Valves in Oxygen Service

Oxygen has typically active chemical properties. It is a strong oxidizing and combustible substance and can combine with most elements to form oxides except for gold, silver, and inert gases such as helium, neon, argon, and krypton. An explosion occurs when oxygen is mixed with combustible gases (acetylene, hydrogen, methane, etc.) in a certain proportion or when the pipe valve meets a sudden fire. The oxygen flow in the pipeline system change in the process of oxygen gas transportation, the European Industrial Gas Association (EIGA) developed the standard IGC Doc 13/12E “Oxygen Pipeline and Piping Systems” divided the Oxygen working conditions for “impact” and “non-impact”. The “impact ” is a dangerous occasion because it is easy to stimulate energy, causing combustion and explosion. The oxygen valve is the typical “impact occasion”.

Oxygen valve is a type of special valve designed for an oxygen pipeline and has been widely used in metallurgy, petroleum, chemical, and other industries involving oxygen. The material of the oxygen valve is limited to working pressure and flow rate to prevent the collision of particles and impurities in the pipeline. Therefore, the engineer should fully consider friction, static electricity, non-metal ignition, possible pollutants (carbon steel surface corrosion), and other factors when selecting an oxygen valve.

Oxygen Properties

• The normal concentration in air is 21%
• Colorless, odorless, and tasteless.
• Cannot be detected by the human senses
• Not flammable but supports and accelerates combustion.
• Flammable materials, including some materials that are normally relatively
• Non-flammable in air, burn very rapidly in high oxygen concentrations.
• Three elements necessary for an oxygen fire are an ignition source, oxygen, and flammable material (fuel) – known as the “fire triangle”

Material Used in Oxygen Valve

Organic materials have ignition temperatures below those of metals. The use of organic materials in contact with oxygen should be avoided, particularly when the material is directly in the flow stream. When an organic material must be used for parts such as valve seats, diaphragms, or packing, it is preferable to select a material with the highest ignition temperature, the lowest specific heat, and the necessary mechanical properties.
Lubricants and sealing compounds should be used only if they are suitable for oxygen service and then used sparingly. Ordinary petroleum lubricants are not satisfactory and are particularly hazardous because of their high heat of combustion and high rate of reaction.
The approximate ignition temperatures in 138 bar (2000 psig) oxygen for a few organic materials are shown in table 1.
Table 1. Typical Ignition Temperatures

MATERIAL	TYPICAL IGNITION TEMPERATURE IN 138 BAR (2000 PSIG) OXYGEN
PTFE and PCTFE	468
70% Bronze‐filled PTFE	468
Fluoroelastomer	316
Nylon	210
Polyethylene	182
Chloroprene and Nitrile	149

Metals

The selection of metals should be based on their resistance to ignition and rate of reaction. Following is a comparison of these two properties for some commonly used valve materials.

Resistance to Ignition in Oxygen

Materials are listed in order from hardest to ignite to easiest to ignite.

1. Copper, copper alloys, and nickel‐copper alloys ‐‐ most resistant
2. Stainless steel (300 series)
3. Carbon steel
4. Aluminum ‐‐ least resistant

Rate of Reaction

Materials are listed in order from the slowest rate of combustion to the most rapid rate of combustion.

1. Copper, copper alloys, and nickel‐copper alloys ‐‐ do not normally propagate combustion
2. Carbon steel
3. Stainless steel (300 series)
4. Aluminum ‐‐ burns very rapidly

Note that stainless steel, once ignited, burns more rapidly than carbon steel. Nevertheless, the austenitic grades (300 series) of stainless steel are considered to be much better than carbon steel because of their high resistance to ignition.

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How Dangerous is Oxygen

Oxygen is not combustible alone. However, if there is a combustion event, high-oxygen content means that combustible materials do burn much faster. Particle impact, rapid pressurization, or compression of materials can result in heating that could cause combustion. Contamination and mechanical energy such as friction can also cause ignition and result in fast, hot fires when more oxygen is present. The higher the concentration of oxygen, the greater the risk of combustion.

Larson pointed out that, while certain precautions must be taken with liquid oxygen, it is even more important to be vigilant when working with it in the gaseous state.

Minimizing Risk

To minimize the risk of fire, it is important to choose highly compatible materials for valves—both metals and soft goods. It is also important to minimize ignition mechanisms. That can be done by minimizing soft goods and limiting the use of lubricants. It is also essential to utilizing best practices—from design to manufacture, to getting the product to the site, to operating it.

How to choose a valve used for oxygen?

Some projects explicitly prohibit gate valves from being used in oxygen pipelines with design pressure greater than 0.1mpa. This is because the sealing surface of gate valves will be damaged by friction in relative motion (i.e. the opening/closing of the valve), which causes small “iron powder particles” to fall off from the sealing surface and easily catch fire. Similarly, the oxygen line of another type of valve will also explode at the moment when the pressure difference between the two sides of the valve is large and the valve opens quickly.