The annular protrusion is symmetrical about the orifice and has a proximal end affixed to the valve body with {HEAD ~ an inwardly directed extension} which provides {TAIL ~ a sealing edge} 
provides
The annular protrusion is symmetrical about the orifice and has a proximal end affixed to the valve body with { HEAD ~ an inwardly directed extension } which provides { TAIL ~ a sealing edge }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL