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Plastic gas tank venting; Expansion and Contraction


David Smid asked:

I have a gas tank with a manual vent. Should the vent always remain open? If the answer is yes, how does one keep rain and other moisture from getting into the tank. I have kept it closed and the tank will balloon in the heat and implode when it is cool. I try to keep the tank full to prevent this however, there are times when the tank is not filled to capacity. As you probably are aware, this is a plastic tank.


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The vent on your gas tank is necessary to prevent a vacuum block which would prevent fuel from being pumped from the tank to your motor. It needs to be open when operating your motor.

During storage, the vent must be kept closed to prevent evaporation and loss of your fuel as well as to prevent dangerous fumes from escaping, which could cause an explosion. This is especially true if your tank is stored in a locker or holding box. Gasoline vapors are heavier than air and can collect in a closed locker. When the right mixture of gas and air are present, any source of ignition can cause an explosion. With the right fuel air mixture, 1 gallon of gasoline has the explosion power of 20 sticks of dynamite. Although it’s difficult to get the right fuel, air mixture, it’s not impossible, so always err on the side of safety.

Plastic fuel tanks are designed and built to allow for expansion and contraction. That’s why fuel should only be carried, transported and stored in containers certified for that purpose. Even gasoline and diesel have different containers.

When you open the vent on an expanded tank, fuel vapors will escape. If the tank is sucked in, opening the vent will allow air into the tank to equalize the pressure. For safety reasons, opening the vent should be done off the boat on a level surface with good air circulation in the area and no ignition source present, even cell phones. Some boats will have explosion proof exhaust fans to vent the area prior to starting the engines and others will have proper ventilation built into the locker design. If in doubt, take the tank off the boat to open the vent and make sure that there are no fuel fumes on the boat prior to starting the motor.

Thanks for your question.

Captain Aurora
Richard Kittar

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