Underground Storage Tanks

When performed according to the manufacturer's specifications, periodic tank tightness testing combined with monthly inventory control can temporarily meet the federal leak detection requirements for tanks (this method does not detect piping leaks). Inventory control alone does NOT meet the federal requirements for leak detection for tanks.

Tightness testing (if conducted at least every 3 years) is also an option for underground piping.

Tank Tightness Testing

How does tank tightness testing work?

Tightness tests include a wide variety of methods. Other terms used for these methods include "precision," "volumetric," and "nonvolumetric" testing.

The features of tank tightness testing are:

• Many tightness test methods are "volumetric" methods in which the change in product level in a tank over several hours is measured very precisely (in milliliters or thousandths of an inch).
• Other methods use acoustics or tracer chemicals to determine the presence of a hole in the tank. With such methods, all of the factors in the following bullets may not apply.
• For most methods, changes in product temperature also must be measured very precisely (thousandths of a degree) at the same time as level measurements, because temperature changes cause volume changes that interfere with finding a leak.
• For most methods, a net decrease in product volume (subtracting out volume changes caused by temperature) over the time of the test indicates a leak.
• The testing equipment is temporarily installed in the tank, usually through the fill pipe.
• The tank must be taken out of service for the test, generally for several hours, depending on the method.
• Many test methods require that the product in the tank be a certain level before testing, which often requires adding product from another tank on-site or purchasing additional product.
• Some tightness test methods require all of the measurements and calculations to be made by hand by the tester. Other tightness test methods are highly automated. After the tester sets up the equipment, a computer controls the measurements and analysis.
• A few methods measure properties of the product that are independent of temperature, such as the mass of the product, and so do not need to measure product temperature.
• Some automatic tank gauging systems are capable of meeting the regulatory requirements for tank tightness testing and can be considered as an equivalent method.

What are the regulatory requirements for tank tightness testing?

• The tightness test method must be able to detect a leak at least as small as 0.1 gallon per hour with certain probabilities of detection and of false alarm.
• Tightness tests must be performed periodically. New UST systems--those installed after December 1988--must have tank tightness tests every 5 years for 10 years following installation. In most cases, existing UST systems--those installed before December 1988--that have spill, overfill, and corrosion protection must have tank tightness tests every 5 years for 10 years following upgrade. There are only a few cases requiring fewer tightness tests. Existing UST systems that have not been upgraded must have tank tightness tests annually until December 1998, after which these tanks must be upgraded, replaced, or closed.
• After the applicable time period noted above, you must have a monitoring method that can be performed at least once per month.

Anything else about tank tightness testing that you should consider?

• For most methods, the test is performed by a testing company. You just observe the test.
• Tank tightness testing has been used primarily on tanks no more than 15,000 gallons in capacity containing gasoline and diesel. If you are considering using tightness testing for larger tanks or products other than gasoline or diesel, discuss the method's applicability with the manufacturer's representative.
• Manifolded tanks generally should be disconnected and tested separately.
• Procedure and personnel, not equipment, are usually the most important factors in a successful tightness test. Therefore, well-trained and experienced testers are very important. Some states and local authorities have tester certification programs.

Inventory Control

How does inventory control work?

Inventory control requires daily measurements of tank contents and mathematical calculations that let you compare your "stick" inventory (what you've measured) to your "book" inventory (what ym your "stick" and "book" inventory is too large, your tank may be leaking.

OUST has produced a booklet available from our web site, Doing Inventory Control Right, that clearly explains how to do inventory control with simple step-by-step directions. The booklet also includes standard forms used to record inventory data.

The features of inventory control are:

• UST inventories are determined daily by using a gauge stick and the data is recorded on a form. The level on the gauge stick is converted to a volume of product in the tank using a calibration chart, which is often furnished by the UST manufacturer.
• The amounts of product delivered to and withdrawn from the UST each day are also recorded. At least once each month, the gauge stick data and the sales and delivery data are reconciled and the month's overage or shortage is determined. If the overage or shortage is greater than or equal to 1.0 percent of the tank's flow-through volume plus 130 gallons of product, the UST may be leaking.

What are the regulatory requirements for inventory control?

• Inventory control must be used in conjunction with periodic tank tightness tests.
• The gauge stick should be long enough to reach the bottom of the tank and marked so that the product level can be determined to the nearest one-eighth of an inch. A monthly measurement must be taken to identify any water at the bottom of the tank.
• Product dispensers must be calibrated to the local weights and measures standards.

Anything else about inventory control that you should consider?

• Inventory control is a practical, commonly used management tool that does not require closing down the tank operation for long periods.
• The accuracy of tank gauging can be greatly increased by spreading product-finding paste on the gauge stick before taking measurements (or by using in-tank product level monitoring devices).
• If your tank is not level, inventory control may need to be modified. You will need to get a corrected tank chart.

Time restrictions on the use of this combined method...

Existing UST systems those installed before December 1988 that have not been fully upgraded with spill, overfill, and corrosion protection must have tank tightness tests annually until December 1998, after which these tanks must be upgraded, replaced, or closed.

The combined method using tank tightness testing every 5 years is valid only after the entire UST system has met spill, overfill, and corrosion protection standards. Following entire UST system upgrade, this combined method may be used for 10 years (or until December 1998, whichever is later) after the date the tank was installed or upgraded with corrosion protection. Note that the end date is based on the compliance status of the tank only, not the entire UST system. As a result, some USTs may not be able to use this combined method for as long as 10 years (see discussion below). At the end of the valid time period, you must use one of the monthly monitoring leak detection choices described in this booklet.

Unique time restriction for some existing USTs...

For some existing USTs those which had corrosion protection before the entire UST system met upgrade standards this combined method of inventory control and tightness testing every 5 years may be valid for less than 10 years.

Federal regulations state that the combined method can be used:

1. until December 1998 or 10 years after the tank is protected from corrosion (whichever date is later), and
2. the period of validity cannot begin until the entire UST system meets upgrade standards.

Therefore, in those cases where the tank had corrosion protection before the UST system met upgrade standards, the period of validity is less than 10 years. The effect of this restriction will be clear in the following example: a bare steel tank upgraded with corrosion prote upgrades were not added system met years after the tank has corrosion protection). In this example, the UST may use the combined method to meet federal leak detection requirements only for three years (from 1995 to 1998).

Correspondingly, when the period of validity is less than 10 years, fewer periodic tightness tests may be required. Check with your implementing agency for guidance.

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