Underground tank sumps allow for access to equipment such as submersible pumps that are directly installed above underground tanks. The most important aspect of the sumps is that they do not leak either fuel to the environment or water to the inside of the sump. Tanks sumps are typically either fiberglass or HDPE plastic, and are circular or rectangular to resist soil loads. Sumps are typically 36” to 42” in diameter and 3’ to 6’ in depth. They connect to the tank either (a) by attachment to a collar which is integrally molded with the tank shell, (b) bolted to a tank manway, or (c) connected with bulkhead fittings through the sump floor to tank riser pipes. Alternative (a) has proven to have superior performance. In California, tank sumps are required to be double walled and continuously monitored. Monitoring of the sump shell is typically by hydrostatic or vacuum methods The most likely cause of water entry into tank sumps are (a) leaking piping penetration fittings, (b) leaking conduit penetration fittings or inflow through unsealed conduit, (c) leaking sump to tank seals, and (d) inflow through the sump led seal.
Fuel Tank Equipment
Manhole frames and covers are designed to protect tank sumps and risers from traffic loads above the tank. The important functional characteristics are (a) resist traffic loads by design to minimum H-20 traffic load, (b) transfer load to concrete pavement and isolate load from tank or sump, (c) direct surface water away from tank sumps, and (d) allow access to the tank by lifting and removing the cover. Proper installation of the manhole frame and cover is critical. The top of the frame should be set about 1-2” above the surrounding pavement elevation with transition to the pavement elevation over an approximate 3 feet radius. Since many drainage manhole frames are set low to collect water at a low point, it is important to indicate for the concrete contractors the importance of this requirement. Another aspect of the installation is to note the isolation of the frame from the tank sump by a minimum of 3”, so that if any settlement occurs, the frame will not bear on the tank sump.
Tank spill containers for underground tanks are designed to accept and spilled product during the disconnect of the fuel delivery hose from the tank. Spill containers range for 5-20 gallon capacity. The tank overfill prevention valve is installed through this same fill pipe connection to close automatically when the tank level reached 90% of capacity. Spill containers should be designed to exclude surface water. The primary detail is to install the frame at 1-2” above the finished concrete grade with a 2-3 feet transition to the slab elevation. In addition, advanced spill containers have a tight seal design to eliminate water inflows. These designs are either a side seal traffic rated cover, or a water tight lid design located beneath a separate traffic rated cover. Overfill prevention valves for underground tanks are designed for gravity delivery of fuel into the tanks. So they are rated at 15-25 PSI capacity. If fuel is to be pumped into the underground tank, then delivery pressures may be up to 100 PSI. A pressure rated overfill valve, as commonly used for aboveground tanks would be recommended in this case. It is highly recommended that manhole covers be constructed of lightweight composite materials so that personnel can remove the covers without injury. An alternative is a cover designed that is hinged with a spring assist.
Tank vents are required to maintain tanks at atmospheric pressure. Air flows into the tank as the fuel level is lowered, and it flows out of the tank when the tank is filled. Aboveground tanks also have air movement through the vent as temperature changes day to night and the fuel expands and contracts. Gasoline fuel tanks are required in many places to have breathing vents which retain vapors for vapor recovery systems. Diesel tanks for emergency generators do not have this requirement and are typically open to the atmosphere. The tank vent is required to disperse vapors that are discharged from the tank. And to exclude water, debris, or animals from entering the vent pipe. Vent pipes are typically required to terminate a minimum of 12 feet aboveground, and a minimum of 10 feet from building openings. In some instances, facility owners take special measures to prevent moisture in the air from entering the tank through the vent. A typical solution for this problem is the use of a desiccant cartridge or vessel installed within the vent pipe, to allow the desiccant material to absorb any moisture in the air. It is important for these systems to have secondary vent devices for both pressure and vacuum conditions to prevent damage to the tank, from a blocked desiccant vessel. Other tank vent accessories include flame arrestors. These are devices installed within the vent pipe on certain flammable liquid storage tanks. They are sometimes required for compliance with local rules on diesel fuel tanks for generators, especially where the vents are on a building roof. The flame arrestor is typically designed to allow air flow through a series of parallel plates that would break up a flame flowing through the vent. Tank vents are normally 2” in diameter for shop fabricated fuel tanks, both underground and aboveground. However where fuel is to be delivered to the tank by a fixed pump or delivery truck pump, then the vent pipe should be increased if need to be equal or greater to the fill line size.
Tank emergency vents are designed to operate during a fire condition to open and relive pressure buildup in a tank. Standard aboveground tanks are atmospheric tanks rated at 2.5 PSI maximum pressure. The normal breathing tank vent maintains the tank at atmospheric pressure by allowing air flow in and out as the liquid level in the tank changes. In a fire condition, fuel generates vapor as it increases in temperature, and the normal vent would be insufficient to allow discharge of these vapors, causing an increase in the pressure of the tank. Emergency vents are sized on the “wetted area” of the tank, that surface area that would be transferring heat into the stored fluid in a fire condition. Based on this surface area a required minimum vent size is determined. It is not necessary for the design engineer to size the emergency vent. The UL listing on the tank specifies the minimum emergency vent capacity required. The emergency vent device should be confirmed to provide that minimum capacity stamped on the tank by the UL requirement. A special circumstance can occur where a local regulation requires emergency vents to be routed to the building exterior. If the discharge point is considerably higher than the top of the tank, then an overfill condition will cause fluid to rise in the vent pipe and increase the liquid head pressure in the tank. In this circumstance it is important to have an additional safety measure of a high level liquid sensor in the base of the vent pipe to act as a fuel system shutdown.
Foot valves are vertical check valves installed into a tank to maintain the prime in the suction piping to a pump. Foot valves are either single or double poppet designed and double poppet designs are preferred. Expansion relief in fuel piping systems is an important consideration when foot valves are used. Many pumps have internal relief valves that relieve downstream fuel pressure back to the suction side of the pump. When foot valves are used, the pump internal relief is still valid for pump protection, however, the suction pipe will be blocked by the foot valve from relieving downstream pipe pressure. An independent path needs to be provided. He extractor valve mounts on the top of the tank to allow removal and inspection / maintenance of the foot valve from the fuel tank. Note that a foot valve extractor is different from a vent valve extractor that is sometimes used on underground tanks for ball float vent restrictors.
Aboveground tanks have equipment mounted on the top of the tank that will require inspection and maintenance. Also it is important to allow manual gauging of the tank contents prior to filling the tanks, as a safety measure in addition to electronic gauges. Smaller tanks may have a stairway at one end to allow filling of the tank from a top of tank connection. Most larger tanks over 2000 gallons have a ground mounted fill station. Access steps and ladders for aboveground tanks must meet the requirements of OSHA and other applicable safety regulations.
Some industrial installations provide full access to the top of the tank using stairs and walkway systems with handrails. This is not typical for commercial applications for emergency generators. Most commercial applications provide a ladder or stairway at one end of the tank, or better – a ladder / stairway and platform at one end of the tank. This allows for visual inspection of the tank equipment, and manual tank gauging at the tank opening adjacent to the tank end. If maintenance of equipment on the top of the tank is required, then mobile access and safety equipment can be setup to provide required safety measures.
Direct reading level gauges are provided for aboveground tanks as a check for tank level / volumes. When an electronic gauge is also used, the manual gauge provides a secondary check. The gauge face must be large enough to be read from ground level.
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