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March 2004: Water Supplies for Fire Protection

When analyzing an existing sprinkler system or designing a new system, one of the key factors is the available volume and pressure of the water supply.

If the occupancy is such that the automatic sprinkler water demand is high, often times a supplemental on-site supply or a booster pump is needed to obtain the required flow and pressures.

Sprinkler systems may be supplied with water from one source or a combination of sources, such as public mains, gravity tanks, reservoirs or wells.

Public Mains

A connection to the municipal underground public water supply is the most common feed for a sprinkler system. It is

usually the first preference due to the fact that the supply is fairly reliable and does not require any maintenance on the part of plant personnel, as does a fire pump or an on-site water supply.

Depending upon the type of distribution within a particular municipality, the public mains can be fed by pumping stations, gravity tanks or a combination of both.

Gravity systems usually have moderate static pressures (usually 60 - 100 psi) with a limited pressure drop when flowing large volumes of water. Large tanks up to 5,000,000 gallons are usually constructed on elevated terrain within a community. If a plant is located fairly close to the public gravity tank, it is not uncommon to have a residual pressure drop of only 10 psi while flowing in excess of 1,500 gpm. If the ceiling sprinkler demand for a given occupancy were such that large volumes of water were needed at relatively low pressures, a supply such as this would be beneficial.

Public water supply distribution systems that rely on pumping stations normally have higher static pressures (80 - 120 psi) with residual pressures that could significantly drop when flowing moderate to high amounts of water. Again, this is based on distance from the pumping station, number of pumps provided, size and age of system piping and other factors.

It is very important to have accurate public water test information prior to designing a new sprinkler system or analyzing an existing sprinkler system. Fluctuations in the available water supply must also be considered, such as during the summer months when lawns are being watered and pools are being filled.

Booster Pump

A booster pump is an electric motor or diesel engine driven pump that takes suction from a public water supply. Its function is to "boost" the available pressure of an existing water supply. As an example, if a booster pump were connected to the gravity system referenced above, the same volume of water would be available at pressures considerably higher than the public has to offer.

If a sprinkler design is such that the volume of water needed from the public water supply is adequate but the pressure is not, a booster pump is an ideal match.

While the booster pump may solve a water supply problem, unlike a public water supply, the pump requires an extensive testing and maintenance program to keep it in proper operating condition. The required testing and maintenance is outlined in NFPA 20, "Standard for the Installation of Stationary Pumps for Fire Protection."

Fire Pump

A fire pump is also an electric or diesel driven pump. It, however, takes suction from an on-site water supply, most commonly a suction tank. A fire pump would be needed if the sprinkler demands of a particular occupancy were greater than the public water supply could deliver, even with a booster pump provided. Fire pumps can be obtained in various sizes/ratings to fit your needs with pumps approved and rated up 5,000 gpm.

Like a booster pump, a fire pump and the associated on-site water supply require an extensive testing and maintenance program to remain in proper working condition. NFPA 20 also covers fire pumps and NFPA 22, "Standard for Water Tanks for Private Fire Protection," addresses the installation and maintenance requirements for various water supply tanks.

Gravity Tanks

Elevated gravity tanks for private use are not as popular as they once were. They are very costly to construct and maintain. Most gravity tanks in service at industrial facilities were built in excess of 30 years ago. While they can be a viable means of fire protection water, pressures are usually low and the volume of water is limited.

Summary

Determining the adequacy of a sprinkler system for a given occupancy takes into account many factors. Considerations include commodity type, storage height and arrangement and a detailed water supply analysis.

Risk Logic, Inc. can help in designing a sprinkler system that will meet all applicable codes and insurance company requirements. Please contact our office and we can work directly with insurance carriers, sprinkler contactors or an in-house design department.