Physical Address
304 North Cardinal St.
Dorchester Center, MA 02124
ASME B31.9-2017 — Building Services Piping
🏢 ASME B31.9-2017, “Building Services Piping,” is the section of the ASME B31 Code for Pressure Piping dedicated to piping in industrial, institutional, commercial, and public buildings, and multi-unit residences. It applies to piping that does not require the range of sizes, pressures, and temperatures covered by B31.1. The Code prescribes requirements for the design, materials, fabrication, installation, inspection, examination, and testing of building-services piping systems, including piping within the building and within the property limits.
💡 B31.9’s role in the B31 family is “practical simplification”: it strips away the stringent power-plant provisions of B31.1 that are irrelevant to building services, so that HVAC, water supply, fire protection, and compressed-air systems can be designed with proper code authority without being over-designed.
1. Scope, Services, and Boundaries 📌
Chapter 900 (Chapter I, Scope and Definitions) first fixes the services covered. The Code applies to the following building services, except as excluded in para. 900.1.3:
- Water and antifreeze solutions for heating and cooling;
- Condensing water;
- Steam or other condensate;
- Other nontoxic liquids;
- Steam;
- Vacuum;
- Compressed air;
- Other nontoxic, nonflammable gases;
- Combustible liquids, including fuel oil.
The Code draws a special boundary for boiler external piping: for steam boilers, up to 15 psig (103 kPa gage) maximum; for water-heating units, 160 psig (1 103 kPa gage) maximum and 250°F (121°C) maximum. Boiler external piping above these pressure or temperature limits falls within the scope of ASME B31.1. Boiler external piping is the piping connected to the boiler and extending to the points identified in Figure 900.1.2.
⚠️ Boundary questions are the most common source of confusion: the same pipe run inside a boiler room may be governed by B31.1 up to the boundary points and by B31.9 beyond them. The boundary points must be clearly marked on drawings, and each stretch designed to its own code, or you risk nonconformances at inspection.
2. Material and Size Limits 🔧
The Code lists the maximum nominal size (and, where noted, wall thickness) for each material — a hard boundary of B31.9 applicability:
| Material | Max Nominal Size | Wall Thickness Limit | Typical Uses |
|---|---|---|---|
| Carbon steel | NPS 48 (DN 1200) | 0.50 in (12.7 mm) | Steam, condensate, heating water, fuel oil |
| Stainless steel | NPS 24 (DN 600) | 0.50 in (12.7 mm) | Sanitary water, chemical additives |
| Aluminum | NPS 12 (DN 300) | — | Compressed air, low-pressure services |
| Brass and copper | NPS 12 (DN 300); copper tubing 12.125 in (308 mm) O.D. | — | Water, heating, cooling lines |
| Thermoplastics | NPS 24 (DN 600) | — | Water supply, drainage, geothermal loops |
| Ductile iron | NPS 48 (DN 1200) | — | Water mains, fire mains |
| Reinforced thermosetting resin (RTR) | 24 in (600 mm) nominal | — | Corrosive-media services |
In addition, the Code sets upper limits on working pressure and temperature: piping systems generally not exceeding 150 psig (1 034 kPa gage) working pressure and 366°F (186°C) working temperature fall within scope; systems beyond these limits should be designed to B31.1, B31.3, or another appropriate code. Chapter III (Materials, para. 923) states general material requirements, and Mandatory Appendix I supplies allowable-stress tables as the basis of pressure design.
💡 For designers, “150 psig / 366°F” and the material size limits are two rulers: the first decides which code the system belongs to, the second how large a given material may be used. Only when both are satisfied is the system within B31.9; exceeding either limit means moving up to a higher-level code.
3. Design Criteria and Wall-Thickness Calculation 📐
Chapter II (Design) is the technical core of the Code, organized in Parts 1 through 6:
- Part 1 (901, 902): Design conditions and criteria — design pressure, design temperature, allowable stress, and load combinations.
- Part 2 (903, 904): Pressure design of piping components — straight pipe wall thickness, bends, tees, reducers, and other components.
- Part 3 (905–908): Selection and limitation of components — pipe, fittings, bends and intersections, valves, flanges and blanks, gaskets and bolting.
- Part 4 (910–917): Selection and limitation of joints — welded, flanged, mechanical/proprietary, threaded, flared/flareless/compression, bell and spigot, brazed and soldered.
- Part 5 (919–921): Expansion, flexibility, and support — thermal expansion, flexibility analysis, and support design.
- Part 6 (922): Design requirements pertaining to specific piping systems.
Pressure design of straight pipe uses the general formula common to the B31 family:
t = P·D / 2·(S·E + P·Y)
where t is the calculated wall thickness (before allowances), P the design pressure, D the pipe outside diameter, S the allowable stress at design temperature, E the weld/joint quality factor, and Y a coefficient dependent on material and temperature. The calculated thickness must then be increased by manufacturing tolerances, thread or groove depth, and corrosion/erosion allowances, and rounded up to a commercial wall thickness.
⚠️ A common mistake is to assume that a “150 psig system” can simply use Schedule 40 pipe without calculation. B31.9 requires pressure design checks for every system; especially for large-diameter carbon steel and hot-water systems, the calculated wall thickness often differs noticeably from what intuition would pick.
4. Joints, Expansion, and Supports 🔩
Chapter IV (Selection and Limitation of Joints) states the conditions under which each joint type may be used. Building piping relies heavily on threaded, brazed/soldered, and mechanical compression joints, and B31.9 restricts where and under what pressure, temperature, and media these are permitted. For example, bell-and-spigot joints are typical for ductile-iron water pipe; brazing and soldering apply to copper systems with requirements on filler metal, joint clearance, and heating practice; mechanical and proprietary joints must be used within the manufacturer’s ratings and installation procedures.
On expansion and flexibility, the Code requires flexibility analysis for systems subject to thermal expansion or restraint, favoring natural compensation (bends, expansion loops) and permitting expansion joints or bellows where needed. Supports must carry pipe and content weight and allow for thermal displacement, so that supports neither bind nor impose excessive reactions and secondary stresses. In high-rise buildings, vertical risers also need support accounting for inter-floor differential settlement and, where applicable, seismic loads.
🚨 Threaded joints are a chronic leak source in building systems. B31.9 restricts end preparation, sealant, and engagement for threaded connections, and threaded joints are not appropriate for high-temperature steam or vibrating services; where space is tight or frequent maintenance is expected, prefer flanged or mechanical joints.
5. Fabrication, Assembly, and Installation 🛠️
Chapter V (927–935) covers fabrication, assembly, and erection: welding of metals (qualification of welders, welding procedure specifications, post-weld heat treatment), brazing and soldering, bending, forming, fabrication of nonmetals, and assembly. The Code requires welding inspection proportionate to system criticality — building services generally do not demand B31.1-level NDT percentages, but critical pressure-retaining welds (for example, those within the boiler external piping boundaries) still require examination per the design documents.
Chapter VI (936, 937) covers inspection, examination, and leak testing. Leak testing is the key pre-commissioning activity: systems are generally required to undergo a hydrostatic or pneumatic leak test at a test pressure not lower than 1.5 times the design pressure (per the Code’s specific rules), held for the required time, and checked for leaks. Where a system cannot be filled with water, a gas test is permitted but only with strict precautions against pressure hazards.
💡 Practical note: pneumatic testing is far more dangerous than hydrostatic testing; the Code sets strict rules on pressurizing procedure, guarded zones, and pressure relief. In occupied buildings with confined spaces, prefer hydrostatic testing unless there is a genuine need for a gas test.
6. Engineering Practice Essentials 🧭
- 📋 Boundary-point management: Mark the boundary between B31.9 and other codes (B31.1, B31.3) on design drawings and as-built records, so that cross-code stretches are not built to the wrong requirements.
- 🎯 Material table checks: Verify the four elements — material, size, pressure, temperature — against Chapter 900 for every line, especially the upper limits of thermoplastic and RTR pipe.
- 🛡️ Joint selection: Write the applicable boundaries for threaded, brazed, and mechanical joints into the line specification and brief contractors accordingly.
- 📏 Flexibility analysis: For long straight runs and high-temperature water systems, always perform thermal displacement and support-load checks to prevent “locked” supports causing excessive secondary stress.
- 🧾 Test records: Retain the pressure-time record and leak-inspection reports of the leak test as completion and acceptance evidence — the most frequently missing, and most frequently challenged, documentation for building piping.
7. Frequently Asked Questions ❓
❓ Q: How exactly do we draw the line between B31.9 and B31.1?
A: By a combination of system parameters and location. Steam boiler external piping at ≤15 psig, and hot-water boiler external piping at ≤160 psig / 250°F, fall under B31.9; above those limits they fall under B31.1. In addition, B31.9’s overall ceiling is 150 psig and 366°F. Systems above the ceiling, or located within power plants and industrial plant boundaries, should normally use B31.1 or B31.3.
A: By a combination of system parameters and location. Steam boiler external piping at ≤15 psig, and hot-water boiler external piping at ≤160 psig / 250°F, fall under B31.9; above those limits they fall under B31.1. In addition, B31.9’s overall ceiling is 150 psig and 366°F. Systems above the ceiling, or located within power plants and industrial plant boundaries, should normally use B31.1 or B31.3.
❓ Q: Can building piping skip pressure design calculations and just use common wall-thickness schedules?
A: No. B31.9 requires pressure design per paras. 903/904. Common schedules are only commercial references; whether they satisfy the Code depends on design pressure, diameter, allowable stress, and allowances, and must be checked item by item.
A: No. B31.9 requires pressure design per paras. 903/904. Common schedules are only commercial references; whether they satisfy the Code depends on design pressure, diameter, allowable stress, and allowances, and must be checked item by item.
❓ Q: What leak test pressure is used, and can we test with air only?
A: Per para. 937, the hydrostatic test pressure is generally not less than 1.5 times the design pressure (subject to the Code’s specific rules). Pneumatic testing is permitted only under the Code’s conditions, with strict pressurizing procedure and safety measures. Hydrostatic testing is preferred.
A: Per para. 937, the hydrostatic test pressure is generally not less than 1.5 times the design pressure (subject to the Code’s specific rules). Pneumatic testing is permitted only under the Code’s conditions, with strict pressurizing procedure and safety measures. Hydrostatic testing is preferred.
❓ Q: Do B31.9 piping welds require nondestructive examination?
A: B31.9 does not mandate B31.1-level radiography percentages for ordinary building services, but critical areas such as boiler external piping still require examination per the design documents and the Code. Welder qualification and welding procedure qualification requirements remain in force — they are not waived.
A: B31.9 does not mandate B31.1-level radiography percentages for ordinary building services, but critical areas such as boiler external piping still require examination per the design documents and the Code. Welder qualification and welding procedure qualification requirements remain in force — they are not waived.
