Evaportative Emissions

Studies show that the evaporative emissions generated in flammable liquid fuel storage tanks are true, and that the volume lost is significant. The recovery of hydrocarbons (air-vapor mixture), without using a processor and using only a pressure and vacuum valve, can achieve up to 50% recovery of what would go to the atmosphere.

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In uncontrolled stations, that is, without any vapor recovery or retention system, air enters the tank through the vent with a volume equivalent to the liquid dispensed. Any refueling will introduce air into the tank, resulting in the creation of evaporative vapors (lean vapors). And on top of rebalancing, this air will generate significant emissions. Evaporative vapors are those that are created as gasoline undergoes a change from a liquid to a vapor phase. This occurs when the equilibrium vapor concentration in the space above the liquid is re-established. This vapor concentration occurs below natural levels by ingesting air into the tank during vehicle filling and truck unloading. when this natural equilibrium vapor concentration is momentarily reduced, liquid gasoline evaporates until the equilibrium concentration level is reached again. The volume of steam generated rapidly increases the internal pressure of the tank, with relatively small amounts of liquid evaporated.

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In some cases, the volume of air entering the vent is greater than the liquid displaced. In this case, the combination of the additional gas volume and the low concentration of hydrocarbons will result in rapid pressurization and vapor emissions to the atmosphere. The valve open pressure on the breather is limited by the allowable pressure of the tank - see NFPA 30. Generally this maximum allowable pressure value in a fuel tank is too low, and the valve opening pressure value should be below. The pressure in a tank is a consequence of filling and is a relatively slow process. The thermal effect vacuum is very fast.

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Pressure and vacuum valves in tank ventilation can produce efficient vapor recovery when it is kept in balance inside the tank, eliminating the complex and expensive vapor recovery system.

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One gallon of liquid gasoline expands approximately 520 gallons of steam at 40% of the hydrocarbon concentration. In the case of diesel, for example, by preventing the entry of uncontrolled air through the tank vent, we will be avoiding the diesel's self-hydration.