： +8618066321396 ：xuran1603@gmail.com WhatsApp：+8618066321396
English
Views: 0 Author: Site Editor Publish Time: 2026-11-02 Origin: Site
A purchase order specifies a stainless steel weld neck flange: NPS 2, Class 150, raised face. The supplier still asks for the pipe schedule. Why is that information missing from an apparently complete description?
The flange’s bolted connection and its pipe-welding end have different requirements. Size and class alone do not settle the weld neck flange bore. This guide explains how pipe wall thickness affects that bore and what to specify before manufacture.
Specify the mating pipe’s dimensional standard, size and wall thickness alongside the flange requirements.
At a fixed pipe outside diameter, a thicker wall produces a smaller nominal inside diameter.
Keep pipe schedule, flange pressure class and flange plate thickness separate.
Use the approved drawing to define weld-end dimensions, preparation and any transition.
Treat a calculated nominal pipe ID as a reference, not an automatic flange acceptance limit.
A weld neck flange joins the pipe through a butt-welded connection at its neck. The pipe end and flange weld end must form the joint required by the design.
The flange’s bolt circle and facing address a different interface. Correct bolt alignment cannot demonstrate that the pipe-welding end is suitable.
When ordering a stainless steel weld neck flange, provide the mating pipe details with the flange specification. The supplier needs enough information to establish the intended bore and weld-end geometry.
An incomplete description can conceal a mismatch until fabrication. For example, a flange supplied for one pipe wall may meet a pipe with a different internal diameter.
This is especially easy to miss when their outside diameters agree. External alignment can look correct while an internal step remains at the joint.
Class 150 and Class 300 are flange rating designations. Schedule 10S, 40S and 80S identify pipe wall-thickness series within the relevant dimensional system.
These designations answer different questions. Neither “Class 150 means Schedule 40S” nor “Class 300 means Schedule 80S” is a valid general ordering rule.
Flange plate thickness is another separate dimension. Do not use the plate thickness or the broad hub section to infer the pipe’s wall thickness.
The pipe wall and flange rating must each meet the system design requirements. A bore comparison does not replace either selection.
For a circular pipe, nominal inside diameter can be calculated from its nominal outside diameter and wall thickness:
Nominal pipe ID = nominal pipe OD − 2 × nominal wall thickness
Keep all inputs in the same units. The result describes nominal pipe geometry, before considering manufacturing tolerances or special end preparation.
The following example uses nominal metric pipe dimensions associated with ASME B36.19. It illustrates the effect of wall thickness, rather than defining finished flange dimensions.
NPS 2 pipe schedule | Nominal pipe OD | Nominal wall thickness | Calculated nominal pipe ID |
|---|---|---|---|
10S | 60.3 mm | 2.77 mm | 54.76 mm |
40S | 60.3 mm | 3.91 mm | 52.48 mm |
80S | 60.3 mm | 5.54 mm | 49.22 mm |
The pipe OD stays at 60.3 mm across these three rows. Increasing wall thickness reduces the opening inside the pipe.
The nominal ID difference between the 10S and 40S examples is 2.28 mm. That is a diameter difference, not a permissible weld-joint offset.
Consequently, “NPS 2” alone does not identify the internal geometry the flange will meet. The schedule or specified wall thickness supplies an essential missing input.
The calculated values above are not universal machining instructions for a weld neck flange. The finished bore and weld end must follow the governing requirements and approved drawing.
Actual pipe dimensions can vary within the applicable product tolerances. End preparation or an approved transition may also affect the local geometry.
For inspection, establish which dimension is being checked and where it applies. A nominal pipe ID and a finished flange-bore limit are not interchangeable entries.
ASME B36.19 covers welded and seamless wrought stainless steel pipe dimensions. Its S-suffix schedules distinguish that dimensional series from the B36.10 series.
Some corresponding wall thicknesses agree at certain sizes. That does not make the series identical throughout their size ranges.
If the piping specification calls for Schedule 40S, retain that designation in the enquiry. Include the dimensional standard and applicable edition rather than shortening the description to “40.”
Likewise, clarify an instruction such as “standard bore.” It may reflect a supplier’s stock convention without identifying the intended pipe dimensions.
Ask for the proposed schedule, nominal wall and drawing reference in the quotation. Resolve any difference before approving the order.
Not every mating pipe is best described by an ASME schedule. A project may specify an outside diameter and wall thickness directly under another dimensional system.
In that case, provide those dimensions and their governing requirements. Do not assign the nearest schedule merely to complete an order field.
For an existing line with uncertain records, establish the pipe specification before choosing the flange bore. Measurements can support that work, but one reading cannot reconstruct the original design requirements.
A suitable bore reference is only part of the connection. The weld end also needs the specified outside geometry, end thickness and preparation.
ASME B16.25 addresses buttwelding-end preparation, including bevels and internal or external shaping. Apply it where the governing flange and project requirements call for it.
The opening visible at the flange face may not describe every section along the bore. A drawing may include a local counterbore, taper or transition.
Identify the weld-end plane and any transition length explicitly. Do not substitute a measurement at the face opening for a specified dimension at the welding end.
Use the flange measurement guide for general dimension checks. For bore approval, add the section drawing and weld-end requirements specific to the order.
A difference in pipe and flange end thickness does not automatically establish either acceptance or rejection. Assess the joint against the applicable design and welding requirements.
Where a transition is permitted, its geometry must be defined. “Grind to fit” is not a sufficient substitute for an accepted detail.
Avoid applying a single allowable internal-offset value to every project. The governing code, component geometry and approved fabrication requirements determine the applicable criteria.
This also means that two equally sized nominal openings do not prove a complete weld-end match. End preparation and permitted dimensional variation still need consideration.
Give the flange and pipe information together, preferably against one controlled drawing or piping-specification reference.
Information group | What to provide |
|---|---|
Flange | Type, standard and edition, NPS or DN, class or PN, facing, material and quantity |
Mating pipe | Dimensional standard, nominal size, nominal OD, schedule or specified wall, and material specification |
Bore and weld end | Required bore definition, dimension locations, tolerances, end preparation and any transition drawing |
Approval and records | Drawing revision, required dimensional report, identification method and approval responsibilities |
For an NPS 2 stainless steel line, the pipe portion of an enquiry might read:
Mating pipe: ASME B36.19, NPS 2, Schedule 40S; nominal OD 60.3 mm and nominal wall 3.91 mm. Confirm the proposed flange bore and weld-end detail on the approval drawing.
This example supplies the pipe geometry. It still needs the flange specification, material requirements, applicable editions and project-specific acceptance details.
Do not present “52.48 mm bore” from the calculation as a complete replacement for that information. A drawing should establish the actual finished requirement and tolerance.
If the connection needs a special bore, discuss custom flange manufacturing using the approved component and joint drawings. Confirm the design basis before machining a non-standard feature.
Review the quotation, drawing and purchase order for the same pipe-wall designation. A correct enquiry can still become an incorrect manufacturing instruction if the schedule changes during order entry.
At inspection, check the specified bore and weld-end features at their defined locations. Keep the results associated with the component or batch identification required by the order.
Distinguish bore diameter from roundness, end thickness and local end preparation. One diameter reading does not establish every aspect of the joint geometry.
Where permitted pipe variation could affect fit-up, review representative mating-pipe measurements under the approved inspection plan. Do not redefine the entire batch from a single unidentified offcut.
For mixed-schedule orders, keep line items distinguishable through packing and dispatch. Similar external flange dimensions can make the wrong bore difficult to notice.
If a mismatch appears, hold the affected parts for technical disposition. Enlarging a bore removes material, while reducing it is not an equivalent machining operation.
Any proposed rework therefore needs an accepted final geometry and reinspection plan. If the required joint cannot be achieved, supply the correctly specified component.
The right weld neck flange bore starts with the mating pipe’s dimensional standard, size and wall thickness. Flange class and nominal size alone leave that interface incomplete.
SDFL supplies stainless steel weld neck flanges and custom flange manufacturing. Providing the pipe specification and required weld-end drawing helps establish the intended bore before production.
Preparing a weld neck flange enquiry?
Send SDFL your flange and mating-pipe requirements. Include the standard, size, rating, facing, material, pipe schedule or wall thickness, quantity and required drawing or inspection records.
Pipe schedule informs the pipe-end geometry and required bore specification. For an otherwise unchanged standard flange configuration, it does not by itself select a different bolt pattern. Confirm both interfaces on the approved drawing.
Do not assume every marking contains the complete bore requirement. Match the component identification to the purchase order, approved drawing and inspection record. If the wall designation remains uncertain, resolve it before fabrication.
Not necessarily. The approved design may include a counterbore or transition. Identify the required diameter at each relevant section, especially the welding end, rather than assuming one measurement describes the entire internal profile.
No. A slip-on flange receives the pipe through its opening, while a weld neck flange forms a butt-welded pipe-end connection. Their bore requirements serve different assembly geometries. Use the dimensions specified for the actual flange type.
No. Bore compatibility is one dimensional check. Pressure suitability also depends on the applicable flange rating, material, temperature, pipe design and complete joint requirements. Matching openings does not independently establish system suitability.
