Storage Tanks
Underground

Both fiberglass and steel tanks are widely used for emergency generator applications. Both are built to UL standards and are inspected and certified by UL procedures. The steel tanks are typically a composite design that includes a fiberglass secondary containment that also provides corrosion protection.

Fiberglass tanks are manufactured by 2 primary US manufacturers: Xerxes Corporation, and Containment Solutions (the old Owens Corning brand). Steel tanks are manufactured by a relatively large number of manufacturers, typically local or regional firms, and typically members of the Steel Tank Institute. Modern Welding is the only national steel tank manufacturer, and Highland tank is a prominent manufacturer serving the East and Midwest.

In general, fiberglass manufacturers promote corrosion-proof designs, and steel tank manufacturers promote higher strength and chemical compatibility for future fuels.

The price comparison between fiberglass and steel tanks varies in time and place, based on oil chemical prices, steel prices, and freight prices. In general they are comparable.

1. Corrosion-proof materials. This is the traditional selling point since older steel tank designs failed because of corrosion. New composite designs of steel tanks have addressed this issue. However freedom from internal corrosion is still an advantage. Water from condensation accumulates in the bottom of tanks and can assist in the internal corrosion of steel tanks.
2. Interstitial Monitoring System. The hydrostatic method of monitoring fiberglass underground tanks is widely used, and recognized as a positive method of leak detection by California regulations.

1. Wide Availability. Steel tanks are widely available from various manufacturers that provide competing price and delivery.
2. Installation Resiliency. Steel tanks are more tolerant of variation in installation handling and backfill procedures.

The code requirements for underground tanks are typically 5 feet from building walls and 15 feet from property lines. Consideration should be given to the access for fuel delivery trucks to the tank area, and remote fill stations should be planned if needed.

The tanks may be located in paved areas, since properly installed they are not damaged from vehicle traffic. If located in unpaved areas or in areas of bituminous paving, it is typically important to install concrete collars around the fill point and manways. The concrete collars work to prevent damage to tank top equipment, and provide a barrier for small volumes of fuel that may be present from tank fill or maintenance operations.

Mechanical designers will typically locate the underground tanks in locations optimum for operational performance. This allows for a minimum of pipe length from the tanks to the building wall or point of use. However the excavation aspects of tank installations can have important impacts on the project costs and schedule, and these should be considered as well. The tank installation is often the deepest excavation associated with the construction, so it is worthwhile to do some preliminary planning. Some aspects of the tank excavation are as follows:

1. Large tank installations squeezed between buildings and property lines can restrict access to the building for construction operations.
2. Tank installations can impact building wall stability where the tank excavation is below the building footing wall elevation. Shoring, even where practical and cost effective, can increase the risk of building settlement. Buildings on adjacent properties can also be impacted in the same way.
3. Tank installations behind retaining walls can impact the stability of these structures if they are within the soil zone of influence for the wall footings.
4. Existing underground utilities in the tank area may require relocation or support, and the tank installation may increase the risk of utility service to the building.
5. A high water table at a proposed tank installation may require dewatering which can increase the installation costs, and increase the risk of ground settlement. Handling of dewatering discharges can also be problematic and may require special processing. 
6. A high rock elevation at a facility will increase the costs of excavation and tank installation. And the construction aspects of rock removal can cause vibration that is detrimental to adjacent buildings or utilities.

Most State regulations require that a specially licensed contractor install all underground fuel tanks and piping. The licensing is typically predicated on initial testing and periodic re-testing or training as a condition of maintaining the license. States may also require that the licensed contractors are covered by liability insurance for environmental claims. Tank. Piping, and monitoring equipment manufacturers also have training and certification requirements for the installers of their equipment.

Besides the licensing of firms, many States also require the licensing of individuals who must be present at all or at least critical points of the installation. Besides the licensing requirements familiarity with special materials, tools, and techniques is typically required for underground tank installation.

Installation of underground tanks requires permits from States and / or local regulatory agencies. This is the start of an extensive set of inspection and approval procedures that are required throughout the installation. Typical inspection requirements are:

1. Inspection of the tank unloading and setting. This sometimes includes holiday testing of steel tank coatings and inspection of hydrostatically charged tanks.
2. Pressure testing of the primary tank, and / or pressure or vacuum testing of the tank secondary space.
3. Pressure testing of primary fuel piping after installation and prior to backfill.
4. Pressure testing of secondary containment fuel piping after installation and prior to backfill.
5. Liquid tightness testing of tank sumps or vacuum testing of double wall sumps.
6. Electrical inspections of conduit and wiring.
7. Mechanical inspection of complete fuel system.
8. Testing of high level alarms for tank fill overfill protection. Testing of leak detection sensors. Testing of remote leak alarms annunciation and monitoring.
9. Final inspection of fuel system including all safety interlocks, such as pump shutdown upon leak detection and emergency stops.

All states regulate underground fuel tank installation and operation. The regulations have their basis in the federal underground tank rules that were first established in 1988 and phased in through 1998. The regulations include technical requirements for fuel tanks and piping as well as financial responsibility requirements. Technical requirements include: leak detection, corrosion priotection, and spill and overfill protection.

Many states have special technical requirements that are in addition to the standard regulations. California rules would be inclusive of most of the special requirements that can be encountered. Their regulations include:

1. Pre-qualification approval of all equipment and materials used in underground fuel tank systems.
2. Full secondary containment of all tanks and piping.
3. Fill pipes and spill container located within secondary containment sumps. All tank openings including tank gauging within secondary containment sumps.
4. Periodic testing of secondary containment piping, sumps, tanks, and tank monitors.
5. Double wall construction for tank sumps and piping transition sumps.
6. Double containment of vent piping.
7. Continuous positive monitoring of tank, pipe, and sump secondary containment by vacuum, pressure, or hydrostatic method.

Precision leak detection testing after installation and backfill using tracer gas method.

Most underground tank installations require the specification and installation of these devices:

1. Tank hold down straps. The straps are specified and provided by the tank manufacturer to resist bouyancy loads from groundwater.
2. Tank sumps and penetration seals for pipe and conduit. Tank sumps are typically fiberglass or steel. They are typically single wall except in California where they are double wall. The sumps are bonded to a collar on the tank shell that surrounds the tank openings or it is bolted to a tank manhole cover. Rubber assemblies of various designs are used to seal pipe and conduit penetrations into the sump.
3. Manway frames and covers protect the sump risers and other tank equipment risers. They are typically designed to resist vehicle loads and to transfer these loads to the surface concrete slab or manway collar. The manway frames and covers are isolated from the sump risers which are not themselves load bearing, and they also allow for minor settlement of the slab at grade without causing a load on the tank sump. Specialized manyway frames and covers incorporate fill pipe access and spill containers. Manway covers are commonly steel, however composite lightweight or hinged and lift-assist covers are increasingly used for ergonomic considerations.
4. Fill Point Adapters / caps, spill containers, and overfill prevention valves. Fill adapters allow for the connection and sealing of fuel delivery hoses. The fill connection point is located within a 5-15 gallon spill container which is typically a bellowed plastic construction with a water tight load bearing cover. The overfill prevention valve is a mechanical float type valve in the fill pipe within the tank. The float closes the fill pipe at a maximum 90% tank fluid level.
5. Leak sensors for tank interstice, tank sumps, and piping sumps, tank level transmitters, and sometimes electronic line leak detectors are installed at the tank and wired to the tank monitor control panel.
6. Submersible fuel pumps where used are installed through a 4 inch tank opening with a short riser pipe.
7. Suction systems typically use foot valves to maintain pump suction prime, and extractor fittings at the top of the tank for footvalve servicing.

The primary causes of failure for underground tanks have historically been (a) corrosion failures to the tank shell, or (b) leakage through top openings when the tank is overfilled. UST regulations enacted in 1988 addressed these common causes of failure, so that they are no longer prevalent, so the common problems have changed.

When installed in accordance with the manufacturers requirements, problems with tanks themselves are not common. The problems are more likely to occur with tank sumps, tank accessories, and piping.

1. Water intrusion into tank or interstice through improper pipe connections on top of tank.
2. Water intrusion into sump through (a) improperly installed or failed sump penetration fittings, (b) conduit that is not internally sealed, c) improper installation or failure of tank to sump seals, (d) surface water entry through top of sump where manways not raised to divert water, or backfill around sump is not open and free flowing.
3. Damage to tank or sump from surface loads where manway covers not properly isolated from tank sumps.
4. Flotation of tanks during installation because of inadequate ballast procedures.
5. Water intrusion at fill pipe spill containers and internal leakage of spill containers.
6. Piping leaks from (a) ground movements, (b) thermal changes, c) ground freeze / thaw, (d) high water table fluctuations, (e) surface loads, (f) ground settlement, (g) damage from construction

To minimize these problems:

1. Specify quality components.
2. Confirm that contractor submittals address the compatibility of tank sumps, sump penetration seals, and piping.
3. Confirm that contractor submittals address tank ballast calculations, along with procedures for ballasting tanks during construction.
4. Require installation and inspection checklists.
5. Require re-testing of fuel system components after backfill, when damage or movement is most likely to occur.

Require inspection and certification of system by contractor prior to end of warranty period.

Underground tanks have very few moving parts that would require maintenance and inspection. However, the components are continually under stress from ground movements, surface loads, and thermal changes. Even though these stresses are small, they are what cause the tank / sump / pipe containment systems to develop points of leakage.

Most States have enacted regulations for the regular, typically annual, inspection and testing of underground tank / sump / pipe containment systems. The inspections must typically be performed by certified tank installation contractors