Estimating Needed Fire Flow And How to Cut Up the Pie
departments
The Volunteers Corner
Water supply is the most important factor in developing a successful strategy to extinguish any structural fire that is beyond the capability of a portable extinguisher.
Water supply is the basis of two of the most important decisions a chief officer has to make in sizing up a fire. He has to determine the fire flow required to extinguish the fire and he has to determine the available fire flow. Fire flow, of course, is the gallons of water per minute that either is required to extinguish a fire or can be delivered—by whatever means—on the fireground.
Determination of the required fire flow also is important in developing a pre-fire plan. It is entirely possible in pre-fire planning—just as on the fireground—that you will find the building being considered requires a larger fire flow than is available. For example, you might determine that a specific building requires a fire flow of 4000 gpm to extinguish an extensive fire involvement. On the other hand, you may find that 2500 gpm is the maximum available flow from your hydrant system.
Available options: You now have two options both on the fireground and in pre-fire planning. You can augment your hydrant system supply with water from pumpers at draft or with a large diameter hose pumper relay from a larger, distant main, or you have to initiate a defensive operation and protect exposures. In this problem, a tanker shuttle would be unable to supply the needed extra 1500 gpm to the fireground.
Obviously, if there is no way of increasing your water supply, then a defensive operation is necessary. You have to protect exposures and, hopefully, have some water available to apply to the fire. Eventually, the fire will begin to diminish as it runs out of fuel. When the fire burns itself down to 2500-gpm fire flow size, you can then—theoretically at least—proceed to extinguish it with your available 2500-gpm fire flow.
Estimating the required fire flow for a building requires judgment bred by experience. There is a formula for determining required fire flow that was published in 1959 by Keith Royer, supervisor of the Fire Service Extension at Iowa State University and Floyd W. Nelson, for many years a fire instructor at Iowa State. The formula states that the rate of flow in gpm equals the cubic feet of the fire area divided by 100. In pre-fire planning, the formula is applied to the largest undivided area in the building. If the fire flow is properly applied, the fire should be darkened down in 30 seconds.
While use of this formula can provide a base for determining the fire flow, the chief officer also must consider additional streams needed for the protection of exposures, the effectiveness of an automatic sprinkler sys tem in limiting the volume of fire-and therefore the volume of water needed from hose lines-and the type of building con struction.
Experience is vital: This is where your experience can be vital. Consider the expo sures. More water will be needed to protect a wood frame building than a brick building the same distance from the fire building. The number of windows in an exposed building also affects the protection needs.
If the fire building has an automatic sprinkler system, the chief must consider the volume of water that will be used by the sprinkler system. Many a large factory has been lost because the use of too many hose lines robbed water from a sprinkler system fed by the same main. If the sprinkler sys tem is fed by a fire pump drafting from a river or pond, the chief should be aware of this because now he can use the entire fire flow of the main as he thinks best-including feeding the sprinkler system.
The chief also must consider the effec tiveness of hose lines both on the fireground and in pre-fire planning. Some industrial complexes are so wide that even if there are no interfering interior walls, streams may be unable to reach the center of the structure. Factories with blank walls present a similar problem of stream penetration. Breeching an exterior wall takes time and there is always a possibility that you may breech a wall only to find an obstruction on the interior side of the wall. These are good arguments for the installation of a sprinkler system before the building plans are approved or when windows are about to be blocked up.
Cutting up the pie: All these consider ations gets us to the problem of how to cut up the pie of fireground water supply-or fire flow. Let’s go back to our example of the fireground with a fire flow of 2500 gpm. The chief has to recognize this limitation. He can’t order the use of fire streams that total a flow of 3000 gpm because there will be a deficit of 500 gpm and the effectiveness of the streams will suffer in quality.
However, the chief can cut up that fire flow pie any way he wants as long as the total of all the streams does not exceed the available fire flow -2500 gpm in this example. He can have a 1000-gpm deluge set, an 800-gpm ladder pipe, a 300-gpm hand line and two 200-gpm hand lines in operation, and they will each have a good quality stream because their total discharge volume is 2500 gpmthe amount of water available.
Not only does the chief have to consider the best strategy for handling a fire, but he also must modify his strategic planning when he faces a limited fire flow.
