Oil storage tank underground Carson Dunlop Associates How Water Contamination Gets into Buried (Underground) Oil Tanks

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This article explains how water gets into buried oil tanks or underground oil storage tanks.

This article series explains the problems caused by water accumulation in oil tanks, how water gets into the oil tank, how to measure water in the oil tank, how to remove water from oil storage tanks regardless of whether the oil tank is indoors, outdoors above ground, or buried, and how to prevent water from getting into an oil storage tank.

We explain how to test for or visually check for water in a buried or above-ground oil storage tank, and how to get water out of an oil tank. Extensive free un-biased oil storage tank inspection and testing advice for property buyers and owners is provided at this website.

This website provides detailed information about underground (buried) oil storage tanks (USTs), aboveground oil storage tanks (ASTs), above ground fuel storage tanks, reporting and cleaning up oil tank leaks, and choosing among oil tank leak testing methods. The buried oil tank illustration (above) is provided courtesy of Carson Dunlop Associates.

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How Does Water Get Inside a Buried Heating Oil Storage Tank

Photograph of an oil tank filler flush with the ground.
  • Water enters a buried oil storage tank by condensation: as temperatures vary moisture-containing air may be drawn into and then out of an oil tank.

    Air leaves the tank as oil is consumed; air enters and leaves the oil tank through the oil tank vent. Moist air entering the tank from outside can bring water which, on entering the cooler tank interior, condenses out of vapor form into water droplets which can, over time accumulate.
  • Water can enter an underground oil tank through the filler pipe: from roof spillage onto the tank or filler top (particularly and obviously if the filler cap is left off), or from ground or surface runoff entering the oil storage tank (particularly and obviously if the filler cap is near, at, or below ground surface level).

    Leaving off an outside oil tank filler cap for a few days is not itself a likely source of a problem unless the filler was exposed to heavy rain, roof runoff spillage onto the open filler pipe, or surface runoff entering the tank (such as for a filler pipe flush with the ground).
  • Water leaks into a buried oil tank from an actual tank perforation that admits ground water, or from a bad plumbing fitting on the tank.

    When oil levels in the tank is below an oil tank perforation or a leak in oil tank piping, it is possible for ground water to leak into a buried oil tank just heating oil may leak out of the oil tank when it is filled above the perforation or leak point.
Oil tank fill cap below ground (C) Daniel Friedman
  • Water leaks into the buried oil tank from a faulty filler cap gasket around the oil tank filler pipe plug, especially at installations whose oil tank filler pipe is located in a box whose top is flush with the ground surface (see our photo, left).
  • Water leaks into the buried oil tank from an oil tank vent pipe that is missing a cap to protect against rainwater or roof runoff spillage into the tank, especially if the vent (or filler pipe) are located under the roof's drip-line.
  • Water leaks into a buried oil storage tank from leaky oil piping fittings. R.W. Beckett recommends that [Quoting]
    All pipe fittings must be tightened securely and have the threads sealed with a resilient compound that can endure the environmental variables in temperature. Any loose or poorly sealed [oil tank fuel piping, fill, or vent piping] joints can permit ground water to infiltrate the [oil] storage tank.
  • Water is delivered to the oil tank along with the heating oil fuel: This is not common, but it is possible to get a delivery of "bad" heating oil that is water contaminated, especially if the oil truck happens to fill-up at the oil storage depot when an oil barge is unloading oil since during that operation water which is normally kept in the bottom of oil depot storage tanks may be stirred-up.

    Most oil companies know to avoid this problem and some also have water filters installed at their oil trucks. No oil company is going to admit that they picked up and delivered water-contaminated oil to your home so don't waste time asking them if they are guilty of this crime.

    R.W. Beckett referred to this [quoting] "possible but rare contributor due to storage and maintenance practices in the [heating oil] distribution system. Large bulk [oil storage] tanks must be checked for condensation accumulation and bled periodically.

    If this is not done conscientiously, then it is possible for water to be drawn out with the oil when the stocks are extremely low or when the bulk storage is filled and the water is forced into temporary suspension.

    Fortunately, [the heating oil sales and delivery] industry has implemented effective maintenance measures and this has been virtually eliminated. However, if water and sludge are found in a tank that does not have a leak, then it must be thoroughly pumped out, cleaned, and treated with a chemical additive that disperses water, sludge and neutralizes bacterial growth."




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