By Andy Soccodato
Fire hydrants are responsible for providing a water supply for a large percentage of fire departments in the United States. Firefighters are introduced very early on in their recruit training to how to perform basic connections to these invaluable water sources.
This usually involves learning how to “wrap,” flush, connect, and charge the hydrant with a single supply line attached. But what about those larger scale or “once-in-a-career” fires that require the full flow potential of the hydrant?
These fires will almost always require the pump operator to perform a heavy hydrant hookup to maximize the full flow potential of the fire hydrant (photo 1). This process, also known as “dressing the hydrant,” involves connecting large-diameter hose (LDH) to multiple ports on the fire hydrant. Over this two-part article, we will examine what effect heavy hydrant hookups have on the availability of water to the supply pumper and why it is so important for maximizing the water available for large-scale firefights. Part 1 of this series will focus on identifying the variables that affect a hydrant’s rated capacity, while Part 2 will focus on tactics that fire departments can employ to achieve the full rated capacity of a hydrant.
1 Heavy hydrant hookups allow the supply pumper to move the hydrant’s full flow potential. (Photos by author.)
The 20-PSI Rule
National Fire Protection Association (NFPA) 291, Recommended Practice for Water Flow Testing and Marking of Hydrants, assigns a color marking to fire hydrants based on their flow rating. Using this system, hydrants are categorized into one of four classes based on their flow capacities (Table 1).1 What is critically important to remember is that these flow ratings are established while maintaining a residual main pressure of 20 pounds per square inch (psi) and while all hydrant outlets are being used.1,2 It is only under these conditions that firefighters can expect maximum flow rates from fire hydrants.
The 20-psi rule is one that often stirs much controversy within the fire service. Most firefighters are taught to never take their intake gauge below 20 psi when operating from a positive pressure source. The primary reason this is taught is because if the pressure in the municipal water supply system drops below 20 psi, there is a risk of water mains collapsing or creating a backflow of polluted/contaminated water into the municipal water system.1,2 Failure to follow this rule could result in people getting sick. While there is obviously merit to this guideline, it only paints half the picture. To truly understand the 20-psi rule, firefighters must understand the difference between the residual main pressure and the residual intake pressure.
2 This TFT hydrant gate valve has a built-in pressure gauge on the intake side of the valve, which allows the operator to monitor the remaining pressure at the hydrant.
3 The master intake gauge in this photo is reading roughly 15 psi. Although the residual intake pressure is below the “20-PSI Rule,” the operation is still within safe parameters because the pressure at the hydrant is above 20 psi.
4 The gauge at the hydrant shows a reading of 30 psi. Although the residual intake pressure at the hydrant is only 15 psi, this is still considered a safe operation because the pressure at the hydrant is well above the 20-psi requirement.
Residual main pressure refers to the remaining pressure in the water main when water is flowing through the system. Residual intake pressure refers to the remaining pressure at the eye of the supply pumper’s impeller when water is flowing and is measured using the master intake/compound gauge on the supply engine’s pump panel. It is critically important to understand that these two values are not the same.
There is no way to truly measure the residual main pressure on a fire scene. This is simply because there is no pressure gauge arrangement that extends to the main feeding the fire hydrant being used. The closest we can get on the fire scene to measuring the residual main pressure is to place a gauge on the hydrant being used. Some manufacturers, like Task Force Tips (TFT), manufacture combination hydrant gate valves with built-in pressure gauges on the inlet side of the valve (photo 2). This allows the operator to have a better idea of what the residual main pressure actually is.
It is important to remember that fire hydrants are rated to flow their rated capacities when the residual main pressure—not the residual intake pressure—is 20 psi. Since friction loss is present any time water moves through hose, fittings, plumbing, and appliances, the pressure reading on the pumper’s master intake gauge will be lower than the pressure at the hydrant, especially at higher flow rates. It is for this reason that it is completely acceptable during high-flow operations for the supply pump operator to be operating with a residual intake pressure below 20 psi as long as the pressure at the hydrant remains above 20 psi (photos 3, 4). The operator can only make this determination if he places some sort of gauge arrangement on the hydrant initially.
5 This Pigeon Forge (TN) Fire Department pumper is maximizing the flow potential from this hydrant by employing a heavy hookup.
6 Double tapping should be considered the minimum action operators should take when connecting to a hydrant.
The goal during any high-flow pump operation from a hydrant should be to get the residual intake pressure and the residual main pressure to match as closely as possible. The only way this is possible is to use multiple outlets on the fire hydrant. This is primarily because it splits the flow several ways and tremendously reduces the friction loss in the system. The less friction loss in the system, the higher the resulting residual intake pressure on the supply engine’s master intake gauge (photo 5).
Using Multiple Hydrant Outlets
As stated above, fire hydrants are rated to flow their maximum volume with a residual main pressure of 20 psi and with all their outlets open. The most common dry-barrel hydrant configuration in the United States is a three-outlet hydrant that has one steamer port and two side ports. On most dry-barrel hydrants, the steamer port measures 4½ inches in diameter and the side ports measure 2½ inches in diameter each. The two most common internal hydrant main valve sizes are 4½ inches and 5¼ inches.
During an Insurance Services Office (ISO) Public Protection Classification Review, municipalities receive maximum credit for hydrants within their jurisdiction that are equipped with a 4½-inch steamer port, two 2½-inch side ports, and a 5¼-inch internal main valve.3 In addition, the American Water Works Association (AWWA) also recommends, as a best practice, that a 6-inch branch/lateral be used to connect a hydrant to the main that supplies it.2 This dry-barrel hydrant configuration will be used as a reference moving forward.
The Continuity of Flow equation can be used to explain how the volumetric flow rate from a hydrant can be maximized. This equation states as follows:
Q = A × V
Where:
Q = the gallon per minute flow rate
A = the area of the opening
V = the velocity of the water
Using this equation, it becomes apparent that the flow rate can be increased one of three ways: by increasing the area, by increasing the velocity of the water, or by increasing both. The pump operator can realistically only increase the area of the hydrant opening when dealing with a fire hydrant. This is mainly because fire hydrants are not equipped with individual underground pumps that are capable of increasing the velocity of the water when the area remains constant.
Increasing the area of the hydrant opening refers to using multiple hydrant outlets. To truly understand the advantage of this tactic, we must convert the diameter of the hydrant main valve and outlets into area values. Using calipers to get an exact measurement of the hydrant outlet internal diameters, the following values were obtained:
- 4½-inch steamer port = 4.576-inch
- 2 ½-inch side port = 2.576-inch
Table 2 lists the conversion values when the hydrant outlet and main valve diameters are converted into their respective area values. When we make this conversion, we see that a 5¼-inch hydrant main valve equals approximately 21.65 in2. It also becomes apparent when looking at the area values that when a 4½-inch steamer port and one 2½-inch side port are used together, the sum of their areas equal 21.65 in2. The major takeaway here is that the steamer port and at least one side port must be used together for the outlet area to equal the internal area of the hydrant main valve. For this reason, “double tapping” the hydrant by using the steamer port and at least one side port should be the minimum standard for maximizing hydrant flows (photo 6).
Further, when the steamer port and both side ports are used, the outlet area of the hydrant becomes 26.86 in2. When we compare this outlet area to the area of the hydrant main valve, we see that there is a 24% increase in the outlet area compared to the area of the main valve. However, the 6-inch branch feeding the hydrant still has a larger area measuring 28.27 in.2 This means that when we configure our dry-barrel hydrants using all three outlets, the limiting factor becomes the area of the hydrant main valve and not the pump operator’s connection configuration. During large flow operations, our goal must be to increase the outlet area as much as possible. The only way this can be done is to use multiple hydrant outlet ports, which takes full advantage of the flow potential within the hydrant itself.
In this article, we have examined the variables that affect a fire hydrant’s rated flow capacity. It becomes evident that to truly maximize a fire hydrant’s flow potential, the operator must perform a heavy hydrant hookup. This allows the hydrant main valve to be the true limiting factor regarding flow when the residual main pressure is reduced to 20 psi. Part 2 of this series will examine how the supply line diameter affects the full flow potential from a hydrant and how heavy hydrant hookups ultimately influence the residual intake pressures on the supply engine.
Endnotes
1,. NFPA 291, Recommended Practice for Water Flow Testing and Marking of Hydrants, 2022. National Fire Protection Association, 2021.
2. American Water Works Association. (2006). Installation, Field Testing and Maintenance of Fire Hydrants (M17): AWWA Manual of Practice (4th ed.). American Waterworks Association.ation.
3. Insurance Services Office, Inc, & Insurance Services Office, I. (2012). Fire Suppression Rating Schedule. Insurance Services Office.
ANDY SOCCODATO has served in the fire service for the past 15 years. He spent nine years of his career as a firefighter and driver/operator for the Charlottesville (VA) Fire Department. He is a full-time fire instructor II at the Tennessee State Fire Academy, where he oversees the driver/pump and aerial programs. Soccodato is the owner of The Water Thieves, LLC, which specializes in delivering street smart driver/pump operator courses.
