Best Practice for a Tank Fire?
Volume 22, No. 6
Where and when do you use cooling water during a tank fire? This question has caused many heated discussions among industrial fire fighting professionals over the years.
Fire chiefs and officers have spent their lives asking for more water pumping capacity, more fire trucks and more fire fighting equipment for the big fire. So it is natural that everyone wants to use it during the largest and longest type fire, namely tank fires. Using it the wrong way will not only waste the water resource but often it causes more problems than it solves.
It takes very good fire ground control to stop excessive water use at a fire that can last over 24 hours. So where should it be used.
There are only three main reasons to use a lot of cooling water at a tank fire -- direct flame impingement, radiant heat and trying to stop the vertical tank wall on the burning tank from folding in. Let's look at each one in order of importance.
1. DIRECT FLAME IMPINGEMENT: The first step of the fire assessment by the first responder is to look for flame impingement on adjacent uninvolved tanks, piping and pumps. If there is direct flame impingement to other equipment, cooling water should be started quickly and directly where the flames are touching the other equipment. A flame impingement requires immediate and precise cooling at the point of impingement.
2. RADIANT HEAT: Cooling water should be used to cool the adjacent tanks and piping exposed to the radiant heat not requiring immediate attention. Radiant heat exposure protection means spraying the cooling water directly onto another tank, piping or pumps. You must apply the water to the object to be cooled. Years ago someone designed a spray nozzle to send a fan spray of water up in the air. This was called a water curtain. It was thought this would protect exposures if it was put between the fire and the exposure. Unfortunately, it does not work enough to stop the very large amount of radiant heat generated by a tank fire. If you want to cool something, put the water directly on it, not into the air.
3. TANK COLLAPSE: One of the biggest and most wasteful uses of cooling water is spraying water on the upper portion of the tank shell above the burning liquid level, in an effort to try to hold up the tank shell above the liquid product level. The main reason to try to keep the wall vertical is to prevent it from drooping over into the tank and ultimately dipping into the burning liquid. If the shell does bend over far enough to touch the surface, it will produce a tunnel of fire that the foam, when applied to the surface, will have to spread through if the tank is to be extinguished.
Keeping the wall from folding in was very important when the foam being used was protein based. These foams do not easily spread through one of these tunnels. If an AFFF type foam is used, it is not as important to hold up the tank wall. AFFF foams are very fluid and spread through these tunnels much easier. Protein based foams are much more rigid than AFFF foams.
It is almost impossible to keep the wall of a burning tank upright. The thing to remember is the side wall of the tank is made out of very heavy steel. When it is heated red hot by the flame in the tank, it looses all its strength, becoming like putty but still weighing the same. It is very hard to keep vertical and can be a big waste of water supplies.
Cooling Water on Crude Oil Tank Fires Causes
Additional Life Treating Hazards
Spraying cooling water or foam into a crude oil tank that has been burning for some time is very dangerous. It is very possible that even a small amount of water may cause a very large slopover that would top regular dikes. Combining crude oil heat waves and cooling water is asking for an explosion. If you have never seen a real slopover or boilover, no words of caution are enough.
Before the cooling water or foam is applied, the dike walls should be built higher and, if possible, a wave deflecting coping should be installed on top of the dikes. The dike wall on the side that exposes other tanks or other valuable equipment should be the first side to be built up or modified.
Cooling water could be cautiously and intermittently sprayed into the burning crude oil to:
1. Test for a heat wave,
2. Purposely cause the heat layer to froth over the side of the tank to break up the heat layer,
3. To test for a slopover just before the foam attack.
What are the consequences of too much cooling water?
A. Putting water into a burning tank adds to the water in the bottom of the tank. It will also raise the liquid level, possibility causing the burning product to discharge over the tank wall and spill into the dike.
B. If there is pooled water in the dike area, any spilled burning liquid will quickly spread over the water surface, directly impinging on other non fire effected items like pumps, piping, and other tanks. The spreading speed of burning liquid product on water is very fast, because it is just floating across the water surface. If foam lines are not pre-deployed, it can easily be so fast that firefighters will not be able to react before piping and other tanks start exploding.
C. On an internal floating roof tank where the external roof has not blown off, any water sprayed on top of the roof can easily get into the tank through the air vent holes on the roof. If the internal roof is still intact, the water will pool on the top of the roof, eventually causing it to dip down on one side and then flip up product vapors into the vapor space. This could cause another explosion in the tank if the vapors are ignited by any fire in the tank or dike.