{HEAD ~ Control features} prevent {TAIL ~ the burner} from firing unless there is liquid flow through the shell 
prevent
{ HEAD ~ Control features } prevent { TAIL ~ the burner } from firing unless there is liquid flow through the shell
HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH