{HEAD ~ Apparatus} and method for stopping the propagation of {TAIL ~ ignited flammable gas} in a conduit 
{ HEAD ~ Apparatus } and method for stopping the propagation of { TAIL ~ ignited flammable gas } in a conduit
HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH