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Domed External Floating Roof Tank next to Vertical Low Pressure Tank.

Petroleum Storage Tank Facilities – Part 1

In today’s fire service we tend to focus our pre-planning and training for the “bread and butter” operations that we encounter daily.

These ‘bread and butter’ operations are generally single and double residential structure fires, multiple family structure fires, hi-rise commercial and office fires, and assorted emergencies that we encounter frequently. We are creatures of habit and feel most comfortable with what we respond to most frequently. While these incidents are usually high frequency/low to medium risk incidents, this tends to be our focus. We are sure that most jurisdictions have industrial and commercial facilities that the firefighters pass frequently without giving them a passing glance. It is fires and emergencies at these facilities that we classify as low frequency/high risk events.

One type of facility that we would like to focus on in this series of articles is the petroleum storage tank facility. We rarely hear of a storage tank fire today, because the industry has improved on the design, construction, and fire protection requirements for these facilities. It is interesting to note that over the years the number of fires in storage tank facilities has decreased, but the actual tank sizes have increased. The larger tank sizes actually increase the hazard. A fire in these larger tanks can be extremely disruptive to business continuity, costly in terms of property damage, create environmental issues, affect interstate commerce and create negative public opinion. When a storage tank fire involves the full surface of the product contained within the tank (hereafter called ‘full surface fire’), it will require a large commitment of resources, both equipment and human, and will require an extensive logistics structure. On a positive note, the improvements to the various codes and standards developed and maintained by the American Petroleum Institute (API) and the National Fire Protection Association (NFPA), along with proper enforcement of these standards and codes by the authorities having jurisdiction, have reduced these incidents to the lowest levels in decades.

The above mentioned bulk storage tank facilities can be located almost anywhere, from large refinery and petro-chemical plants to smaller bulk storage plants with loading terminals. In between we may have large marine terminals and smaller ethanol refining facilities. In this series of articles we will focus on the types of storage tanks, fire suppression systems, firefighting operations, and pre-incident response planning.

Types of Storage Tanks
In this article we will focus on above ground atmospheric petroleum storage tanks. These tanks range from diameters of 3.048 meters (10 feet) to over 106.68 meters (350 feet). In some instances, there are tanks exceeding 121.92 meters (400 feet). Average heights for above-ground tanks are 13.72 meters (45 feet). Tanks may be in individual dike areas or may have multiple tanks within one dike. Dikes, or bunds as they are also referred to are physical barriers or dividers used to prevent the spread of tank contents in the event of a tank overflow or tank rupture.

There are several types of above ground atmospheric storage tanks. For this article we concentrate on the following types, typically found at bulk storage facilities:

  • Cone roof and dome roof tanks
  • Open top floating roof tanks
  • Covered floating roof tanks including geodesic domes
  • Vertical low-pressure storage tanks
  • Horizontal Storage Tanks

The types of tanks used to store flammable and combustible products are generally determined by the physical characteristics of the product being stored, however, this may not always be the case. There have been instances where products have been stored in tanks not intended for the particular product.

Cone or Dome Roof Tanks
Cone roof and dome roof tanks are similar with the difference being the shape of the roof. Cone roofs have a cone shape, but depending on the slope of the roof, the cone shape may not be evident from the ground. These tanks will have a vapor space between the product surface and the underside of the roof. If this vapor space is in the explosive range and an ignition source is introduced, an explosion will occur. Generally, these tanks are used to store liquids with a flashpoint of 37.8 degrees C (100 degrees F) or higher, however, there have been instances where liquids with lower flashpoints have been stored in such tanks and the vapor space has ignited.

These tanks are equipped with a pressure/vacuum relief device to allow the internal pressure to nearly equal the external atmospheric pressure. They may also have open vents. These devices allow the tank to “breathe” during loading, unloading, and extreme changes in temperature resulting in a change of the pressure in the vapor space. Cone and dome roof tanks will also have a weak roof-to-shell seam. In the event of an incident such as internal overpressure from an explosion or similar incident, the roof will separate from the vertical shell, thus preventing the failure of the bottom seams and a resultant tank rocketing event. Flame arrestors may also be found to prevent the introduction of a spark to the vapor space through vents or pressure/vacuum relief devices.

Cone Roof Tanks – note the difference in roof shapes.

Cone Roof Tanks – note the difference in roof shapes.

Open Top (External) Floating Roof Tanks
Open top floating roof tanks are vertical steel cylinders with a roof that floats on the surface of the liquid in the tank but it is open to the atmosphere above. The roof moves up and down inside the tank shell with the product. This floating roof’s advantage is that there is no vapor space between the liquid and the roof as in a cone roof tank. These roofs float on pontoons or have a double-deck for floatation on the liquid’s surface.

These tanks can be distinguished from a cone roof tank by the presence of a wind girder that rings the top of the tank. The wind girder acts as a stiffening ring for the top of the tank, giving it additional structural support. Between the shell of the tank and the roof edge, a rim seal will be provided to prevent vapors from escaping to the outer air. The rim seal area is considered to be the space between the tank shell wall and the floating roof edge. This distance may be 0.30 meters to 1.21 meters (1 foot to 4 feet). Open top floating roofs generally carry low flash point liquids which have high vapor pressures.

While the roofs of these tanks are designed for carrying a specific live load plus additional loads created by rain and snow, they can fail if the load exceeds the designed limits. To prevent the excessive load, the roofs are designed with a drainage system to remove normal rain water from the roof to the ground where it can be collected inside the dike area. In the event the drain system fails, or is overwhelmed by severe weather, the load can partially or fully sink the roof. When this happens the product is exposed to the atmosphere and vapors are released, subjecting them to possible ignition.

Open Top Floating Roof Tank – note the presence of the wind girder just under the open top.

Open Top Floating Roof Tank – note the presence of the wind girder just under the open top.

Covered (Internal) Floating Roof Tanks
These tanks exhibit the same basic construction features as the open top floating roof tanks but with the added feature of a fixed roof at the top of the tank. The fixed roof may be self supporting or may have vertical supports within the tank. These tanks also have a rim seal to prevent the escape of vapors from the liquid. The fixed roofs of these tanks are freely vented with the expectation that any vapors in the space above the floating roof will be below the flammable limit.

Covered floating roof tanks have distinguishing “eyebrow” vents at the top of the tank shell. These vents allow air to escape and enter the inside space between the fixed roof and the internal floating roof as it moves up and down inside the tank shell.

Covered Floating Roof Tank – note the ‘eyebrow’ vents near the tank top.

Covered Floating Roof Tank – note the ‘eyebrow’ vents near the tank top.

Domed External Floating Roofs
Domed external floating roof tanks are similar to covered floating roof tanks but instead of a steel roof, a much lighter roof structure is installed on an existing open top floating roof tank. These roofs are often referred to as geodesic dome tanks. The dome serves to provide a barrier to the wind and rain and may also provide environmental control with respect to fugitive emissions.

Vertical Low Pressure Storage Tanks
These tanks have relatively simple features. They are cylindrically shaped with a top and bottom. They will have some form of pressure/vacuum device. These tanks are generally smaller than cone roof tanks and generally used in process areas or specialty storage areas.

Horizontal Storage Tanks
Above ground horizontal storage tanks are normally smaller capacities, 151,400 liters (40,000 gallons) or less and are used primarily for storing flammable and combustible liquids.

Locations of Storage Tanks
The above-mentioned storage tanks can be found at many locations within fire districts. Locations that are most common are refineries, petro-chemical facilities, bulk storage plants, airports and marine terminals. This list is not all inclusive and many smaller facilities may have storage of flammable and combustible liquids. This series of articles focuses on larger facilities where the number and spacing of tanks, or the volume of the tanks creates a severe fire hazard.

Too many times these facilities with their tanks become just “part of the landscape” within our districts. As mentioned previously the fire service tends to focus on our bread and butter operations, and ignore the low frequency events. It is imperative that we notice these facilities, pre-plan them, and learn as much as we can about the facility, product, processes, and the fire protection or lack thereof in a facility.

Fire Hazards
The following are some of the hazards associated with the various types of storage tanks:

Fixed (Cone Roof) Tanks

  • Vent fire
  • Overfill ground fire
  • Unobstructed/obstructed full surface fire

Open Floating Roof Tanks

  • Rim seal fire
  • Overfill ground fire
  • Unobstructed/obstructed full surface fire

Internal (Covered) Floating Roof Tanks

  • Vent fire
  • Overfill ground fire
  • Obstructed rim seal fire
  • Obstructed full surface fire

Domed External Floating Roof Tanks

  • Vent fire
  • Overfill ground fire
  • Obstructed rim seal fire
  • Obstructed full surface fire

Vertical Low Pressure Tanks

  • Vent fire
  • Overfill ground fire
  • Obstructed full surface
  • Tank explosion and failure preceded by ground fire
  • Tank explosion and failure with resulting ground fire

Horizontal Tanks

  • Vent fire
  • Overfill ground fire
  • Tank explosion and failure preceded by ground fire
  • Tank explosion and failure with resulting ground fire

The above listings may occur alone or in combination with each other. For instance, you may have a full surface fire in a tank and at the same time have a ground fire in the dike area. Our next article will focus on the types of fires in depth and the fire protection methods available followed by a third article which will give firefighting strategies and tactics as well as pre-incident planning guidelines.

For more information, go to www.worldsafeinternational.com

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Craig is a 45-year veteran of the fire service. He served with the FDNY for 26 years retiring as the Chief of Marine Operations. Currently, Craig is an Assistant Chief with Industrial Emergency Services (IES) and Manager of Marine Operations. In addition, he is the CEO of World Safe International.