SPRINKLER SYSTEM WATER SUPPLY ANALYSIS
An automatic sprinkler system is only as good as its water supply. Just because a building is equipped with a sprinkler system doesn’t mean the sprinkler system will be effective—it must be supported by a water supply that can meet the demands of both sprinklers and hose streams during a firelight.
To evaluate the adequacy of a sprinkler system, it is important to evaluate the water supply that is available to feed the system as well as calculate the amount of water needed to supply the sprinkler system and hose streams. It is equally important that the available water supply be checked periodically (at least yearly) for adequacy in meeting the original sprinkler/hose stream demand. Finally, the building the sprinkler system protects must be evaluated to ensure that its occupancy/hazard type or storage arrangement (in the case of warehouses) has not changed to render the sprinkler system inadequate.
EVALUATING WATER SUPPLIES
The same units of water measurement used to describe the foreground hoseline are used in the measure of sprinkler system water supplies: Gallons per minute, pounds per square inch, and friction loss are terms common to both.
Water flowing through city mains, sprinkler supply pipes, and branch lines loses pressure due to friction induced by the pipe’s interior surface. In addition, changes in piping elevations raise/lower water pressure under flow and no-flow conditions.
Having considered these hydraulic “influences” in the design of a sprinkler system, a sprinkler system designer turns to the water supply itself to determine its adequacy. The performance and reliability of a water supply for fire protection is best determined by water flow testing, which determines a water supply curve that plots available flow vs. pressure. (See sidebar on page 70.) Testing and associated calculations are based on fundamental principles of water flow through pipes and discharge from circular orifices. The results are accurate within five to 10 percent.
After the supply is figured, you then calculate the sprinkler system and hose stream demands to determine if the supply is adequate for the building.
SPRINKLER SYSTEM DEMAND
Sprinkler demand, expressed in gpm, is the amount of water per unit time that is required for adequate sprinkler protection. Theoretically, sprinkler demand is equal to density (gpm/ft*) multiplied by the demand area (ft’).
Density. This is the amount of water applied by the sprinklers per square foot over the demand area per minute. Densities range from approximately 0.05 to 0.6 gpm/ft depending on the hazard level of an occupancy and the demand area.
Demand area. This ranges from 1,500 to 6.000 square feet, depending on the hazard level of the occupancy and to a certain extent the type of building construction It is essentially a designated rectangular area that is the “predicted” fire size. The water supply to the sprinkler system is designed to supply a specific number of heads that have opened on the system—not every head opening on the entire system.
In some cases, a demand area and density will be specified, such as for the more-hazardous storage or operations facilities. For most cases, however, the designer has the flexibility to pick a demand area and density according to design tables found in such documents as NFPA 13 (figure 2-2. 1(b)) and the Factory Mutual Engineering and Research Loss Prevention Data Sheets. The tables allow the designer to select a higher density over a smaller demand area or vice versa for the type of hazard involved.
Demand. When determining the total water demand (gpm needed) to protect a facility, it is critical to consider sprinkler demand and hose stream demand. The density, demand area, and duration of demand vary depending on the combustibility of a building’s construction and occupancy. For example, note the variables in Tables 1 and 2, which show the total water demand for paper-processing and plastics-processing occupancies. For the paper-processing facility, the minimum sprinkler flow required (the demand point) is density multiplied by demand area: 0.15 X 2,500 = 375 gpm. The hose stream demand is 250 gpm.
HOSE STREAM DEMAND
The hose stream demand and duration estimates in Tables 1 and 2 are based on Factory Mutual Engineering and Research analyses of losses in fully sprinklered buildings and fire test data. Both hose stream demand and duration may be increased up to 50 percent based on other conditions. For example, water demand duration may be increased for areas that are relatively inaccessible for manual firefighting efforts, for persistent fires, and for areas where lack of drainage is a concern (where fuel cannot be flushed out or extinguished by sprinklers and it may be necessary to wait until the fuel is burned up).
Table 1. Paper Processing
Table 2. Plastics Processing
Hose stream demand may be increased for a particular property if there are expected areas with shielded fire potential. These could include combustible, concealed spaces such as those found in walls and ceiling ducts. In hollow-joisted or woodframe construction, hose stream demand should be increased by 250 gpm. Hose stream demand also may be increased for locations where fire departments are required to use more than the usual number of hose streams.
SUPPLY VS. DEMAND
Once the hose stream demand has been determined, its flow is subtracted from the available water supply line, found by flow testing (see sidebar on page 70), to determine the actual water available for the sprinkler system. For the paper-processing facility example, pick a point on the supply line and move 250 gpm to the left and plot a point. Do this at least twice and draw a line through the new points. This is the water supply with hose stream deduction line. (See graph, top right.)
Since the available water supply for the automatic sprinkler system and the sprinkler demand have been determined, the results can be compared. The top right graph plots the sprinkler demand point for the paperprocessing facility (assuming that the sprinkler operating pressure is 55 psi). Since the demand point falls below the available water supply line with a hose stream deduction, the water supply is adequate for both the sprinklers and the hose stream demand.
If the demand were above the water supply with hose stream deduction line, the end result likely would be an uncontrolled fire, even though the building is fully sprinklered. This would be the case if the same water supply protected the plastics-processing facility. The graph below shows the total water demand point, which is above the available water supply line with hose stream deduction.
KEEPING PACE WITH INDUSTRY
Calculating available water supply and sprinkler demand often is not seen as the fire service’s responsibility. However, as part of each prefire planning session, the fire department should know whether the sprinklered building is adequately protected. This information has a significant impact on safe and effective foreground operations.
In addition, research continues into automatic sprinkler systems to keep pace with new fire challenges that arise as industries continue to grow. It is important for the fire service to keep pace with evolving fire protection technology. This can be done through training and technical documentation, such as Factory Mutual Engineering and Research’s Loss Prevention Data Books for the Fire Service.