Back Pressure Valve 4 : A valve designed to control flow rates in such a manner that upstream pressure remains constant. May be operated by diaphragm, spring, or weighted lever. (WT)
The Mudblogger (Oilfield, Energy and Related Photographs), Oil Field Glossary, TX, USA 2014. http://mudblogger.com/oilfield-glossary/, (Oil - Amer) {777}