Comparing Four- and Five-Inch Large-Diameter Hose

By Paul Shapiro

I think it is pretty accurate to say that the majority of municipal fire departments are using either four- or five-inch large-diameter hose (LDH) for their water supply lines.

There are many statistics that departments need to consider when deciding what size LDH to purchase. Some of the more common stats involve nonperformance facts such as the weight and the cost of the hose. These are important, but if a department makes a decision based totally on the weight and cost of the hose, it could be limiting its water delivery capabilities.

The most important factor for a fire department to look at when making this decision should be water delivery capabilities. Many factors go into analyzing the hose’s water delivery capabilities, including water supply demands, hydrant performance and spacing, the amount of pumping apparatus available for water delivery operations, and water delivery operation tactics.

Here are some simple facts about four-and five-inch LDH:

  1. There are two types of LDH: supply hose and attack hose. Supply hose has a maximum operating pressure of 185 pounds per square inch (psi). Attack hose has a maximum operating pressure of 275 psi.
  2. Five-inch hose (100 feet) weighs approximately 105 pounds empty. Four-inch hose weighs approximately 80 pounds. Some manufacturers offer lightweight versions for four- and five-inch LDH that weigh several pounds less.
  3. LDH can be used for either supply lines or discharge lines.
  4. Five-inch hose flows twice as much as four-inch hose.

Before comparing four- and five-inch hose in specific supply line evolutions, users need to understand the fire hydrant system itself and how it supports a supply line evolution. A fire hydrant is supported by water mains filled with water under pressure. The system has two basic pressure ratings. The first is called the static pressure. It measures the water pressure at rest, or water that is not flowing. Many people think the static pressure is the number to look at when determining how good a fire hydrant is. This is misleading because it is measuring the water at rest. Residual pressure is the measurement of the water at the hydrant while water is flowing. It is the residual pressure that determines how well the supply hose can deliver the required flow. Finally, the volume of water has to be available whether the pressures are high or low.

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1 LDH can be used for either supply lines or discharge lines. (Photos by author.)

Supply Line Evolution

Let’s take a look at the most common supply line evolution deployed by fire departments: a house fire scenario where a department lays a single supply line from a hydrant. In most cases, the flow will be 500 gallons per minute (gpm) or less. The hydrant being used in this example has a 60-psi static pressure. After flowing 500 gpm, the residual pressure is down to 40 psi. This is the pressure that will move the water through the supply hose to the pumper at the fireground. The friction loss in four-inch hose flowing 500 gpm is approximately five psi. Dividing 40 psi by five psi equals eight. This means that this hydrant will flow 500 gpm for 800 feet using four-inch supply hose.

Now let’s see what five-inch hose will do with the same hydrant. The friction loss for five-inch line flowing 500 gpm is roughly two psi. In reality, this would not show up on a discharge gauge, but the number will serve for this comparison. Divide 40 psi by two psi and the result is 20. This means that five-inch hose will move 500 gpm with the 40 psi residual hydrant pressure 2,000 feet. This is a little bit more than twice the distance of the four-inch hose.

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2 This 600-foot five-inch supply line is supplied from hydrant pressure. It would have performed better being pumped from the hydrant.

Relay Operations

Is there a possibility of doing a single supply line evolution connected directly to the hydrant with a flow requirement of 500 gpm that requires a distance of greater than 800 feet? If so, a department needs to answer the following question: Does it do relay pumping to boost the pressure and flow in its supply lines? If the answer is yes, then four-inch hose will work. On the other hand, if a department wants to rely on hydrant pressure for its supply line evolutions, then the answer is five-inch hose.

Keeping with the 500-gpm flow, how far can a department move 500 gpm in four- and five-inch LDH set up in a relay pump operation based on supply hose with an operating pressure of 185 psi? A four-inch LDH relay will flow 500 gpm 3,700 feet while the five-inch relay will go a whopping 9,200 feet. This is only a two-engine relay, one at the water supply and one at the fire. Based on this example, four-inch hose will be sufficient.

To efficiently fulfill water supply demands, a department needs to do more than just lay a line from a hydrant. Departments need to set up multiple-engine water supply evolutions from numerous water supplies. Relay pump operations are the only way to fulfill the required flows by boosting the pressure to move the water through the supply line. This holds true for both four- and five-inch hose. This should not be a freelance operation. A water supply officer needs to be in charge of setting up these evolutions, which involve deploying the lines, spotting apparatus, and choosing the best water sources to get the required flow.

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3 Sometimes crews just need to work a little harder, even with LDH.
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4 To efficiently fulfill water supply demands, a department needs to do more than just lay a line from a hydrant. Departments need to set up multiple-engine water supply evolutions from numerous water supplies.

Large Flows

So far we’ve really only talked about low-flow operations. Based only on this information, it would be tough to recommend five-inch over four-inch-especially when it costs more money and is heavier. Some folks may choose four-inch hose based on their not responding to large fires. This type of thinking will only get you in trouble. Fire departments and firefighters are sworn to protect against all fires-not just the small ones. And when the big one hits and you’re not prepared because you purchased the wrong equipment, did not implement the proper hose evolutions, or both, you’re going to be in big trouble-or at least should be.

Now let’s step up the flows to see how the four- and five-inch perform. The goal is to reach 1,000 gpm. Most fire departments that use four- and five-inch LDH lay directly from the hydrant to the fire no matter how long the lay is or what the flow requirement is. Here are two misleading trains of thought that justify this action. First, “We have an excellent hydrant system to support our hose evolutions, allowing us to use hydrant pressure only to meet our demands.” A good hydrant system is definitely a plus, but remember it’s what comes after the hydrant that causes restriction, mainly hose. As an example, a flow test on a hydrant without anything connected to it revealed a flow of 2,060 gpm. After connecting 300 feet of five-inch hose to it, the flow at the end of the line was only 1,250 gpm. The reason for this is the restriction in the five-inch hose.

Here is another misleading assumption: “Laying LDH supply line is like laying an above-the-ground water main or like taking the hydrant right to the fire.” Think about these statements. They’re basically saying that LDH is a cure-all, and simply hooking it to the hydrant will bring in as much water as is available.

I think these statements came about years ago when departments were first trying LDH and comparing it to their 2½- and three-inch supply lines. With the smaller supply lines, firefighters knew what to do to bring in as much water as possible. They would lay dual and sometimes up to four supply lines and pump from the hydrant in a relay pump operation.

To better show the difference between LDH and small-diameter hose, look at this comparison between three-inch and five-inch hose. It would take four three-inch supply lines to move as much water as a single five-inch supply line. It’s no wonder firefighters in those days thought LDH was the best thing to happen to them since motorized apparatus. The fact is that LDH, as good as it is, will probably not max out the water supply from a decent hydrant by using just hydrant pressure. This especially holds true for four-inch hose.

Let’s take a look at another example. In this scenario we are going to compare four- and five-inch hose in a 600-foot supply line connected directly to the hydrant. The target flow is 1,000 gpm, and the residual pressure in the hydrant after flowing 1,000 gpm is 60 psi. Divide the 60 psi residual pressure by 20 psi, the friction loss for four-inch hose at 1,000 gpm, and the result is that 60 psi will only move 1,000 gpm 300 feet. Using the same 60-psi residual hydrant pressure and five psi for the friction loss in five-inch hose at 1,000 gpm, a department will be able to move 1,000 gpm a total of 1,200 feet, successfully delivering the water in the 600-foot supply line example. Keeping these numbers in mind, if a department operates from hydrant pressure only, then its choice should be five-inch hose.

Weight Considerations

As mentioned above, the weight of an empty 100-foot length of five-inch line is 105 pounds and 80 pounds for four-inch hose. A department should use weight as criteria for deciding between the two diameters after all other performance considerations have been decided. The weight issue is usually only a problem when personnel use the wrong techniques for repacking the hose. One thing to consider if the choice is five-inch hose is to purchase it in 50-foot sections. Yes, the cost will go up somewhat because of the increased number of couplings, but the weight will go down by half.

Based on the water delivery capabilities of supply lines supported only by hydrant pressure, five-inch hose would be the proper choice. However, if a department agrees with supporting supply line evolutions with relay pump operations as well as multiple supply lines from the water sources as needed, both four- and five-inch hose will be able to move any amount of water required as long as the water volume is available from the water source and there are enough equipment and personnel to set up the proper evolutions. So basically, the difference between four- and five-inch is that a four-inch operation would have to work harder than the five-inch operation to accomplish the same goal. The choice is yours.

PAUL SHAPIRO is director of Fire Flow Technology. He is a nationally recognized instructor on large-flow water delivery. He is also a retired engineer from the Las Vegas (NV) Fire Department. He has authored numerous articles for fire trade magazines. He has been in the fire service since 1981, is author of Layin’ the Big Lines, and produced the first in a series of videos on large-flow water delivery.

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