Full Bore vs Reduced Bore Ball Valve: Pressure Drop, Cv and Pigging

In most process plant service, the pressure drop through a single reduced bore ball valve is negligible compared to the pressure drop in the pipe, fittings, and control valves in the same system. However, pressure drop becomes significant when: (1) gas service at high velocity (above 10 m/s) — the constriction velocity in the reduced bore increases and creates significant noise and pressure drop; (2) multiple reduced bore ball valves in series — cumulative pressure drop from several valves can be significant in low-pressure systems; (3) close-coupled to a control valve — turbulence from the reduced bore entering the control valve disturbs the Cv; (4) viscous fluid service — reduced bore significantly increases pressure drop for viscous oils and polymers. The pressure drop across a fully open reduced bore ball valve can be calculated from the valve Cv (available from the datasheet) using the standard Cv-to-pressure-drop formula. Vajra provides Cv values for all ball valve sizes in both full bore and reduced bore configurations.

No — using a full bore ball valve on a pipe size one class smaller (e.g., a full bore DN150 ball valve with DN100 flanges and pipe) creates an abrupt bore expansion inside the valve body where the ball bore (DN150) is larger than the pipe bore (DN100). This creates reverse flow separation, turbulence, and potential erosion at the bore transition. The correct approach is to always match the ball valve bore and flange size to the pipe inside diameter: (1) for full bore service: the valve bore equals the pipe ID at the valve's nominal size; (2) for reduced bore on the same line: install a standard reduced bore ball valve of the same nominal size as the pipe — the ball bore will be approximately one size smaller than the nominal pipe ID, creating a smooth reductionfrom the pipe ID to the ball bore and back. Never install a full bore valve of a larger nominal size onto a smaller pipe to achieve a 'full bore' connection — specify full bore at the correct nominal pipe size.

API 6D (Specification for Pipeline and Piping Valves) defines full bore (FB) and regular bore (RB) ball valves separately. API 6D does not mandate full bore for all applications — the choice between FB and RB is specified by the purchaser in the purchase order. Full bore is typically required by the project specification when: (1) the pipeline operator requires pig-compatible valves for inspection and maintenance pigging — the pipeline designer specifies 'full bore per API 6D' in the piping specification; (2) the line operates under DOT/PHMSA Part 195 or Part 192 regulations requiring pig compatibility; (3) custody transfer metering stations with electromagnetic or ultrasonic flow meters (full bore ensures meter calibration is valid); (4) high-velocity gas lines where reduced bore creates unacceptable noise or erosion. For process plant piping not covered by transmission pipeline regulations: API 6D regular bore (reduced bore) is acceptable and commonly specified to save cost and weight. Always check the project's piping class specification and the line class designation (piggable vs non-piggable) before selecting bore type.