Petroleum Storage Tank Facilities – Part 3
In previous installments of this series we talked about the types of petroleum storage tanks, their locations, common fire hazards, described the types of fires, and fire suppression systems that storage tanks may have.
In this third and final article we will discuss firefighting strategies and tactics as well as pre-incident response planning basics.
Firefighting Strategies and Tactics
Firefighting strategies and tactics begins with a well planned and tested pre-incident response plan. This will be discussed later in the article. Storage tank fires are complex events. These fires will require the implementation of plans, preparation, proper utilization of resources, and an extensive logistics section to ensure the resources are available and arrive on scene in a coordinated and timely fashion. The following strategies and tactics for firefighting presume that the planning and preparation stages have been performed by plant and fire department personnel. Experience tells us that successful and safe extinguishment of tank fires can only be achieved when based on planning and preparation, with all associated participating in all aspects of the process as well as the exercising of the plan. Exercising the plan can be conducted with table top scenarios as well as periodic full scale exercises.
As soon as a fire department receives notification of an incident, size up and intelligence gathering should be started. Information should be gathered quickly to begin the development of firefighting strategies. The following should be considered:
- Rescue of personnel in the immediate area
- Life safety hazards to site personnel
- Environmental impact
- Community impact
After the immediate issues are addressed, we need to identify the type of fire present:
- Vent fire
- Seal fire
- Piping-connection fire
- Full surface involvement fire
- Overfill fire
- Tank and dike (bund) fire
- Multiple tank fires
Once we have examined the above information we can then begin to develop our resource list and incident action plan (IAP). Remember that the type of product involved will also impact our resource needs and tactics. The following are various types of fires and firefighting tactics:
Ground Spill or Dike Fires
These fires can be viewed as simple pool or spill fires. Calculate the area (length x width) and use the correct application rate based on NFPA 11, Standard for Low-, Medium-, and High-Expansion Foam. Knowing the product will also give you the correct type of foam concentrate and application method. Alcohol products will require a gentle application method. Firefighters should not enter the dike area unless safe to do so and approved by the Incident Commander in consultation with the incident’s Safety Officer. Atmospheric testing should be conducted prior to and during entry. Exposures such as tanks, associated piping, and pumps should be protected with water via ground or fixed monitors. Ground fires should be extinguished first, then using dry-chemical equipment, valves and flanges extinguished. The most effective equipment for these combined fires would be hydro-chem™ technology whereby foam/water solution as well as dry-chemical can be delivered simultaneously through the same nozzle.
Rim Seal Fires
Rim seal fires can usually be extinguished using the fixed or semi-fixed foam systems if installed and properly maintained. On external floating roof tanks, if the fixed or semi-fixed fire protection systems are not present, manual firefighting will need to be performed. Under the protection of a water spray, a firefighting crew will ascend to the gauging platform with hand-held foam equipment. The primary method should be the use of foam wands to capture the fire (Photo 1- Foam Wand) which allows the placement of specialized monitors to be placed on the lip of the tank. (Photo 2- Specialized portable monitor) The monitors can then be used to extinguish the rim seal fire using the reach of the monitor so that hoselines and personnel are not operating from the wind girder away from the ladder. If this equipment is not available, then foam hoselines could be used from the wind girder. This is a hazardous operation, and only undertaken if there is a structurally safe wind girder with handrails. (Photo 3- Foam chamber and Wind Girder) Personnel should be secured to prevent falling.
In some instances, elevated streams from fire vehicles have been used. This is not a primary method of extinguishment. It has been noted that there is always a chance of sinking or tilting the roof under the excess water/foam solution, thus creating a larger problem, which may include an obstructed /unobstructed full surface fire.
On tanks fitted with internal floating roofs, these fires may be considered rare, but they do occur. They will be extremely difficult to extinguish unless fixed or semi-fixed fire protection systems are installed. Foam chambers and foam dams are the most effective, and the design of the system should be calculated on a full surface fire, especially if the pan below is aluminum.
The most difficult method of extinguishment in a covered floating roof tank will be to shoot foam water solution through the eyebrow vents. Using hydro-chem™ into these vents has proven effective in the past.
Full Surface Fires
Staffing requirements for a major tank fire will vary depending on the type of tank, location, water supplies, nature of the incident and the availability of trained personnel. Attacks on these fires will predominantly use the Type III “Over the Top” method of extinguishing agent delivery. The product involved will determine the required foam application rate and percentage of concentrate to water flowed. The size of the tank will also determine the application rate. For larger tank diameters a larger application rate is required. The chart below is accepted by industry experts to be the minimum application rates based on the tank diameter:
Table 1– Application rates
- Foam solution (foam concentrate + water) flow rates to be established are based on the following formula:
- Foam solution flow rate = Tank surface area x application rate (as noted in table 1)
- Tank surface area = 3.14 x radius2
- Application rate = as per table below
Foam Concentration Flow Rate (lpm [gpm] of foam concentrate)
- Foam concentrate flow rate = Foam solution flow rate x foam %
- Foam % = 1%, 3%, 6% depending on type of foam, product on fire and manufacturer’s recommendations
Foam Concentrate Quantities
- Foam concentrate flow rate (lpm or gpm) x duration
- Duration = 65 minutes for Type III (over the top) applications
Please note that these quantities are for extinguishment purposes. For vapor suppression after extinguishment it is an accepted practice to double our extinguishment supplies to maintain the suppression of vapors and prevent the possible reignition of the product.
Some of the above flows may be well in excess of 37,854 lpm (10,000 gpm) and will require large capacity delivery devices such as large trailer mounted monitors and large portable pumps.
Now that we know our flow rates and foam concentrates required we need to also look at other factors such as:
- Position and condition of roof drains
- Volume of the product
- Status of tanks and valves
- Depth of water bottoms
- Structural condition of tank
- Product in tank and its physical properties
- Is there room in the tank to accept the total foam solution without causing an overfill
- What other tanks, piping, or structures may be exposed
- Wind direction
- Weather conditions (present and expected)
In any fire situation we want to involve the local facility personnel in our planning section as technical specialists. They may also be at the command post advising the Incident Commander directly. These fires are not our ordinary ‘bread and butter’ operations and should be treated as an incident that can change rapidly and unexpectedly, often with severe consequences. Do not attempt to extinguish a full surface fire without all necessary resources on the scene. Cooling of adjacent tanks would be a tactic to be used prior to all foam delivery and personnel resources are on the scene. The cooling of the tank that is on fire is not recommended unless complete 360 degree cooling can be accomplished, which is rare. Also, when cooling a tank, use only the amount of water necessary. When the cooling water stops turning to steam, you may shut down the streams and start them up again when necessary. This will conserve water supplies for extinguishment and reduce the water flowing into the dike areas. Generally between 1,893 lpm (500 gpm) and 3,785 lpm (1,000 gpm) will be required for each tank cooled. In addition to the proper delivery devices and foam supplies, we need to be sure that our foam solution delivery zone on the surface will be able to spread out once it hits the surface and cover the complete surface area. According to the National Fire Protection Association (NFPA) foam can travel effectively across at least 30m (100 ft) of burning liquid. We believe that for calculating foam runs, this number should be reduced to 24m (80 ft), ensuring that our landing zones travel and overlap each other. Firefighters should be aware of the distance a master stream can reach as well as the landing zone length and width. These can be obtained from the manufacturer of the monitors and nozzles and verified in the field during drills and exercises. By knowing this information we can pre-plan the positioning of our master streams. Range finders can be used during operations to gauge distances to the tank to assist with monitor positioning. There are a few occurrences that can happen at a storage tank fire that the fire service should be aware of. These are:
This event can happen when a water stream is applied to the hot surface of burning oil, provided that the oil is viscous and the temperature exceeds the boiling point of water. It causes a short duration of slopping of froth over the rim of the tank with a minimum of intensity.
Frothover is a steady, slow moving froth over the rim of a tank without a sudden and violent reaction. Frothover may occur when the tank is not on fire and water already inside the tank comes in contact with hot viscous oil which is being loaded. An example is when hot asphalt is loaded into a tank car and comes into contact with water in the tank, causing the product to froth over the top. During a fire with crude oil it may also happen when the heat wave created by the burning crude oil reaches the water layers (stratums) in the crude oil. This heat wave will convert the water to steam, causing a frothover.
This event is a sudden and violent ejection of crude oil from the tank due to the reaction of the hot-layer and the accumulation of water at the bottom of the tank. The light fractions of crude oil burn off, producing a heat wave in the residue. The residues with their associated heat wave sink towards the bottom of the tank. This heat wave will eventually reach the water that normally accumulates at the bottom of the tank, and when the two meet the water is superheated and subsequently boils, expanding explosively causing a violent ejection of the tank contents and fire. The expanding contents being expelled can travel the distance equivalent of ten tank diameters. Careful consideration should be given during pre-incident response planning of the location of the command post, staging areas, rehab, equipment placement, etc.
Pre-Incident Response Planning
When planning for a response to a petroleum storage tank facility it is best that the information gathered is done on-site and with the assistance of facility personnel. While on site, access roads that you may use to access the area and position fire apparatus (appliances) should be driven by the vehicles that will be used during an incident. Many times the turning radius of apparatus is too great to make the turns needed in the facility. Swales or culverts may also impede apparatus. If the apparatus chassis is too long and or low, it may hang up or ground while traversing a swale or culvert. Bridges on site may not allow the weight limit of new apparatus, preventing its use at an incident.
During pre-incident response planning information that should be gathered includes the following:
- Tank types, dimensions, contents and capacities
- Pipe isolation valves, locations, and operating mechanism
- Fixed fire protection systems available
- Access points to facility and tank
- Contact phone numbers
- Locations and operation of emergency shutdown devices (ESD’s)
- Availability of firefighting resources
- Water supplies
- Pumping requirements
- Foam concentrate requirements
- Mutual/automatic aid available
Other information can be obtained based on the needs and requirements of your department. While these articles are not totally inclusive of all information that a fire department needs to know, it is a good start. Other resources are listed at the end of this article. It is important that firefighters attend classes on this specialized firefighting, pre-plan these facilities, and exercise the pre-plans. Don’t allow these facilities to become part of the landscape. Visit these facilities and ask questions!
For more information, go to www.worldsafeinternational.com
Top Image:- Foam wand being placed during training. Note the protective hose stream in place.