Explosion mitigating cover

A manhole cover adapted to behave as a relief valve to relieve pressure, said manhole cover movable from a seated position to an unseated position with respect to a frame, the frame having a peripheral wall defining an access opening, said manhole cover comprising:

• a cover portion having an upper surface, an outer peripheral edge, and an underside opposing said upper surface;
• a control leg extending from said underside of said cover portion and including a foot extending radially outwardly from said control leg to a position underneath said frame, said foot spaced vertically below said frame to permit at least a portion of said cover proximate said control leg to have a range of movement, such that said portion of said cover is capable of moving within said range of movement upon a pressure level on said underside reaching a threshold, said foot contacting said frame at a limit of said range of movement wherein said portion of said cover clears said frame to permit fluid flow from said access opening;
• and
• a first leg extending from said underside of said cover portion, said first leg spaced from said control leg, said first leg including a foot extending radially outwardly from said first leg to a position beneath said frame, said foot on said first leg contacting said frame to prevent a portion of said cover proximate said first leg from clearing said frame.

A manhole cover adapted to behave as a relief valve to relieve pressure, said manhole cover movable from a seated position to an unseated position with respect to a frame, the frame having a peripheral wall defining an access opening, the peripheral wall including a cover receiving flange said manhole cover comprising:

• a cover having an upper surface, an outer peripheral edge, and an underside opposing said upper surface, said cover defining an opening extending through the cover from said upper surface to said underside;
• a first leg extending through said opening in said cover, said first leg including an upper end and a lower end and defining a length therebetween, wherein said upper end includes a lug and said lower end includes a foot extending outwardly from said leg and underneath a portion of said frame, wherein said first leg is slidably movable within said cover opening to a closed position wherein said lug engages said cover and said foot is spaced at a vertical distance below said underside of said cover with said foot extending radially outwardly underneath the frame such that in response to increased pressure a first portion of said cover has a range of movement with respect to the cover receiving flange that is capable of providing a vertical clearance between said first portion of said cover and the frame to enable the escape of fluids through the access opening, wherein said foot contacts the frame at a limit of said range to prevent said first portion of said cover from moving beyond said limit of said range;
• and
• a second leg extending from said underside of said cover, said second leg spaced from said first leg, said second leg including an upper end and a lower end defining a length therebetween, said upper end affixed to said underside of said cover and said lower end including a foot outwardly from said second leg and underneath a portion of said frame, said length of said second leg being shorter than said length of said first leg to prevent a second portion of said cover from vertically clearing the frame and causing said cover to open at an angle with respect to the frame in response to movement of said first portion of said cover within said range of motion.