What Is a Full Bore Ball Valve?

Quick Answer

A full bore (FB) ball valve has an internal bore diameter equal to the pipe’s inside diameter, creating zero flow restriction. A reduced bore (RB) ball valve has a bore one size smaller than the nominal pipe size. Full bore is mandatory for piggable pipelines and preferred when minimal pressure drop is critical.

Full Bore vs Reduced Bore

The bore of a ball valve is the hole through the ball. In a full bore valve, this hole matches the pipe ID. In a reduced bore valve, the hole is typically one nominal size smaller (e.g., a 6” reduced bore valve has approximately a 5” bore opening).

Parameter	Full Bore (FB)	Reduced Bore (RB)
Bore size	Equal to pipe ID	One size smaller than nominal
Bore ratio (typical)	100% of pipe ID	~70-80% of pipe ID
Pressure drop	Negligible	Moderate (restriction at bore)
Flow velocity through valve	Same as pipe	Higher (due to smaller bore)
Pigging capability	Yes	No (pig cannot pass)
Ball diameter	Larger (more material)	Smaller
Valve weight	Heavier	Lighter
Face-to-face length	Per ASME B16.10	Per ASME B16.10 (may be shorter)
Cost	20-40% more than RB	Lower
Standards	API 6D, API 608	API 6D, API 608
Typical use	Pipelines, piggable lines, critical process	General process, utility

When Full Bore Is Required

Piggable pipelines: any pipeline that uses pigs (pipeline inspection gauges) for cleaning, batching, or inline inspection must have full bore valves throughout. A reduced bore valve will trap or damage the pig.

Minimum pressure drop: for long-distance gas transmission pipelines, even small pressure drops at each valve accumulate over hundreds of valves. Full bore eliminates this loss.

Erosive or viscous service: higher velocity through a reduced bore accelerates erosion in sand-laden production fluids. Full bore keeps velocity constant through the valve.

Metering stations: flow measurement accuracy depends on a uniform flow profile. Full bore valves upstream and downstream of meters prevent disturbance.

When Reduced Bore Is Acceptable

Reduced bore ball valves are acceptable and cost-effective for general process piping where pressure drop is not critical, utility services (cooling water, instrument air, nitrogen), and small-bore connections. The one-size reduction typically adds less than 0.1 bar pressure drop, which is negligible in most process applications.

Bore Dimensions Example

Nominal Size	Full Bore ID (approx.)	Reduced Bore ID (approx.)
4”	102 mm (4.026”)	78 mm (~3”)
6”	154 mm (6.065”)	102 mm (~4”)
8”	203 mm (7.981”)	154 mm (~6”)
10”	255 mm (10.020”)	203 mm (~8”)
12”	305 mm (12.000”)	255 mm (~10”)

Pro Tip

Check the project piping specification (pipe class) for bore requirements. Most EPC pipe class specifications mandate full bore ball valves on hydrocarbon process lines and allow reduced bore only on utility services.

Both full bore and reduced bore designs are available in floating and trunnion mounted configurations, rated per ASME B16.34.

Read the full guide to valve types