Overview - ISO 11621:1997 (Gas cylinders - Procedures for change of gas service)
ISO 11621:1997 defines general requirements and step‑by‑step procedures to be followed when refillable gas cylinders are transferred from one gas service to another. It applies to seamless steel, aluminium‑alloy and welded steel cylinders of all sizes (including large cylinders, water capacity > 150 L) for permanent and liquefied gases. The standard excludes cylinders for dissolved acetylene, radioactive gases and certain toxic/corrosive gases (group G in the standard’s grouping).
Key topics and technical requirements
- Gas grouping and transfer matrix: Gases are grouped by reactivity and service risk (e.g., inert, oxidizing, flammable, embrittling, carbon monoxide, toxic). Tables in the standard prescribe required actions for each From→To transfer.
- Mandatory preparatory actions: External inspection, verification of contents and identification, valve and pressure‑relief device checks, owner authorization, test‑date review and retest if required, safe pressure reduction (blow‑down), and removal/re‑marking of labels.
- Contamination control and cleaning: Internal inspection for liquids, hydrocarbons and corrosion; cleaning and drying procedures (particularly before oxygen/oxidizing service); requirements for sealing after cleaning to prevent recontamination.
- Moisture control: Visual checks for liquid water and moisture tests (dew point) for steel cylinders; drying where necessary to prevent condensation in service.
- Materials compatibility: Verify cylinder, valve and accessory materials are compatible with intended gas (see ISO 11114‑1).
- Non‑destructive testing (NDT) and hydrotesting: NDT (e.g., angle‑beam ultrasonic, acoustic‑emission) to detect internal defects, laminations or corrosion; hydrotests where indicated - especially if cylinder may have been exposed to water or incompatible service.
- Safety measures for evacuation: Evacuation, purging or water fill/drain to reduce flammable gases below lower flammable limits before internal work or valve removal.
- Special risks: Attention to embrittling gases (hydrogen), stress‑corrosion cracking risks (e.g., CO service), and restrictions on cylinders that are incompatible with current or prior services.
Practical applications - who uses ISO 11621
- Cylinder filling and exchange facilities
- Gas manufacturers and distributors (industrial, medical, specialty gases)
- Cylinder maintenance, repair and requalification organisations
- Safety officers, inspectors and compliance teams
- Laboratories, welding and fabrication shops managing cylinder pools
ISO 11621 provides a practical, safety‑focused framework to determine cleaning, inspection, NDT and retest needs when repurposing cylinders - reducing contamination risks, avoiding material incompatibility, and ensuring regulatory compliance.
Related standards (normative references)
- ISO 11114‑1 - Compatibility of cylinder/valve materials with gas contents (metallic materials)
- ISO 5145 - Cylinder valve outlets: selection and dimensioning
- ISO 10156 - Gases and gas mixtures - fire potential and oxidizing ability
- ISO 6406, ISO 10460, ISO 10461 - Periodic inspection and testing of gas cylinders
Keywords: ISO 11621:1997, gas cylinders, change of gas service, cylinder inspection, NDT, purging, moisture test, materials compatibility, oxygen service, hydrotest.
