Benzene releases from gasoline stations - Implications for human health: Mitigating measures

There are a number of mitigating measures for vapour releases at gasoline stations based on the Environmental Code of Practice issued by the Canadian Council of Ministers of the Environment (CCME 1991). These measures, which include Stage 1 vapour recovery (Figure 6), were designed to reduce the release of gasoline vapours from gasoline stations, thus lowering concentrations of volatile organic compounds (VOCs) and ozone in the lower atmosphere. Certain provinces and municipalities have enacted requirements for Stage 1 vapour recovery at gasoline stations. however, the majority of provinces and territories do not currently have any such requirements.

Another potential vapour mitigation measure at gasoline stations is the installation of pressure/vacuum (p/v) valves on the top of vent pipes. These valves maintain the underground storage tank over-pressure or under-pressure within set limits. In addition to reducing emissions during gasoline unloading, these valves also reduce evaporative losses from the storage tanks during daily gasoline station operations. The presence of p/v valves is not currently regulated in Canada, but their use in gasoline station storage tank vent pipes is mandated in the United States (USCFR 2021).

Figure 6. In the presence of Stage 1 vapour recovery in the tanker truck, as liquid gasoline is deposited into the storage tank, the vapour in the storage tank is predominantly drawn back into the delivery truck
Figure 6. Text version below. Figure 1 is a graphic showing gasoline being delivered by a tanker truck with the presence of Stage 1 vapour recovery.
Figure 6 - Text description

Figure 6 is a schematic representation of gasoline being delivered by a tanker truck with the presence of Stage 1 vapour recovery. The tanker has an evacuation tube at the bottom, from which it empties liquid gasoline into an underground storage tank. Gasoline vapour forms on top of the liquid gasoline in the tanker. There is pre-existing gasoline vapour in the underground storage tank, in addition to residual gasoline liquid. The top of the underground storage tank (where the gasoline vapour accumulates) is connected by a second tube to the top of the tanker truck (at a location where the gasoline vapour accumulates). The underground storage tank also has a vent pipe (at a location where the gasoline vapour accumulates) connecting it to the atmosphere.

As the tanker empties liquid into the underground tank, most of the gasoline vapour from the tank is directed back into the tanker through the Stage 1 vapour recovery tube. Some gasoline vapours may be released to the atmosphere through the vent pipe to balance the pressure inside the storage tank and the tanker truck.

Graphic: Washington State JLARC Report on Gas Vapor Regulations

It is estimated that the use of Stage 1 vapour recovery on a tanker truck during gasoline unloading, as shown in Figure 6, could capture 50% to 90% of the vapour released during the truck unloading event (CCME 1991; Statistics Canada 2012) and up to 99% if p/v valves are present on vent stacks (Statistics Canada 2012). The recovered vapours in the tanker truck are returned to the loading facility (for example, bulk plant or refinery) for processing/recovery.

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