A fairly common debate I have with folks is the “normal venting” and “emergency venting” requirements for “Inside Flammable Liquid Storage Tanks.” The bottom line…
These tanks MUST be vented outside the building for normal and emergency venting, except for Class IIIB liquids.
Tanks Inside Buildings (TINBIDS) requirements in Chapter 57 of the IFC become applicable when any Atmospheric Aboveground Storage Tank (AST) containing Class I, II or IIIA liquids is installed INDOORS.
Normal Venting
In addition to the requirements for tank construction, volume limits, and overfill protection, the IFC has requirements for terminating a Tank Inside Buildings (TINBIDS) normal vent and emergency vent. The purpose of the normal vent is to maintain the pressure inside the tank when liquids are
- introduced into or
- withdrawn from the tank
All storage tanks must be designed to handle the vacuum and pressure generated when liquid is introduced into or withdrawn. Improperly sizing a tank’s normal vent or obstruction of the vent can cause excessive negative pressure to generate inside the tank, causing the tank to collapse. IFC Section 5704.2.7.3 has a number of provisions to ensure the normal vents are properly terminated. For Class I, II, and IIIA liquids, the IFC requires the termination of the normal vent outside the building. It MUST be located at least 12′ above the finished ground level and at least 5′ from building openings and lot lines that can be built upon. The 12′ elevation of the vent is necessary to ensure that the surrounding air mixes with the vapor being exhausted from the tank so the atmosphere is maintained below 25% of the flammable liquid’s Lower Explosive Limit (LEL).
Pressure/Vacuum vents are required by IFC Section 5704.2.7.3.2 on the normal vents of Tanks Inside Buildings (TINBIDS) containing Class IB or IC liquids to limit the potential release of flammable vapors. A Pressure/Vacuum vent only operates when the product is withdrawn or added to the tank. In place of a Pressure/Vacuum vent, the IFC now permits the installation of an in-line flame arrestor designed and installed per API 2028.
Emergency Venting
With the exception of an Atmospheric Aboveground Storage Tank (AST) that is:
- LARGER THAN 12,000 gallons
- contains a Class IIIB flammable liquids
- located so it can NOT be affected by a release of Class I or II liquids
the IFC, Section 5704.2.7.4, requires ALL Tanks Inside Buildings (TINBIDS) to be equipped with a means of emergency venting. Emergency venting is a pressure relief device designed to protect the tank from being over-pressurized beyond its design limits so it does not rupture. The IFC requires the emergency vent to be installed and maintained in accordance with NFPA 30, Section 22.7, Emergency Relief Venting for Fire Exposure for Aboveground Storage Tanks.
Installation of Tanks Inside Buildings (TINBIDS) introduces additional requirements for the tank’s emergency vent. The IFC PROHIBITS the discharge of an emergency vent inside a building. The primary reason for this provision is the emergency vent’s function. When an emergency vent opens, it depressurizes the storage tank by relieving vapor generated by a fire. If not discharged outside the building, this vapor could add vaporized fuel to an unwanted building fire. If enough vapor is released before it finds a fire or ignition source, the resulting flash fire could create a vapor cloud explosion, causing severe damage to the tank and building and potentially injuring or killing building occupants.
An issue that impacts the design of emergency venting systems terminated outside a building is that the addition of pipe and fittings beyond the outlet of the storage tank can create backpressure inside the tank. This back pressure results from the friction of the liquid-vapor moving across the interior of the pipe and fittings.
NFPA 30 Section 22.7.4, Extension of Emergency Vent Piping, requires that piping extended more than 12 inches beyond the Atmospheric Aboveground Storage Tank (AST) emergency vent opening be evaluated for this pressure loss. Analysis routinely finds that the pipe and fitting diameters may need to be increased beyond the diameter of the tank’s emergency vent opening to accommodate this backpressure.
Because of the importance of emergency vents and the additional design challenges that arise for vents protecting Tanks Inside Buildings (TINBIDS), the IFC was revised to permit the termination of the emergency vent inside the building when combustible liquids are stored in PROTECTED aboveground storage tanks. A protected Atmospheric Aboveground Storage Tank (AST) is defined in IFC Section 202 as:
A tank 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 pool fire exposure. The tank may provide protection elements as a unit or may be an assembly of components, or a combination thereof.
Exception 2 of IFC Section 5704.2.7.4 states:
Emergency vents on PROTECTED Atmospheric Aboveground Storage Tank (AST) complying with UL 2085 containing Class II or IIIA liquids are ALLOWED to discharge inside the building.
NOTE: A UL 2085 tank is an aboveground, shop-fabricated, secondarily contained vessel designed to store flammable and combustible liquids and constructed according to Underwriters Laboratories Standard 2085. It is intended to limit the heat transferred to the primary tank during a fire and protect the tank from physical damage.
For Class IIIB combustible liquids, the emergency vent can be discharged indoors.
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 a building without changing the occupancy to a “Hazardous Occupancy.” The requirements in the IFC permit up to 3,000 gallons of fuel oil inside a building when:
- The fuel oil is stored in a PROTECTED Atmospheric Aboveground Storage Tank (AST) ,
- The entire floor housing the TINBID is protected by an automatic sprinkler system meeting NFPA 13,
- The fuel oil piping system is designed and constructed in accordance with the International Mechanical Code and
- The PROTECTED Atmospheric Aboveground Storage Tank (AST) is no more than two stories below the building’s grade plane.