Overview
Ground water monitoring
Vapor monitoring
Secondary containment with interstitial monitoring
Automatic tank gauging systems
Tank tightness testing and inventory control
Manual tank gauging
Leak detection for underground suction piping
Leak detection for pressurized underground piping
Statistical inventory reconciliation
More information

Tank Tightness Testing and Inventory Control

Will I be in compliance?

When performed according to manufacturer's specifications, periodic tank tightness testing combined with monthly inventory control can temporarily (as described below) meet the Federal leak detection requirements for new and existing UST's. In addition, you should determine if State or local requirements have limitations on the use of these methods or requirements different from those presented below.

These two leak detection methods must be used together, because neither method alone meets the Federal requirements for leak detection for tanks. Tightness testing is also an option for underground piping, as described in the later sections on leak detection for piping.

Because they must be used together, both tank tightness testing and inventory control are discussed in this section. Tank tightness testing is discussed first, followed by inventory control.

How does it work?

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

Operation

Graphic of tank tightness monitoring system There are a few methods that do not measure the level or volume of the product. Instead, these methods use a principle such as acoustics to determine the physical presence of a hole in the tank. With such methods, all of the factors in the following may not apply.

Most tightness test methods are "volumetric" methods in which the change in product level or volume in a tank over several hours is measured very precisely (in milliliters or thousandths of an inch).

For most methods, changes in product temperature also must be measured very precisely (thousandths of a degree) concurrently with 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 temperature-induced volume changes) 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 6 to 12 hours, depending on the method.

Many test methods require that the product in the tank be at a certain level before testing, which often requires adding product from another tank on-site or purchasing additional product.

Variations

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.

There are several different acceptable ways to measure product temperature: mixing the product so it is all one temperature; using a sensor that calculates an average temperature by measuring temperature throughout the depth of the product; and using at least 3 temperature sensors at different product levels to calculate an average temperature.

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.

What are the regulatory requirements?

The tightness test method must be able to detect a leak at least as small as 0.1 gallon per hour. By December 1990, the tightness test method must also be able to meet the Federal regulatory requirements regarding probabilities of detection and false alarm. Tightness tests must be performed periodically as shown in the following table:

MINIMUM TESTING FREQUENCY
New tanks Every 5 years for 10 years following installation
Existing tanks, upgraded * Every 5 years for 10 years following upgrade
Existing tanks, not upgraded Every year until 1998
* Upgraded tanks have corrosion protection and spill/overfill prevention devices.

After the applicable time period listed above, you must have a monitoring method that can be performed at least once per month. See the other sections for allowable monthly monitoring options.

Will it work at my site?

Tank tightness testing has been used primarily on tanks less 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.

How much does it cost?

There are no capital costs for test equipment. The total cost per test is highly variable. The prices quoted by testing companies range from about $250 to $1,000 per tank, with most between $500 and $800. These prices are for a simple test with no problems. The final cost for a tank tightness test can be significantly higher. Some factors that would add to the cost of a test that you should ask about are:

Costs can be reduced if a large number of tanks are to be tested and if you check the tank and do the necessary repairs and replacements before the test crew arrives.