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Taking the guesswork out of the numerous variables that impact the water delivery time of dry-pipe fire sprinkler systems.
FOR AUTOMATIC DRY-PIPE fire-sprinkler systems, NFPA 13, Installation of Sprinkler Systems, requires a maximum water-delivery time of 60 seconds for all systems with a capacity larger than 750 gallons (2,840 liters). This time is measured from the moment the inspector's valve is opened at the location farthest from the water supply until water flows from the valve, theoretically providing a worstcase condition.
Until recent advancements were made in computercalculation methods, however, the numerous variables affecting the water-delivery time of dry-pipe fire-sprinkler systems couldn't be anticipated because it was difficult to show how they caused other factors, such as water supply, volume, and air pressure, to change, as well. Only after the system had been installed could the required water-delivery time actually be verified.
Fortunately, computer software is now available to calculate dry-pipe system performance, and the 2002 edition of NFPA 13 allows designers to submit these calculations as acceptance testing, rather than perform a traditional field-validation test.
The advantage of dry-pipe fire-sprinkler calculation software becomes evident when we look at the complicated interaction of the four basic variables that affect waterdelivery time in a dry-pipe fire-sprinkler system: air and water pressure; system capacity and piping configuration; the size of the test orifice; and the dry-pipe valve-trip ratio.
Air and water pressure includes system air pressure; static water pressure; and residual water pressure and flow. System air pressure is the pressure in dry-system piping that keeps the dry valve closed to prevent water from entering the system. This pressure is dictated by the static water pressure and the dry-pipe valve design.
Static water pressure is the pressure at the base of the sprinkler riser when it isn't flowing into the fire-sprinkler system.
Residual water pressure and flow are the pressure of the water at a given flow rate. Each water supply follows a unique curve in which the pressure drops as the flow rate increases. This pressure drop significantly affects the time required to establish a steady discharge of water from the test connection after the dry-valve trips.
The total volume of all piping on the system side of the dry-pipe valve is the system...