Photograph of  parts this leaking oil tank Oil Tank Leaks or Oil Tank & Tank Piping Failure
& Oil Leak Odor Causes

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Causes of oil storage tank leaks & failures:

This document explains the common reasons for both buried and above ground heating oil tank leaks. We explain the many causes of leaky oil tanks, and by those descriptions give suggestions on both where to look for leaks and how to prevent them.

Oil tank leaks are caused by corrosion, mechanical damage, soil conditions, and quite a few other factors which are explained here. We cite expert studies including one that indicates that the majority of oil tank storage system leaks occur in the piping system due to corroded threads and fittings rather than in perforation leaks of the oil tank itself.

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Oil Storage Tank Failure Mechanisms - What Causes Fuel or Heating Oil Storage Tank Leaks?

Outdoor oil tank problems (C) D FriedmanHere we describe the causes of oil storage tank leaks. In a companion artilce we also document the frequency or rates of occurrence of oil storage tanks & piping at OIL TANK FAILURE RATES.

[Click to enlarge any image]

Oil tank leaks, depending on the location and size of the leak, can lead to costly environmental contamination and cleanup costs outdoors or indoors in buildings. In addition, oil tank leak smells or fumes from indoor leaks or oil spills are a source of building occupant complaints that need to be addressed.

Photo of four oil tank lines coming off of a single outdoor oil storage tank is provided courtesy of Arlene Puenes.

External oil tank rust, unless very heavy, isn't highly correlated with internal rust, corrosion, and tank leaks.

Most oil tank failures are due to rust perforation from the inside of the tank. This means that if you see any indications of even a pinhole or leak on an oil tank, be careful! The steel may be quite thin and can be easily punctured even though from outside it may look pretty good.

Article Contents

Categories or Types of Causes of Leaks in Oil Storage Tanks Discussed Here

Here we will discuss the following categories of oil storage tank leak causation:

Oil Tank Rust Perforation: Usual Reasons for Oil Storage Tank "Bottom" Leaks

Underground fuel storage tanks that leak from an actual tank perforation (as opposed to a piping or fitting leak) usually fail from rust perforation from the inside of the tank, due to several effects of water inside the tank including, in the case of heating oil, combination of water with sulphur in the fuel.

So if a test shows that there is a lot of water in a buried oil tank one would be more pessimistic about its remaining life.

Water in home heating oil joins with sulphur in this case to become acidic and corrosive. It causes tank failure by rust penetration from the inside. Also, there may be a bacteria living in tanks, existing at the water/oil interface, digesting organics and excreting acids.

The corrosiveness of this activity is often most significant at the water-oil interface in the tank, which explains why some tank leaks will develop not at the very bottom of the tank (but look there too) but instead, a few inches up, along the side of the tank.

The height of this corrosion line along the sides of the inside of the oil storage tank depends on the amount of water in the tank and thus the location of the water/oil interface line on the side of the tank.

At REFERENCES we cite research on the role of bacteria and fungi in oil storage tank corrosion and leaks.


How Water Gets into Oil Storage Tanks & What Problems are Caused by Water in Heating Oil?

Oil tank water entry (C) A Puentes D FriedmanBad oil tank filler location

Water, enters a heating or fuel oil tank from a poorly sealed fill box which is flush with the ground or which is located below a roof edge, from missing fill pipe or vent pipe caps, from loose pipe fittings, and less commonly, from water delivered with fuel from an improperly maintained bulk storage facility.

Photo courtesy of Arlene Puenes.

Condensation inside oil tanks and groundwater leaks into oil tanks

In outside above-ground tanks water also often enters the fuel oil tank from condensation as temperatures change, particularly when the tank is not kept filled.

Above ground tanks in cold climates may be exposed to temperature variations drawing and expelling air from a partly-empty tank. When warm moist outdoor air is drawn into a steel tank, moisture in the air may condense and accumulate in the tank.

Water can also leak into a tank from ground water when the oil level is low if the tank is damaged. [R.W. Beckett Corporation, Technical Information Bulletin, October 15, 1990.]

Delivery of bad oil that contains water contaminants

Most oil delivery companies store their product in very large storage tanks near a rail or waterway delivery point. Large oil storage facilities may deliberately keep water in the bottom of those tanks so that if the large (above ground oil storage depot tank) begins to leak, water will leak out first and thus be observed before there is a large oil spill.

An oil truck which is filled when oil has just been delivered to the oil depot tank might in unusual circumstances pick up excessive amounts of water in the oil it obtains, thus potentially delivering it to its customers. Equipment is often used to attempt to trap and filter water in oil and other fuel delivery systems to reduce this risk.

How Improper Oil Tank Installation Later Leads to Leaks

Placing a tank in cinders or ash or acidic soil: tanks set in beds of cinders or ashes cause outside-in corrosion and leaks.

Tanks may also be damaged by being dropped or pushed into the excavated hole rather than being carefully lowered by a rope.

Burying an indoor-rated tank: That an installer would bury an indoor-rated 275-gallon tank outdoors may come as a surprise to some readers but I've been surprised at how often we've found either completely or partially buried indoor-tanks.

At least on older oil tanks (perhaps before 2000), the UL label affixed to the tank explicitly says "indoor use only" right there for the installer to read.

I contacted a few manufacturers and wrote to UL to ask if it was a building code violation to bury a tank with this indoor use rating. I haven't received a reply, but interestingly, newer 275-gallon steel oil storage tank UL rating labels simply dropped the wording that indicated where the tank could be used.

Our concern was that an indoor-rated tank may lack the thickness of steel or an anti-corrosion coating that a buried tank needs.

Placing an indoor-rated tank outside with no weather protection also risks water entry in the tank (from roof spillage or condensation), gelling of the oil and loss of heat in cold climates, and perhaps extra corrosion.

Mechanical damage to oil storage tanks

Oil tanks may be damaged during their installation, such as being dropped or gouged against a rock or other item which scratches or dents the tank, increasing its vulnerability to outside corrosion.

While it's speculative (we have no field data on this item) tanks or tank piping may be damaged by vehicle traffic in some locations as well.

Oil Storage Tank Manufacturing Defects as a Leak Source

Manufacturing defects such as defects in the coating of a steel tank to be buried or poorly-welded seams may result in underground oil leakage. We have not found significant reports of this occurrence - this may be a theoretical rather than a significant risk.

Does Type of Petroleum Product Stored Affect the Storage Tank Failure Rate?

We think the jury is out on this question, in part because the type of petroleum fuel stored in a tank may correlate with the UST location and with a concomitant exposure of the oil or other petroleum fuel storage tank to stresses of vibration, nearby vehicle traffic, frequency of re-filling, and other local environmental factors.

Nevertheless, Heck reported very interesting results that relate the type of petroleum product storage tank contents to the tank leak-failure rates. Heck concluded that "... systems containing heating oil for on-site consumptive use are no different than any other underground storage tank and do not experience a significant percentage of failures."

Table of Petroleum Product Storage Tank Leak Rate by Fuel Type

Tank Contents / Fuel Number Tested Number of Failures Failure Rate %
No. 2 home heating oil
Diesel fuel
K-1 (kerosene) fuel
Waste Oil Storage
Source: A Case Study of a Large Scale Precision [oil or fuel] Tank Testing Program", Diane H. Heck [3]

How Does Urban vs Rural Location Affect Underground Oil Storage Tank Leak Rates?

Heck observed that the majority of petroleum product storage tank failures (leaks) in her Marlyland UST study occurred in the Batlimore County area, a densly populated urban area around the city of Baltimore.

She suggested that both urbanization effects on USTs (listed below) and the local geological enviornment might be factors in UST leak rates, but she found that "By far, the major influences compromising [heating oil, gasoline, kerosene, or waste oil] [underground] tank ingegrity were those characteristic of urban areas... influences ... noticeably absent from most of the rural locations..." and she noted that [3]

The [UST oil, kerosene, gasoline storage] tank system failure rate was 50% in urban areas and only 25% in rural areas.

Urban factors impacting underground storage tank leak rates included:

How Soil Conditions & Chemistry are a Contributor to Underground Oil Storage Tank Leaks

Above at Geological environment Heck explained the role of soil conditions in the conduction of electrical currents that in turn contribute to corrosion at oil tanks or oil tank piping and fittings and she cites clay soil as more conductive (less resistant) to electrical currents.

In addition to the role of urban electrical currents in UST tank system corrosion and leaks, soil chemistry may play a role. If a soil is acidic and particularly in areas where there is also a lot of groundwater or surface or roof runoff around a buried oil tank, the corrosivitiy of the soil combined with its historic moisture level appear to be a factor in oil tank external corrosion and ultimate leakage.

How Improper, Damaged or Corroded Heating Oil Fill, Vent, or Distribution Piping can Cause Heating Oil Spills & Leaks

Oil tank piping materials or connections can lead to oil leaks. Heck reported that in her Maryland study 82% of petroleum product storage tank leaks occurred in the tank piping system. [3]

If you didn't know, residential oil tanks are usually filled under pressure. The oil filler nozzle is actually linked to the top of the inlet pipe and the truck pumps into the tank at pressure - most-likely to speed the delivery process. This fill-pressure can be considerable and can cause leakage or even catastrophic tank failure and leaks into a building if the tank piping is improper or if the tank is damaged.

Health Hazards of Exposure to Heating Oil Fumes

OSHA's position is that heating oil fumes are a nuisance and may not pose a hazard to a healthy individual.

Our field investigation experience as well as a review of oil spill guidelines from several sources provide anecdotal and other evidence that the elderly, infants, or people who are in fragile health, as well as people who suffer from multiple chemical sensitivity, allergies, asthma, and some other respiratory conditions may experience more serious symptoms including asthma attacks and other complaints.

Several U.S. states including the Connecticut department of health provides a fact sheet on home heating oil spills that includes the advice that homeowners should avoid both breathing heating oil fumes and skin contact with heating oil.

The Wisconsin Department of Health Services offers similar advice. The US ATSDR (Agency for Toxic Substances and Disease Registry) also provides a Public Health Statement for Fuel Oils and related documents including Heating Oil Exposure Health Effects and ATSDRs section on Heating Oil Chemical Properties.

A typical No. 2 home heating oil MSDS document includes the hazard identification information for home heating oil that we list below. The same document provides information about toxicity levels - the exposure necessary for serious medical effects to be at risk or to actually occur.

Exposure Limits for No. 2 Home Heating Oil

The following workplace exposure limit for heating oil is quoted from ATSDR.

The Occupational Safety and Health Administration (OSHA) and the Air Force Office of Safety and Health (AFOSH) regulate levels of petroleum products in the private sector and Air Force workplaces, respectively.

The maximum allowable amount of petroleum products in the workroom air during an 8-hour workday, 40-hour workweek, is 400 parts of petroleum distillates (naphtha) per million parts of air, or more simply stated, 400 ppm.

See OIL SPILL CLEANUP / PREVENTION for advice on how to prevent heating oil leaks & spills. See OIL TANK LEAK ADVICE for our detailed advice on handling leaky oil tanks as well as links to oil tank leak regulations for U.S. states and Canadian provinces. Readers should also see BOILER NOISE SMOKE ODORS for a discussion of flue gas leaks, smells, and hazards from the combustion products of oil burning heating appliances.


Continue reading at OIL TANK FAILURE RATES or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

If your oil tank is leaking see OIL TANK LEAK ADVICE.

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OIL TANK FAILURE CAUSES at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.


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