Fires happen uninvited, unannounced and at unpredictable places. Buildotech has carried discussions on fires in homes and the precautions or fire fighting measures that can be taken to control and kill fires. However, what does one do when a storage tank is on fire? The natural reaction would be to douse it with water. “Not necessarily,” say experts. An interesting discussion…..
When a storage tank is on fire, is it necessary to cool the tank as long as it contains the product? Nycil P, HSEF Manager, Essar Oil Limited, Jamnagar poses a question. As a point of reference, Nycil states a conceptual scenario of a tank filled with a product and on full surface fire. In such a scenario, the situation is highly dangerous as the product within the tank will absorb the heat of fire. The question therefore is: if the steel is internally wetted with product, is it practically required to cool it with water?
Glenn H. Director at Horton and Horton Fire Limited, London recommends identifying the nature of fire — Rim seal fire/full/partial and the involvement of fuel surface/type of tank involved.
Assuming the tank of origin has a serious fire, and that there is a significant time loss between the arrival of fire service and an attack on the involved tank considering the time taken to assemble the necessary resources to mount a full and sustained attack on the tank, it is important to undertake the task of cooling the adjacent tanks almost immediately.
Since the product within the tank already wets/dampens the interiors of the tank, it is not absolutely necessary to wet the exteriors of the tank. The liquid inside will act as a coolant. However, the tank walls above the coolant level will require cooling in an effort to prevent collapse of the tank sides which can then make the application of foam compound to the entire burning surface difficult/impossible. Clearly, if the fire continues to burn for an extended period, then the product level inside the tank will reduce and of course with floating roof tanks the level of product in the tank will also influence the amount of tank wall that requires cooling.
Another key issue for significant fires is cooling of adjacent tanks/plant to prevent fire spread. Nonetheless, if possible, one should remove the product as far as practicable and the fire will eventually burn out. Planning is the biggest pre-requisite for these situations.
Dwight H, Promoting Fire Prevention & Safety Planning, Albany, New York Area states that another consideration is the potential presence of water in the lower portion of the tanks, especially in crude oil tanks. If the crude oil burns long enough, the product and the water will heat up. When the water reaches its boiling point, it can send the burning oil out of the tank, causing a significant increase in the burning rate and endangering everyone for a much greater distance from the tank than prior to the event.
Himat Khimani, Chartered Chemical Engineer, Kuwait recommends going through the API 2021: Management of Atmospheric. In case of storage tank fires, one should evaluate application of three tactics (Passive defensive, Defensive and Offensive Tactics) to mitigate LAST fire. There have been many incidents where tank fire resulted in more serious damage due to wrong tactics applied to control or extinguish tank fire. E.g. for Rim fire if people pour a lot of water on the roof of the tank, it will sink and result in full surface fire.
Dick G, Principal Engineer at Fire Engineering Solutions Pty Ltd, Brisbane Area is of the opinion that it is important to use the fixed fire protection systems to combat fires in tanks. Fighting tank fires by hand is inherently very dangerous, which is why many jurisdictions insist on fixed fire protection. Having said that; if a fixed foam protection system is fitted to the tank, it should be used as soon as possible. There is little benefit in cooling the shell of a tank on fire, except perhaps to protect the foam pourers whilst the foam attack is being set up.
Vakil J, Insurance Accredited Agency for Certification of Fire Protection Systems and Fire Risk Mgt Consultant, Mumbai notes that as per the Indian Code, all tanks in the same dyke and those within 15m of each other are to be water sprayed at 10.2LPM/m2 of tank surface area.
Neeraj Dubey, Manager (HSEF), Essar states that even when the tank is on Full Surface fire, the tank’s cooling system must be on. All tanks having ‘A’ or ‘B’ fuel have fixed cooling systems available. The cooling system has to be kept on to maintain the integrity of Tank Structure which is very important.
Hemang P, Sr. Fire inspector at GACL-Dahej agrees that it is necessary to cool the tank on fire as long as it contains the product. However, it is important to take into consideration the type of material in the tank and the boil over point of that material.
Secondly, time lag is a very important factor while considering fires of any kind. Once the fire is out of control, flash takes over (everything in the path of the fire will be destroyed). Every second matters in such cases. Therefore, it is very important to have a multiple action plan ready which should include speedy fire fighting activity, deployment of fire fighting staff, communication systems, combined foam, water application at different places, wind direction position, fire appliances position, pump status, fire water reservoir status, etc. The jumbo curtain system is another useful tool to protect and prevent fire loss.
Iswar I, Fire Protection Professional, Kuwait explains that water spray systems provide for absorption of heat to keep the tank cool. This prevents the liquid from boiling away. If the liquids within the tank boil away, the heat will not be transferred away from the shell. This could cause the shell to rupture in the case of direct flame impingement.
Provision should be made for the application of cooling water to fixed roof tanks containing products adjacent to those containing Class I or II (petroleum) or wherever there is less than 1 x diameter of the largest tank between adjacent tanks. In particular, there should be cooling wherever there is less than 15 metres access between tanks.
In the case of tanks exposed during fire, they will possibly have to be cooled. It is now an accepted theory that you should not cool the tank on fire unless you can cool it evenly all around for 360°. Uneven cooling will allow the tank shell to fail in the area where the uneven cooling was applied.
In the late stages of extinguishment, cooling water applied on the area above the liquid level would help the foam stay in contact with the tank shell. The cooling streams should be stopped when foam attack has started to conserve water and to concentrate on extinguishment.
Software for Full tank fire analysis which includes wind simulations are easily available in the market.