YQA Fuel Storage Tanks in Buildings
4.01 What type of storage tank is used in a building?
Tanks inside buildings are steel tanks of UL 142 or UL 2085 fire rated construction. They are typically horizontal tanks, although vertical tanks are sometimes used because of the available dimensions of the room.
Tanks may be cylindrical or rectangular. Rectangular tanks are advantageous in that they are somewhat more compact. But more importantly their dimensions can be readily modified to fit through doorways, between columns, and into narrow rooms.
Tanks may be single wall or double wall. Single wall tanks are used where the room itself provides the required secondary containment capacity. Otherwise double wall tanks are more typical.
Fire-rated tanks are sometimes required by local fire marshalls. Often the fuel tank room is required to be constructed for 4 hour fire resistance, and the fire rating on the tank would be unnecessary. Also the greater weight of the fire rated tank might require additional structural characteristics, or special procedures during installation.
04.02 What is the largest tank capacity that can be used?
Fuel storage volumes inside buildings can range up to over 100,000 gallons – as the main storage on the lowest level of the building.
Fuel storage volumes – above the lowest level of the building are highly restricted. Local building codes may limit the volumes to 50, 250, 550 or other restricted volume. The limitation may be on the tank size or the total volume in the room. The importance of the limitation is to limit the size of the generator day tanks
04.03 How is fire protection provided?
The primary means of fire protection is the fire rated construction of the fuel storage rooms. Beyond this requirement the room would typically be sprinkler protected. In some cases foam fire suppression systems may be required by local code or fire marshall.
Fuel storage rooms with sprinklers, create the question of what happens to the water generated, since it may be mixed with fuel. Some codes address the issue by requiring a containment capacity for the fuel volumes plus the fire protection water, and this volume can be problematic. Other regulations may require that the discharge from the room be handled appropriately such as being directed to a remote holding area or underground tank
04.04 Should the room be ventilated?
The issue of ventilation in tank rooms will be addressed in local building codes. Typically the ventilation for a tank room is separated from the other ventilation systems for the building. The issues are these: (a) since people enter the tank room, ventilation is required, (b) since fuel is present in the room, air quality monitoring such as a hydrocarbon sensing is often required, c) since fuel vapors are heavier than air, exhaust systems need to be effective at the floor level, (d) if there is a fire in the room the ventilation should stop to prevent air from feeding the fire, and (e) since the exhaust air may have fuel vapors it should be separated from the other ventilation for the building.
04.05 Does the electrical work need to be explosion-proof?
Diesel fuel commonly used for generators is #2 Diesel which has a flash point of about 120 degrees F. This would classify diesel fuel in NFPA standards as a combustible and not a flammable liquid. Explosion proof electrical construction is required for flammable liquids, but not typically combustible liquids. So fuel rooms in buildings are typically ordinary electrical aras and not explosion proof.
However, local fire codes – or the facility insurance requirements, may specify that the fuel storage room is a hazardous electrical area. This classification may be either Class 1 / Division 1 meaning the conditions for fire / explosion are always present, or Class 1 / Division 2 meaning the conditions are present under upset conditions.
Class 1 / Division 2 Construction can be accomplished by (a) control panels and devices to be constructed of all C1D2 approved components, (b) explosion proof motors, (c) all contactors and spark potential devices located within NEMA 7 explosion proof enclosures, (d) control panels and devices to be constructed of all C1D2 approved components, (e) intrinsically safe or explosion-proof instrumentation and sensors, (f) rigid steel conduit with seals at the area boundaries.
Class 1 / Division 1 Construction would not allow item (a) above. Control panels would need to be in explosion-proof enclosures – or more commonly, located outside of the room in an ordinary electrical area.
Lighting, utility outlets, and other electrical devices in the rooms also need to be addressed for the hazardous electrical environment.
04.06 What is required for doorways and access?
Access and egress for fuel tank rooms typically has the same requirements as other rooms in the building. Rooms over a certain size may require 2 means of access.
The 4 hour fire rating for the room construction can sometimes be problematic, because standard door constructions may not be available in 4 hour rating. In some instances, doorways are constructed as double door entry to fulfill the requirement.
Another aspect of access egress is that the fuel storage room has a floor and lower wall space that provides secondary containment. Stairs are often required to allow access over the containment wall, and these steps are sometimes problematic to layout given the limited room dimensions and size of the tanks.
In some cases, the only entry into a tank room is through the roof / ceiling. Manway covers should be designed as hinged or lightweight construction. Ladders are sometimes required to be non-sparking construction.
04.07 Is a tank room an OSHA confined space?
A confined space is defined by OSHA as a space with limited or restricted means for entry or exit, and not designed for continuous employee occupancy. So properly lit and ventilated tank rooms with adequate access and egress may avoid the confined space designation.
A tank storage room may be constructed like a vault with access only through roof openings, and this type of room, would typically be considered a confined space. Because the room has fuel in it, the space would further be classified as a permit space with additional safety requirements for personnel entry.
04.08 How are tank vents terminated when tanks are located in buildings?
Tank vents are required to be located outside of buildings and typically above roof lines and minimum 5 feet from building openings. Additional considerations for the termination points are that (a) 5 feet would seem inadequate if the building opening in a ventilation air intake, and (b) 5 feet may be inadequate as a separation from the potential high heat of generator engine exhaust.
The normal breathing vent for a tank lets air in as the tank level deceases, and let sair out of the tank when the tank is filled. So it is pretty obvious that this vent should terminate outside because it will regularly discharge fuel vapors. Emergency vents are normally closed and would operate only in a fire conditions, so the need to terminate the emergency vent outside is sometimes debated.
The termination point for emergency vents is an important consideration. While a normal vent for a 10,000 gallon tank would be 2” piping, the emergency vent would be 10” piping, and the emergency vent for the secondary space would be another 10” pipe. In some places it would be required to terminate all of these vent pipes outside the building, and sometimes for architectural reasons this means through the roof of a 3-100 story building. It can be a highly significant cost issue, and also be a highly significant space and cost issue because of the riser space required. It is typically a local requirement and should be checked early in the design process. In some cases the vent piping is several times the cost of the fuel supply, fuel return, and vent piping.
Another aspect of the emergency vent termination is that if the tank is overfilled, and the fuel rises up through the vent piping, then the tank could experience a liquid head pressure that is beyond its design pressure, causing it to burst. Where vent piping is used for emergency vents, a vent pipe liquid sensor should be installed to function as an emergency stop.
04.09 How are tank accessories installed with low ceiling heights?
Tanks inside buildings are often installed in rooms with limited ceiling heights. This can be a problem for the installation of tank equipment. For example, there may be 3 feet of clearance above the tank and with a tank diameter of 8 feet a level transmitter installed into the top of the tank may be 10 feet in total length/
One solution is to provide floor openings and access doors in the floor slab above the tank, but in most cases this is impractical.
Level transmitters are available from several manufacturers in flexible models which can bend to be inserted into the tank. Drop tubes and suction pipes may be installed as segmental pipes which are joined as they are installed into the tank.
It is recommended that gauges and instruments be installed at the end of the tanks where they can be readily inspected and maintained without having to have a person gain access to the top of the tank.