How to read a flange drawing, and download one for your size
A flange drawing answers three questions: will it bolt up, will it weld to the pipe, and is it the flange the specification asked for. This guide shows how to read one, and where to draw and download any size we make.
A flange drawing shows the flange in two views: a section through its axis, which gives the thickness, hub and bore, and a view on the face, which gives the bolt circle and holes. Read each dimension by the standard's letter (O, tf, Y, X and W in ASME B16.5; D, C, H2, N1 and K in EN 1092-1) and check whether the thickness includes the raised face, because the two standards measure it differently. On this site you can draw any ASME B16.5 Class 150 or 300 flange from NPS 1/2 to 24, or any EN 1092-1 PN 10 or PN 16 flange from DN 15 to 600, and download it as a PDF sheet or a full-size DXF.
The two views
The section view is cut through the flange's axis. Metal the cut passes through is hatched, and the bore is left open. The bolt holes do not lie on the cutting plane, so they are rotated into it and drawn with hidden lines; that is how the thickness at the bolt circle can be read. The face view looks at the gasket face and shows the outside diameter, the raised face, the bore, the bolt circle as a chain line and every bolt hole.
On a weld neck flange the hub tapers from X at the back of the flange to A at the weld end, then runs straight for a short land before the weld bevel. A equals the pipe's outside diameter and B its inside diameter, so B tells you which pipe schedule the flange is bored for: 108.2 mm for NPS 4 Sch 10S in the drawing below.
Line types and what they mean
| Line | Looks like | Shows |
|---|---|---|
| Continuous wide | Visible edges of the flange | |
| Dashed | Hidden edges: the bolt holes in the section | |
| Chain (long dash, dot) | Centre lines and the bolt circle | |
| Thin with arrowheads | Dimension and extension lines | |
| Thin at 45° | Hatching: metal cut by the section |
These are the usual drafting conventions, set out in ISO 128-2 internationally and in ASME Y14.2 in the US, so a flange drawing reads the same as any other mechanical drawing.
Dimension letters in ASME B16.5 and EN 1092-1
| Dimension | ASME B16.5 | EN 1092-1 | Note |
|---|---|---|---|
| Outside diameter | O | D | |
| Flange thickness | tf | C1 (type 01), C2 (type 11), C4 (type 05) | ASME without the raised face, EN with it |
| Length through hub | Y | H2 (type 11) | Measured the same way as the thickness |
| Hub diameter at the back face | X | N1 | |
| Hub diameter at the weld end | A | A | The pipe outside diameter |
| Bore | B | B1 (type 01) | On a weld neck, A minus twice the wall |
| Raised face diameter | R | d1 | |
| Raised face height | 1.6 mm (0.06 in) for Class 150 and 300 | f1, 2 to 5 mm by size | |
| Bolt circle | W | K | |
| Bolt holes and bolts | Number, hole diameter, bolt size in inches | Number, L, metric thread (M16) |
Current ASME B16.5 gives tf and Y without the raised face. Older editions and many catalogs, our own 2015 catalog among them, include it, so an NPS 4 Class 150 weld neck reads 22.2 mm (0.88 in) thick in one table and 23.8 mm (0.94 in) in another. EN 1092-1 includes the raised face in C: a PN 16 DN 100 type 11 flange is 20 mm thick, 3 mm of which is raised face. A drawing that does not say which way it measures is not finished; ours state it on every sheet.
What the title block tells you
| Field | Example | Why it matters |
|---|---|---|
| Title | Weld neck flange, ASME B16.5 Class 150 RF | Type, standard, class and face in one line |
| Size and bore | NPS 4 (DN 100), bore Sch 10S, ASTM A182 F316L | The bore must match your pipe; the grade your specification |
| Notes | Source of the dimensions, raised face convention, reference status | Tells you how the numbers were measured and what governs |
| Dimension table | Every dimension in inches and millimetres, and the bolting | No need to scale off the drawing |
| Drawing code | RIC-B165-WN150-4-10S | Standard, type, class, size and bore; quote it in an RFQ |
| Scale, units, date | Scale 1:2.5 on A4 | Holds when printed at actual size |
| Links and contact | The page it came from and the quote form | Anyone who receives the PDF can find the source |
Using the PDF and the DXF
The PDF is a vector A4 sheet. Print it at actual size and the scale in the title block holds; on screen it stays sharp at any zoom. The DXF is drawn full size in millimetres or inches, as you chose, in DXF R12, the version almost every CAD program reads. Insert it into a piping layout or a nozzle detail, or lay it over the mating equipment flange to check the bolt pattern before you order.
| Layer | Contents |
|---|---|
| OUTLINE | Visible edges and the raised face |
| HIDDEN | Bolt holes in the section |
| CENTER | Centre lines and the bolt circle |
| DIMENSIONS | Dimension and extension lines, arrowheads |
| HATCH | Section hatching |
| TEXT | Dimension values and view labels |
| TITLE | Title, notes, bolting and source; switch it off to keep only the geometry |
Draw a flange
Choose the type, class, size and bore. This one starts on an NPS 6 Class 150 weld neck bored for Sch 10S in 316L, a common technology cooling system header flange. "Add to RFQ" collects each line in our quote form, so a whole flange schedule can be built while you draw.
| Letter | Dimension | mm | in |
|---|---|---|---|
| O | Outside diameter | 279.4 | 11.00 |
| R | Raised face diameter | 215.9 | 8.50 |
| f | Raised face height | 1.6 | 0.06 |
| tf | Thickness (min), without raised face | 23.8 | 0.94 |
| Y | Length through hub, without raised face | 87.3 | 3.44 |
| X | Hub diameter at base | 192.1 | 7.56 |
| A | Hub diameter at weld end (pipe OD) | 168.3 | 6.62 |
| B | Bore (pipe ID, Sch 10S) | 161.5 | 6.36 |
| W | Bolt circle diameter | 241.3 | 9.50 |
| h | Bolt holes, 8 off | 22.2 | 0.88 |
| Bolts | 8 stud bolts of 3/4 in; holes 0.88 in (22.2 mm) | ||
Line item Weld neck flange, NPS 6 (DN 150), ASME B16.5 Class 150 RF, bore Sch 10S, ASTM A182 F316L
The drawing shows NPS 6, Class 150. Turn on JavaScript to choose another size, rating or face and to download it.
The same tool, with the dimension tables, is on the weld neck, slip-on, blind and lap joint flange pages, and EN 1092-1 drawings are on the EN 1092-1 page.
Checking a supplier's flange drawing
- The standard, its edition and the class or PN are named in the title block.
- The drawing says whether thickness and hub length include the raised face.
- For a weld neck, the bore equals your pipe's inside diameter for its schedule or wall.
- Bolt circle, hole count and hole size match the mating flange or equipment nozzle.
- The face matches the mating flange: a flat-faced cast iron or plastic flange needs a flat-faced steel flange (see flat face vs raised face).
- The material grade is the one your specification names, and the certificate type is stated (see our sample mill test certificate).
Welded-on flanges are normally set so the bolt holes straddle the vertical and horizontal centre lines, with no hole on a centre line; fitters call it two-holing, and our face views are drawn that way. EN 1092-1 writes the same off-centre rule into the standard for its integral flanges (type 21).
One pipe, two standards: NPS 4 Class 150 and DN 100 PN 16
Projects in India, the Middle East and Europe often carry both systems: American chillers and CDUs with ASME nozzles, European pumps and valves with PN flanges. The two flanges below fit the same 114.3 mm pipe and still do not bolt together.
| Dimension | NPS 4, Class 150 | DN 100, PN 16, type 11 |
|---|---|---|
| Outside diameter | O 228.6 | D 220 |
| Bolt circle | W 190.5 | K 180 |
| Bolts and holes | 8 × 5/8 in, holes 19.1 | 8 × M16, holes 18 |
| Raised face | R 157.2, 1.6 high | d1 158, f1 3 high |
| Thickness | tf 22.2, raised face not included | C2 20, raised face included |
| Length through hub | Y 74.6, raised face not included | H2 52, raised face included |
| Hub at the back face | X 134.9 | N1 131 |
| Bore | B 108.2 (Sch 10S) | 107.1 (neck wall S 3.6) |
The EN drawing uses the standard's own letters and measures C2 and H2 from the top of the raised face. Both tools are on this site: ASME B16.5 Class 150 and 300 and EN 1092-1 PN 10 and PN 16, which follows the current 2018 edition.
Frequently asked questions
Is the PDF flange drawing to scale?
Yes. Each sheet states its scale, from 1:1 for the smallest flanges to 1:10 for NPS 24 (DN 600), and keeps it when printed at actual size on A4, or on US Letter from our US site. Printing with "fit to page" changes the scale. The DXF is drawn full size.
Which CAD programs open the DXF files?
The files are DXF R12, the most widely read version of the format. AutoCAD and AutoCAD LT, BricsCAD, DraftSight, LibreCAD, QCAD and most CAM programs open them. The geometry is split into layers, and the title and notes sit on their own layer so you can switch them off in one step.
Why do two drawings of the same flange show different thicknesses?
Usually because of the raised face. Current ASME B16.5 gives the thickness without it, older editions and many catalogs include it, and EN 1092-1 always includes it. An NPS 4 Class 150 weld neck is 22.2 mm (0.88 in) thick without its 1.6 mm raised face and 23.8 mm (0.94 in) with it.
Can I use your drawing in a submittal or with an order?
Yes, as a reference drawing of the standard flange; the purchase order and the standard govern. For anything non-standard, such as a special bore or drilling, send your own drawing or a marked-up copy of ours with the request for quotation.
Do the drawings give weights, stud bolt lengths or tolerances?
No. They show the standard dimensions and the bolt size. Tolerances come from the standard, and stud lengths depend on the gasket and the mating flange, so neither is printed on the sheet.
Sources
- ASME B16.5, Pipe Flanges and Flanged Fittings, NPS 1/2 through NPS 24 Metric/Inch Standard (dimension letters; thickness and hub length without the raised face)
- Rajendra Industrial Corporation, product catalog, 2015 web edition, flange dimension tables (raised face included in C and Y)
- CEN, EN 1092-1:2018, Figure 4 Note 1 (thickness C includes the raised face), clause 5.6.4 (bolt holes) and Table 13 (PN 16)
- ISO 128-2:2022, Technical product documentation, general principles of representation, Part 2: Basic conventions for lines
- ASME Y14.2-2014 (R2020), Line Conventions and Lettering