ASME B31.5

Covers — The engineering design, materials selection, fabrication, installation, examination, and testing of refrigerant and secondary-coolant piping and heat-transfer components. This includes:

• Piping and components for volatile refrigerants (e.g., ammonia, carbon dioxide, freons).
• Secondary coolant systems (e.g., brine, glycol solutions) used for heat transfer.
• Systems subject to the low-temperature and cyclic conditions typical of refrigeration cycles.
• Condensers, evaporators, and cooling coils (as components of the piping system).

Does NOT cover (exclusions) — the following are governed elsewhere:

• Process piping in refineries/chemical plants → ASME B31.3; power piping → ASME B31.1.
• Water piping (other than secondary coolants) → ASME B31.9 or plumbing codes.
• Self-contained/packaged refrigeration units and the overall safety standard for refrigeration systems → ASHRAE 15.
• Pressure vessels/heat exchangers themselves (the shell/tube boundary) → ASME BPVC Section VIII.

Jurisdictional Boundary: B31.5 strictly governs refrigerant piping. However, where refrigerant service is just an incidental utility to a larger chemical or process plant, you must carefully confirm whether B31.3 or B31.5 has jurisdiction based on the owner's specifications and the plant's main code.

B31.5 sets strict minimum requirements tailored for refrigeration piping:

• Materials — Demands rigorous material selection focused on low-temperature toughness (impact testing) to prevent brittle fracture, and chemical compatibility with specific refrigerants and lubricating oils. Use of certain metals like copper, brass, or aluminum must be carefully matched with the refrigerant type (e.g., ammonia reacts aggressively with copper).
• Design — Pressure design of pipe and components must account for the maximum and minimum cycle pressures and temperatures, including transient states (defrosting, standby, or compressor surging). Includes rules for expansion and flexibility to handle thermal cycling.
• Fabrication & Installation — Specific joining requirements (brazing, soldering, or welding) suitable for preventing leaks of low-molecular-weight refrigerants. Requires cleanliness during assembly to prevent internal contamination.
• Examination & Testing — Mandates thorough leak and pressure testing (often pneumatic with dry nitrogen or tracer gases) appropriate for refrigerant service, as standard hydrostatic testing introduces unwanted moisture into the system.

ASME B31.5 is the specialized B31 code dedicated to refrigeration, secondary-coolant piping, and heat-transfer components, broadly covering both commercial HVAC and heavy industrial refrigeration systems (like cold storage or food processing).

While it utilizes the familiar baseline B31 pressure-design framework, it introduces highly specific rules to address refrigeration's unique hazards: the absolute necessity of low-temperature material toughness to avoid sudden brittle failure, strict chemical compatibility rules between metals and the chosen refrigerant, and the ability to withstand continuous cyclic pressures and wide temperature variations inherent in a refrigeration cycle.

It sits alongside its siblings B31.1 (power) and B31.3 (process) and is intended to work in conjunction with ASHRAE 15 (Standard for the Safety of Refrigeration Systems) and ASME BPVC Section VIII (for vessels and heat exchangers). Where a refrigerant system is embedded within a larger industrial plant, clear engineering judgment is required to confirm whether B31.5 or the plant's main piping code has jurisdiction over those specific lines.

Engineering Note: When designing under B31.5, always select and qualify materials for the absolute lowest temperature the refrigerant might reach under any condition (including transient or fault conditions, not just normal operation). Additionally, do not reflexively apply B31.3 to a refrigerant line simply because it is in a process plant — B31.5's specialized low-temperature and refrigerant-specific rules generally make it the more appropriate governing code where it has jurisdiction.