SLIDE 9 www.energyinst.org
Look-up tables
Component Potential for accidental discharge Pollution prevention control measures Further useful information Underground storage tanks (USTs) [typical historical single- skin systems and modern double- skin systems] Single-skin USTs Fuel leakage from USTs is largely due to corrosion of the single-skin steel variety without secondary containment in place, based on industry experience. A release will always be to ground for the single-skin USTs. For older UST installations without secondary containment and/or leak detection systems in place, the volume of a release can be significantly higher than for USTs conforming to current best practice guidelines. This is due to the likely time delay between the leak occurrence and detection of stock loss, which results in an extended leak duration, alongside the absence of secondary containment. Leak detection systems, such as monitoring wells, are sometimes used to monitor a leak should the UST fail. The outside of the external tank skin is typically polyurethane-coated to resist corrosion, and the coating is certified for consistency and uniform thickness by the tank manufacturer. Automatic Tank Gauge (ATG) systems provide accurate data for use in wet-stock management, with tank ullages, delivery and sales volumes often remotely monitored by specialist 3rd parties. Dynamic reconciliation or Statistical Inventory Reconciliation (SIR), based on ATG data, may also be used to further limit the volume of release and provide early warning of component failure. Tank construction: Double-skin tanks represent recognised good practice. Tanks constructed to standards set out within EN12285-1 are considered best practice due to the high level of quality assurance achieved through certification. The USTs should be tested following installation. Corrosion protection: Certified corrosion protection for applicable tanks is recognised good practice. Certified corrosion protection with cathodic protection is considered best practice. Note GRP tanks are not susceptible to corrosion. Corrosion protection does not improve the risk management, but may extend the life of the tank. Leak containment: Leak containment class 3 is considered good practice (vapour/hydrocarbon sensors monitoring interstitial space). Class 1 or 2 considered best practice (interstitial monitoring using liquid, gas or pressure) as detects breach in either inner or outer tank. Leak detection systems: Leak detection class 4a/b/c (approved ATG with inventory reconciliation) considered good practice. Leak detection class 6a/b/c (approved ATG with statistical inventory reconciliation) considered best practice, potentially supplemented by leak detection class 5 (monitoring wells with discriminating liquid/vapour sensors). See sections 8.2, 8.3, 8.6, 11.2-11.6 (EI and APEA 2011) and wetstock monitoring under site operation and maintenance. Double-skin USTs Double-skin USTs have significantly lower probability of releasing fuel to ground as any leaks will be identified by the leak detection system(s) and then adequately contained whilst the source of the leak is rectified. Double-skin tanks are used to significantly reduce the probability of releases to ground, with the second skin acting as secondary containment. The interstitial space between tank skins can be continuously monitored by a proprietary leak detection system.
PFS Training pack