{HEAD ~ The vent} acts to prevent {TAIL ~ ambient air} and thus, oxygen from entering the inner passageway of the pipes of a system when positive pressure, such as that caused by high temperature and fire, is present in the inner passageway of the pipes 
acts to prevent
{ HEAD ~ The vent } acts to prevent { TAIL ~ ambient air } and thus , oxygen from entering the inner passageway of the pipes of a system when positive pressure , such as that caused by high temperature and fire , is present in the inner passageway of the pipes
HEAD HEAD HEAD HEAD HEAD HEAD REL REL 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 OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH