ASME B31.3 (Process Piping): What It Covers: and How Crestflo Components Comply
ASME B31.3, Process Piping, is the American Society of Mechanical Engineers code covering the design, materials, fabrication, assembly, erection, examination, inspection and testing of process-piping systems in petroleum refineries, and in chemical, pharmaceutical, textile, paper, semiconductor and cryogenic plants. It sits within the broader ASME B31 Code for Pressure Piping. Engineers, EPC contractors, panel fabricators and QA/QC inspectors turn to it constantly, whenever they need to confirm that a process line, along with the components inside it, meets a recognized pressure-boundary standard.
One rule matters throughout this page: ASME B31.3 is the process-piping code, not power piping (ASME B31.1) and not the pipeline codes (B31.4 for liquid, B31.8 for gas). Crestflo supplies the instrumentation tube fittings, valves and tubing used within a B31.3 system: small-bore impulse, sample, instrument-air and hydraulic lines. These are built from B31.3-listed materials, with the traceability and test evidence that the code's material-control and examination programs require. Crestflo doesn't design, fabricate or certify the piping system itself. That job stays with the owner, engineer and fabricator.
Also known as: ANSI/ASME B31.3, B31.3 and the Process Piping Code
Project specifications reference this code in several forms. It's written formally as ASME B31.3 or ANSI/ASME B31.3 (an American National Standard), shortened to B31.3 or "asme b31 3," and titled in full as the ASME Code for Pressure Piping, B31.3. In prose, people just call it the Process Piping Code, or sometimes the B31.3 process piping standard. ASME revises it roughly every two years. Recent editions include ASME B31.3-2022. Always design and procure to whatever specific edition your project specification or piping class actually names.
What ASME B31.3 covers: scope and applications
B31.3 governs metallic and nonmetallic process piping that carries chemicals, petroleum products, natural gas, steam, water, air, cryogenic fluids and other process media. Its scope covers pressure design of components, material selection, fabrication and welding, examination and leak testing. Piping owned by other B31 sections and by the ASME Boiler and Pressure Vessel Code falls outside it, by design.
| In scope of ASME B31.3 | Out of scope (owned elsewhere) |
|---|---|
| Process piping in refineries; chemical, pharmaceutical, paper, semiconductor and cryogenic plants | Power-plant piping → ASME B31.1 |
| Metallic and nonmetallic piping for process media | Liquid pipeline transportation → ASME B31.4 |
| Pressure design, materials, fabrication, examination, testing of that piping | Gas transmission and distribution pipelines → ASME B31.8 |
| Instrument, impulse, sample and utility lines within the plant | Boiler external piping within BPVC scope |
Process lines built to B31.3 show up across the industries Crestflo serves, including petrochemical and chemical, downstream refining, pharmaceutical and biotech, semiconductor plants, and LNG and cryogenic facilities.
Code vs standard: and is ASME B31.3 mandatory?
B31.3 is a code: it lays out requirements that a jurisdiction can adopt into law or that a contract can make binding. Left on its own, it's just a voluntary consensus document. But once an owner's engineering specification, a purchase order or a state or local regulation invokes it, compliance stops being optional, and that happens routinely on refining, chemical and pharmaceutical projects. People often call it a "standard" out of habit. Formally, though, it's a code, and it carries whatever force the adopting authority gives it.
ASME B31.3 fluid-service categories
What B31.3 demands, from how much examination to which leak test to how conservative the design has to be, comes down largely to the fluid-service category assigned to each line. That category tracks the hazard and duty of the service.
| Fluid-service category | Meaning | Effect on examination & testing |
|---|---|---|
| Category D | Nonhazardous service within limited pressure/temperature bounds | Lightest examination; relaxed provisions |
| Normal Fluid Service | The default for most process piping not in another category | Baseline random examination; hydrostatic/pneumatic leak test |
| Category M | Fluid service where a small exposure to a highly toxic fluid can cause serious harm | Most stringent, extensive examination and controls |
| High Pressure Fluid Service (Chapter IX) | Service designated by the owner as high pressure | 100% examination; heightened design rules |
| High Purity Fluid Service (Chapter X) | Piping requiring high-purity/cleanliness controls | Added cleanliness and joint provisions |
| Severe Cyclic Conditions | Service with significant cyclic loading and fatigue risk | Higher examination extent; stricter detailing |
| Elevated Temperature Fluid Service | Service in the creep/high-temperature regime | Additional design and examination provisions |
Category thresholds and the exact numeric limits sit in the applicable edition; assign the category from whichever edition your project invokes.
ASME B31.3 materials and allowable stress
B31.3 works from listed materials, meaning those with published allowable-stress values in its material tables (Table A-1), cross-referenced to ASTM and ASME material specifications. Unlisted materials are permitted only when qualified under the code's provisions. The allowable stress at a given temperature, together with the pressure-design equations, sets a component's required wall thickness. The tube fittings, valves and tubing Crestflo supplies are built from grades within that listed set.
| Component | Listed-material grades | Referenced product-form specs |
|---|---|---|
| Seamless / welded tubing | 316/316L, 304/304L, 904L, duplex/super-duplex, nickel alloys | ASTM A269, ASTM A213, ASTM A312 |
| Bar/forgings for fittings & valve bodies | 316/316L through 6Mo and high-nickel alloys | ASTM A182, ASTM A479, ASTM A276 |
For sour service, listed materials are constrained further by NACE MR0175 / ISO 15156 where invoked. See the 316/316L material page, Super Duplex 2507 or the full materials library for grade detail. Allowable-stress values are per the applicable edition of B31.3; material selection is customizable to requirement.
ASME B31.3 pressure design, examination and testing
Pressure design under B31.3 sets the size of the pressure boundary: wall thickness, branch reinforcement, and flexibility or stress analysis, all pulled from the listed-material allowable stresses. Welding follows ASME BPVC Section IX (qualified WPS/PQR/WPQ), and bending, forming, and heat-treatment rules stack on top of that. From there, the extent of examination and the leak test scale with the fluid-service category. Nondestructive examination (NDE) draws on the usual methods: visual (VT), radiography (RT), and, where specified, liquid penetrant (PT) and magnetic particle (MT, on ferromagnetic materials). The required percentage is set by the service category.
| Service category | Examination extent | Leak-test basis |
|---|---|---|
| Category D | Primarily visual examination | Leak test per the code's Category D provisions |
| Normal Fluid Service | Random examination (a defined percentage of welds) | Hydrostatic test, or pneumatic where permitted |
| Category M | Extensive / heightened examination | Test per Category M provisions |
| High Pressure (Ch. IX) | 100% examination | Test per Chapter IX provisions |
The default closure is a hydrostatic leak test at a pressure not less than 1.5 times the design pressure, adjusted for temperature by the stress ratio. A pneumatic or sensitive leak test is permitted where the code allows and hazard controls are met. Exact examination percentages and test-pressure formulas come from whichever edition is in force, but the principle stays constant across editions: more hazardous service means more examination and a tougher test. For the wall-thickness math behind this, Crestflo's guide to tube sizing and wall thickness covers it in more depth.
Tube fittings, tubing and valves in a B31.3 system
A B31.3 system isn't just large-bore pipe. Small-bore instrument, impulse, sample and utility lines fall within the code too, and B31.3 covers flared, flareless (compression) and other tube-fitting joints for that kind of service, plus listed-material tubing. That's exactly the scope Crestflo manufactures.
| Component | What it does in a B31.3 line | Product page |
|---|---|---|
| Compression / double-ferrule tube fittings | Flareless joints on impulse, sample and instrument-air lines; performance-qualified to ASTM F1387 | Tube fittings |
| Instrumentation tubing | Small-bore process, impulse and utility lines in listed grades | Instrumentation tubing |
| Instrumentation valves & manifolds | Isolation, gauge-root, bleed and double block & bleed duty | Instrumentation valves |
A note on qualification basis: compression tube fittings earn their performance qualification under ASTM F1387, while valves are qualified separately, against the applicable valve standards. That means ASME B16.34 and the relevant API valve standards, not F1387, since F1387 is a tube-fitting standard rather than a valve standard.
How Crestflo components comply with ASME B31.3
Crestflo manufactures instrumentation fittings, valves and tubing, and its products come from the B31.3-listed materials the code references, backed by the material-control evidence a B31.3 program expects. The table below maps that evidence to each component. The boundary stays the same: Crestflo supplies conforming components, not a certified B31.3 system.
| Evidence a B31.3 program needs | What Crestflo supplies |
|---|---|
| Listed materials | 304/304L, 316/316L, 904L; duplex 2205, super duplex 2507, 254 SMO, AL-6XN, Alloy 20; Inconel, Monel, Hastelloy; Titanium Gr2 |
| Traceability | EN 10204 MTC 3.1 standard (3.2 on request); heat/lot number on every part; see material traceability |
| Material verification | Positive material identification (PMI) on delivered product |
| Examination / test | In-house NDT and destructive testing at ISO/IEC 17025-accredited labs; 100% seat and shell leak testing on manifolds and valves; third-party inspection on request, see testing & inspection |
| Fitting qualification | Compression tube fittings performance-qualified to ASTM F1387 |
| Sour service | NACE MR0175 / ISO 15156 compliant where specified |
Origin is Indian, with US and European melt available on request, and full material traceability. TAA and domestic-melt questions get honest answers at origin & compliance. Crestflo holds ISO 9001 / 14001 / 45001 certification and is PED certified.
Request a quote for B31.3-ready fittings, valves or tubing, send your line list, tubing schedule or spec, or request a sample.
Related standards: the ASME B31 series and referenced codes
B31.3 sits inside the ASME B31 Code for Pressure Piping, and it references a family of companion standards for the components that go into a process line.
| Standard | What it governs | Relation to B31.3 |
|---|---|---|
| ASME B31.1 | Power piping | Sister section; power plants, not process plants |
| ASME B31.4 / B31.8 | Liquid pipelines / gas transmission | Sister sections; pipeline transport between facilities |
| ASME B16.11 | Forged socket-weld / threaded fittings | Referenced component dimensional standard |
| ASME B16.34 | Valve pressure-temperature ratings | Referenced for valves in the system |
| ASME B1.20.1 | NPT pipe threads | Referenced thread standard |
| ASME BPE | Bioprocessing / high-purity equipment | Applied on high-purity B31.3 Chapter X service |
| ASTM A269 / A213 / A312 | Tubing product forms | Listed-material specs for tubing |
| NACE MR0175 / ISO 15156 | Sour-service metallic materials | Constrains material choice where H₂S service applies |
The B31 series covers B31.1 (power piping), B31.3 (process piping), B31.4 (liquid pipelines), B31.5 (refrigeration), B31.8 (gas transmission) and B31.9 (building services), among others. Each is its own section, with a separate scope, its own basis for allowable stress and its own testing rules.
Why Crestflo for B31.3-ready components
Crestflo is the instrumentation division of a Government-of-India-recognized export house with four decades behind it and a track record delivering into the Middle East. That heritage in critical-service supply now backs a component program serving US customers. Here's the offer: ready stock for standard items, made-to-order for specials, and 6–8 week delivery. Listed-material grades run from 316/316L through duplex and high-nickel alloys. Every part carries EN 10204 MTC 3.1 (3.2 on request) with heat-number traceability. PMI, in-house NDT and ISO/IEC 17025 destructive testing are standard, and F1387-qualified compression fittings are part of the lineup. Private-label and made-to-print work round it out. We publish the material, traceability and examination structure in full, so your engineers can check the evidence before a print ever gets sent. We Engineer Confidence.
Frequently asked questions about ASME B31.3
Is ASME B31.3 legally required?
It's a voluntary consensus code on its own, but it becomes mandatory once a jurisdiction, an owner's engineering specification or a purchase contract adopts it. That's common on refining, chemical and pharmaceutical projects. Confirm which edition your project invokes.
Is ASME B31.3 a standard or a code?
It's a code, it lays out requirements that can be written into law or contract. People often call it a "standard" loosely, but that's not quite right. Formally, it's a code.
What is the difference between ASME B31.3 and B31.4?
B31.3 covers process piping inside plants. B31.4 covers pipeline transportation of liquids and slurries between facilities. The two codes differ in scope, in how they set allowable-stress bases, and in their testing rules.
What are the ASME B31 standards?
The B31 Code for Pressure Piping covers a whole family of standards: B31.1 for power piping, B31.3 for process piping, B31.4 for liquid pipelines, B31.5 for refrigeration, B31.8 for gas transmission, B31.9 for building services, and several more besides.
What is the latest edition of ASME B31.3?
ASME revises B31.3 on a roughly two-year cycle. Recent editions include ASME B31.3-2022, so check and buy against the specific edition your project calls out.
Where can I get a copy or PDF of ASME B31.3?
The code is published and sold by ASME; buy the official PDF or print edition from asme.org. Copies circulating on free download or PDF-sharing sites are typically outdated or unauthorized, so procure the current edition from ASME and design to the edition your project specifies.
Does ASME B31.3 cover instrument tubing and tube fittings?
Yes. Small-bore instrument, impulse and sample lines fall under B31.3, and the code covers flared, flareless and compression tube-fitting joints for that kind of service. Crestflo supplies B31.3-listed-material tube fittings and tubing for these lines.
What materials does ASME B31.3 allow?
Crestflo stocks listed materials with published allowable-stress values (Table A-1), cross-referenced to common ASTM specs. Unlisted materials get in only after they're qualified per the code. Within that listed set, Crestflo carries grades running from 316/316L through duplex, 6Mo and nickel alloys.
What examination and testing does ASME B31.3 require?
Category M and High Pressure service push the examination and leak/pressure test requirements higher. Closure typically means a hydrostatic test at no less than 1.5 times design pressure, though pneumatic or sensitive leak testing is allowed in certain cases. The exact percentages and pressures depend on whichever edition is currently in force.
Are Crestflo fittings and valves ASME B31.3 compliant?
Crestflo supplies components built from B31.3-listed materials, backed by EN 10204 3.1/3.2 MTC, heat-number traceability, PMI, and F1387-qualified compression fittings, that's the evidence a B31.3 material and examination program calls for. System-level B31.3 certification stays the responsibility of the engineer and fabricator.
How do I get B31.3-ready components and what is the lead time?
Crestflo can quote against your line list, tubing schedule, or spec. Standard items ship from ready stock, specials are made to order, and delivery runs 6–8 weeks. Every part comes with MTC and heat-number traceability.
Related standards and resources
- Standards library: every governing spec mapped to conforming alloys and products
- ASME B16.34 · ASME B16.11 · ASME B1.20.1 · ASME BPE
- ASTM A269 · ASTM A213 · ASTM A312 · ASTM F1387 · EN 10204 MTC
- Tube fittings · instrumentation tubing · instrumentation valves
- Material traceability · testing & inspection · origin & compliance