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