ASME B31.3 vs ASME B36.19: What’s the Difference?

Short answer: ASME B31.3 is the Process Piping code. It sets the rules for design, materials, fabrication, examination, inspection and testing of a piping system, and it does not contain its own pipe size tables. ASME B36.19 is the dimensional standard for stainless steel pipe: outside diameters and wall thicknesses for Schedules 5S, 10S, 40S and 80S. B31.3 lists B36.19 as an accepted component standard, so pipe supplied to B36.19 sizes fits a piping system designed to B31.3.

A customer recently asked us: "Why is your standard B36.19 when our engineer's sizes are B31.3?" The question is common, and the answer is that the two documents do different jobs.

What does ASME B31.3 cover?

B31.3 is the Process Piping section of the ASME B31 Code for Pressure Piping. ASME describes it as covering "materials and components, design, fabrication, assembly, erection, examination, inspection, and testing of piping" in refineries, chemical, pharmaceutical, hydrogen, semiconductor and cryogenic plants and related terminals (ASME, B31.3 product page). As of October 2026, the edition shown on the ASME site is B31.3 2024.

The code tells the designer how thick the pipe wall must be for a given pressure and temperature (para. 304.1, pressure design of straight pipe), which materials are allowed, how welds are made and examined, and how the system is pressure tested. It does not publish its own list of pipe diameters and schedules. Instead, para. 300(c)(4) requires that "piping elements shall conform to the specifications and standards listed in this Code" or be qualified under the code (ASME B31.3 2018 preview, Chapter I). Those listed standards sit in Table 326.1, Component Standards.

What does ASME B36.19 cover?

B36.19 "covers the standardization of dimensions of welded and seamless wrought stainless steel pipe for high or low temperatures and pressures" (ASME B36.19 2022 preview, section 1). Its Table 2-1 gives, for each nominal pipe size (NPS) and DN, the outside diameter, the nominal wall thickness for each schedule and the plain end weight.

A few points from the standard itself:

• The 2022 edition dropped the "M" from the title. Older drawings and older B31.3 editions say B36.19M; it is the same standard series.
• For NPS 12 (DN 300) and smaller, the outside diameter is larger than the nominal size (NPS 2 is 60.3 mm OD). From NPS 14 (DN 350) up, the OD equals the nominal size.
• Schedules 5S and 10S are thin walls developed for stainless steel, which is usually joined by welding. The standard states that 5S and 10S walls do not permit threading to ASME B1.20.1.
• Dimensions apply to pipe made to ASTM specifications (section 3). The ASTM material specification, for example ASTM A312, sets chemistry, mechanical properties and tolerances.

Does B31.3 reference B36.19?

Yes. B31.3 Table 326.1 lists ASME B36.10M (Welded and Seamless Wrought Steel Pipe) and ASME B36.19M (Stainless Steel Pipe) among the accepted component standards, together with B16.5 for flanges and B16.9 for factory made wrought buttwelding fittings (OpenExamPrep B31.3 study guide; Formación Industrial, ASME piping standards). The official table of contents of B31.3 confirms Table 326.1 "Component Standards" under para. 326 "Dimensions and Ratings of Components".

In practice the engineer works like this:

1. Calculates the minimum wall thickness for the design pressure and temperature under B31.3, then adds corrosion allowance and the mill under tolerance.
2. Picks the nearest standard schedule from B36.19 (or B36.10) that meets that thickness.
3. Writes the result on the line list or datasheet, for example "NPS 4, Sch 10S, ASTM A312 TP316L".

So when an engineer says "our sizes are B31.3", the sizes are almost always B36.19 or B36.10 schedules chosen under B31.3 rules.

B31.3 vs B36.19: comparison table

How do "S" schedules compare with B36.10 schedules?

B36.19 adds the suffix "S" to separate stainless schedules from the schedules in B36.10, which covers welded and seamless wrought steel pipe in carbon and alloy steel. For most sizes, 10S, 40S and 80S walls match Schedule 10, 40 and 80 in B36.10. The standard names the exceptions: Schedule 10S for NPS 14 to 22, Schedule 40S for NPS 12, and Schedule 80S for NPS 10 and 12 differ from B36.10 (ASME B36.19 scope). From NPS 14 up, 40S and 80S equal STD and XS (9.53 and 12.70 mm), so they are thinner than Schedule 40 and 80 in B36.10. B36.10 also contains other walls (for example Sch 20, 30, 60, XXS) that are commercially available in stainless steel.

Example values (mm), taken from B36.19 Table 2-1 as reproduced by Wermac and cross checked with Instemi pipe schedule data (B36.10 2022 / B36.19 2022):

This is why "Sch 40" and "Sch 40S" on a 12 inch line are not the same order. Always write the "S" when you mean B36.19.

Where do fittings and flanges fit in?

Buttweld fittings follow ASME B16.9 for dimensions and tolerances. Their wall thickness at the weld end is ordered to match the connecting pipe schedule, so a Sch 10S line needs Sch 10S elbows, tees and reducers. Flanges follow ASME B16.5 (NPS 1/2 to 24) with the bore of a weld neck flange matched to the pipe schedule. Both B16.9 and B16.5 are listed in B31.3 Table 326.1, in the same way as B36.19.

How should a buyer write the order?

Give the supplier four things: the size (NPS or DN), the schedule with the "S" if it is B36.19, the ASTM material grade, and any project requirements from the B31.3 design such as extra examination or PMI. A line like "NPS 6 Sch 10S, ASTM A312 TP304L, ends beveled, MTR to EN 10204 3.1" leaves little room for error.

HuaboPipe is a manufacturer of stainless steel fittings and forged flanges, also supplying stainless steel pipe. Our cold formed buttweld fittings are made to ASME B16.9 dimensions. Standard sizes DN15 to DN1200; larger sizes are made to drawing, up to DN3000. Wall thickness Sch10s to XXS. We supply pipe to ASME B36.19 and B36.10 standard dimensions. MTR is available on request, and PMI inspection is available. See our buttweld fittings and forged flanges, or read how to interpret ANSI flange dimensions.

FAQ

Is B36.19 a replacement for B31.3? No. B31.3 is a code for the whole piping system. B36.19 only gives stainless pipe dimensions. A B31.3 project still needs a dimensional standard for its pipe, and B36.19 is one of the standards B31.3 lists.

Can pipe made to B36.19 be used in a B31.3 system? Yes, when the material and wall thickness meet the B31.3 design. B36.19 appears in B31.3 Table 326.1, so its dimensions are accepted. The engineer still checks the wall against the pressure design.

What does the "S" in Sch 10S mean? It marks a stainless steel schedule from B36.19. Most S walls equal the B36.10 wall of the same number up to NPS 12, except 40S at NPS 12 and 80S at NPS 10 and 12. From NPS 14 up, 10S (to NPS 22), 40S and 80S all differ from Schedule 10, 40 and 80.

Is B36.19M different from B36.19? It is the same standard series. ASME dropped the "M" in the 2022 edition. Older B31.3 editions and drawings still write B36.19M.

Does B31.3 set pipe wall thickness? B31.3 gives the formula for the minimum required thickness. The pipe you buy is then chosen from the standard schedules in B36.19 or B36.10 that meet that minimum.

Can I thread Sch 10S stainless pipe? B36.19 states that Schedule 5S and 10S walls do not permit threading to ASME B1.20.1. Use 40S or heavier for threaded joints, or weld.

Takeaway

B31.3 tells you how to design, build and test a process piping system. B36.19 tells you the exact size of the stainless pipe in it. An order "to B36.19" for a "B31.3 project" is correct and normal. Ask your engineer for NPS, schedule and material grade, and send them to lucas@hbpipe.com for a quote on fittings, forged flanges and pipe.