The Compressed Gas Association (CGA) and European Industrial Gases Association (EIGA) have recently harmonized key documents providing guidance on nonmetal oxygen compatibility:

• CGA G-4.4 / EIGA Doc 13: Oxygen Pipeline and Piping Systems
• CGA G-4.14 / EIGA Doc 200: Design, Manufacture, Installation, Operation, and Maintenance of Valves Used in Liquid Oxygen and Cold Gaseous Oxygen Systems

These publications outline suggested criteria and tests for qualifying nonmetallic materials for oxygen service, and the industry looks to these documents for the most relevant guidelines on designing and operating safe oxygen piping systems, including the rapidly growing sector of oxygen from electrolyzers.

In this article, the oxygen safety experts at WHA International Inc. outline the contents of the new guidance and which tests are recommended for different scenarios.

Importance of Using Oxygen-Compatible Materials

Using oxygen-compatible materials is not just a recommendation, it’s a crucial step for ensuring the safety and reliability of oxygen systems. Incompatible materials can pose serious risks, leading to oxygen fires in high-pressure enriched oxygen environments.

Plastics, elastomers, composite materials, and lubricants can be especially vulnerable to ignition.

Selecting materials that have been specifically tested and approved for oxygen use is paramount in preventing potential accidents and ensuring the integrity of oxygen systems.

Nonmetals Compatibility Test Methods

The CGA/EIGA documents reference several standardized tests to evaluate oxygen compatibility. These tests subject materials to extreme conditions of heat, pressure, and/or mechanical shock to determine their ignition sensitivity and fire-damage potential.

1.  Heat of Combustion Test

This test involves exposing the material to a pressurized oxygen environment while measuring the heat released during combustion. By quantifying the thermal energy produced, oxygen equipment designers can evaluate the material’s ability to initiate a kindling chain and create a larger fire within an oxygen system.

During the test, a sample of the material is placed in a controlled chamber filled with pressurized oxygen. An ignition source is then applied to initiate the sample’s combustion, and sensors measure the total heat generated as the material burns. The data collected allows for calculating the heat of combustion per unit mass, providing valuable insights into the potential fire propagation stemming from the material in service.

2. Auto-Ignition Temperature (AIT) Test

This test involves subjecting the material to escalating temperatures in a controlled setting to pinpoint the exact temperature at which it auto-ignites (without an external ignition source) in high-pressure oxygen.

During the AIT test, the sample is exposed to increasing temperatures while being monitored for signs of self-ignition. By identifying the precise temperature at which ignition occurs, researchers can evaluate the material’s ignition sensitivity when exposed to elevated temperatures in oxygen systems.

3. Liquid Oxygen Mechanical Impact Sensitivity (LOXMIS) Test

This test assesses the ignitability of nonmetallic materials in environments with extremely cold temperatures in liquid oxygen (LOX). It simulates the scenario where materials may be subjected to severe mechanical energy, such as impacts or vibrations, during operation in LOX.

During the test, samples of the material are immersed and chilled in liquid oxygen at atmospheric pressure. They are then individually subjected to a sudden mechanical force delivered by the impact of a falling plummet. Researchers can then observe the sample’s behavior, looking for evidence of ignition via light emission, explosive-like sound release, or charring of the sample during posttest sample inspection.

VIDEO: Watch a liquid oxygen mechanical impact sensitivity (LOXMIS) test.

4. Pneumatic Impact Tests

Pneumatic impact testing is designed to assess the ignition sensitivity of nonmetals to compression heating that develops during an oxygen pressure surge (also known as pneumatic impact, gaseous fluid impact, and oxygen shock). This test simulates real-world conditions where nonmetals, including lubricants, may be rapidly pressurized with oxygen during system operation.

In the pneumatic impact (oxygen pressure surge) test, a sample of the nonmetal is placed in a sample cup positioned at the end of a surge tube. An upstream closed valve, holding high-pressure oxygen on its upstream side, is quickly opened to introduce oxygen to the surge tube, which causes pneumatic impact and compression heating at the sample, mimicking the transient pressurization that can occur in an operational setting depending on pressurization rates. Researchers observe how the nonmetal reacts to these conditions, looking for any signs of ignition or other undesirable outcomes.

Key Takeaways for Nonmetallic Materials

The CGA/EIGA documents provide the following guidelines for nonmetal selection intended for use in oxygen:

Step 1. Verify the mechanical and chemical suitability, (other than oxygen compatibility) of the nonmetallic material based on design and operating conditions (including transient conditions).

Step 2. Determine the fire-damage potential and ignitability of the nonmetal by performing the oxygen compatibility testing and considering the following guidelines:

• Heat of combustion (HoC) should be less than 2500 cal/g (4500 Btu/lb)
• Auto-ignition temperature (AIT) should be at least 300°C (572°F) at a test pressure of 103 bar (1500 psia) per ASTM G72, or 120 bar (1740 psia) per ISO 21010.
• AIT should provide a margin of at least 100°C (212°F) above the operating temperature.
• Materials may be subjected to gaseous fluid impact or mechanical impact tests.
• For liquid oxygen (LOX) service, nonmetals shall pass a mechanical impact (LOXMIS) test in LOX. This is a mandatory requirement.

Special Considerations for Lubricants and Locking Compounds

The CGA/EIGA documents provide additional considerations for lubricants and locking compounds:

1. Auto-ignition temperature (AIT) should be at least 400°C (752°F) at the maximum operating pressure in gaseous oxygen or at 103 bar (1500 psia) if the maximum operating pressure is below 120 bar (1740 psi).
2. Lubricants must pass a pneumatic impact test (gaseous fluid impact sensitivity or GFIS test) in gaseous oxygen at a pressure of at least 50 bar (725 psi) higher than the maximum operating pressure.

Additional Recommendations for Non-Metals Compatibility Testing

WHA also emphasizes the following recommendations from CGA G-4.14 / EIGA Doc 200:

• Minimize the quantity of nonmetals used
• Evaluate possible ignition with a kindling chain and take account of heat dissipation in the design by embedding the nonmetallic part in an adequate mass of burn-resistant metal (where necessary) that will act as a heat sink
• Avoid locating non-metals directly in the gaseous or liquid oxygen stream, where possible
• Prevent excessive friction or mechanical impact of the non-metallic component

Many factors can influence oxygen compatibility including, but not limited to, polymer resin quality, manufacturing, mechanical properties, porosity, impurities, and oxygen cleanliness. Therefore, WHA test results apply to the customer-supplied material, and it is the customer’s responsibility to adequately control these types of factors to ensure oxygen compatibility test results can be applied to the ongoing production of the specific material.

Furthermore, WHA recommends testing the final configuration of the material (i.e., oxygen-cleaned part) so the oxygen compatibility test results can be applied to the specific end-product.

When performing oxygen compatibility tests on materials consisting of an assembly (e.g., energized seals (polymer ring with metal spring), binary non-metallic materials (e.g., plastic-coated elastomer) or non-metallic coating on a metal substrate), the results only represent the oxygen compatibility of the assembly and do not evaluate the oxygen compatibility of the non-metallic material(s) alone.

Are Your Materials Safe for Use with Oxygen?

By conducting these tests on the final product configuration, you can navigate the oxygen compatibility requirements and select nonmetallic materials suitable for your oxygen piping system application.

Consult the full CGA/EIGA documents for additional details and considerations.

Contact us for additional information and to inquire about oxygen compatibility testing and other oxygen safety services from WHA International.