Table 31 - Typical Number of Vacuum Breakers, NF6 and Roof Drains,a NF7

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Tank diameter D (feet) bNo. of vacuum breakers, NF6Pontoon roofDouble-deck roofNo. of roof drains, N F7 double-deck roof c5011110011115022220032325043530053735064d40074da This table should not supersede information based on actual tank data.b If the actual diameter is between the diameters listed, the closest diameter listed should be used. If the actual diameter is midway between the diameters listed, the next larger diameter should be used.c Roof drains that drain excess rainwater into the product are not used on pontoon floating roofs. They are, however, used on double-deck floating roofs, and are typically left open.d For tanks more than 300 feet in diameter, actual tank data or the manufacturer's recommendations may be needed for the number of roof drains.

                    
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