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Keeping it simple

Dec. 15, 2010

As I did the math in my current FlashPoint column (December 2010) on the topic of calculating required fire flow, I noted my discovery that the Iowa State formula for a three-metre ceiling is identical to Paul Grimwood’s basic formula of floor area in metres, multiplied by four, to give required flow in litres per minute. Let me show you what I mean:

Flow (Iowa State)   = length x width x height x 4/3
                            = area x 3 x 4/3
                            = area x 4

So, as long as we are willing to assume a minimum ceiling height of three metres for our calculations on the fire ground, Grimwood’s formula is the most mathematically simple, and it is validated by the work that went into the development of the Iowa State formula as well. I would like to make one further assumption here, and that is to assume a higher fuel load right off the bat and use Grimwood’s formula of area multiplied by six. This not only helps with the problem of increased and increasing fire loads, but it gives another unexpected mathematical advantage. For any buildings with ceilings higher than three metres, we can simply bump up the multiplier by adding two for every metre of ceiling height above three metres. As an example, a 60-square-metre workshop or garage with a ceiling height of four metres would use this formula:

Flow   = area x (6 + 2)
         = 60 x 8
         = 480 litres per minute

In the column, I also stated that I was assuming 100 per cent fire involvement and not considering exposures. These are both features of the U.S. National Fire Academy (NFA) method for calculating flow. Let’s see if I can address them here.

I would always want enough flow to cover 100 per cent involvement, even if the fire compartment is less than fully involved. Logically, if 50 per cent fire flow will extinguish 50 per cent involvement, then 100 per cent fire flow will also extinguish 50 per cent involvement. Why short change yourself? Having an overwhelming advantage over the advancement of the fire can only be a good thing.

As for exposures, I am going to take a tactical mindset on this and state the obvious fact that you cannot protect an exposure with the same attack lines with which you are fighting the primary fire. Therefore, if there is a threatened exposure it must be treated as a separate required flow to be calculated and prepared independent of the main attack. If you have calculated that two 38-millimetre handlines are needed in the fire apartment, then you need to have two lines ready in case of extension into the adjacent apartments, the apartment above and the apartment across the hallway. Before we turn this into a hose manufacturers’ convention, note that I said have two lines ready. You won’t likely have extension in multiple directions, and you can move those lines around as needed.

So here’s my current recommendation: area times six, and increase the multiplier by two for every metre of ceiling height above three metres.

I am going to consider this a work in progress, and I would appreciate your input, suggestions and comments. In the meantime, enjoy the last few weeks of 2010 with your family and friends and have a safe and happy 2011!