Butterfly valves are quarter-turn valves that use a rotating disc to regulate or isolate flow, popular for their light weight and compact design. Lug- and wafer-style versions typically comply with API 609 for design, face-to-face dimensions, and testing requirements, which often reference MSS SP-67 for high-performance specifications. Pressure ratings adhere to ASME B16.34. Inspection and testing are governed by API 598 or ISO 5208, mandating a hydrostatic shell test and a stringent seat leakage test, particularly for resilient-seated or high-performance types, to confirm tight isolation against the disc.