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