Investigation of HEPA filters subjected to tornado pressure pulses: Initial structural testing

Initial results from a program to determine response of 610 x 610-mm (24 x 24 in.) HEPA filters to tornado-induced pressure transients are described. Filters were structurally tested by subjecting them to 20.7 kPa (3-psi) pressure differentials at pressurization rates from 6.9 kPa/s (1 psi/s) to 34.5 kPa/s (5 psi/s). Two sizes of filters were used in the tests: 149 mm (5 7/8 in.) thick and 292 mm (11 1/2 in.) thick. The 149 mm (5 7/8 in.) thick filters failed catastrophically at 20.7 kPa (3-psi) pressure differentials. Pressure drop across the filters, not pressurization rate, appears to be the primary cause of failure. The 292 mm (11 1/2 in.) thick filters were also tested with the above pressurization rates. These units failed by breakage of the folded downstream ends of the fiber mat. Of four 292 mm (11 1/2 in) thick filters tested, a 20.7 kPa (3 psi) pressure differential produced failure at the lowest and highest pressurization rates, but not at two intermediate rates.