{HEAD ~ A check valve} in the vent inhibits the flow of fugitive gases to the vent and allows {TAIL ~ the escape} of the fugitive gases to the vent if the pressure of the fugitive gases exceeds a predetermined value 
allows
{ HEAD ~ A check valve } in the vent inhibits the flow of fugitive gases to the vent and allows { TAIL ~ the escape } of the fugitive gases to the vent if the pressure of the fugitive gases exceeds a predetermined value
HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH