When it is desired to open the valve, {HEAD ~ the pressure} in {TAIL ~ the annulus} is raised at the surface until rupture disc 62 fails 
in
When it is desired to open the valve , { HEAD ~ the pressure } in { TAIL ~ the annulus } is raised at the surface until rupture disc 62 fails
OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH