Flashpoint: December 2010
By Peter Sells
Elsewhere in this issue, Chief Bill Stewart describes the tactical problems encountered and overcome by Toronto Fire Services at a wind-driven fire on an upper floor of a downtown highrise. Our industry has come up with various methods over time for calculating the required flow of water for fire protection.
By Peter Sells
Elsewhere in this issue, Chief Bill Stewart describes the tactical problems encountered and overcome by Toronto Fire Services at a wind-driven fire on an upper floor of a downtown highrise. Our industry has come up with various methods over time for calculating the required flow of water for fire protection. I’d like to present a few of them to you and discuss the following question: What is the correct method for calculating a tactical fire flow?
Some of you may think, “Hey, I thought FlashPoint was an opinion column, not a technical forum,” and you are right. Don’t worry; I won’t deviate too far from my usual path.
Fortunately, the fire service has, over time, developed simple flow calculation methods. Unfortunately, there are several of them, and they don’t always generate the same result. Also unfortunately, many fire services don’t make use of any flow formula, preferring to rely on standard procedures or judgment calls, which are much more likely to result in too little flow (too small a hose line) than too much. We have all either been at fires or heard stories of fires at which a later arriving incident commander must give orders to replace 38-millimetre lines with the 65-millimetre lines that perhaps should have been laid in the first place. The use of a quick and easy flow calculation, along with knowledge of the characteristics of the nozzles and appliances we drag into place, should result in more appropriate initial tactical postures.
Chronologically, here are three flow formulas to consider, using the Toronto highrise fire as an example (note that we will consider only the fire apartment, and assume 100 per cent involvement and ignore exposures, to keep the math simple):
- The Iowa State formula was developed in the 1950s. Intuitively, you might expect that it may give too low a flow, given that it reflects the much lighter fire loads that were present in Ward Cleaver’s or Ozzie Nelson’s homes or offices (note to generation X and Y: look up those names in Wikipedia). The formula (converted into metric units) is: Flow = Length x Width x Height x 4/3. So, for our 50-square-metre apartment with 2.5-metre ceilings, we get: Flow = Area x Height x 4/3 = 50 x 2.5 x 4/3 = 167 litres per minute (lpm).
- Next, let’s use the U.S. National Fire Academy (NFA) formula, developed in the 1980s, again converted to metres and litres: Flow = Length x Width x 3.6 = Area x 3.6 = 50 x 3.6 = 180 lpm
- The NFA formula is simpler by one factor in that there is no need to estimate the ceiling height. Intuitively, this would also mean that there is no way to adapt this formula to higher ceilings, perhaps a shortcoming of this method. On the other hand, it did give us a higher flow than the Iowa State formula.
- Lastly, and most recently, let’s use the Grimwood formula, developed by retired United Kingdom fire officer Paul Grimwood after analyzing actual effective fire flows at structure fires in London in the early 1990s. Grimwood keeps it exceedingly simple: Flow = Area x 4 for normal fire loads, or Flow = Area x 6 for high fire loads. So for the Toronto apartment fire, we have: Flow = 50 x 4 = 200 lpm or Flow = 50 x 6 = 300 lpm.
As Chief Stewart noted, the fire load was very high, so of all of these results, we would expect the highest – Grimwood’s Area x 6 figure of 300 lpm – to be the most appropriate. Looking a bit deeper though, 38-millimetre lines (at least 360 lpm) and 65-millimetre lines (950 lpm) were inadequate. A ground monitor capable of flows in excess of 2,000 lpm was needed to get the job done, requiring perhaps 10 times the flow that would be calculated using the standard Iowa State, NFA or Grimwood formulas, and approximately seven times that which would come from Grimwood’s formula for high fire loads.
Here’s the rub: none of these tools were intended for highrise use, and Paul Grimwood makes specific note that his formula is not adequate for wind-driven fires. The initial attack met any of the flows that could have been calculated, but the extraordinary fire conditions rendered those calculations useless. So here is your homework: there is a lot of new material available on wind-driven fires; find it and learn it, because it could quite literally save your life.
Interesting note: As I did the math for this column, I discovered that the Iowa State formula for a three-metre ceiling is identical to Grimwood’s basic formula. I will explore that in more detail in an upcoming blog at www.firefightingincanada.com.
Retired District Chief Peter Sells writes, speaks and consults on fire service management and professional development across North America and internationally. He holds a B.Sc. from the University of Toronto and an MBA from the University of Windsor.