Know Your Supplier

Issue 2 and Volume 12.

A fire hydrant is a fire hydrant, right? It is in the ground, connected to a pipe, and it will give us as much water as we need when we need it, and it is fine to take whatever flow we need from it, correct? Just because it is there, it doesn’t mean that it will do anything that we want it to do anytime we need it to do that. Firefighters must understand a number of things about their hydrants to use them effectively when needed.

Water Supply

First, firefighters must understand where the water supply to the hydrant comes from. Is it from a gravity tank system, a pumped system, or some other arrangement? Unless it is a private industrial or some other specialty system, the water supplying the fire hydrant system also provides the regular nonfire water for residences and commercial and industrial facilities in the area.

There are plenty of small-water systems out there with a few hundred-thousand-gallon capacity or less. These systems normally see increased usage during daytime hours with total capacity reduced by the evening hours and the system refilled/replenished on the overnight hours when usage levels decline. This may be termed as the “recovery” period for the water system. However, if there are one or more significant industrial or commercial users on the system, fire departments should understand during what hours those uses occur and what impact they will have on the flow capacity of hydrants.

Departments also need to understand both the capacity and flow that water systems will permit to reduce to as a minimum for firefighting. In combined systems, there may not be a minimum reserved for fire protection use. Is the system refilled from wells, a stream, a creek, or a river? How are those supplies doing? As of November 2016, almost 50 percent of the United States was in some form of drought, with 12 percent of the United States in a severe, extreme, or exceptional drought. Almost 86 million people were affected by severe, extreme, or exceptional drought.1 Extreme or exceptional droughts were being experienced in 13 states. In drought areas, it may be difficult to refill the water systems feeding hydrants to full capacity on a regular basis. Other factors, including pump problems and leaks, can also limit both flow and total capacity of a hydrant system. If fire departments drain a limited system, the other users on the system will not have any water supply. Fire officers need to maintain current knowledge of what is occurring with hydrant systems in their area.

Two adjoining hydrants on two separate systems. What flows are available from which one?

System Pressure

When evaluating how much water is available for hydrant systems, look at both the static pressure and the residual pressure on the system. The static pressure is the pressure measured when no water is flowing from the system through hydrants. The residual pressure is measured when a flow is taken from the system, generally through a hydrant. The pressures and flow are graphed and will denote the available flow at the point where the static and residual pressure is measured. (While I won’t go into the details of how to perform flow testing, you can find more on this topic in Paul Shapiro’s article “Measuring Water Flow for Fire Suppression” in Fire Apparatus.2)

An important part of the flow evaluation is to leave a minimum of 20 psi residual pressure in the water supply system from the hydrant. Twenty psi residual pressure is an important number that is used to analyze water supply piping, as some pressure needs to remain in the supply system to avoid cross-contamination of the system and the potential to damage the piping if pressures are reduced too far.3 It is critical for pump operators operating from fire hydrants to watch their intake gauge and not allow it to go below about 20 psi when on pressurized sources unless they have been specifically instructed otherwise by a local authority.

Sprinklered Buildings

When dealing with a private water supply system, such as that for an industrial or commercial complex that has its own water supply system, or when dealing with a sprinklered facility on the public supply, understanding the supply is even more important. Buildings that are sprinklered will have a specific sprinkler flow demand that is normally intended to control the fire with a hose allowance for fire department use to finish the job. The fire department “outside” hose stream allowance may be 250 gpm or 500 gpm but is typically not more than this. (Find more information on fighting fires in sprinklered buildings in FireRescue’s “Firefighting in Sprinklered Buildings.”4)

The important thing to understand is what fire flow is available after the flow for the sprinkler system is accounted for. You can start with flowing the predetermined outside hose stream, but firefighters need to be aware that if they want to flow more than this, they need to understand where the water is coming from and ensure that the flows will not rob the sprinkler system of the proper pressure and flow that it needs to function. In particular, some are concerned about the potential flows that firefighters might use from steamer (large-diameter) outlets on newer fire hydrants that may be in place. Many older style hydrants on industrial/commercial properties had dual 2½-inch outlets that were intended to have hoselines connected to the hydrant to directly fight the fire within the restrictions of the hose stream allowances.

Sites with sprinkler systems may have their own independent fire protection loop, supplied from an on-site gravity tank, ground tank and pump, or pond/impoundment with a fire pump. While fire pumps are normally set to automatically come on with pressure drop, they are normally set to run until manually shut off. Firefighters should understand this system and how it works, as pumps may need to be manually started or valves changed if something is not functioning properly. At the very least, at the conclusion of the incident the fire department should work with facilities personnel to shut down any fire pumps that are running and restore the on-site fire protection system to full service. All of this should be trained on in advance of an incident occurring.

One key function that should be done at any fire in a sprinklered building is to connect to the building or property fire department connection and pump into it. This can help overcome potential concerns about “robbing” water supply from the sprinklers to be used for manual firefighting. This is a key factor supporting the need for incident commanders to understand the water supply at these facilities so that proper tactical decisions can be made. Knowing about water supplies that are not directly supporting the sprinkler system and how to tap into them (hose relays or tanker shuttles) may be key to properly attacking a significant fire in a sprinklered building.

An old-style industrial hydrant with only two 2½-inch connections. This hydrant for some reason was installed in the ground and is difficult to use.

Hydrant Understanding

One final key issue regarding hydrants is to understand that hydrants that are considered private but tied to the public system may be metered with a charge assessed to the owner if water is flowed from them. Firefighters wishing to use these hydrants, particularly for high-flow operations, should understand that there may be a cost for the property owner associated with the use of these hydrants.

Fire departments need to have a clear understanding of what flows and capacities are available from their hydrant systems under all conditions. They also need to work together with their local water departments to help plan for future changes/improvements to the water system to continue to improve fire protection for their coverage areas.

One place to start is to obtain the list of fire flow requirements for buildings throughout your coverage area. This can be obtained from the ISO.5 The ISO information is generally good and will show where significant fire flows are needed and what may be available. Any areas where adequate flows are not available are opportunities to work on for improvement, in both protection and with fire protection ratings for the community. Every hydrant is not the same, and understanding the water supply system can greatly help firefighters and officers to best use that system to protect their community.


1. The National Drought Mitigation Center, Drought Condition (Percent Area): United States, http://droughtmonitor.unl.edu/Home/TabularStatistics.aspx.

2. Shapiro, Paul, “Measuring Water Flow for Fire Suppression,” Fire Apparatus, www.fireapparatusmagazine.com/articles/print/volume-21/issue-4/features/measuring-water-flow-for-fire-suppression.html.

3. ISO, Water-Supply Evaluations, https://firechief.iso.com/FCWWeb/mitigation/ppc/3000/ppc3010.jsp.

4. FireRescue, Firefighting in Sprinklered Buildings, www.firerescuemagazine.com/articles/print/volume-8/issue-9/strategy-and-tactics/firefighting-in-sprinklered-buildings.html.

5. ISO, Requesting More Information from ISO, https://firechief.iso.com/FCWWeb/mitigation/ppc/5000/ppc5009.jsp.