Back Pressure Valve 3 : A one-way check valve that is installed through the X-mas tree, into the tubing hanger, and prevents well fluids from flowing out of the well. (Spec 6A)
The Mudblogger (Oilfield, Energy and Related Photographs), Oil Field Glossary, TX, USA 2014. http://mudblogger.com/oilfield-glossary/, (Oil - Amer) {777}