Secondary Containment Vacuum Monitoring

By Jay Walsh

Currently most new fueling installations in both the United States and other countries include a double-wall underground storage tank with connected double-wall pipe. Often, containment sumps for each product are monitored to detect the presence of liquid in the interstitial space. Further protection often includes leak detection for the primary line by way of either electronic via pressure transducer or a mechanical line leak detector. Even though each of these methods waits for a leak to develop versus actually preventing a leak to the environment, applying advanced protection schemes can be effective in signaling a leak in the underground piping. Where they fall short is in confirming the integrity of the interstitial space of the pipe, which is the key to preventing leaks.

For the European market, the majority of new fueling systems are required by regulation to be installed with double-wall pipes as well as continuous interstitial monitoring. In the United States, California has been requiring continuous interstitial monitoring of piping since July 1, 2004. Interstitial monitoring is being applied in other areas, including underground storage tanks. This article is focused on the benefits of vacuum monitoring, which prevents leaks in underground piping systems.

Leak Detection
When a leak develops in the primary underground piping, the systems installed today rely upon the interstitial space to move the liquid to a containment sump (at the submersible pump or under dispenser containment) where a liquid sensor may exist. This method of leak detection falls short when there is a problem with the integrity of the secondary. Liquid leaking from a glued joint in the primary for example, enters the interstitial of the pipe, and finds an exit to the environment through the void in the secondary, prior to accumulating in a containment sump where it can be detected.

Continuous vacuum monitoring of double-wall piping eliminates this possibility. Flaws in the secondary are signaled immediately as opposed to being detected by an annual test (or possibly not detected even annually, where annual testing is not required). All of this protection can be achieved in continuously active fueling systems. There are many methods of primary line leak detection, including electronic options which have a high level of sensitivity and precision. These systems monitor the pressure in the primary pipe when the system is inactive to determine that the pressure is holding (indicating no leak). The weakness is that they rely on a period of inactivity to determine if there is a leak. In today's busy convenience stores, systems are often continuously busy, making it impossible to make a leak assessment. With a continuously monitored secondary, there is no need for a period of inactivity. This enhances protection, as leak detection is done all the time, and improves compliance since many busy sites find it difficult to achieve passing line tests as required by current monthly/annual line requirements.

With continuous monitoring of the double-wall pipe, even vapor leaks are eliminated, due to the secondary being at a constant vacuum. With today's systems, leaks of a very small magnitude may not be detectable. Often, leaks start out small and get larger over time. Small leaks could cause a vapor-rich environment in the pipe that could seep into the backfill if there was a problem with the secondary integrity. This environmental contamination possibility is serious and avoidable with secondary containment vacuum monitoring.

How Vacuum Monitoring Works
Common systems in use today employ the submersible pump's vacuum capability to draw an initial vacuum in the piping's interstitial space. When the proper vacuum level is achieved (typically 5-8 inch Hg), a solenoid closes and the vacuum level is continuously monitored to ensure the integrity of the secondary. If there are changes in barometric pressure that cause the vacuum level to drop (not a leak), the monitoring system will “refresh” the vacuum level, again using the submersible pump's vacuum port. If a leak were to develop in the secondary, the Secondary Containment Monitor will quickly identify it through a continued loss of vacuum. Once it surpasses a leak threshold, an alarm is posted. It is conceivable that ground water could enter the interstitial of the pipe and not cause a loss of vacuum. In commonly available systems, this condition is immediately detected since the volume of the interstitial space gets smaller. Upon installation, the vacuum monitor “learns” the volume of the interstitial space. The groundwater ingress reduces the interstitial volume and will cause an immediate reduction in this space, and the appropriate alarm will be posted. Other systems rely on an additional liquid float sensor installed at the submersible pump vacuum port to signal the presence of liquid in the secondary.

Vacuum monitoring as a method of leak detection should be used in addition to primary line leak detection and sump sensors by station owners who are concerned with preventing leaks versus detecting them after they occur.

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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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