TRACKING/ESTIMATING WATER SUPPLY ON THE FIRE GROUND
There are two main ways most firefighting publications (articles, books, etc.) offer to determine the remaining water supply we have on the fire ground when operating off permanent water supply via a municipal hydrant system. In this article, I will briefly outline these two methods and then present what I feel is a much more practical method we can utilize on the "real world" fire ground.
1) Percentage Method
To calculate (or better said, "estimate") your remaining water supply using the Percentage Method, you'll need to calculate the drop in pressure between your initial static pressure and your residual pressure (on your intake gauge) as a percentage, hence the name… The Percentage Method. For example, if you have 80psi static pressure and then open a deck gun flowing 500 GPM with 60psi residual pressure, in theory, you would have a 25% drop in pressure.
Once you know that percentage, you can run it against the following matrix:
0-10% = 3X initial target flow
11-15% = 2X initial target flow
16-25% = Same as initial target flow
In the above illustration, you would have roughly another 500 GPM available for firefighting operations.
To read more on the Percentage Method, check out this article on Fire Rescue 1 - https://www.firerescue1.com/fire-products/water-supply/articles/water-supply-basics-hydraulics-for-pump-operators-MEXfQw4e4aH1rJkn/
2) First Digit Method
To calculate (or better said, "estimate") your remaining water supply using this method, you would start by noting the difference between your initial static pressure and subsequent residual pressure once water is flowing. If the difference is less than or equal to the first digit in the static pressure reading, the theory says you can provide three times your initial target flow. If the difference is less than or equal to two times the first digit, you should be able to provide two times the initial target flow. If the difference is less than or equal to three times the first digit, you should be able to provide the same initial target flow.
Again, here is the matrix:
Less than or equal to first digit = 3X
Less than or equal to 2X first digit = 2X
Less than or equal to 3X first digit = 1X
Again, to read more on the First Digit Method, check out this article on Fire Rescue 1 - https://www.firerescue1.com/fire-products/water-supply/articles/water-supply-basics-hydraulics-for-pump-operators-MEXfQw4e4aH1rJkn/
In my opinion... none of these are remotely practical when the shit hits the fan.
When we pull up at a three-story, twenty-four-unit apartment complex with a working fire on the third floor that has extended into the attic, I don’t see any of these as very practical. There is simply too much going on, and too much being demanded from our Engineers to run these formulas.
So what can we do… practically?
3. “Hashing Your Intake”
STEP 1 - To utilize this method, we start the same as we have in the previous two methods — by noting the static pressure on our compound gauge. As a rookie I was taught to utilize a grease pen (aka “china marker”) to create a small “hash” mark of this (and subsequent) reading…. hence the name of this method.
As a quick refresher, in order to read static pressure, we need two things to be true; First, we need to have a permanent water supply established. And second, we need to ensure that no water is leaving the discharge side of the pump (i.e. no lines flowing). If both of these are true, we simply need to look at what psi our compound gauge is reading.
NO STATIC PRESSURE?… NO PROBLEM!
If you missed grabbing your Static reading, here are the steps for “backing into it:”
STEP 1 - Note the flowing (Residual) pressure on your intake gauge with the first line flowing
STEP 2 - Place another nozzle delivering the same GPM into operations (say, your backup line) and note the drop in pressure.
STEP 3 - Divide the drop in pressure by 2 and add this to the flowing pressure that was noted in step 1.
Source - Lawrence W. Evern, Fire Company Apparatus And Procedures 2nd ed.
STEP 2 - Next, open the desired discharge and place a hash mark with your china marker where the compound gauge is now reading (i.e., residual pressure). Now, it's critical to know the GPM (i.e., target flow) you are flowing through that individual discharge. In fact, without a knowledge of your target flow, none of these methods work.
STEP 3 - Take note of the difference between these first two pressure readings. This is the "cost of doing business." for each [YOUR TARGET FLOW/GPM] that you put into play — UP TO 1,000 GPM with a single five-inch supply line. So, if the first line you put into play is a 1.75" hand-line flowing 160 GPM, and the difference between your static and residual pressure is 10psi… then each additional 160 GPM you discharge will cost you 10psi on your compound gauge (up to the first 1,000 GPM you are flowing).
Here is an example:
Static Pressure = 85psi
Residual pressure = 70psi while flowing 500 GPM through a 1 3/8" Smooth Bore Tip.
Using this method, you can anticipate that for every 500 GPM you put into play, it will cost you 15psi, up to 1,000 GPM with a single 5" supply line.
Why is there an emphasis on "up to 1,000 GPM with a single 5" supply line?"
The simple answer is… friction loss on the intake side of your pump (in your supply hose). The reality that many of us tend to overlook is that once you start pushing over 1,000 GPM on the discharge side of your pump, you start to see a major jump in friction loss in your 5" supply hose. This will translate into larger drops in pressure on your intake gauge and, therefore, something we need to develop a solution for.
You'll notice that if we are under 1,000 GPM, our friction loss in 100' of supply hose is between 0 and 6psi. Once we move into the 1,250 and higher flows, we see a significant rise in our friction loss curve.
So, the easy way to combat unnecessary friction loss in our supply setup is to drop a second 100' of 5" to our hydrant and run it off of another discharge on our hydrant. Because we've split the volume load between two 5" hose, our friction loss for the same 100' distance looks like this now...
We've effectively doubled our volume on the discharge side and kept minimal friction loss in our supply hose. Why does this matter? Because if we don't address the rapid increase in friction loss above 1,000 GPM, this method will quickly run away from itself as we put increased volume into play. Once a second supply hose is connected, you're back in business… up to 2,000 GPM.
Now, it's important to note that the friction loss in our supply hose is only part of the reason we see a difference in Static and Residual pressures. Another factor is our municipal water system's volume capacity and current demand. Despite this and additional considerations, I've used this hashing method very consistently over the past decade with great success. IMO, it is a much more useful, quicker, and more dependable method for estimating available volume when Drivers are operating on the fire ground.
A few things we need to be mindful of as we wrap this up:
Always leave yourself margin. This is not an exact science.
There is no set drop between Static and Residual that you can anticipate every time. There are too many variables to create a "a crosslay flowing 160 GPM will always result in a 5psi drop" rule of thumb. Every hydrant will be different.
Train, train train… and then go out and train some more. See if this works for you. If not, test and practice the initial two methods we describe so that you can effectively deploy them when the bell actually hits.
Hold Fast & Raise The Bar,
