The object of firefighting has always been to put out fire, in a timely fashion, limiting damage from the fire and our suppression efforts. What has snuck up on us as an industry is that our structure fires are reaching hotter temperatures faster, requiring more water for knockdown. At the same time, we’re typically arriving with fewer firefighters to deliver the water. Fortunately, we’re developing new equipment and adjusting our strategies and tactics to ensure we’re delivering the best possible service to our customers.
As an engineer, there’s a concept that I find makes structure firefighting very easy to put into perspective. In simple terms, structure firefighting is simply fighting fire inside a box. If we get enough water inside that box, then the fire goes out. For the most part, the fire goes out so fast that it can be measured in seconds. We call it delivering the “critical fire flow”–and it’s really fun to do.
Critical Fire Flow For a long time we have held as truth that there’s an amount of water (fire flow) that can tap the fire in less than a minute when applied inside the burning box (Iowa rate of flow). My totally unscientific hunch is that in the last 10 or 20 years, the amount of water needed to do this in some “boxes” has nearly doubled due to modern fuels, lightweight construction (the new “ordinary”) and faster flashover. These factors have created the mindset in our industry that sometimes the best bet is to punch the fire really hard from the outside and then chase it down interior. So long as we are not talking rescue, a heavy-hitting transitional mindset in many cases helps us to stay safe and maintain a high success rate–and also do a really good job of property conservation.
But this shift has ratcheted up the expectations of firefighters and fire officers looking to the driver/engineer to deliver the punch. For a long time, so long as something wet came out of the nozzle, we were doing our job. Today, when a firefighter pulls a nozzle off an engine into a fire, they have a clear expectation that the correct amount of water will come out of it.
As I outlined in the first two benchmarks, engineers can make their lives easier by simply putting lines on the discharge gauges for every preconnect so we can accomplish the goal of pumping any number of discharges perfectly without hardly thinking. Just open the valve enough to bring the needle to the line and we are good. But doing that one simple preplanning step opens up a world of incredible new possibilities.
The Third Benchmark The world of the engineer can now shift from thinking in terms of psi to thinking in terms of gpm. Why do we care? Because fire suppression efforts, water supplies and everything else water related is always described in terms of gpm.
The third benchmark is knowing and communicating how much water you’re currently flowing and your available water supply. You can determine additional available water supply by keeping an eye on pressure drop on the intake manifold pressure gauge, using one of two methods:
The “First Digit Method”: If the drop is less than or equal to the first number in the static pressure reading, then there is three times more of what you just opened. Twice the first digit in pressure drop equals two times more available, and three times the first digit in drop tells us we have approximately the same amount of water as the line you just opened.
The “Percentage Method”: If the difference in intake pressure before and after a 200-gpm handline is opened is less than 10%, then three times the 200 gpm is available; 11—15% pressure drop equals two times and 16—25% drop equals one time the 200 gpm is available. The formula is “pressure drop as %” = (static — residual)(100)/static.
Let’s look at an example: I open a crosslay flowing 200 gpm. There’s a 10% drop in intake pressure off the hydrant. Cool!!! “Hey Captain, I still have 600 gpm in the hydrant, you want the blitz at 500 gpm?” The first two or three times you say that the “Cap” is going to look at you a little funny, but watch out, soon enough everyone is going to start expecting it. Well, the really exciting thing is it doesn’t have to stop there.
Your engine carries a number of handlines–200 gpm, 300 gpm and 500 gpm. There’s also a deck gun. How many of them can you supply with water? In the last example, the engine was hooked to a hydrant flowing 200 gpm with an additional 600 gpm available. It’s an 800-gpm hydrant. In that scenario, there are two water supplies, the hydrant at 800 gpm and the water tank on the engine–but at what gpm?
Tank-to-Pump Flow Rate For every water supply, we need to know the flow rate in gpm. Especially with a transitional fire attack, we need to know at what rate the water in the tank can get pulled through the pipe. With a standard 3″ line from the tank, roughly 600—700 gpm can be delivered. With a 4″ line, nearly 900 gpm can be delivered. (Note: In the state of Washington, there’s an Administrative Code on the books that says all fire engines will get an annual service test [pump test] which includes a tank-to-pump flow rate test. It wouldn’t hurt to ask around and see if you can put your hands on a copy of the pump test report for your engine. The current standard is NFPA 1911 2007 edition Chapter 18.7.14. For those of you in Washington State the code is WAC 296-305-04507.)
The question remains: What can you supply and for how long? Because we’re pumping to the lines on the gauges, we can now talk freely in gpm and time. “Hey Cap, I have 800 gpm on the hydrant or I have 900 gpm for 30 seconds from tank water.”
At some point the captain is going to have a lightbulb turn on inside his head that if you can supply 900 gpm for 30 seconds from tank water, you can do that anywhere and within 60 seconds from air brakes/arrival on the fireground. Well, isn’t that nice to know. Hmmm, through an attic ridge vent within seconds from arrival ….
The Fourth Benchmark The fourth benchmark is thinking outside the box. If we think of the intake manifold of the pump as a hollow metal box that we bring water into through intake valves, then what’s outside the box are all of your water supplies. Each one has a pressure and a flow that is described in gpm. The fourth benchmark of a modern engineer is to be able to open up more than one intake valve and have the water wind up going into and out of the box in the directions you are intending.
For most fires, we open up one valve to our water supply and pump it until the intake hose is sucked down to 20 psi residual. Remember the captain who’s over by that burning building, staring at that really nice deck gun on top of your engine? The reality is the captain’s creativity at solving the fire problem isn’t limited by the fact that the one valve you have open is connected to a 500-gpm hydrant. The captain would really like for you to be able to open up another valve and get enough water to that deck gun to put out the fire.
If the weak link in the water delivery chain is the size of the hose connected to the hydrant, the solution is easy: Stretch another line and connect it, and away you go. If the hydrant’s water main is too small, however, more hose to the hydrant is not going to improve the situation. If you’re pumping from a water tender rather than a hydrant, the exact same principle applies: There’s a 500-gpm pump on the tender that you are nursing off of. If the weak link in the water delivery chain is a small hose from the tender, connect another hose. If the weak link is the pump size, more hose will not help.
Remember: What puts out the fire is the gpm delivered into the hollow metal box called the intake manifold of the attack engine and then delivered out of attack hoses and/or the deck gun. Back to the captain still staring at you with that shiny deck gun over your head and only one valve opened on your intake manifold. You’re limited by a 500-gpm fire hydrant as a water supply.
But what if we added another water supply? What if the fire engine that’s already connected to the hydrant was also supplied by a water tender, and it pumped to the attack engine’s intake manifold?
This will of course give us a greater water supply, but when you open a second valve, allowing more water into the manifold, something very unexpected might happen. If the water tender with the 500-gpm pump is pumping at a pressure higher than the hydrant pressure then the tender could very easily pump dirty tender water directly into the potable water system of the hydrant. This is really not good and technically would necessitate all the radio stations and TV channels broadcasting a public service announcement for residents to boil their drinking water. This is much too high a risk and our local water department might take back our hydrant wrenches if they caught us doing this.
Pressure Balancing Keep in mind that the intake manifold doesn’t know where you want the water to go. The water will simply move from wherever there’s a higher pressure to where there’s a lower pressure. After our brief time in a “gpm world,” we realize that for advanced pump operations revolving around water supply and the intake manifold, we must be incredibly clear about the volume of water available in gpm and the pressure available in psi.
We’ve been keeping the captain waiting over there by that burning building for way too long and it’s time we finally did something. Remember: We’re connected to a 500-gpm hydrant. There’s one valve open–the intake valve from the hydrant–and there’s a really big deck gun that we want to use. But it won’t even reach the burning building until we develop 750—1,000 gpm.
The hydrant has hydrant pressure. The supply hose takes away a bit of that pressure with friction loss. Ultimately, the intake pressure gauge on the hollow metal box shows 20 psi residual because we are pumping the hydrant for all it has and we don’t pump any more because if we throttle up we will pull more pressure out of the hose and it will start to collapse the hose.
One technique to put a smile on the captain’s face: Open a second valve. The “tank to pump” valve connects the hollow metal intake manifold to the water tank on the engine. The reason why water from the hydrant doesn’t flow backward out of the intake manifold and into the water tank is that there is a one-way check valve or “flapper” that only allows the water to move one way from the tank by gravity into the hollow metal intake manifold if the pressure in the manifold is about 0 psi. If we throttle the pump up and pull the hydrant hose down to an intake manifold pressure of 0, the hose collapses.
Looking up at the deck gun (and a sheepish probie firefighter who’s aiming it at the huge fire), you say, “Water coming at you.” The fireground shifts strategy to defensive and we shut down the handlines. When we open up the deck gun, a wimpy 500-gpm of hydrant water lands somewhere short of the building (thankfully not on the captain, who’s now burrowing holes into you with a very intense stare).
What do we do? We shut down the hydrant (trust me) and, as the hydrant pressure goes away inside the intake manifold, the pressure governor throttles up the diesel engine and the 500 gpm is now coming from tank water. So we throttle up the engine to whatever our tank-to-pump flow rate is and the deck gun finally falls on target at 700 gpm for a 3″ tank-to-pump line or 900 gpm for a 4″ tank-to-pump line. Finally, the captain turns to the building and evaluates the attack to see if we’re achieving knockdown.
Not being satisfied with only producing 900 gpm, we put a finger on the throttle and grab the valve to the soft suction from the hydrant and simultaneously throttle up the engine and slowly open the valve. We open the valve only enough to allow 500 gpm to come into the manifold and only enough to pull the hose pressure down to 20-psi residual on the soft suction hose (you’re not going to see it on the intake pressure gauge, you will feel it in the hose). We throttle up enough to throw 1,400 gpm into the fire–900 gpm from the tank and 500 gpm from the hydrant.
The captain whips around with a look of, “Where is all that water coming from?” but we don’t notice because it’s really busy over here at the moment. Quickly the water tank runs out, we hit idle, command calls “knockdown,” we open the handlines back up and they go in for overhaul on 500 gpm of hydrant water. If the handlines aren’t taking the full 500 gpm from the hydrant, we can sneak the tank fill line open and refill the tank for another hit, if need be.
Eventually the captain taps you on the shoulder and asks with a silly grin on his face, “What was that?” Smoke and mirrors, Cap.
Juggling Act It’s simply juggling. Start with a couple balls and then work up to keeping as many balls in the air as you have valves on the hollow metal box. With lots of good practice, this juggling becomes faster and easier. Setting the intake valve to keep hoses with 20-psi residual becomes easier. Eventually the probie on the deck gun will even get good at keeping most of the water off the sidewalk or front yard and delivering it into the fire.
For those of you frustrated with my example because the closest hydrant isn’t even in your fire district, have no fear. The fire pump really doesn’t discriminate between chlorinated/fluorinated drinking water from a hydrant, nursing swampy tender water or even the salty Pacific Ocean through a relay. 500 gpm is 500 gpm when it’s coming into your intake manifold.
In fact, some of the best examples of the fourth benchmark of a modern-day engineer are found on a fireground far away from a hydranted water utility with millions of gallons of water at our disposal. You know you’re there when a firefighter is handing you a water supply hose from the tender who can’t get to the other drop tank that was just set up in the wrong place, another engine is just throttling up to give you tank water and another tender has just dumped to your first drop tank. Oh, and you remember the captain? He just had the probie pull the deck gun and set it up with a 4″ LDH ground base in the front lawn and instructed the probie to take off all the tips down to the 2″ tip.
Start juggling … Hard suction valve in the full drop tank closed. Second hard suction connected and in the empty second drop tank, valve closed. Other engine supplying tank water at pressure, valve is open and is my current attack water to handlines. Water now coming from nurse tender (“Throttle it up and give me everything you have”), open up the second hard suction line enough to back-fill the inaccessible drop tank but not cause problems on the nurse tender line (20-psi residual). OK, we’re set. When the nurse tender is almost empty, both hard suctions wide open, deck gun discharge wide open, throttle up hard on the engine, gate down nurse engine and nurse tender to keep the hoses at 20 psi. Shut down the deck gun? But I only just got everything set! Oh, that’s right, we’re here to put out the fire, and before I even got all the valves set, the fire was knocked down.
It took me many years of juggling practice to become smooth with water supplies. I started with two and then added in more slowly as I found the rhythm. Why? Because gpm puts out the fire. In my earlier days, I produced 40,000 gallons of water worth of steam (8,840,000 ft3 if you run the math) and still burned a small house to its foundation.
Remember: It’s not just the wet stuff; it’s the correct volume of wet stuff in gpm that puts out the fire. As engineers, what we do is put the correct volume of wet stuff in gpm into every nozzle pulled into the fire. If the fire isn’t going out, then we need to be able to do our magic–and deliver the impossible.
The Benchmarks In an article in the April issue of FireRescue (“Delivering the Impossible,” p. 52), I explained why yesterday’s performance benchmarks for engineers are no longer sufficient, and I outlined four new benchmarks for us to shoot for when at the pump panel today:
Get beyond just “making water.” You must not only deliver water to the nozzle, but also deliver the correct amount so that the nozzle can achieve its peak flow, helping to ensure the success of the firefighters holding on to it.
Deliver the correct volume of water to the second handline without the first handline noticing any change in its fire flow.
Know and communicate how much water you are currently flowing (in gpm) and how much more water is available in your water supply (in gpm or minutes of fire flow).
Receive multiple water supplies simultaneously into your fire engine under varying pressures and/or vacuum while remaining in complete control of where the water goes.