Patent ID: 8033753

Claim:
Apparatus for preventing downward flow of substantial amounts of surface water into an underground ventilation duct communicating upwardly to a surface opening having a fixed length between opposing sides of the opening, comprising: a support having a top opening for fluid communication with said surface opening, a support floor, and an opening in a lower portion of the support higher than the support floor for fluid communication with a proximate portion of said ventilation duct, said support supporting one or more sets each comprising a seat and a paired water buoyant gate disposed to rotate upwardly toward said seat, each said set being arranged in said support to provide airflow from said lower portion opening to said surface opening that is a nominal fraction of the airflow from said ventilation duct to said surface opening absent presence of said apparatus, said fraction having the numerator 1 and a denominator which is the sum of 1 plus the number of said sets, the seat of at least one set being mounted under said top opening spaced from one of said opposing sides a distance nominally equal to said fraction applied to said fixed length, said buoyant gate having a seat engagement height nominally equal to said fraction applied to said fixed length, being positioned lower than said seat, and responsive to water rising in said support by floatingly upwardly until engaging said seat and blocking passage for air flow under said seat.