The dry pipe system is one in which water is turned on the main pipes that supply the pipes in the building, but an air pressure greater than the water pressure is maintained in the distributing pipes. When a sprinkler is open the air in the pipe system immediately begins to escape. The air pressure is thus lowered and water automatically flows into the system and escapes through the open sprinklers. The dry system is desirable only in places where wet pipes will freeze. The general Underwriters' requirements for proper installation of sprinkler systems call for the use of two independent water supplies, in order to secure the minimum rate of insurance, which may be from 30 to 50 per cent. reduction on the total insurance rates. One of these supplies must be automatic, and one should furnish water under heavy pressure. The Underwriters accept the following combinations: Public waterworks and duplex steam pump; public waterworks and air-pressure tanks; elevated gravity tank and duplex steam pump; public waterworks and elevated gravity tank; public waterworks and rotary pump; elevated gravity tank and air-pressure tank; elevated gravity tank and rotary pumps. In most of the foregoing combinations it might be possible to substitute the air-pressure tank in lieu of the gravity tank. The choice of these supplies is determined by the Underwriters. A steam pressure of not less than 50 lb. should be maintained at all times on the pump, and an automatic regulator should be applied to the steam pump, so that it will start automatically when a sprinkler is unsealed, furnishing the system with a full supply of water. Fire-pumps built according to the Underwriters' pump specifications of the Associated Factory Mutual Insurance Companies, dated May 20, 1893, have given the best satisfaction and are recommended. The 500-gal. pump, which is 10 X 7 X 12 (ordinary size for small mills), has 8" suction, 6" discharge, 3" steam, 4" exhaust, and 2" hose outlets. The 750-gal. pump, which is 16 X 9 X 12 (ordinary size for general use), has 10" suction, 7′′ discharge, 3" steam, 4" exhaust, and 3" hose outlets. The 1,000-gal. pump, 18 X 10 X 12 (usual size for large factories), has 12" suction, 8" discharge, 4" steam, 5" exhaust, and 4" hose outlets. A water gauge and a pressure gauge should be connected to the end of the tank, with try cocks and blow-off drips. Air-pressure tanks should be constructed according to the Underwriters' rules, and their capacities determined by the Underwriters. These tanks should be located in the upper story of the building and must be kept full of water, and an air pressure maintained (never less than 75 lb.) that will give not less than 15 lb. pressure on the highest line of sprinklers when all water has been discharged from the tank Fig. 4 shows the pressures and quantities of water in pres sure tanks at different water levels. The sizes of pipes for a sprinkler system will in all cases be determined by the Underwriters having jurisdiction. In December, 1896, at a meeting of the Underwriters, held in New York City, a committee was appointed to deal with the subject of automatic sprinkler installations and establish uni. form rules for same. The result of their work is indicated in the third and fourth columns of the following table: For branch lines on which there are more than 6 sprinklers the above pipe schedule shall, after passing the sixth sprinkler, apply to the next larger size of piping. When the sprinklers required on any single floor exceed the number allowed to a 6" pipe, then a number of supply pipes are used, either 4", 5", or 6" pipes. The Underwriters' schedules must be strictly adhered to by the constructing engineer, except where special permission is granted by the Underwriters to make changes. The size of the risers is determined by the number of sprinklers required on each floor of the building or in any section divided by fire walls. Belt, stair, or elevator towers should be treated as one room, and the pipe sizes arranged accordingly. In no case should risers or supply pipes for sprinkler systems be tapped for domestic purposes. |