Knowing Subsurface Safety Valve- API14A

A flapper-valve mechanism is less susceptible to malfunction than a ballvalve
assembly and offers several advantages during operation. Debris in the
flow stream and solids buildup from scale or paraffin are less likely to prevent
closure of a flapper valve than a ball valve. A ball valve can be damaged
more easily by a dropped wireline tool or other equipment lost in the wellbore.
Fluids can be pumped through flapper valves without damage to the flapper
sealing surface. The primary function of a subsurface safety valve is to close
and block flow when emergency conditions require halting well production.
The API has set an acceptable leakage rate of 5 scf/min [0.14 m3/min] for
newly manufactured subsurface safety valves. This is considered sufficient to
contain the wellbore pressure.
Charlie Chong/ Fion Zhang
https://www.slb.com/~/media/Files/resources/oilfield_review/ors02/win02/p52_64.pdf