Area of the operator piston exposed to the close operating pressure, divided by the cross-sectional area of the piston shaft exposed to wellbore pressure.
Source: API Specification 16A, Specification for Drill-through Equipment, Fourth Edition, April 2017. Global Standards
Closing Ratio (CR)
The area of the operating piston exposed to the close operating pressure, divided by the cross sectional area of the piston shaft exposed to well bore pressure.
Source: API STD 53, Blowout Prevention Equipment Systems for Drilling Wells, Upstream Segment, Fourth Edition, November 2012. Global Standards