Importance of Storage Tank Venting
With exception of Above ground Storage Tanks larger than 12,000 Gallons, which contain Class IIIB liquids that are located where they cannot be affected by a release of Class I or II liquids, IFC Section 5704.2.7.4 requires all Tanks Inside of Buildings to be equipped with a means of Emergency Venting. An emergency vent is a pressure relief device designed to protect your Above ground Fuel Storage Tank from being unpressurized beyond its design limits so it doesn’t rupture. The International Fire Code requires an emergency vent be installed and maintained per NFPA 30, Section 22.7.
When installing a tank installed inside of a building it introduces additional requirements for the tank’s emergency vent. The IFC prohibits discharge of an emergency vent inside a building. The reason for this provision is the emergency vent’s function. When it opens, it depressurizes the storage tank by relieving vapor generated by a fire. This vapor, if not properly discharged outside a building, adds vaporized fuel to an unwanted building fire. If enough is released before it finds an ignition source, it can result in a flash fire and create a vapor cloud explosion. This can cause severe damage to your fuel storage tank and building. Potentially injuring or killing occupants as well.
One issue that impacts the emergency vent design terminated outside a building is an addition of pipe and fittings that go beyond the outlet of the fuel storage tank. This can create a back pressure inside the tank. This back pressure results from friction of the vapor moving across the interior of the pipe and fittings. NFPA 30 Section 22.7.4 requires that your piping is extended more than 12-inches beyond the Above ground storage tank emergency vent opening and be evaluated for this pressure loss. It is routinely found that pipe and fitting diameters need to be increased beyond the diameter of the emergency vent opening to accommodate for this back pressure. The calculations are based on a derivative of the Darcy-Weisbach equation and as a result, the design of your emergency vent extension piping should be supervised by a petroleum engineer or contractor.
Because of the importance of emergency vents and the design challenges that arise for vents protecting Tanks Inside of Buildings, the 2012 International Fire Code was revised to permit the termination of the emergency vent inside the building where combustible liquids are stored in a protected above ground storage tank. A protected AST (aboveground storage tank) is defined in International Fire Code Section 202 as: A tank that is listed in accordance with UL-2085 consisting of a primary tank provided with protection from physical damage and fire-resistive protection from a high-intensity liquid fire exposure. Your tank may provide protection from the elements as a unit or assembly of components, or combination thereof. With Exception 2 of IFC Section 5704.2.7.4 which allows the emergency vent to be terminated inside the building when the tank is storing Class II or IIIA combustible liquids. For Class IIIB combustible liquids, the emergency vent has always been permitted to be terminated indoors.
The code was revised based on calculation of vapor pressure of ultra low sulfur diesel stored in a UL 2085 AST. As a condition of listing a protected aboveground storage tank, UL 2085 prohibits the thermocouple measuring the primary containment from exceeding a maximum temperature of 400°F. Vapor pressure calculations determined that at 400°F, the vapor pressure of the diesel is below the 2.5 PSIG opening pressure specified in NFPA 30, Section 220.127.116.11.1. Based on the fire-resistance and insulating quality of the materials used in the fabrication of protected ASTs, the emergency vent for these tanks storing Class II and IIIA liquids will not operate inside a building.
The change in Section 5704.2.7.4 was developed in response to a code change that was approved in 2009 IFC. IFC Section 603.3.1 was modified to allow increased quantities of fuel oil inside of a building without changing the occupancy to a Hazardous occupancy. The requirements in the 2009 IFC permit up to 3,000 gallons of fuel oil inside a building when:
- The fuel is stored in a Protected aboveground storage tank.
- The entire floor housing the Tanks Inside of Buildings is protected by a NFPA 13 compliant automatic sprinkler system.
- The fuel oil piping system is designed and constructed in accordance with the International Mechanical Code.
- The PAST is located not more than 2 stories below the building’s grade plane.
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