The ball valve.

The closure member is a sphere with a hole bored through it, held between two annular seats. A quarter turn of the stem swings the bore into line with the pipe, or presents the solid face of the ball to the flow. The pattern is an isolation valve, and sustained throttling chews the seats at the part-open edge.

The geometry buys speed and certainty. One ninety-degree stroke closes the line in a second, and the lever reads open or shut from across the plant. Resilient seats wipe the ball as it turns, giving the drop-tight shutoff the family is specified for. Where a valve is cycled often, interlocked or actuated, the quarter-turn stroke is the point. The multi-turn alternative for undisturbed block duty is the gate valve.

A full-bore ball carries a hole the same diameter as the pipe: pressure loss is close to nil, and pigs, scrapers and inspection tools pass straight through. That is why pigged pipelines and metering runs specify full bore. A reduced-bore ball is one size smaller through the middle: lighter and cheaper, at a modest extra loss.

Neither is the default: the enquiry has to say which. Where the line will never see a pig, reduced bore saves real money across a project’s valve count. Where it will, the saving is false economy. If the schedule is silent, the counter asks before quoting.

BS 5351 covers steel ball valves for the petroleum industries. API 6D is the pipeline valve specification: trunnion-mounted transmission and station valves are ordered against it. API 607 is the fire type test above. API 598 governs shell and seat testing before despatch, and the class markings trace to ASME B16.34, tabulated in the pressure class reference.

Designations are published reference data, not a claim about any particular valve. The governing edition is always the one your purchase order names. Confirm ratings against the manufacturer’s data for the size and seats quoted.