1. Room Fires etc 2. Porch Fires etc 3. Nozzles etc 4. Flame Length etc 5. Cockloft Fires etc 6. GPM and Safety Margins etc
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6)GPM and Safety Margins etc

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1st draft 30Aug22 Latest rev 02Aug24
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Three-story, wood-frame residential buildings — Fig. 6-1 How much water through each line for interior attack?

What gpm should be used for interior attack at common residential fires?

The more-is-better argument is popular in the U.S. right now, but any bandwagon should welcome scrutiny. It’s okay to revisit any question. Is 150 gpm better than 100? 185 better than 150? Would 200 be better still?

To see why all three answers are "no", break the issue into two queries.

What minimums do you need?
How much extra do you want as a margin of safety?

To tackle either question, we need to realize that:

There are multiple safety margins in play.
The margins don’t have the same minimums (reference points).
Increases to a margin have diminishing value as the margin grows (i.e. increases have less value for margins that are already big).
There are always tradeoffs. Margin increases come at a price.

The main gpm safety margins are for knockdown and flow reduction. You want to be okay if knocking the fire somehow requires more water than expected. You also want to be okay if a kink or some other mishap reduces the flow to the nozzle.

Stream from a nozzle that is useless because of a kink in the hose — Fig. 6-2 Kinks ruin your stream

The reference point for the flow-reduction margin (RP-kink) is the bottom of the nozzle’s operating range.A nozzle's operating range is the range of flows that will produce a usable stream. Precise limits are arguable, but comparisons are valid. It’s the smallest flow that will provide a usable stream.A usable stream is one that has enough reach but not too much reaction force.

The reference point for the knockdown margin is the smallest flow that will black the fire out. Any flow less than RP-Knockdown will fail to kill the fire. Any flow more than RP-Knockdown will give you a margin of safety. The actual number varies from fire to fire, so for planning purposes we take 30 gpm as an adequate residential RP-Knockdown. It would easily cope with all but the largest residential rooms.

As with most safety margins, distance above the reference point will limit the benefit of a margin increase. If your RP is 30, a flow increase from 40 to 60 would be valuable. A boost from 140 to 160 would not. It’s unlikely that the higher numbers would ever come into play.

Flow safety margins for automatic and smooth-bore nozzles at 100 and 150 gpm. Knockdown margins very large in all cases. Kink margins are big for automatic, but only 29% for smooth-bore regardless of size. — Fig. 6-3 GPM Safety Margins — 4 Examples

Figure 6-3 shows that, when some departments pushed residential target flows from 100 gpm to 150, they expanded knockdown margins that were already large (over 230%). Any risk reduction was tiny. Kink margins got worse if they changed from adjustable-flow to fixed-flow nozzles (e.g. automatic to smooth-bore); got better if they kept adjustable-flow; and stayed the same if they kept fixed-flow. So marginal benefit from the flow increase went only to departments that had adjustable-flow nozzles and stayed with them. They saw a modest decline of their risk from kinks.

Garden apartment building — Fig. 6-4 How much water through each line for interior attack?

Comparing benefits to costs, we see several tradeoffs, but two stand out:

Applying more water requires more exertion.
Applying more water requires more water.

More exertion should trigger questions about cardiac events, about using your air supply faster, reduced work time, and possibly exhaustion. Using more water can of course cause you to run out of water, but the fear of running out of water can also have a ripple effect on other decisions and cost you time.

You're safer and more effective if you take a disciplined approach to exertion efficiency and water efficiency.

But don't stop there. Study more tradeoffs: maneuverability, stream reach, adaptivity, false sense of security, misdirection of training, water damage, water weight. See the whole board. Think beyond room knockdown—it's often the easy part.

Remember that the tradeoffs are real and the bandwagon should spark skepticism:

Rowhouse condominiums — Fig. 6-5 How much water through each line for interior attack?

Be skeptical of recommendations that add difficulty or complexity to an operation.

Be skeptical of the combat metaphor. Talking about “meeting the fire with overwhelming force” is not a substitute for thinking about how to put the fire out.

Be skeptical of anxiety-based language. A nozzle team is unlikely to ever need or benefit from “every drop of water that you can get.”

Be skeptical of comparisons to the past. They’re often false.

Be skeptical of training prop magic shows. Ask the basic simulation questionsWhat exactly is being simulated?
How is this simulation different from the corresponding real-world situation? early and often. Be wary of vague answers. Be wary of a fast presentation.

Be skeptical of knee-jerk objectives. “Apply as much water as possible” is a hollow objective that betrays a fundamental lack of understanding.

Questions

The RP of 30 gpm is a lot lower than some published calculations of required flow. Why the discrepancy?

Published numbers have been creeping up for decades. Anxiety played a role, but bad science and carelessness added to it.

Some authors assumed that nozzle teams extinguish fire by absorbing the heat being released (HRR)Heat Release Rate . Not true.Fires are killed mostly by wetting the burning fuels—choking off the supply of pyrolysis gases. Room Fires etc

Some authors assumed that todays fires need more water than those that were studied in the past. Not true.See Room Fires etc

Some authors confused target flows with required flows. A common example is adding safety margins to NFPA’s 100gpm target flow. Not needed since generous margins were already built in.

And some field experiences can be misleading when mental models are flawed. Nozzle teams who fail to kill a fire because they can't hit the burning fuels sometimes misread the problem Cockloft fires provide a common example. See Flame Length Etc and Cockloft Fires etc as not enough water.

Is added exertion really a problem? Many departments are flowing the bigger numbers and seem to be doing okay.

Large, single-family residential building — Fig. 6-7 How much water through each line for interior attack?

If exertion were the only issue, it would be unimportant at most fires. Most fires are easy and quick, and most FFs are not close to a cardiac event when they open the nozzle.

But flowing wastefully has other costs; and a small percentage of fires can make you pay dearly for a lack of discipline.

We should balance all the costs against the lack of any benefit from the bigger gpm numbers.

Can a safety margin analysis be applied to cooling on approach (COA)?

Yes and no. Trying to identify margins and tradeoffs is worthwhile, but reference points are tough to pin down. It's not just that the physics and chemistry are hard (most real world situations are complex and their variety is staggering.) For many fires, COA will likely do more harm than good; and when it might help, it’s value as a protective measure also depends on which other protective measures are available.

The relevant threats to FFs are:

heat transfer to FFs (without flame)
flame propagation through airborne fuel
flame spread over combustible surfaces
flame extension (i.e. flame becomes longer without propagating through additional fuel).

Each of these could have a margin in some fires but a lot of work needs to be done before we could know reference points—or even when the tactic will hurt or help.

(When looking at tradeoffs don’t forget perception. Once you open the nozzle you become visually impaired and hearing impaired.)

Study the whole board and be skeptical of the bandwagon.