Document jyVJ85J7d71adJeggd289J2kO

Fic 41-31 --The principle of a dry-pipe system is illustiatcd by these simplified drawings of a dry-pipe valve Compressed air in the sprinkler system holds the dry valve closed, preventing water from enter ing the sprinkler piping until the air pressure has dropped below a predetermined point Covrteut National Ftre Protection Association where piping is exposed to heezmg tempeiatuies It is essential, however, that the drypipe valve and water-supply line be located m a heated enclosure In the dry-pipe system, the piping contains compressed an that holds back the water by means of a dry-pipe valve (Fig 43-31) When a sprinklei nozzle opens, the au is re leased, the piessuie drops, and the diy-pipe valve opens to admit watei into the piping system These sequential actions delay the actual wetting Because of this delay, extra hazard occupancies are difficult to protect with a dry-pipe system In general, moie watei damage results with the dry-pipe system be cause more sprmkleis open than with the wetpipe system, that is, the fiie progresses fur ther, hence trips more sprmkleis, before the extinguishing action of the water takes effect To reduce this delay, quick-opening devices, such as exhausters or accelerators, can be added to dry-pipe systems to expel the air moie lapidly The system air pressure should be kept 15 to 20 psi above the normal tupping pressure, highei pressuie delays the action of the drypipe valve The system should not lose moie than 10 psi an piessure per week, which would requne repressurizmg oftenei than once a week, equipment that leaks more is not acceptable (See NFPA Standards 13 and 13A foi testing dry pipe systems ) It is essential that all parts of a dry-pipe system be installed so that they can be thioughlv diamed Therefore, where it is necessary to use inverted (pendant) spnnkleis or "drop piping," a special type of pend ant sprinkler may be used oi the piping filled with an antifieeze solution Preaction. Preaction systems aie similar to dry-pipe systems, but react faster and, hence, minimize water damage m case of fire or mechanical damage to spnnklers or piping Supervision of the system against mechanical damage can be accomplished by connecting to the piping an automatic, low-pressure air supply that compensates for minor system leakage A rapid reduction m pressure re sulting from, sav, an accidental breakage of 1365