Sprinklers hydraulics

A deluge sprinkler system is a sprinkler system designed to NFPA 13 with open sprinkler heads. A water spray system is hydraulically designed with open spray heads to protect a specific hazard. Water spray systems are discussed in Section 7.4.8. 

Water supplies for sprinkler systems can be provided by public or private systems, or a combination of both. In both instances, it is customary to also provide for a fire hose flow allowance. This allowance is particularly important with fire control sprinkler systems. Public systems can be augmented with fire pumps typically powered by a diesel or electric driver. Private systems are usually supplied by automatic fire pump(s) taking suction from a tank, reservoir, or natural body of water such as a river. The water supply should be capable of meeting the hydraulic requirements of the sprinkler system, have an adequate duration, and be considered reliable. In some cases, it may also be necessary to have two sources of water to improve the reliability of the system. This is particularly relevant in areas having an unreliable public water supply. Fire pumps should be automatic starting. If a reliable electrical supply is not available, a diesel driven fire pump should be used. 

System Piping—A system of piping progressively increasing in size in proportion to the number of sprinklers from the most remote sprinklers to the source of supply. The pipe size and distribution are determined from standards or hydraulic calculations as outlined by the NFPA, F.M. insurance underwriter, or other authorities for the hazard being protected. 

There are two methods for designing sprinkler systems. One is hydraulic design. The other works from tables, charts, and data provided in the code. The code specifies the water pressure required at each head. Achieving those pressures at each head will ensure the proper water flow rate at the heads. Selection of sprinkler heads helps get the water where needed and makes sure all areas or locations have protection. 

Hydraulic Design In hydraulic design, calculations must show that a system achieves head pressures at all heads when water at some pressure and flow rate enters the system inlet. Hydraulic designs will usually reduce system components and costs compared to the table method. Computer programs help analyze sprinkler designs to determine whether they meet... 

In modem sprinkler systems (shown diagrammatically in Figure 4.2.10) water is supplied at pressure from tire town mains which charges a pressure tank. When a sprinkler head mptures, the loss of water pressure in the supply pipe triggers the alarm valve. The alarm is actuated by the alarm valve either by diverting part of tire water flow to a hydraulic gong or by actuating an electrical alarm. The pressure tank acts as a buffer to absorb any water hammer effects and to provide a temporary water supply if the mains flow is intermpted. 

The protection area per sprinkler tor hydraulically designed systems with densities below 0.25 gal/min fF may exceed 100 fF.but shall... 

NFPA 318 took an interesting approach for hydraulics on duct sprinklers. They allow the use of /8" heads. This is desirable in smaller ducts. It will reduce the amount of airflow restriction the operating sprinkler would create in the duct. By using less water and finer spray, the spray from the 3/8" head is more effective in extinguishing the duct fire. By putting less water into the ducts, there are less problems with drainage. 

Automatic sprinklers for cleanrooms or clean zones shall be in accordance with NFPA 13 and shall be hydraulically designed for a density of 0.20 GPM/square feet over a design area of3000 square feet. Note that this density exceeds the Ordinary Hazard Group 2 occupancy design density required by UFC 1003.2.5.3 (per NFPA 13, Fig. 5-23) of 0.17 GPM/square feet over 3000 square feet (NFPA 318 2-1.2.1). 

Sprinklers installed in duct systems shall be located at 12-foot intervals, and at changes in direction. The sprinklers shall be hydraulically designed to provide 0.5 GPM over an area derived by multiplying the distance between the sprinklers in a horizontal duct by the width of the duct. A separate indicating control valve shall be provided for sprinklers installed in ductwork. Drainage shall be provided to remove all sprinkler water discharged in ductwork. The sprinklers shall be accessible for periodic inspection and maintenance. (UMC 609.7, NFPA 318 2-1.2.6.1, NFPA 318 2-1.2.6.2, and NFPA 318 2-1.2.6.5). 

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