Flange bolt circles: a short checklist
A flange looks like the easiest part in the catalogue: a disc, a bore, a ring of holes. Yet most of the flanges I have had to redraw were wrong in one of the same few places. Here is the checklist I now run before calling one finished.
1. Start from the standard, not from a sketch
Outer diameter, pitch circle diameter (PCD), hole count and bolt size come as a set from the flange standard for a given nominal size and pressure class. Mixing a PCD from one table with a hole count from another is the classic way to get a flange that fits nothing.
| Size, PN16 | D | PCD | Holes | Bolts |
|---|---|---|---|---|
| DN25 | 115 | 85 | 4 × Ø14 | M12 |
| DN50 | 165 | 125 | 4 × Ø18 | M16 |
| DN100 | 220 | 180 | 8 × Ø18 | M16 |
Values above are the usual EN 1092-1 numbers I keep in my template. Always confirm against the edition your project specifies.
2. Straddle the centre lines
Bolt holes sit between the main axes, never on them. With four holes the first one is at 45°, with eight holes at 22.5°. In general the first hole is at 180° / n. This lets two flanges line up with valves and pumps whose bolts also straddle the axes.
3. Clearance holes, not bolt diameter
The hole is larger than the bolt: Ø18 for M16, Ø22 for M20, Ø26 for M24. Modelling the hole at the nominal bolt diameter is a mistake that only shows up during assembly.
4. Leave room for the nut
Check that a nut and a socket actually fit between the hole and the hub or the pipe weld. A quick way is to sketch a circle the size of the nut across corners around one hole and see if it hits anything.
5. Drive it all from one table
In my template the nominal size is the only input. A lookup table fills in D, PCD, hole count, hole diameter and thickness, and a circular pattern places the holes with the 180° / n start angle. Changing DN50 to DN100 then takes one edit instead of six — more on that in parametric habits.