The use, storage, and handling of flammable liquids or chemicals require appropriate measures to protect people from fires or explosions.
Experience has proven that poor safety practices when handling combustible or flammable liquids can be a major cause of explosions or fires that can cause injuries and at times death (Hildebrand and Gregory 21).
Before looking at this subject, one must know various important terms relating to the subject. The flash point of a liquid describes the lowest temperature at which the liquid generates enough vapor to form an ignitable mixture with the air close to the liquid’s surface (NFPA 6-197).
The flash point is very important in the classification of combustible and flammable liquids. The boiling point of a liquid on the other hand describes the temperature at which the saturated vapor pressure of the liquid matches the atmospheric pressure (Shelley 63).
Finally, the vapor pressure of a liquid describes the force exerted by the vapor released by a liquid or chemical in a closed container.
Classes of flammable and combustible liquids per NFPA 30
A flammable liquid can be described as a liquid that has a flash point that is below 1000F (37.80C) and has a vapor pressure that is less than or equal to 40 pounds per square inch at 1000F (NFPA 6-198).
A liquid mixture that has a component with a flash point that exceeds 1000F and that constitutes 99% or higher of the total mixture is not considered as a flammable liquid. Flammable liquids are subdivided into three classes:
- Class 1A includes those liquids with flash points below 730F and a boiling point that is below 1000F.
- Class 1B includes those liquids that have flash points below 730F and a boiling point that is above 1000F
- Class 1C includes those liquids that have a flash point that is equal to or above 730F and boiling points below 1000F
Combustible liquids on the other hand describe those liquids that have a flash point that exceeds 1000F (NFPA 6-198). Combustible liquids are also subdivided into three classes:
- Class II includes those liquids that have a flash point equal to or exceeding 1000F and below 1400F
- Class IIIA includes those liquids that have a flash point equal to or exceeding 1400F and below 2000F
- Class IIIB includes those liquids that have a flash point equal to or exceeding 2000F
There are two main classifications of flammable and combustible liquids: Hydrocarbon (insoluble) and polar solvent (soluble) (API 6). Hydrocarbons include non-soluble petroleum products such as gasoline, crude oil, and jet fuels. Polar solvents on the other hand are composed of soluble liquids such as esters, ketones and alcohols.
In most industrial complexes, you will find storage tanks that are specially made for the storage of flammable liquids. Examples of these industries include but not limited to power plants, oil refineries as well as car manufacturing plants.
Most of this facilities store their flammable liquids in atmospheric storage tanks. The sizes of these tanks may vary with some having a diameter as large as 350 feet and capable of holding quantities exceeding 1.5 million barrels of liquid (Hildebrand and Gregory 18).
The classification of storage tanks is mainly based on the type of roofs that they have. There are several types of storage tank that may include; internal (covered) floating roof, domed external floating roof and open top floating roof (Crawford 25).
While choosing a storage tank several things are considered including the location as well as the liquid that will be stored in the tank. For those liquids that are flammable, it is advised that they be stored in large cone roof tanks. If this is not possible, they may be stored in underground tanks or tanks that are lowly pressurized.
Flammable liquids are mainly stored in internal floating roof tanks or open top floating roof tanks when in large quantities and in small low-pressure horizontal or vertical tanks and underground tanks (Shelley 63).
- Open-top floating roof tanks
These types of tanks are those that are used and have no fixed roof. Their roof is made to float on the liquid in the tank and the roof can rise or fall according to the level of the flammable liquid in the storage tank. “The tanks are fitted with rim seals to prevent vapors from escaping to the atmosphere” (Hildebrand and Gregory 33).
- Internal floating roof tanks
These types of tanks are made having two roofs where one is a permanent roof and the other is a floating roof. Internal floating roof tanks are usually equipped with vertical supports for the permanent roof (Hildebrand and Gregory 33).
Just like the open –top floating roof the internal roof will float on the liquid and will rise and fall as the level of the liquid in the storage tank changes. This roof will usually float on pontoons and other times it will be fixed with a double deck that helps it to float on the liquid.
In order to allow the space between the two roofs to breathe, the fixed roof is usually constructed with air vents. These vents are fitted since the vapor space of the fixed roof is considered lower than the flammable limits (Shelley 64).
In the attempt to avoid fumes escaping from the tank, seals are fitted in the rim seal spaces of the tank. The internal floating tanks are mostly used to store liquids that are very flammable such as petrol.
- Fixed roof tanks
These are vertical cylindrical structures made of steel and have a permanent attached roof (Shelley 64). In industries that mostly deal with petrol the roofs are cone shaped and this gives them their name fixed cone roof tanks.
In fixed roof tanks, the roof is usually slightly domed or flat to allow a vapor space between the underside of the roof and the surface of the liquid and to prevent the accumulation of water (NFPA 7-16).
In the construction of the tanks, API standards are followed and the tanks are fitted with a weak roof that helps to shell seam. This allows the roof and the main structure shell to separate in case of internal over pressure arising from explosion or other situations.
Storage tank fire: Causes and Prevention
Every year there are very many cases of storage tank fires in the world. The causes of this fire can be as a result of human errors, inappropriate safety policies or simple design errors. The following section presents various causes of storage fires and the appropriate suppression, prevention techniques.
- Overfill fires
These fires occur above ground in the dike surrounding the storage tank due to leakages in the pipe or tanks (Crawford 33). These types of fires can occur on any storage tank that is above the ground. Most of these fires are caused by operators’ errors and at times the malfunction of equipment or both.
This leads into the spillage of the flammable or combustible liquid into the dike above ground. It is important to identify the source of the spillage and isolate all source of ignition from the spillage in order to prevent overfill ignition.
An example of overspill fires is the Buncefield oil depot fire that occurred in December 11, 2005 in Hertfordshire, UK (Crawford 35). The oil depot was struck by lightning leading to the malfunction of the safety system responsible for stopping the flow of unleaded gasoline into the storage tank.
Over 300 tons of gasoline overflowed from the tanks and began filling the containment dike. Finally, the vapor cloud from the spill flowed over the dyke and spread to surrounding areas in the facility.
Sometime later, an explosion originated from a nearby car park and spread quickly to other areas of the facility and finally reaching the dikes. It took over five days for the fire to be extinguished completely.
- Vent fires
These are the types of fires resulting from the ignition of fugitive vapors exiting tank vents, mainly during tank filling (Hildebrand and Gregory 35).
In most cases, the fires usually are as a result of lightning but can also be caused by static discharge, electrical arcing and human activities near the location of the tank. Vent fires can occur in any type of storage space tanks apart from external suspended-roof tanks, which do not hold any vents.
In order to prevent static discharge in the storage tanks, API standards have been developed to identify the correct conditions and flow rates.
Another way of suppressing vent fires is through ensuring that the tanks are relatively safe from lightning from applying lightning protectors as per NFPA guidelines (NAFP 20-217) and that any human activities near the tanks are closely monitored.
- Rim-Seal Fires
In floating roof tanks, especially external floating roof tanks, the most common types of fires are rim-seal fires. According to studies, 95% of all rim-seal fires are as a result of lightning strikes. Studies also claim that during any one year, 0.16% of all tanks fitted with rim seals will suffer a rim seal fire.
According to NFPA 780, one of the main methods of preventing rim seal fires is through the installation of roof shunts that enable the dissipation of energy from lightning strikes and thus prevent fires (12).
Recent studies have also shown that rim-seal fires can be mitigated by ensuring that tight-fitting seals (both primary and secondary) have been installed and are effective in restricting the escape of vapors from the tanks.
These fires can also be mitigated by ensuring the submerged grounding cables are in place to connect the storage tank shell and the roof. It is also important to install monitoring systems and other fire fighting systems to detect rim fires and to allow rapid response when dealing with these fires (Ritchie 6).
- Full Surface fires
These types of fires occur when the entire surface of the liquid in the storage tank is on fire (Hildebrand and Gregory 37). Full surface fires can exist either as obstructed full surface fires or as unobstructed full-surface fire. “Obstructed full –surface fires are those fires that have the roof or pan blocking access to the burning part” (Hildebrand and Gregory 38).
These types of fires usually occur due to sinking of the roof or the pan. Roof sinking may arise due to several factors. First, sinking may occur due to accumulation of rain on the roof exceeding the design standards for the tank.
Secondly, sinking may also occur when the pontoon is filled with the contents of the tank mainly due to corrosion or other failure. Finally, sinking may also take place due to inappropriate use of firefighting materials during a rim-seal fire, resulting in the sinking of the roof.
Full surface fires can be suppressed using regular inspection programs. The main of these inspection programs is to identify pontoon failure and drain plugging as per API 653 standards.
Unhindered full surface fires occur when there is a free admittance to the entire tank surface (Hildebrand and Gregory 38). These types of fires usually take place in fixed roof tanks that do not have pans (internal roofs).
In these types of fires, the weak weld between the roof and the shell is broken and the roof separated from the shell mainly as a result of a particular incident. The fires may also be experienced in external floating roof tanks, where the design capacity of the storage tank may be overwhelmed by heavy rains.
For tanks smaller than 45m, unobstructed full-surface fires are usually extinguished easily provided there are enough resources such as foam and water (Ritchie 7). For tanks larger than 45 meters, these fires may prove to be a problem and are usually quite difficult to manage mainly due to the large resources needed.
According to Crawford (49), “the largest successfully extinguished full surface fire took place on 8th June 2001 in Norco, Louisiana.” A tank with a 325,000-barrel capacity, 82m in diameter and 10 meters high was struck by lightning and its contents ignited. The blaze was put out within 65 minutes of repression doings at the end of a 13-hour ordeal. The fire required 50% more water than the Burchfield site fire disaster.
Fire risk mitigation
The risk of storage tank fires is impossible to eliminate however, it can be greatly reduced through proper design, operation, and maintenance practices. Carrying out appropriate inspections as per the API 653 standards is important in recognizing design and maintenance issues with existing tanks (Shelley 68).
API 653 details three main types of inspections: Monthly routine in-service inspections, formal in-service external inspections and out-of-service internal inspections.
Monthly routine in-service inspections involves a visual inspection of tanks and exterior surfaces so as to identify shell distortions, evidence of leaks, paint coating, foundation condition, settlement, corrosion and the nature of insulation system (Shelley 68).
Formal in-service external inspections one the other hand are hand are compulsory checks carried out after every five years or sooner should the remaining corrosion allowance be less than 20 years. “If the corrosion allowance is less than 20 years, inspection must be carried out at a time period that is one quarter of the remaining estimated tank life” (Ritchie 8).
This inspection mainly focuses on the fire protection system utilized, roof, access structure, shell appurtenances, dike, foundation, wind girder, and internal floating deck. The condition and operating capacity of these elements are assessed and a report is prepared as to their effectiveness and level of safety.
The final type of inspections is the out-of-service internal inspections. These kinds of check should be carried out in a minimum of once in 20 years except a risk based check has been placed or at a sector of the storage tank remaining life.
If a corrosion rate for the tanks has not been identified, the inspections should be carried out every 10 years (API 86). “In order to undertake these inspections, the tanks must be emptied and cleaned” (API 87). Apart from visual inspection, a combination of ultrasonic thickness, magnetic flux, and leak testing is also applied.
The main purpose of this inspection is to confirm the continued integrity of the tank through verifying that the bottom of the tank has acceptable levels of corrosion, gathering data for minimum shell and bottom thickness evaluations, and identification and evaluation of any settlement in the bottom of the tank.
Apart from these, this evaluation also seeks to inspect the roof and the interior shell walls for localized pitting and general corrosion. In pontoons are present in the tank, they are inspected to evaluate them for corrosion and fracture that may lead to failure.
Apart from safety controls installed in storage tanks, the use of effective inspection techniques is the most effective means of suppressing flammable and combustible liquids in storage tanks. Inspection enables the identification of anomalies before they escalate into major problems that may lead to catastrophic failure.
The storage of flammable and combustible liquids brings with it the inherent risk of explosion and fire hazards. There are three classes of both flammable and combustible liquids. These classes mainly determine the type of storage facilities to be used for storing these liquids.
When storing flammable liquids there are various storage tanks designs that may be adopted. Each type of storage tank is prone to specific type of fire risk with some risks being shared across the full spectrum of the tanks. Using various standards such as NASP, various design, inspection, and maintenance practices have been developed to reduce the risk of fire or explosion in storage tanks.
Researchers and scientists have also developed various fire fighting equipments and system that are meant to ensure that the flammable and combustible liquids are safely contained.
By identifying the type of fire common in storage tanks, one can chose the best prevention and suppression technique from a list of many methods. This will ensure that damage is kept to a minimum should the worst ever happen. The most important suppression technique is proper inspection as per the laid down standards.
Proper inspection ensures that all the information about the storage tank is continuously evaluated and that subtle changes can be made as required before small problems escalate to massive disasters causing loss of money and at times lives.
Suppression and prevention of fires is important when dealing with flammable and combustible liquids is very important. It should be well planned in any industry or setting that contains storage tanks for these liquids.
Having a suitable suppression system that is based in NASP guidelines can be crucial in deciding the outcomes of a particular event in large companies or industries that are connected with storage of combustible and flammable liquids.
American Petroleum Institute (API). Prevention and Suppression of Fires in Large Above Ground Atmospheric Storage Tanks. Washington DC: API, 2006
Crawford, Edward. Tank Fire Suppression/Tank Overfill Prevention. Washington: American Petroleum Institute, 2008.
Hildebrand, M and Gregory Noll. Storage Tanks Emergencies: Guidelines and Procedures. Anapolis, MD: Red Hat, 1997.
National Fire Protection Association (NFPA). Fire Protection Handbook. 20th ed. Quincy, MA: NFPA, 2008
Ritchie, Ray. “Preventing Storage Tank Fires”. Hydrocarbon Processing. November (2009): 5-11
Shelley, Craig. “Storage Tank Fires: Is your Department Prepared”. Fire Engineering, 11.2 (1998): 63-69