An RFQ for stainless steel piping may describe one item as SCH 40 and another as SCH 40S. Because the two designations can produce the same nominal wall thickness at some sizes, buyers sometimes treat them as interchangeable. That shortcut can create a dimensional mismatch at other sizes or leave suppliers quoting against different assumptions.
The practical answer is simple: do not approve a substitution from the schedule label alone. Identify the governing dimensional standard, nominal pipe size and required wall thickness from the approved piping specification. Then carry the same information into the pipe, elbow, tee and reducer line items.
What Does a Pipe Schedule Number Tell You?
A schedule number identifies a wall-thickness series for nominal pipe sizes. It does not identify the stainless steel grade, manufacturing route or complete pressure capability of a piping system. Those details come from the material specification, applicable design code and project calculations.
Nominal pipe size is also a designation rather than a direct measurement of every dimension. For purchasing, "NPS 4" by itself is incomplete. A quotation line should state NPS together with the full schedule designation or the required nominal wall thickness and units. This gives the supplier a clear dimensional basis and lets the buyer compare the offer with the mating pipe.
Why Are SCH 40 and SCH 40S Different Designations?
ASME B36.10 standardizes dimensions for welded and seamless wrought steel pipe. ASME B36.19 covers welded and seamless wrought stainless steel pipe. The "S" suffix distinguishes schedule designations associated with the stainless steel pipe standard.
This does not mean SCH 40 and SCH 40S always have different walls. At some nominal sizes the listed wall thicknesses coincide; at others they can differ. A verbal statement that the schedules are "basically the same" is therefore not a purchasing control. Check the table in the standard edition required by the project and record the resulting wall thickness.
The project specification remains the authority for the order. A buyer should not change SCH 40S to SCH 40 merely because one designation appears more often in a supplier catalogue. If the requested size is outside a listed schedule range, use the approved nominal wall thickness or drawing requirement rather than inventing a schedule label.
Keep Pipe and Fitting Standards in Their Correct Roles
Pipe dimensional standards and fitting standards answer different questions. ASME B36.10 or B36.19 provides the pipe size and wall-thickness framework. ASME B16.9 covers overall dimensions, tolerances, ratings, testing and markings for factory-made wrought buttwelding fittings. Its scope includes fittings of any producible wall thickness, so citing B16.9 alone does not tell a supplier which wall the buyer needs.
The material specification is another separate requirement. For example, ASTM A403/A403M covers wrought austenitic stainless steel fittings for pressure piping and uses grade and class designations. "ASME B16.9, SCH 40S" does not replace a material grade such as an approved WP grade, and a material grade does not replace the fitting dimensions.
What Should a Stainless Steel Fitting RFQ Include?
Build each line item from the approved piping class, drawing and bill of materials. For an elbow or tee, state the common end size and wall. For a reducer or reducing tee, identify the size and wall requirement for every end; do not assume that one schedule applies to both ends.
- Fitting type: elbow angle and radius, tee type, reducer type, cap or other configuration.
- End sizes: NPS or DN for each connection, with the units or naming system used by the project.
- Wall requirement: the complete schedule label, such as SCH 40S, or the approved nominal wall thickness for each end.
- Material: material specification, grade and any required fitting class.
- Dimensional standard: designation and edition required by the project.
- Welding ends: the applicable end-preparation standard and any project-specific bore transition or drawing detail.
- Commercial scope: quantity, required documents, inspection, marking, packing and delivery destination.
How Should Buyers Compare SCH 40 and SCH 40S Quotations?
Compare the offered dimensions before comparing price. A compliance table makes differences visible:
| Check | Supplier response to record |
|---|---|
| Governing standard | Standard designation and edition used for the offered dimensions |
| Nominal size | NPS or DN for every fitting end |
| Schedule offered | Full designation, including the "S" suffix where specified |
| Nominal wall | Numeric thickness and units for each end |
| Deviation | Any proposed change from the RFQ, listed separately |
If two suppliers use different schedule labels but report the same nominal wall, verify both entries against the specified standard and size. If the walls differ, the quotations are not technically equivalent. Ask for a revised compliant offer or send the alternative to the responsible engineer for review. Purchasing should not accept the thinner or thicker option solely because it is available sooner.
A Clear RFQ Line Prevents Schedule Assumptions
A useful line-item format is: "90-degree long-radius butt weld elbow; NPS [size]; ASTM A403/A403M [approved grade and class]; ASME B16.9 [project edition]; ends to match NPS [size] pipe at SCH 40S or [approved nominal wall]; quantity [number]; documents and inspection per [project specification]." Replace every bracketed item with approved project information.
For reducers, repeat the size and wall for both the large and small ends. For replacement orders, confirm the existing pipe identification and measured dimensions against controlled records. A photograph may help explain the layout, but it cannot establish the required wall thickness.
Check the Purchase Order and Receiving Records
Carry the accepted schedule, numeric wall, standards and approved deviations into the purchase order. When the fittings arrive, check their markings and material documents against the same line item. Receiving personnel should not have to decide whether "40" was intended to mean "40S."
For related products, review Jiede Pipeline's butt weld pipe fittings and stainless steel seamless pipes. When requesting a quotation, send the approved size, complete schedule or nominal wall, material grade, quantity and document requirements so the offered pipe and fittings can be checked on the same basis.