Types of High-Pressure Gas Cylinders: Materials, Uses, and CGA Standards

High-pressure gas cylinders—also known as compressed gas cylinders—are essential for safely storing gases at substantial pressures in industrial, medical, laboratory, and energy applications. While these cylinders may look similar, their materials, construction, and safety features differ in important ways that affect performance, inspection, and service life.

The Compressed Gas Association (CGA) develops the industry’s consensus standards for compressed gas cylinder safety, covering everything from selection and filling to labeling, inspection, and requalification. This guide highlights the major types of high-pressure gas cylinders, where they are commonly used, and the CGA standards that ensure safe handling and compliance.

The Anatomy of a High-Pressure Gas Cylinder

Every high-pressure cylinder comprises a pressure vessel (the shell), a threaded neck/shoulder region, and a valve assembly with a defined outlet connection. Critical standards include:

  • Valves and outlets. CGA V-9 covers cylinder valve design; CGA V-1 defines the valve outlet/inlet connections that minimize hazardous misconnections across different gases.
  • Pressure relief devices (PRDs). CGA S-1.1 provides selection criteria for cylinder PRDs; it’s foundational for safe overpressure protection on portable containers.
  • Marking & labeling. CGA C-7 sets out the guide to classification and labeling of compressed gases (Appendix A is incorporated by reference in U.S. regulations).
  • Safe handling. CGA P-1 is the go-to standard for safe handling of compressed gases in containers.

Seamless Steel Cylinders (Type 1): Durable, Long-Service Cylinders

What they are. One-piece, seamless steel shells (e.g., DOT/TC-3 series, UN 9809 specifications) designed for long service life and broad gas compatibility. They’re widely used for industrial gas, medical oxygen/air, and specialty gas applications.

Why they’re chosen. High durability, broad temperature tolerance, and mature requalification pathways.

Inspection & requalification.

  • Visual inspection: CGA C-6 provides accept/reject/condemn criteria for steel cylinders.
  • Hydrostatic/pressure testing: CGA C-1 details methods for pressure testing cylinders and tubes.
  • Ultrasonic testing: CGA C-20 identifies and describes ultrasonic test methods for these cylinders and in some cases is an alternative to hydrostatic/pressure testing.
  • Special steel designs: Certain high-strength steel designs (e.g., 3HT) have specific requalification provisions in CGA C-8.
  • Internal corrosion guidance: CGA C-17 helps prevent and detect internal corrosion—especially important for oxygen-bearing and moisture-prone services.

Aluminum Gas Cylinders (Type 1): Lightweight and Corrosion-Resistant

What they are. Seamless aluminum alloy shells (e.g., DOT/TC-3AL, UN 7866) prized for corrosion resistance and weight savings. They’re common in medical oxygen, breathing air, calibration gas, SCUBA, and many specialty gas applications.

Why they’re chosen. Lighter weight than steel, good corrosion resistance, and excellent portability for healthcare and field work.

Inspection & requalification.

  • Visual inspection: CGA C-6.1 covers aluminum alloy cylinder inspection.
  • Pressure testing: CGA C-1 applies.
  • Ultrasonic testing: CGA C-20 applies.
  • Valve installation: CGA V-11 provides guidance for installing valves in high-pressure aluminum alloy cylinders.

Composite Overwrapped Cylinders (COPVs: Types 2, 3, and 4): High Strength-to-Weight Performance

What they are. Lightweight cylinders using a metallic or polymer liner with fiber reinforcement (e.g., carbon or glass).

  • Type 2: Metallic liner with hoop-wrap reinforcement.
  • Type 3: Metallic liner with full-wrap reinforcement.
  • Type 4: Polymer liner with full-wrap reinforcement.

These designs deliver high strength-to-weight performance for mobility-critical uses—such as firefighters’ breathing air, aerospace, and hydrogen mobility/storage.

Inspection & requalification.

  • Composite specifics: CGA C-6.2 governs visual inspection and requalification of fiber-reinforced cylinders.
  • Moisture considerations: For composites with 6061 aluminum liners, see CGA C-22 on water corrosion behavior.

Note: Inspection intervals and test methods are specified by regulation for each cylinder design. CGA standards provide the detailed procedures used by requalifiers and owners.

How to Choose the Right Gas Cylinder for Your Application

Your “best” choice depends on the gas and how the cylinder will be used:

  • Gas service & compatibility. Oxidizers, flammables, toxics, corrosives, and inert gases impose different requirements. Start with CGA V-1 (connections) and CGA S-1.1 (PRDs).
  • Weight vs. durability. Aluminum and composites reduce weight (ideal for handheld, medical, or SCBA/SCUBA), while steel maximizes ruggedness. The C-6 series sets fitness-for-service criteria.
  • Operating environment. Temperature extremes, moisture, and outdoor exposure drive material and PRD decisions. See CGA C-17 for corrosion prevention and detection.
  • Regulatory/marking needs. Different sectors (medical, industrial, specialty) require specific labeling. See CGA C-7 for classification and labeling.
  • Filling practices. For nonflammables, use CGA P-15; for flammables, see CGA P-99. Both include pre-fill inspection and documentation guidance.
  • Change of service. CGA C-10 outlines how to safely prepare cylinders for a different gas service.

Inspection, Testing, and Requalification of Gas Cylinders

Regardless of type, high-pressure cylinders must be inspected before filling and periodically requalified.

  • Prefill checks and handling: Guided by CGA P-1 and filling standards (P-15, P-99).
  • Visual inspection: Criteria are outlined in the C-6 series (steel, aluminum, composite), with C-6.4 providing guidance for NGV/HGV containers.
  • Pressure testing: CGA C-1 describes hydrostatic and related methods; CGA C-8 applies to 3HT steel.
  • Ultrasonic testing: The criteria in CGA C-20 can be used in some cases as an alternative to pressure testing.

Tip: Many CGA standards are incorporated by reference (IBR) in U.S. and Canadian regulations. Always verify the edition required for compliance.

Training and Reference Tools from CGA

  • eLearning modules. Role-based courses cover filling industrial cylinders (TM-6), medical oxygen cylinders (TM-5), cylinder valves (TM-7), and periodic requalification (TM-12).
  • Handbook of Compressed Gases (Digital). A complete reference with gas monographs and chapters on cylinders, valves, and safety practices.
  • CGA safety resources. Posters, safety alerts, and topic pages on cylinder and equipment safety provide quick refreshers and links to standards.

Putting It All Together: Safe Cylinder Use Across Industries

If you need maximum ruggedness in harsh conditions, seamless steel is often the first choice. For portability and corrosion resistance, aluminum cylinders deliver. When the application demands high performance (firefighting SCBA, hydrogen mobility, aerospace), composite overwrapped cylinders provide unmatched strength-to-weight ratios—provided inspection and requalification follow CGA C-6.2 and related standards.

No matter the type, safety relies on following CGA standards for valves, PRDs, labeling, inspections, and filling practices. Explore CGA’s publications library and eLearning catalog to ensure your team has the knowledge and tools to manage cylinders safely and in compliance with regulations.

Quick Reference: CGA Standards and Resources