Obtaining large flows from distant water supplies
The purpose of a fireground water delivery operation is to produce and deliver the flow demands of the fire as efficiently as possible. Often on large fires, these requirements cannot be met with the available water in the immediate fireground area. When this happens, distant water supplies need to be tapped into and delivered to the fire as quickly as possible. To help facilitate this type of long-distance operation, large-diameter hose (LDH) needs to be deployed.
If fire hydrants represent the water supply, it’s important to remember its performance capabilities as it relates to moving water. As you know, it takes pressure to move water through fire hose. The reason for this is simple: When water moves through fire hose, there is a pressure loss because of the water moving against the sides of the hose. This is called friction loss. There are two things that affect friction loss in fire hose. One is the flow or gallons per minutes (gpm) and the other is the diameter of the hose. The larger the flow, the higher the friction losses. As a rule of thumb, if the gpm is doubled, the friction loss quadruples. As the diameter of the hose increases, the lower the friction loss becomes. Another rule of thumb states that when the diameter goes up one inch, the water delivery capabilities are doubled. So basically, physics tells us that the largest diameter of hose will provide the highest flow.
Let’s look at a realistic example of what a fire hydrant can do with regard to water delivery through LDH. For this example, we are going to move 1,500 gpm through four-inch hose and five-inch hose to see which one is more efficient. It’s obvious that the five-inch hose will do the best job, but let’s take a look anyway. The hydrant that is to be used in this example has a static pressure of 80 pounds per square inch (psi). Remember that a static pressure is the pressure of water at rest so this is not a true indicator of how good the hydrant is because once the water starts flowing this pressure drops, becoming a residual pressure. It is the residual pressure that moves the water through the hose. For this example, after flowing 1,500 gpm, the residual pressure moving the water is 30 psi. Now let’s analyze how far 30 psi will move the 1,500 gpm through the supply lines. The friction loss for 1,500 gpm in four-inch hose is roughly 45 psi. Divide the 45 psi into the 30 psi and we come up with a number less than one. This represents less than 100 feet of four-inch hose so 1,500 gpm will only go less than 100 feet using four-inch hose based on the 30-psi residual pressure.
Now let’s analyze the five-inch hose. The friction loss in five-inch hose is approximately 15 psi. Divide 15 psi into 30 psi and you come up with two. So, 1,500 gpm can be delivered through five-inch hose 200 feet. Now obviously there would still be a fair amount of water available, but after the 200 feet mark, it would start declining.
Even though based on physics the five-inch hose can flow twice as much as four-inch hose, it’s important to remember that this can only happen when the water supply has the capability in volume as well as pressure. The fact is that most fire hydrants do not flow enough water to allow five-inch hose to perform to its highest capability.
The best way to create the most efficient long-distance supply line when using fire hydrants is to place a pumper at the hydrant to increase the pressure capabilities for moving the water. This is known as a relay pump operation. Let’s look at another example using the 1,500-gpm capabilities of the hydrant again. Based on a 185-psi pump discharge pressure, the maximum operating pressure for LDH of the pumper pumping from the hydrant using four-inch hose, 1,500 gpm will be able to be discharged approximately 400 feet. The same scenario using five-inch hose can be delivered 1,200 feet. So, you can see the advantage that the larger of the two hose sizes has. Remember, this example is a relay so the hydrant pumper only has to develop the pressure to overcome the friction loss and a residual pressure for the receiving pumper.
Hydrant Plan of Action
With this being said, what should our plan of action be when using a hydrant far away? As mentioned previously, the best way to increase the volume of water is to set up a relay pump operation. How many pumpers should be involved with the relay besides the attack pumper, which is on the receiving end of the relay? Here’s an example of a training session conducted in a rural area where hydrants were scarce.
The first-in pumper stopped at the hydrant to lay into the fire scene. This pumper carries 1,000 feet of five-inch hose, and it was predicted correctly that the lay would be much longer than 1,000 feet. So, the plan was to lay the line and go directly to the fire scene even if they ran out of hose—which they did. The scenario involved a quick attack off the tank water, and the next-due pumper was told to make up the difference by laying from the end of the 1,000-foot hoselay to the pumper at the fire scene. They wound up laying 600 more feet of five-inch to complete the supply line. They were next told to go back to the hydrant and set up for a relay pump operation. It is estimated that they delivered approximately 1,200 gpm to the fire scene. The stopping point of the water delivery was based on the unit at the hydrant reaching its maximum operating pressure for five-inch hose, which is 185 psi. Keep in mind that they still had 55 psi left on their intake gauge; however, they just couldn’t flow any more water because of the hose pressure limitations.
From the time the pumper laid the line, spotted the hydrant, started making the hookups, and then charged the line to the pumper at the fire scene, they took approximately 15 minutes. This means that for at least 15 minutes the pumper at the fire scene was operating off booster tank water, which was 500 gallons. Although the relay was a necessity, it did take time to set up when time was of the essence. So far, this action was the correct thing to do. The next flow test they conducted involved placing a second pumper at the end of the 1,000-foot relay and then connecting the 600 feet that went to the pumper at the fire scene and relay pumped. So, we had two pumpers set up in relay, one at the hydrant and one 1,000 feet down the road. Did this improve the flow? Yes, it did. They were able to get approximately 500 more gpm by placing the second pumper in relay. However, instead of taking 15 minutes to make the connections and get water flowing, it took close to 20 minutes because the second pumper set up in relay had to first arrive on scene and then make the connections and coordinate the operation with the other two pumpers involved. In 20 minutes with an unchecked fire, especially a large one, you can imagine how much fire extension there would be.
The alternative to the second pumper relay, in my opinion, would be to lay a second supply line from the front suction of the pumper at the fire scene and reverse out to a second hydrant, even though it was also a long distance from the fire. Not only would this operation bring in the most amount of water as compared to the other evolution, it would also take less time to get the initial water since it would be coming from the first relay, which got a head start in being set up. Even though the two-pumper relay maximized the first hydrant, the trade-off was an extended amount of time to get the water moving. Remember, volume is important; however, the speed of getting water to the fire in this case trumps the water volume.
There are certain things that can be done when developing a relay pump operation that will expedite its deployment. First, when laying a line itself, don’t baby the hose. I have seen departments laying an LDH line at about two miles an hour simply because of the size of the hose. Most departments have a rule of eight to 10 mph when laying an LDH line, which I think is very sufficient. When making the hookups to the hydrant, I recommend using 50-foot sections of LDH stored in a donut roll (both ends of the hose at the end of the roll). This makes the deployment of the hose much easier than if it was in a single role because the size and weight of a 50-foot section of hose are extremely difficult to just roll out with what I call the bowling ball throw. If possible, after unrolling the hose, make the first connection to the intake of the pumper so that, after the hydrant is connected, you can charge the line from there instead of having to go back to the pumper to first make the connection and then back to the hydrant. If the receiving pumper is not ready for water, the relay laying can be precharged to a point where either you can’t see the hose anymore or just before it reaches the pumper. This can cut off two to three minutes of time for deployment.
Moving water in a difficult situation like I have been discussing needs to involve training, analyzing the scene to see what decision to make, and understanding the skills involved in this type of operation. It’s important to not get tunnel vision and focus totally on the amount of water you can deliver. Determine the needs of the fire suppression operation with regard to getting the water to the fireground, or at least enough to get started. Don’t wait for the fire to test your skills. Go out with the troops and train.