The main valve is biased in the opposite direction from that urged by the inlet pressure, so as normally to keep it open, but the main valve snaps shut when {HEAD ~ the inlet pressure forces} overcome the sum of {TAIL ~ the downstream gas pressure forces} and the biasing forces 
The main valve is biased in the opposite direction from that urged by the inlet pressure , so as normally to keep it open , but the main valve snaps shut when { HEAD ~ the inlet pressure forces } overcome the sum of { TAIL ~ the downstream gas pressure forces } and the biasing forces
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 HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH