Types of Atmospheric Storage Tanks, Safety Challenges, and How to Overcome Them
For decades, refineries and chemical process industries have relied on storage tanks to safely store hazardous and flammable hydrocarbon liquids. Their popularity is evident with the numbers. For instance, nearly 542,000 underground storage tanks in the USA store petroleum and hazardous substances.
This article explores atmospheric storage tanks, along with their associated challenges and how to mitigate them.
What is an Atmospheric Storage Tank?
Atmospheric storage tanks store fluid at ambient pressure and are vented directly to the atmosphere. These tanks are used to store liquids for various chemical engineering and other purposes for varying durations. They are designed to operate at pressures ranging from the atmosphere's pressure through 0.5 PSIG when pressure measurement is taken at the top of the tank.
All underground tanks at gasoline stations are atmospheric. On the other hand, pressurized tanks store materials that need to be stabilized and stored under pressure. These pressure vessels are used for storing fluids like liquid nitrogen and LPG.
What does an Atmospheric Storage Tank contain?
Atmospheric tanks are primarily used for storing potentially volatile substances like gasoline, crude oil, other hydrocarbons, and hazardous chemical products. These tanks are typically much larger than pressure tanks, and some of them are capable of holding thousands of gallons.
Types of Atmospheric Storage Tanks
Typically, aboveground tanks are mainly classified into two categories, based on the tank roof design.
Fixed Roof Tanks
A fixed roof design includes a cylindrical shell with a permanently welded roof that can be dome-shaped, conical, or flat. In this type of tank, the roof and shell remain static. They store fluids with True Vapor Pressure (TVP) of less than 10kPa (absolute). TVP is the absolute pressure when the liquid-vapor is in equilibrium with the fluid at a constant temperature. These solutions are used to store gas oil, fuel oil, and sometimes kerosene in cold regions.
Fixed roof tanks are further categorized into umbrella type, cone type, and dome type tanks. Dome roof tanks can store materials with vapor pressures more than the atmosphere but less than justifying a sphere or bullet-shaped tank.
Floating Roof Tanks
The design consists of floating roofs that float on the material surface and fall and rise with changes in the liquid level. The tanks are ideal for storing petroleum products with a TVP of 10.3 to 76.5 kPa absolute. Perfect for storing hydrocarbon liquids with a flashpoint below 37.8oC, they reduce evaporation and control breathing losses while filling. These tanks are a long-term, reliable, and economical solution.
Floating roof tanks are further categorized into various types, and some of these tank classifications include:
External Floating Roof Type
They consist of a vertical cylindrical tank with a floating roof on the material surface. Though it drastically reduces losses due to evaporation and hazards related to large and open combustible vapor space, such tanks are exposed to external weather elements such as rainwater, snow, and ice. They are further divided into three categories.
● Pan-Type Roofs: It's the least expensive option in this category and consists of a single-deck roof that comes in contact with the tank-material surface. Any leak in the deck will make it sink. The tank has no additional buoyancy besides that provided by the deck, and snow or rain can deform this type of tank.
● Pontoon Floating Roof: In this type, pontoons occupy around 20-40% of the roof area, and the tanks have increased buoyancy and stability.
● Double Deck Floating Roof: In this category, trusses and bulkheads separate the upper and lower decks. The space between these decks is divided into liquid-tight compartments. They have a better loading capacity and are recommended for tanks with diameters 60 meters and below 12 meters.
Internal Floating Roof Type
These systems are gaining popularity with stringent environmental requirements. They offer several advantages over the above solutions and are more expensive. They are used in applications that require to exclude the possibility of air-contamination. or water, such as storing jet fuel.
These tanks are ideal for regions that receive heavy snowfall as snow can sink the conventional tank roof. As the inside vapor can be vented via emission control equipment, they are also helpful in reducing VOC emissions. These systems need to be adequately ventilated.
Closed Floating Roof Type
These tanks are similar to internal floating roof tanks, except that they utilize pressure vacuum vents and gas blanketing instead of natural ventilation. These are ideal solutions when there is an additional emission control need as they allow the collection of emissions for treating them further in an emission abatement system.
Other types of atmospheric tanks include:
These are ideal for performing more than one duty for a relatively short duration, such as intermediate and component tankage, or determining the need for product/feedstock. They save significant capital costs.
They are popular due to their low-costs and primarily offer water services such as cooling water, service water, and firewater. They are made of steel, concrete, or glass-reinforced plastic.
Safety Challenges in Atmospheric Tank Applications
The inflammable nature of the tank contents warrants the utmost caution when these tanks are used. There is a range of challenges associated with the tanks that store oil or hazardous chemicals.
Fire poses the most significant hazard to storage tanks. A recent study revealed that environmental (38%), equipment (20%), operational (15%), and maintenance (10%) factors were prime initiators of fire accidents in atmospheric tanks, while the domino effect caused 13% of fires.
Electrostatic charges can cause ignition. Protecting a tank includes bonding and grounding the tank, piping, and ancillary equipment and using dip-pipe or bottom loading to minimize material splashing.
Overpressure or Under pressure
Emptying, filling, or mixing materials in atmospheric tanks that contain vapors of near-flashpoint organic materials can result in a flammable organic or air mixture. Temperature variations due to natural causes, impurities, or light gases like hydrogen can cause a sudden change in pressure inside vessels.
In a fixed roof tank, an increase or decrease in pressure beyond a certain limit can lead to severe tank damage.
Besides proving expensive, volatile, and hazardous, product evaporation can be harmful to the environment, so you should aim to minimize it.
Excessive Vapor Generation
Temperature variation or routing materials that are more volatile than usual can cause excessive vapor generation. For tanks with internal heaters, maintain an adequate level above the heater's surface to avoid vapor generation due to overheating the tank contents.
Tanks spills can happen due to various reasons. Common reasons include accidental overfilling, high-level alarm failure, roof collapse due to mechanical failure, withdrawal of water from the tank bottom, and accumulation of a large volume of snow, ice, or water leading to roof collapse. In floating roof tanks, the collapse of the roof due to wedging or other reasons can lead to tank spills.
Froth over or Boil over
If the vessel temperature increases and causes the water to boil, it can form a foam of steam and organics. The consequences of violent froth formation can include froth over of ignitable organics and major fires.
Things like overpressure, overfilling, loss of integrity of tank walls or wall weld, and constant heat exposure can result in tank rupture and massive loss of contents.
How to Overcome Challenges Associated with Storage Tanks
The National Fire Protection Association (NFPA) has laid down tank specifications for aboveground storage tanks and other types of tanks, and your crew should stay updated with these regulations.
Installing fire-fighting facilities like individual tank foam-systems, hydrants, fire-loops, and monitors in the storage area, and deluge spray systems for cooling the exposed tank surfaces can help contain the situation in the event of a fire break out in an adjacent tank.
Besides these, the following measures can help offer fire-protection.
1. Keep adequate spacings between the tanks,
2. Avoid the use of air to mix flammable materials.
3. Use fire-resistant insulation for piping, critical vessels, valve actuators, outlet valves on tanks, essential electrical facilities, and instrument lines.
4. Install deflagration and detonation flame arresters on atmospheric vents. They can prevent the breakout from the outside of the tank to the vapor space inside the tank.
5. Use automatic, remote-controlled, corrosion-free, and fire-actuated valves to avoid content loss during an emergency.
Also, consider the following solutions that can help overcome safety challenges associated with atmospheric tanks.
Vents: Tanks with fixed roofs need sufficient vents to prevent pressure changes. You need to calculate the pressure vessel’s vent capacity based on tank inbreathing, tank outbreathing, thermal inbreathing, thermal outbreathing, and emergency venting.
Dikes: A tank dike, also called a firewall, offers a secondary containment around tanks and prevents content from spilling and spreading to other areas.
Double-walled Tanks: Double-walled tanks offer a reliable secondary containment in a broad range of chemical engineering and other domains. They integrate the containment into the outer tank shell and eliminate the need for a dike.
Tanks in Concrete Vaults: Concrete vaults are widely used in utility companies for preventing transformer oil leaks. Though a conventional option, concrete vaults can develop cracks due to temperature variation, and carbonation components in moisture and rain can cause it to ebb and pit over time.
Frac Tanks and Spill Berms: Frac tanks and berms are reliable solutions for storing volatile chemicals during maintenance and offer containment during potential spills. Double-wall frac tanks store jet fuel, de-icing fluids, and flammable contents with secure self-containment. Additionally, such tanks are easy to clean and come in handy for environmentally sensitive projects.
Timely maintenance and proactive safety measures can go a long way in increasing your atmospheric storage tank’s life and functionality. You will also need to partner with safety and containment experts to mitigate the impact of spillage or leaks.
At Adler Tank Rentals, we offer several application-specific frac tank and spill berm product options. With rapid turnarounds and expert support, we have successfully delivered containment and fluid storage tanks for various industries.
Contact us to know more about temporary fluid storage and emergency services.