Three-way and four-way valves are multiport valves used for diverting, mixing, or selecting flow between multiple lines, typically utilizing a modified ball or plug design. They are governed by the general design codes of the base valve type, such as API 608 or API 599 (Metal Plug Valves), and dimensional standards often refer to ASME B16.34. Inspection and testing procedures must be specifically developed to confirm seat tightness across every available flow path and in every switching position. API 598 or equivalent testing protocols are adapted to verify the seal integrity against all ports using hydrostatic and pneumatic seat leakage tests.