Which option is typically not a part of overfill prevention techniques for underground storage tanks?

Overfill prevention for underground storage tanks is a critical aspect of environmental protection and safety, aimed at preventing spills and leaks during the filling process. The correct answer, external venting, is typically not considered a direct method for preventing overfills.

Overfill alarms, secondary containment, and automatic shut-off valves are all specifically designed to mitigate the risk of overflow. Overfill alarms alert operators when a tank is nearing its capacity, allowing timely intervention. Secondary containment systems provide an additional layer of protection to capture any spills that may occur. Automatic shut-off valves can halt the filling process once a certain level is reached, directly preventing overflow.

In contrast, external venting is primarily involved in managing pressure within the tank and does not actively prevent overfilling. While it is important for allowing air to escape during the filling process and can help maintain safety by preventing pressure buildup, it does not serve to alert the operator of an impending overfill or to directly stop the filling process. Thus, it does not fall under the category of techniques specifically aimed at preventing overfills.