Table 11201 Major Variables Affecting Leak Detection

Variable

Impact

Temperature change Water table

Tank deformation

Vapor pockets

Product evaporation Piping leaks

Tank geometry

Wind

Vibration Noise

Equipment accuracy Operator error

Type of liquid stored

Power vibration Instrumentation limitation Atmospheric pressure

Tank inclination

Expansion or contraction of a tank and its contents can mask leak and/or leak rate.

Hydrostatic head and surface tension forces caused by groundwater may mask tank leaks partially or completely.

Changes or distortions of the tank due to changes in pressure or temperature can cause an apparent volume change when none exists.

Vapor pockets formed when the tank must be overfilled for testing can be released during a test or expand or contract from temperature and pressure changes and cause an apparent change in volume.

Product evaporation can cause a decrease in volume that must be accounted for during a test.

Leaks in piping can cause misleading results during a tank test because many test methods cannot differentiate between piping leaks and tank leaks.

Differences between the actual tank specifications and nominal manufacturer's specification can affect the accuracy of change in liquid volume calculations.

When fill pipes or vents are left open, wind can cause an irregular fluctuation of pressure on the surface of the liquid and/or a wave on the liquid-free surface that may affect test results.

Vibration can cause waves on the free surface of the liquid that can cause inaccurate test results.

Some nonvolumetric test methods are sound-sensitive, and sound vibrations can cause waves to affect volumetric test results.

Equipment accuracy can change with the environment (e.g., temperature and pressure).

The more complicated a test method, the greater the chance for operator error, such as not adequately sealing the tanks if required by the test method in use.

The physical properties of the liquid (including effects of possible contaminants) can affect the applicability or repeatability of a detection method (e.g., viscosity can affect the sound characteristics of leaks in acoustical leak-detection methods).

Power vibration can affect instrument readings.

Instruments must be operated within their design range or accuracy will decrease.

A change in this parameter has the greatest effect when vapor pockets are in the tank, particularly for leak-rate determination.

The volume change per unit of level change is different in an inclined tank than in a level one.

Source: Reprinted, from U.S. Environmental Protection Agency (EPA), 1986, Underground tank leak, detection methods: a state-of-the-art review (EPA, EPA 600-2-80-001, Washington, D.C.).

FIG. 11.20.2 Laser interferometer used to measure level changes. (Source: U.S. EPA.)
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