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Views: 0 Author: Site Editor Publish Time: 2026-11-13 Origin: Site
Flange bolt hole orientation determines where the holes sit around the bore once the flange is fixed in place. Two stainless steel flanges can share the same bolt circle, hole count and hole diameter, yet still refuse to bolt together. This can happen when a replacement spool arrives with its holes turned relative to a fixed equipment nozzle.
The missing detail is the angular position of the pattern, often called clocking. This guide shows how to read it in an end view, where a project reference matters, and what to send with a drawing before machining or fabrication.
Bolt pattern dimensions and orientation are separate. Check both when neither side can rotate independently.
Straddling a centerline puts two adjacent holes on opposite sides of that line. An on-center pattern can put a hole directly on it.
Show the view direction and the reference. A note like “rotate clockwise” is incomplete without a defined starting line and viewing side.
Check the finished spool as well as the loose flange. The part may meet its drawing while the assembly is clocked incorrectly.
A bolt circle describes the circle that passes through the hole centers. Hole count and hole diameter describe how many openings there are and how large they are. Orientation describes where the whole pattern sits relative to a reference, such as the top of a nozzle or the plane of an elbow.
What to Check | What It Tells You | What It Does Not Tell You |
|---|---|---|
Bolt circle diameter | How far the hole centers sit from the bore axis | Where the first hole sits around the circle |
Hole count and diameter | How many openings there are and their size | Whether the two patterns line up at installation |
Orientation or clocking | How the pattern is turned relative to a datum | Whether the pattern dimensions, facing or rating match |
Use the flange measurement guide to identify the physical bolt pattern. When ordering custom stainless steel flanges, add the orientation to the part drawing if the holes must line up with another fixed feature on the part.
A loose, round flange with no other feature that fixes its angle can usually be turned before it is attached. Once it is welded to a spool with an elbow or branch, its bolt pattern has a fixed relationship to that geometry. The spool may no longer be able to turn to suit the mating flange.
Look straight at the flange face and draw a vertical line through the bore center. In a straddled pattern, the nearest holes at the top sit equally to the left and right of the line. In an on-center pattern, a hole can sit directly at the top. The same number and size of holes can appear in both views.
On an equally spaced eight-hole pattern, moving the layout by half the gap between adjacent holes changes one of these views into the other. Moving it by a full gap brings the hole locations back to the same positions. This is why a note that says only “rotate by one hole” may not describe the change the drawing needs.
ASME B16.5 includes bolt-hole provisions for flanges and flanged fittings. For example, the 2020 edition describes pairs of holes straddling fitting centerlines. Use the edition specified for the project.
The equipment and spool drawings still need to establish the installed reference. For a horizontal run, a vertical line through the face may be the reference shown. On a vertical run, the drawing may instead use a connecting pipe axis or a plant grid line. “Top” alone does not define an in-plane direction on a flange face that points upward.
The nozzle cannot turn. The replacement spool may also have a branch, support or elbow that must point a certain way. A matching bolt circle does not resolve a conflict between those two fixed positions.
Obtain the approved nozzle end view and identify its reference feature. If the interface already exists, confirm that the reference and hole positions shown reflect the installed equipment. A face photo can help explain which holes you mean when it shows the reference. Use a dimensioned record to control fabrication.
Each end may fit its own mating flange on paper, yet the finished spool may not fit both at once. A rotation that lines up one end can leave the other end out of position. An elbow or an offset makes this easier to see because rotating the spool also moves its pipework.
Review both end views together. Identify the viewing direction for each, and relate both to the same spool or plant datum. Check the clocking of each end before the flange positions are fixed, then verify the completed spool against the approved drawing.
A custom flange may have a feature that must sit at a particular angle relative to its bolt holes. Examples include a drilled port, a locating feature or a mounting detail. In this case, turning the loose flange also turns the feature. The orientation belongs on the part drawing, even before spool fabrication begins.
A loose flange can provide rotational adjustment where the design uses a compatible backing flange and stub end. That can help align matching holes during assembly. It cannot change the hole diameter, count or bolt circle.
Provide a face view along the bore axis. Mark which side the reader is looking from, such as “view from gasket face toward spool.” Clockwise on one side becomes counterclockwise when the same face is viewed from the opposite direction.
Then give the reader enough information to reproduce the pattern:
Name the datum. Show a plant axis, nozzle reference, elbow plane or another feature that can be identified on the actual assembly.
State the required layout. Show a pair straddling the datum, a hole on it, or an angular offset measured to a defined hole center.
Include the base requirements. Identify the flange standard and edition, size, rating, material and facing. For a custom pattern, provide the controlling dimensions.
Assign the check. State which drawing governs, who checks orientation, and whether acceptance applies to the loose part, the finished spool or both. Use the project’s angular tolerance if one is required.
A simple drawing note for a straddled pattern might read:
View from gasket face toward spool. Datum A is the vertical centerline shown in this view. The two upper bolt holes straddle Datum A equally. Verify at the finished spool against the approved end view.
For an offset pattern, show the direction and angle from the datum to the center of the named hole. Add the corresponding mating view for fixed equipment. A clear sketch and a short note are easier to review than an angle written in an email with no attached view.
Start with the pattern dimensions. If the bolt circle, hole count or hole diameter is wrong, rotating the part will not correct that difference. If those measurements match, compare the hole positions relative to the same datum and in the same viewing direction.
A pattern that appears uniformly turned suggests a clocking issue. If some holes line up while others do not, check for drawing, drilling or measurement discrepancies. Also verify whether the flange centers and faces are positioned correctly; a shifted pipe axis can look like a hole pattern error when viewed at an angle.
Record the mismatch before deciding on a correction. Do not enlarge holes or pull fixed flanges into position with bolts. Refer the discrepancy to the responsible engineering team to determine whether the part, spool or mating arrangement needs revision.
Flange bolt hole orientation is a separate fit-up check. A standard pattern can have the correct dimensions and still sit at the wrong angle on a fixed assembly. Use a labeled end view, a datum that both teams can find, and a clear check on the final part or spool.
SDFL supplies stainless steel flanges and can review drawing requirements for custom flanges. Include the mating interface and state which features must stay in position when the two sides are brought together.
Ordering a flange for a fixed nozzle or spool?
No. Spacing describes the distance or angle between neighboring holes. Orientation describes where the entire pattern sits relative to a reference. A pattern can turn without changing the spacing.
In common shop usage, two-holing refers to setting a pair of holes in relation to a reference, often with one hole on each side of a centerline. Use the drawing to confirm which pair and which reference the project means.
A round blind flange with no indexed feature can often rotate during fit-up if its bolt pattern matches. Orientation matters if a vent, drain, handle or other feature must point in a set direction.
A straight-on photo with a visible datum can help identify whether a hole sits on a line or a pair straddles it. It does not replace a dimensioned drawing or inspection record when a specific angle or tolerance applies.
Yes. Agree on the reference and record to be supplied. For a loose part, check against the approved part drawing. If the flange is welded to a spool later, the spool fabricator must also check the installed orientation against the assembly drawing.
