Closed Containers and Fire (1976)

11 railroad tank cars loaded with propane gas left the tracks yesterday evening in a spectacular fire some witnesses likened it to an atomic blast with the characteristic mushroom cloud earth tremors were felt more than 30 miles away six persons were injured in the fire which caused more than 1 million dollars in damages firefighters from 9 surround hired a gas terminal near here killing two men and injuring three others has been blamed on a malfunctioning refrigeration unit those on the scene reported flames roaring 150 feet in the air and debris flying in all directions however the flames did not reach several nearby tanks containing chemicals they were getting 100 thousand dollars in damage was caused because it truck backfired the truck had just been filled with gasoline from the bulk storage tanks behind it and was pulling away when it reportedly ignited fuel spilled on the ground when it was over all five gasoline and diesel storage tanks were destroyed fortunately no one was reported injured to the been here for the past two hours trying to contain the blaze it's so widespread that major efforts are concentrated only where they're most needed and although the fire appears to be nearly under control firefighters say anything can still happen it's not out yet so we'll keep you posted on any major developments what do all these reports have in common they all involve storage tanks or containers that were closed containers in a position to rupture when heated by a fire a container can be of any size or any shape can contain almost any kind of material liquid or gas flammable or non flammable and it can be in any kind of environment but in a fire the fact that a container is closed makes it dangerous there are two general classes of closed containers the first is the ordinary container it can be of any shape or any size its primary uses to hold liquids and their vapors under normal temperatures and pressures however some volatile liquids exert enough pressure with normal temperatures that they would over stress an ordinary container these liquids and other materials which are normally a gas require a second class of closed container the pressure container it's a stronger specially designed container characterized by its rounded ends and it usually has a safety device for relieving excess pressure at first glance the pressure container and the ordinary container appear different and in many respects they are different but in the heat of a fire they both behave as pressurized containers this is what happens as the temperature when a closed container rises due to excessive heat molecular activity increases and the liquid begins to boil and vaporize into gas as a result the gas exerts more pressure against all the internal surfaces of the container as heat is added boarding continues to increase the pressure but the morning they begin to slow as pressure and temperature began to reach a balance point ordinary containers are not designed to hold the increased pressure and will eventually rupture what are the warning signs that pressure is building up in an ordinary container one sure sign is the rounding out of any flat surfaces such as the end the round shape more equally distributes the added stress as the ordinary container rounds out and the flat areas are stressed metallic pinging noise may be heard as internal pressure increases the areas under greatest stress will give out first this generally occurs near the seams at either end of the container depending on the amount of heat applied and any weakness in the material the container may rupture the extent of the rupture may vary a small area the container may crack open and fendt in such a venting the height of the flame and it's sound will give clues to the amount of internal pressure involved the lower or higher the flame the lower or higher the internal pressure the same can be said for the sound it makes the lower or higher the pitch the lower or higher the pressure just as pressure will rise when heat is applied to a liquid pressure will fall when it's cool reducing the height of the flame and lowering the pitch of the sound when a container has only partially ruptured the pressure is only partially released if the flames around it are hot enough to build up enough internal pressure it may erupt you're completely or explode releasing all the pressure at one time this is not an explosion due to internal combustion but rather an explosion due to the pressure behind the expanding vapor when such a container holds any amount of liquid at a high temperature the sudden boiling of that liquid may add to the force behind the expanding vapor this is called a fluffy a falling liquid expanding vapor explosion here's how a BLEVE develops all liquids have a normal boiling point at atmospheric pressure as the pressure in the container Rises the boiling point of the liquid also rises the pressure limits boiling even though the liquid may be at a temperature well above its normal boiling point when such a container bursts and the pressure suddenly drops the liquid vaporizes with explosive force if the vapor is flammable it may be ignited by the surrounding flames this is the characteristic fireball scorching everything and everyone within its area and perhaps starting new fires in analyzing the same explosion note where the vapors are ignited at the start of the burst the vapor expansion is so violent that it extinguishes the surrounding fire once the vapor is mixed with air it can be ignited by an outside source the flames then follow the expanding vapor front until it is entirely consumed this is the most dangerous type of closed container rupture when flammable liquids and their vapors are involved many liquids in closed containers are not flammable this does not mean they won't burst or fluffy even though the expanding vapor may not ignite a real danger is present from flying debris large objects can carry further from the explosion than the expanding paper and in any direction this barrel took off at a speed of 200 feet per second yet yet first at an internal pressure of only 75 pounds per square inch as an ordinary container it was not designed for that pressure on the other hand pressure containers are usually designed to withstand internal pressures of 100 psi or more to withstand these higher pressures they are designed with rounded corners and ends and they generally have some type of relief device to vent off excess pressure here's how a relief valve works in a fire situation when the internal pressure of a container reaches the point at which the relief valve is set to operate it will open to relieve the pressure the pressure then begins to drop and the boiling increases on some smaller pressure containers the relief valve will pop out and vent continuously until the internal pressure is equalized with atmospheric pressure on larger tanks when the internal pressure falls below the relief valve setting the valve will close until the pressure builds up again as long as the relief valve is operating properly the internal pressure will be controlled a working relief valve will vent at intervals the length of each interval depends on the amount of heat being applied to the container the short of the interval between each venting the more heat is being applied also the more heat applied the longer the period of venting during intermittent venting the height of the flame should stay relatively constant if enough heat is applied printing may be continuous indicating that the internal pressure is building as fast as it is being relieved in this case as with ordinary containers the flame height and its sound can be clues as to the amount of heat applied in either case the relief valve should keep pressure from rising beyond the design strength of the container but a relief valve alone is not enough to keep a container from rupturing it may still flow flames impinging on a container may weaken it enough for it to burst and this can occur at any time even before the relief valve vents of critical importance is whether the flame impinges above or below the level of the liquid inside the container when flames impinge below the liquid level the liquid absorbs much of the heat but flames hitting above the liquid level rapidly raise the temperature of that part of the container most closed containers are made of some kind of metal usually steal when the temperature of ordinary steel is raised high enough it loses all its strength under sufficient tensions deal will begin to stretch at somewhat lower temperatures it stretches and eventually breaks with a combination of high temperature weakening the metal above the liquid level and increasing pressures any container will pose and eventually rupture even before the metal is hot enough to blow again as with ordinary containers pressure containers may merely crack open and bent the height of the flame and the pitch of the sound will give clues to the amount of pressure involved most pressure containers when heated sufficiently will burst open rather than bet the destructive force of an expanding vapor explosion can be great this LP tank contained only a small amount of liquid before it was torn open the combination of high internal pressure and the external weakening of the metal above the liquid level cause this bulging and Terry all pressure containers regardless of their size are subject to these same forces causing them to bulge and tear apart in random fashion in this situation several liquid propane tank cars have exploded when they ruptured the prime dangers were from heat and from flying debris in all situations chief officers must make operational decisions and firefighters need to know how to minimize their risk in doing what's necessary in fighting such a fire the prime safety factor is distance relative safety increases as the square of the distance from the fire for example here at 100 feet from the center of the fire a firefighter would have been in extreme danger at 500 feet or five times further away his relative safety would be five squared or 25 times greater at 1,000 feet or 10 times the original distance away his relative safety would be 10 squared or 100 times greater in any closed container fire distance is your greatest ally the further away you are the safer you are also the less time you spend close to such a fire the safer you are the exact moment when a container will burst cannot be predicted and if you are close when it blows there is no time to escape your only hope then is your personal protective gear but this doesn't mean that some control measures can't be taken to stabilize the situation with a minimum of risk deleuze guns or automatic water distribution appliances may be set up under cover of hand lines this allows continuous cooling with short-term exposure to personnel if such appliances aren't readily available makeshift methods can also serve the same purpose in sizing up a situation two primary questions arise what dangers should firefighters be exposed to and how can each firefighter minimize the risk decisions involving the degree of risk must be based on the importance of the mission for example our other lives in danger to justify go again if little is to be gained then little should be risked the answer to the second question how can a firefighter minimize the risk is based on what is known about how closed containers may react in a particular situation the likelihood of unanticipated occurrences enclosed container fires emphasizes the need for full protective clothing back up lines the use of natural barriers for shielding and positive communication any decision that is made or action taken should be based on a continuous exchange of data gathered from all sources this means careful observation by everyone using both eyes and ears watch for the effects of flame impingement sometimes a fire burn so cleanly that it has no color and is difficult to see so look for the effects of the flames flaking paint bulging or a dull blow of the metal also look for venting either through a relief valve or through a partial rupture in the container since the venting may not be ignited in some cases you should also listen listen for the roar of venting pressure and for the pinging sounds of a container rounding out these clues not only indicates the possible danger but also can indicate how effective your control measures are you should be aware of these clues in any fire because a closed container can be in any fire if you know what's happening you'll have a better idea of how close to go if you don't know then don't go it may blow you