Water Supplies for Sprinkler Systems

Sprinkler Systems Quick Courses

Overview

Automatic sprinklers are the most reliable and effective fire protection devices available today — provided they operate effectively. To determine if a sprinkler system can adequately protect a property, underwriters should be familiar with the components, operation, and testing of the systems, as well as potential causes for failure.

A sprinkler system consists of the following components:

Overhead piping specifically designed or hydraulically designed overhead piping consisting of a riser, cross mains, and branch lines.
A water supply and fire department connection
Underground and aboveground pipes
Valves and drains
Alarms and test connections
Automatic sprinklers

Every sprinkler system requires at least one automatic water supply of adequate pressure, capacity, and reliability. The water supply needs to be capable of supplying all designated sprinklers that operate during a fire (sprinkler demand) for the required duration, as well as accounting for the hose stream demand water used by the fire department operations during the fire.

Types of water supplies

Water supplies to sprinkler systems can consist of one or more supply sources:

Public/private mains
Elevated storage tanks (gravity tanks)
Ground-level suction tanks
Embankment-supported fabric tanks
Static storage—such as a lake, pond, or reservoir—with pumps
Pressure tanks

A hydrant connects to public water supply

Static storage for an office building

Public/private water supply mains

The most common and least expensive water source for sprinkler systems is the public water supply system. If the public water supply is the only source of water for the sprinkler system, it must be reliable and always maintain adequate pressure and volume.

To establish the capabilities of water supply systems, historical hydrant flow test data (within 5 years) can be obtained, or actual hydrant flow tests can be conducted as needed at or near the specific building location or locations. These tests can be obtained from or conducted by the local fire department during public protection gradings, the water company, or by reputable fire sprinkler contractors. Verisk does not conduct hydrant flow tests but will witness them upon request.

It is recommended that these hydrant flow tests follow the procedures and criteria as outlined in NFPA 291, Recommended Practice for Fire Flow Testing and Marking of Hydrants, or AWWA Manual M17, Fire Hydrants: Installation, Field Testing and Maintenance.

The purpose of the hydrant flow test is to measure the volume of water, in gallons per minute (GPM), and the associated pressure, in pounds per square inch (PSI), of the water distribution system available at a specific location. The testing process can identify partially closed or closed valves and any changes in the carrying capacity of the mains due to increased local area water demands or deterioration of the internal conditions of the piping itself.

A fire hydrant connects to the public water supply system

Hydrant flow testing should be conducted by qualified personnel and the proper equipment.

Elevated storage tanks

An elevated storage tank (or gravity tank) of adequate capacity and elevation is an acceptable water supply source for a sprinkler system. Elevation of the gravity tank determines the water pressure available. Elevation is directly related to water pressure, as every foot of elevation provides 0.433 PSI. For example, if the top level of an elevated water tank is nominally located 100 feet above the ground, the water pressure available would be 0.433 psi/ft * 100 ft = 43 psi. The capacity required depends on the intended use of the tank and the number of gallons available from the tank. For example, if the elevated tank was providing a water supply for a single sprinkler system, its capacity would be determined by the demand of the sprinkler system required for a specific duration. Standard sizes of elevated steel gravity tanks range from 30,000 to 500,000 gallons.

Ground-level suction tanks

Ground-level suction tanks are another acceptable source of water for a sprinkler system. Since these tanks do not have the benefit of high elevation to provide adequate pressure for their intended purpose. They require a UL/Listed fire pump to increase pressure to meet the demand of the system. The elevation of a ground suction tank is high enough to provide a positive suction pressure to the inlet side of the fire pump.

A qualified fire protection engineer or similar expert bases the tank size on the total volume of water required to meet the demand of the fire sprinkler system for the duration of time required specified in NFPA 13. This demand volume is determined by adding the sprinkler system demand plus the associated hose stream demand (accounts for FD use of hose streams) in GPM, and the duration in minutes. Fire pump selection is a process consisting of many variables and must take into consideration the most demanding use of a fire protection system, choosing the best combination of rated flow and pressure for a specific application.

Suction tanks are usually made of steel, and standard sizes range from 50,000 to 1,000,000 gallons. To prevent freezing and subsequent pump failure, such tanks circulate heated water or use embedded coils.

It’s very important that water stored in the tank doesn’t freeze. During freezing weather, water in the elevated tank may become unavailable because of an ice plug forming within the piping that supplies the fire pump and could burst the pipe, causing subsequent pump failure or extensive time being out of service due to maintenance.

There are three basic methods of heating tank water:

Gravity circulation of hot water
Steam coils inside the tank
Direct discharge of steam into the water

The type of heating method depends on the tank’s height, construction, size, and shape—and on the lowest temperature of exposure expected.

Embankment-supported fabric tanks

Embankment-supported fabric tanks can also function as suction tanks. The tank is usually composed of a reservoir liner with an integral flexible roof. A specifically prepared excavation supports the tank with embankments of earthen berms.

Tank capacity ranges up to 1,000,000 gallons in increments of 100,000 gallons. The tank can be located underground with the top of the tank at grade level or above ground with the earthen berms supporting the entire tank. To prevent freezing, many of the tanks use a water recirculation system with a heat exchanger.

Static storage with pumps

Water for sprinkler systems can also come from natural or man-made bodies of water, such as wells, lakes, ponds, or reservoirs. Those static sources need to deliver the water at the volume and duration required by the system and the anticipated hose streams. The water must be available 24x7x365.

To ensure that water is available for firefighting, a hydraulic engineer evaluates the water source. The engineer conducts a detailed study of the water source to determine if the required volume and capacity will be available, even during a 50-year drought in the area.

If the water comes from a dynamic source, such as a well, engineers must “proof” the water. They conduct tests to ensure that the water supply can keep up with the demand and draw of the fire pump. Safeguards must also be in place to make sure the pump won’t clog with debris, fish, or silt.

Hydraulic engineer evaluating the water source

Water supply source for sprinkler system

Pressure tanks

Pressurized water tanks also supply water to sprinkler systems. They're often located in rural locations where city water is unavailable and the demand on the sprinkler system is relatively light. Tank capacity is generally 5,000 to 20,000 gallons. Two-thirds of the tank is water, and one-third is air or inert gas. The air pressure in the tank is approximately 100 psi (pound-force per square inch).

Pressure tanks have very limited capacity and are designed to suppress room fires. Once the water in the tank is gone, there's no more water available to the sprinkler system. Well pumps, which don't have sufficient capacity to keep the sprinkler system flowing at the designed rates, usually fill the tanks. Monitoring of the water supply is critical for a pressure tank to work properly. It's important to monitor and maintain tank water level, water temperature, and air pressure.

Ensuring adequate water pressure

In addition to a water supply that has the capacity to supply the volume of water to all sprinklers that may activate in a fire, the water supply must be able to deliver that volume at an adequate pressure.

In locations where the volume of supply is adequate, but the water pressure isn’t high enough to deliver the required demand or provide for other fire protection needs, a fire booster pump is provided to produce the water pressure needed. Fire pumps don’t create water; they create pressure. They provide the additional pressure (energy) needed to overcome the friction losses associated with the volume of water needed to be delivered to the demand point as well as overcome the frictional losses associated with the piping and any elevation difference.

Electric motors or diesel engines can power fire pumps. Pumps operate automatically and should contain alarms indicating whether the pump is running or has become inoperative because of power failure or other conditions.

Fire pumps are specifically manufactured and listed for fire protection service. Each pump has a nameplate that states the rated flow (GPM), rated pressure (PSI), rated speed, and other information specific to that pump.

Fire pump powered by an electric motor

(Photo courtesy of APi Group, Inc.)

Fire pump powered by a diesel engine

Fire department connections (FDC)

Sprinkler systems may require additional water pressure during a fire. Fire department connections, also called Siamese connections, are located on the outside wall of the building or near the street and are specifically designed to supplement the fire sprinkler system. Such a connection allows the fire department to connect its pumper apparatus directly to the building’s sprinkler system to increase water pressure, as well as provide additional water to the system. In a larger fire with many additional sprinklers operating that might exceed the original design, supplying the FDC would help maintain higher pressures to the activated sprinklers to augment the degrading water supply to provide the minimum flows needed to control the fire during FD suppression operations.

Fire department connections must be:

Easily located
Accessible for use by the fire department
Clear of debris
Properly connected to the sprinkler system
Maintained in working order at all times

Fire department connection with caps in place

Fire department connection without caps and a bottle in a hose connection

Water supplies for sprinkler systems summary

A well-designed sprinkler system is a highly effective method of minimizing property damage from a fire. But simply installing a sprinkler system doesn't guarantee adequate fire protection. Proper determination of the water supply, as well as the availability and adequacy, is critical.

For example:

A reliable water supply is required for the system to be effective.
A water supply needs appropriate capacity, quality, pressure, and temperature.
Regular inspection, maintenance, and testing of water supply sources are vital to ensure that the water supply is adequate and available at all times.

Verisk has trained field staff to evaluate the design, installation, and condition of sprinkler systems anywhere in the country.

Sprinkler Assessment Report