understanding of the vent emission patterns and magnitudes.

Key specifications for vent emission measurement systems include:

• Flow rate range: Typically 0 to 100 m³/h, with an accuracy of ±1% of the reading
• Operating temperature: -40°C to +60°C
• Explosion protection: Suitable for use in hazardous areas (e.g., ATEX, IECEx, or NEC certifications)
• Data logging: Continuous recording of flow rate, pressure, and temperature data

Correlation between Tank Pressure and Vent Flow Rate

Studies have shown that there is a correlation between the tank pressure and the vent flow rate. Spearman correlation coefficients between these two parameters were found to be moderately and strongly correlated, respectively, for two gas stations studied.

This relationship highlights the importance of simultaneously monitoring both tank pressure and vent flow rate to gain a comprehensive understanding of the fuel tank venting dynamics. By analyzing this correlation, operators can better predict vent emissions and implement more effective mitigation strategies.

Conclusion

Enhanced fuel tank venting is a complex and multifaceted process that requires the integration of various measurement technologies and data analysis techniques. By accurately quantifying tank pressure, vent flow rates, and fuel deliveries, operators can ensure the safety and environmental compliance of their fuel storage systems, while also gaining valuable insights into the venting and fugitive emissions patterns.

This comprehensive guide has provided a detailed overview of the key aspects of enhanced fuel tank venting, including venting and fugitive emissions estimation, tank gauging, pressure measurement, vent emission measurements, and the correlation between tank pressure and vent flow rate. By understanding and implementing these best practices, facility managers can optimize their fuel storage operations and contribute to a safer and more sustainable environment.

References

1. Guideline PNG035 – Estimating Venting and Fugitive Emissions v.1
2. The Engineer’s Guide to Tank Gauging | Emerson
3. 40 CFR Part 79 — Registration of Fuels and Fuel Additives – eCFR
4. Vapor pressure build up in fuel tank – ThirdGen.Org
5. Vent Pipe Emissions from Storage Tanks at Gas Stations – NCBI