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196 BRADLEY August
scientific auspices over the last few years have proved conclusively that such a limitation of vent to film load is dangerous, that it will produce pressures on the borderline of explosions, and that a rupture of the vault may and frequently does result. In case of such a rupture, neighboring vaults are involved in the losses, as actual incidents on record indicate.
A vent of 1 square inch for not more than 5 pounds of film is recommended ; or to state the matter another way where vaults are already constructed, not more than 5 pounds of film for each square inch of vent should be allowed. With such a ratio, no serious pressure is likely.
Let us examine this typical vault still further but this time in terms of the arrangements of film cans, and the habits of (nitrate) film fires on which recent experiments have thrown much light. For example, the typical vault is designed for open-rack storage with the cans placed on edge. Upright partitions extending from floor to ceiling divide these racks into sections 36 inches wide each. This arrangement provides for the storing of some 20 or 25 cans (depending on their thickness) side by side on each rack in each section. There are no horizontal separations between racks so that an open space exists from the floor to the ceiling. Experiments have shown that if the film in one can ignites, the fire will spread sideways to the adjacent cans at intervals of less than five seconds. The fire will also spread vertically to floor and ceiling. In this case, the section is the unit of risk. But that is not all; the fire may jump across the aisle and involve the film on the other side of the vault and cross-fire back and forth until the entire contents of the vault are involved. If the water system should fail, the duration of the fire is generally less than three minutes. If the water system does not fail and the sprinkler heads operate properly, the spread of fire will be slower but may, and frequently does, involve the entire vault. If, by chance, some of the film escapes the fire, it is likely to be damaged by water.
In such a circumstance, the gamble is that there will be no fire; for it can be seen that if a fire occurs, the protection is negligible. Or, granting the possibility of a fire, the gamble is that the water sprinklers will operate, that the progress of the fire will be retarded, that the pressure will not rupture the vault, and that the loss will be limited to the original vault involved. Let it be repeated that only when the material stored represents tertiary values or items that can be easily and cheaply replaced, could such a risk be justified.