Flange Bolt-Hole Orientation

ASME B16.5 specifies that bolt holes shall straddle the natural centerlines of the flange. In other words, no bolt hole sits directly on the vertical or horizontal centerline — they are equally spaced on either side of it.

This single rule governs every flanged connection in a piping system and ensures that flanges from different manufacturers, fabricators and contractors always mate correctly on site.

For flanges on horizontal piping (flanges in the vertical plane), bolt holes straddle both the vertical and horizontal centerlines of the flange face.

This means the top bolt hole is never at the 12-o'clock position — instead, two holes sit symmetrically on either side of the top-dead-center line.

For flanges on vertical piping (flanges in the horizontal plane), bolt holes are oriented so that they straddle the Plant North centerline.

Plant North is the reference direction defined on the plot plan and is not necessarily geographic north. Every project establishes a Plant North arrow, and all horizontal flange bolt-hole patterns are aligned to it.

• Interchangeability — Equipment manufacturers, spool fabricators and field crews all work to the same orientation convention. A flange made in one shop will align with its mating flange fabricated in another.
• Bolt accessibility — Straddling the centerline keeps bolts clear of obstructions such as pipe supports, structural steel and adjacent piping.
• Gasket alignment — Consistent bolt patterns ensure uniform gasket compression and reliable sealing.
• Maintenance — Standard orientation makes it easier to remove and replace flanged joints during shutdowns without re-working adjacent steelwork.

Deviations from the standard orientation are rare and should only be made with explicit engineering approval. Common exceptions include:

• Connections to equipment nozzles where the equipment vendor has fixed a non-standard bolt pattern.
• Tight spaces where bolt clearance requires a rotated pattern.
• Specific customer or project specifications that override the default convention.

In all cases the deviation should be clearly documented on the isometric drawing and approved by the lead piping engineer.