Leak Prevention By Interstitial Monitoring

By Joie L. Folkers

Conveying hazardous fluids such as gasoline underground has inherent concerns for contamination of soil and water resources, as well as for public safety at the site, most commonly a service station.

Increasingly strict regulations have been enacted in most areas of the United States over the past 40 years to improve the protection of our environment, including equipment to measure the surrounding material to detect leaks and requirements for secondary containment of tanks and piping. While these improvements have had beneficial effects, the approach to the issue has been flawed. Rather than reacting to leaks that already have breached the system, technology exists that will react to any communication between layers of underground tanks and piping and prevent a leak from reaching the soil.

All underground equipment includes a primary carrier, i.e., single-wall tank or pipe, and most now have both a primary carrier and a secondary containment layer. The space between these layers is called the interstice and references to it use interstitial to describe it.

V-P-H Systems
The U. S. Patent Office has issued patents for a variety of designs intended to identify changes in the interstitial space between double-walled equipment. These systems generally can be categorized into three classes: Vacuum, Pressure or Hydrostatic (V-P-H).

Vacuum systems operate on the principle of drawing a vacuum on the interstitial space, then closing off the vacuum source to measure whether vacuum is lost over time. Pressure systems operate similarly in that air pressure is placed in the interstice, then the source is closed off and sensors watch for losses in pressure.

Vacuum and Pressure (compressed air) systems have inherent problems induced by ambient changes in temperature and barometric pressure. These systems must allow a change “threshold” in their operation to avoid false alarms. They measure pressure, whether negative (vacuum) or positive from one point in time to another and activate only if the difference exceeds the threshold level. If the change is small enough, the system ignores it and is allowed to replenish itself for the next “check-in” period. Small leaks that do not trigger the alarm can go undetected indefinitely.

Hydrostatic systems operate on the principle of monitoring a liquid level, typically brine or glycol, to avoid evaporation or freezing. These systems are much less sensitive to changes in pressure or temperature, as most liquids are incompressible and have low coefficients of thermal expansion. Hydrostatic systems are “charged” by filling the interstice with fluid to a level above the tank or piping and into a reservoir. The liquid level in the reservoir is monitored with very commonly used, well-proven sensors.

An example of the operation can be seen in monitoring of a double-wall pipe with the primary carrier filled with gasoline under pressure, and the interstitial space filled with brine. If there is a leak in the containment layer, the brine would escape from the containment, and the fluid level in the reservoir would go down. If there is a low-level sensor operating on the reservoir, the loss of fluid (not pressure, and that is very important) would be detected, and the system would go into alarm.

Similarly, if for any reason there may be communication between the primary carrier and the secondary containment layers, gasoline would flow into the interstice (since it is under pressure and the interstitial fluid is not) and cause the liquid level to go up. Inclusion of a high-level sensor on the reservoir would then notify the operator of this condition.

Significantly, the hydrostatic system described here will identify a leak regardless of the rate that the leak is occurring. Since the liquid level is being measured, the effects of the leak are cumulative, continuously compared relative to a starting condition. No leakage, regardless of the rate, would go undetected.

Reliability Rating
The U. S. Environmental Protection Agency (EPA) requires a 95 percent reliability rating on leak-sensing equipment. This is approached in two ways: If there is a leak, the system is to go into alarm 95 percent of the time (19 out of 20 occurrences); if there is an alarm, the system should be leaking 95 percent of the time. In other words, the sensors should not miss leaks and should not sound false alarms.

A note of concern was raised by the State Water Resources Control Board of California with regard to the above system. If there were simultaneous leaks in both the primary carrier and secondary containment layers of a pipe system, each leaking at the same rate, such a condition would cause the level monitor to miss the simultaneous leaks. While this is highly improbable, a solution has been developed. If the brine in the interstice is pressurized to a level higher than the gasoline in the primary carrier, communication through either layer would cause the reservoir level to go down. (Note: The system is still measuring fluid level, not pressure.) By this design, simultaneous leaks would reinforce each other, rather than offset. Problem solved. If it was necessary to identify which layer was communicating, the same system could be operated without interstitial pressure to see if the liquid level went up (primary layer breached) or down (containment layer integrity lost).

In February 2007, the EPA enacted guidelines mandating each state to require either Secondary Containment or Financial Responsibility for all sites. Part of the secondary containment requirements identified continuous monitoring. While the requirements do not go so far as to require a specific system, the requirement for reliability is intended by these guidelines.

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The PEI Journal • Fourth Quarter 2008 • Volume 2, No. 4 • Contents are Copyright © Data Key Communications, Inc.
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