Overview
ISO/TR 22694:2008 - "Gas cylinders - Methods for establishing acceptance/rejection criteria for flaws in seamless steel and aluminium alloy cylinders at time of periodic inspection and testing" provides a technical basis for deriving quantitative allowable flaw sizes and acceptance/rejection limits for high‑pressure seamless gas cylinders. The report explains analytical modelling and experimental verification methods to predict how flaws (corrosion, pits, notches, cracks, LTAs) affect cylinder strength at both working and hydrostatic test pressures.
Key points from the report:
- Uses structural integrity and fitness‑for‑service analysis to calculate critical flaw size (CFS) and then derive allowable flaw sizes.
- Analytical approach is verified against experimental burst test data.
- Assumes no free moisture inside cylinders (no active corrosion) when establishing allowable sizes.
- Notes applicability caveat: main conclusions use DOT‑designed cylinder test/working pressure ratio (3:5); ISO‑designed (2:3) requires further calculation.
Key Topics
- Critical flaw size (CFS): flaw dimensions that will cause failure at designated pressures (working or test).
- Allowable flaw size: CFS reduced to account for time‑dependent degradation (e.g., fatigue crack growth) over the inspection interval.
- Modelling methods: API RP 579 (fitness‑for‑service) procedures are used for analytical predictions.
- Verification: comparison of API 579 analysis with hydrostatic burst tests on cylinders with induced flaws.
- Flaw types covered: cuts/gouges, cracks, local thin areas (LTAs), pitting/clusters, general/local corrosion.
- Fatigue consideration: example evaluation used 3 500 pressure cycles (approximate daily fills over a 10‑year retest interval) to adjust CFS to allowable sizes.
- Inspection techniques: supports quantitative ultrasonic inspection and wall‑thickness measurements used in modern periodic testing.
Applications and Users
Practical uses of ISO/TR 22694:2008:
- Establishing acceptance/rejection criteria for periodic inspection programs of seamless steel and aluminium alloy gas cylinders.
- Defining defect acceptance policies when using ultrasonic testing and other non‑destructive evaluation (NDE) methods.
- Supporting safety decisions in cylinder maintenance, recertification, and lifecycle management.
Intended users:
- Inspection and test laboratories, NDE technicians, cylinder manufacturers, regulatory authorities, transport safety agencies, and maintenance organizations responsible for cylinder periodic testing.
Related Standards
- API RP 579 (Fitness‑for‑Service) - analytical methods used in the report.
- National and ISO standards covering cylinder design, manufacturing and periodic inspection (refer to local regulations for pressure ratio differences and formal acceptance criteria).
Keywords: ISO/TR 22694:2008, gas cylinders, allowable flaw sizes, acceptance/rejection criteria, periodic inspection, ultrasonic inspection, fitness‑for‑service, API 579, seamless steel cylinders, aluminium alloy cylinders, hydrostatic testing, critical flaw size.
Open preview (PDF)Gas cylinders — Methods for establishing acceptance/rejection criteria for flaws in seamless steel and aluminium alloy cylinders at time of periodic inspection and testing
