Fire Detection for Tank Storage Farms
Industry | Floating Roof Tanks​
Flammable and combustible liquid storage tanks are found in industrial facilities such as refineries, petrochemical facilities, bulk storage plants and marine terminals. Power plants, airports, local fuel companies and large manufacturing facilities, such as automotive and steel plants, may also have bulk storage of flammable and combustible liquids. Atmospheric storage tanks are used to store or mix flammable and combustible liquids in various ways, depending on the facilities. These tanks can range from 10 feet to more than 350 feet in diameter and have an average height of about 45 feet. Such tanks can hold more than 1.5 million barrels (6 million gallons for crude oil and other petroleum products). Fire detection for tank storage farms is considered critical for a safe operating environment.
In a floating roof tank, a floating roof with a rim seal rises and falls with the level of surface liquid. This minimizes liquid loss through evaporation. However, when crude oil and petroleum products are stored in these tanks, there is a risk of vapor leaks around the rim seal. These vapors could ignite, destroying the rim seal and causing a large fire, unless first detected by a suitable flame detector.Â
Other examples of natural causes include:
- Lightning strikes and the creation of static electricity when tanks are filled or emptied.
- Human error, such as overfilling tanks, ineffective grounding of tanks, and allowing pumping equipment to become overheated are causes where human error can yield ignition.
- Fire sources tied to equipment malfunction include:
- Pipe/valve failures.
- Improperly operating venting devices.
Challenges
Flame detection around the rim seal must occur at the earliest stage to enable prompt activation of suppression, before the rim seal is damaged and more vapors can escape.
Other areas that require continuous monitoring include:
» Tank piping manifolds
» Valves and surrounding areas
» Laboratory and quality control facilities
» Control rooms
» Compressors and turbine enclosures
» Loading/filling stations for rail cars and automotive tankers
Fire Detection for Tank Storage Farms
Industry | Fixed Tanks
Fixed roof tanks are used to store petroleum products with vapor pressures close to atmospheric pressure. Pressure-vacuum valves purged with natural gas prevent air intake into the vapor space. Vapor loss through the pressure vent valve can be very high. Dangerous levels of toxic H2S can also be present.
Typical fire sources include overfill ground fires or dike fires, resulting from leaking pipes or tanks. Sometimes, fires are a result of operation error or equipment malfunction. These incidents are the least severe. In the event of a leak without ignition, all ignition sources should be isolated. If ignition occurs, the fire should be treated as a large pool fire.
Overfill ground fires are common to fixed-cone roof, internal floating roof, external floating roof, and domed roof tanks. Vent fires are associated with fixed roof tanks, such as cone and internal floating roof tanks. The main cause of a vent fire is a lightning strike which ignites fugitive vapors present at the vent.
Obstructed full liquid surface fires can occur in fixed- cone roof, internal floating roof, or external floating roof tanks. They are challenging, as the roof or pan blocks access to the burning surface.
Unobstructed full liquid surface fires are easy to extinguish when the tank diameter is less than 150 feet, there are trained personnel, and sufficient resources. Larger tanks are more challenging, as a large amount of resources are needed in order to control and extinguish a fire with such a large surface area.
Unobstructed full surface fires can occur in fixed-roof
tanks without internal roofs, if the frangible weak seam at the roof-shell joint separates, as a result of an explosion or similar event, leaving a full surface tank. External floating roof tanks are also prone to unobstructed full surface fires in the event of heavy rain. With closed roof drains, the roof can quickly sink, leaving the exposed liquid surface vulnerable to a lightning strike.
Challenges
Perimeter monitoring is required for fixed roof areas,
to check for escaping vapors. Other areas that require continuous monitoring include:
» Tank piping manifolds
» Valves and surrounding areas
» Laboratory and quality control facilities
» Control rooms
» Compressors and turbine enclosures
» Loading/filling stations for rail cars and automotive tankers
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