🔍 Key Concepts

• Tank ullage holes purpose and what can escape from them when a tank is not gas-free • How ignition sources outside a tank can be prevented from igniting vapors inside the tank • Difference between pressure/vacuum protection and flame arresting devices

💭 Think About

• Ask yourself: What is the main safety hazard at an open ullage hole on a tank that still contains flammable vapors? Is it overpressure, backflow, or flame propagation? • Which of the listed devices is specifically designed to stop a flame front from traveling through an opening, while still allowing gas to pass? • Think about where you have seen each option used: Which ones are usually on vent lines, which are on cargo systems, and which are purely informational?

✅ Before You Answer

• Verify which device is specifically intended to prevent flames from passing into or out of a tank opening while allowing gas/vapor flow. • Confirm that PV (pressure/vacuum) valves and stop-check valves are mainly for pressure or flow control, not for stopping flame fronts. • Remember that warning signs do not provide physical protection against ignition or flame travel—only information.