Ball float valves should be able to restrict vapor flow when the tank reaches what percentage of its capacity?

• A. 70%
• B. 80%
• C. 90%
• D. 100%

Answer: (C)

Ball float valves control vapor venting by using a float that rises with the liquid level. As the tank nears full, the float seals more of the vent, restricting vapor flow to minimize emissions during topping off. The typical target is about 90% of capacity, leaving roughly 10% headspace for thermal expansion and to prevent liquid from blocking the vent. This point balances safety and environmental concerns: you reduce vapor release when the tank is nearly full, but still have some venting available if pressure or temperature changes require it. If the restriction happened earlier (around 70% or 80%), venting would be curtailed too soon, potentially causing pressure buildup or unnecessary venting later. If restriction occurred only at 100% capacity, there would be little protection against overflow and vapor loss at the critical near-full moment. That’s why 90% is the best threshold.