Oxygen compatibility

Last updated September 03, 2024

Oxygen compatibility is the issue of compatibility of materials for service in high concentrations of oxygen. It is a critical issue in space, aircraft, medical, underwater diving and industrial applications. Aspects include effects of increased oxygen concentration on the ignition and burning of materials and components exposed to these concentrations in service.

Understanding of fire hazards is necessary when designing, operating, and maintaining oxygen systems so that fires can be prevented. Ignition risks can be minimized by controlling heat sources and using materials that will not ignite or will not support burning in the applicable environment. Some materials are more susceptible to ignition in oxygen-rich environments, and compatibility should be assessed before a component is introduced into an oxygen system.^[1] Both partial pressure and concentration of oxygen affect the fire hazard.

The issues of cleaning and design are closely related to the compatibility of materials for safety and durability in oxygen service.

Prevention of fire

Fires occur when oxygen, fuel, and heat energy combine in a self-sustaining chemical reaction. In an oxygen system the presence of oxygen is implied, and in a sufficiently high partial pressure of oxygen, most materials can be considered fuel. Potential ignition sources are present in almost all oxygen systems, but fire hazards can be mitigated by controlling the risk factors associated with the oxygen, fuel, or heat, which can limit the tendency for a chemical reaction to occur.

Materials are easier to ignite and burn more readily as oxygen pressure or concentration increase, so operating oxygen systems at the lowest practicable pressure and concentration may be enough to avoid ignition and burning.

Use of materials which are inherently more difficult to ignite or are resistant to sustained burning, or which release less energy when they burn, can, in some cases, eliminate the possibility of fire or minimize the damage caused by a fire.

Although heat sources may be inherent in the operation of an oxygen system, initiation of the chemical reaction between the system materials and oxygen can be limited by controlling the ability of those heat sources to cause ignition. Design features which can limit or dissipate the heat generated to keep temperatures below the ignition temperatures of the system materials will prevent ignition.

An oxygen system should also be protected from external heat sources.^[1]

Assessment of oxygen compatibility

The process of assessment of oxygen compatibility would generally include the following stages:^[1]

Identification of worst-case operating conditions.
Evaluation of flammability of system materials. Geometry should be considered as most materials are more flammable when they have small cross-sections or are finely divided.
Assessment of the presence and probability of ignition mechanisms. These may include:
- Chemical reaction: An exothermic reaction between chemicals that could release sufficient heat to ignite the surrounding materials.
- Electrical arc: Electric current arcing with enough energy to ignite the material receiving the arc.
- Engine exhaust
- Explosive charges
- Flow friction: Heat generated by high velocity oxygen flow over a non-metal
  - Note:Flow friction is a hypothesis. Flow friction has not been experimentally verified and should be considered only in conjunction with validated ignition mechanisms.
- Friction between relatively moving parts
- Fragments from bursting vessels
- Fresh metal exposure: Heat of oxidation released when unoxidized metal is exposed to an oxidizing atmosphere. Usually associated with fracture, impact or friction.
- Galling and friction: Heat generated by rubbing components together.
- Lightning and other electric arc discharge
- Mechanical impact: Heat generated by impact on a material with sufficient energy to ignite it.
- Open flames
- Particle impact: Heat generated when small particles strike a material with sufficient velocity to ignite the particle or the material.
- Personnel smoking
- Rapid pressurization: Heat generated by single or multiple adiabatic compression events.
- Resonance: Acoustic vibrations in resonant cavities that cause rapid temperature rise.
- Static discharge: Discharge of accumulated static electrical charge with enough energy to ignite the material receiving the charge.
- Thermal runaway: A process which produces heat faster than it can be dissipated.
- Welding
Estimation of the ignition risk and the consequences of ignition. The further development or dissipation of the fire.
Analysis of the consequences of a fire

Compatibility analysis would also consider the history of use of the component or material in similar conditions, or of a similar component.

Oxygen service

Oxygen service implies use in contact with high partial pressures of oxygen. Generally this is taken to mean a higher partial pressure than possible from compressed air, but also can occur at lower pressures when the concentration is high.

Oxygen cleaning

Oxygen cleaning is preparation for oxygen service by ensuring that the surfaces that may come into contact with high partial pressures of oxygen while in use are free of contaminants that increase the risk of ignition.^[2]

Oxygen cleaning is a necessary, but not always a sufficient condition for high partial pressure or high concentration oxygen service. The materials used must also be oxygen compatible at all expected service conditions. Aluminium and titanium components are specifically not suitable for oxygen service.^[2]

In the case of diving equipment, oxygen cleaning generally involves the stripping down of the equipment into individual components which are then thoroughly cleaned of hydrocarbon and other combustible contaminants using non-flammable, non-toxic cleaners. Once dry, the equipment is reassembled under clean conditions. Lubricants are replaced by specifically oxygen- compatible substitutes during reassembly.^[2]

The standard and requirements for oxygen cleaning of diving apparatus varies depending on the application and applicable legislation and codes of practice. For scuba equipment, the industry standard is that breathing apparatus which will be exposed to concentrations in excess of 40% oxygen by volume should be oxygen cleaned before being put into such service.^[2] Surface supplied equipment may be subject to more stringent requirements, as the diver may not be able to remove the equipment in an accident. Oxygen cleaning may be required for concentrations as low as 23%^[3] Other common specifications for oxygen cleaning include ASTM G93 and CGA G-4.1.^[4]

Cleaning agents used range from heavy-duty industrial solvents and detergents such as liquid freon, trichlorethylene and anhydrous trisodium phosphate, followed by rinsing in deionised water. These materials are now generally deprecated as being environmentally unsound and an unnecessary health hazard. Some strong all-purpose household detergents have been found to do the job adequately. They are diluted with water before use, and used hot for maximum efficacy. Ultrasonic agitation, shaking, pressure spraying and tumbling using glass or stainless steel beads or mild ceramic abrasives are effectively used to speed up the process where appropriate. Thorough rinsing and drying is necessary to ensure that the equipment is not contaminated by the cleaning agent. Rinsing should continue until the rinse water is clear and does not form a persistent foam when shaken. Drying using heated gas – usually hot air – is common and speeds up the process. Use of a low oxygen fraction drying gas can reduce flash-rusting of the interior of steel cylinders.^[2]

After cleaning and drying, and before reassembly, the cleaned surfaces are inspected and where appropriate, tested for the presence of contaminants. Inspection under ultraviolet illumination can show the presence of fluorescent contaminants, but is not guaranteed to show all contaminants.^[2]

Oxygen service design

Design for oxygen service includes several aspects:

Choice of oxygen compatible materials for exposed components.^[5]
Minimising exposed area of materials which are necessary for functional reasons but are less compatible, and avoiding high flow velocities in contact with these materials.^[5]
Providing effective heat transfer to avoid raised temperatures of components.^[5]
Minimising the possibility of adiabatic heating by sudden increases of pressure - for example by using valves which cannot be opened suddenly to full bore, or opening against a pressure regulator.^[5]
Providing smooth surfaces in contact with flow where practicable, and minimising sudden changes in flow direction.
Use of flame arrestors/Flashback arrestors/Oxygen firebreaks in flexible hose

Oxygen compatible materials

As a general rule, oxygen compatibility is associated with a high ignition temperature, and a low rate of reaction once ignited.^[6]

Organic materials generally have lower ignition temperatures than metals considered suitable for oxygen service. Therefore the use of organic materials in contact with oxygen should be avoided or minimised, particularly when the material is directly exposed to gas flow. When an organic material must be used for parts such as diaphragms, seals, packing or valve seats, the material with the highest ignition temperature for the required mechanical properties is usually chosen. Fluoroelastomers are preferred where large areas are in direct contact with oxygen flow. Other materials may be acceptable for static seals where the flow does not come into direct contact with the component.^[6]

Only tested and certified oxygen compatible lubricants and sealants should be used, and in as small quantities as is reasonably practicable for effective function. Projection of excess sealant or contamination by lubricant into flow regions should be avoided.^[5]

Commonly used engineering metals with a high resistance to ignition in oxygen include copper, copper alloys, and nickel-copper alloys, and these metals also do not normally propagate combustion, making them generally suitable for oxygen service. They are also available in free-cutting, castable or highly ductile alloys, and are reasonably strong, so are useful for a wide range of components for oxygen service.^[6]

Aluminium alloys have a relatively low ignition temperature, and release a large amount of heat during combustion and are not considered suitable for oxygen service where they will be directly exposed to flow, but are acceptable for storage cylinders where the flow rate and temperatures are low.^[5]

Applications

Aerospace – Term used to collectively refer to the atmosphere and outer space
Medical – Diagnosis, treatment, and prevention of illness
Manufacturing – Industrial activity producing goods for sale using labor and machines
Underwater diving – Descending below the surface of the water to interact with the environment

Research

Hazards analyses are performed on materials, components, and systems; and failure analyses determine the cause of fires. Results are used in design and operation of safe oxygen systems.

Related Research Articles

Nitrox refers to any gas mixture composed of nitrogen and oxygen. This includes atmospheric air, which is approximately 78% nitrogen, 21% oxygen, and 1% other gases, primarily argon. In the usual application, underwater diving, nitrox is normally distinguished from air and handled differently. The most common use of nitrox mixtures containing oxygen in higher proportions than atmospheric air is in scuba diving, where the reduced partial pressure of nitrogen is advantageous in reducing nitrogen uptake in the body's tissues, thereby extending the practicable underwater dive time by reducing the decompression requirement, or reducing the risk of decompression sickness.

A rebreather is a breathing apparatus that absorbs the carbon dioxide of a user's exhaled breath to permit the rebreathing (recycling) of the substantially unused oxygen content, and unused inert content when present, of each breath. Oxygen is added to replenish the amount metabolised by the user. This differs from open-circuit breathing apparatus, where the exhaled gas is discharged directly into the environment. The purpose is to extend the breathing endurance of a limited gas supply, while also eliminating the bubbles otherwise produced by an open circuit system. The latter advantage over other systems is useful for covert military operations by frogmen, as well as for undisturbed observation of underwater wildlife. A rebreather is generally understood to be a portable apparatus carried by the user. The same technology on a vehicle or non-mobile installation is more likely to be referred to as a life-support system.

The autoignition temperature or self-ignition temperature, often called spontaneous ignition temperature or minimum ignition temperature and formerly also known as kindling point, of a substance is the lowest temperature at which it spontaneously ignites in a normal atmosphere without an external source of ignition, such as a flame or spark. This temperature is required to supply the activation energy needed for combustion. The temperature at which a chemical ignites decreases as the pressure is increased.

A breathing gas is a mixture of gaseous chemical elements and compounds used for respiration. Air is the most common and only natural breathing gas, but other mixtures of gases, or pure oxygen, are also used in breathing equipment and enclosed habitats. Oxygen is the essential component for any breathing gas. Breathing gases for hyperbaric use have been developed to improve on the performance of ordinary air by reducing the risk of decompression sickness, reducing the duration of decompression, reducing nitrogen narcosis or allowing safer deep diving.

<span class="mw-page-title-main">Backdraft</span> Rapid or explosive burning of superheated gasses in a fire

A backdraft, backdraught or smoke explosion is the abrupt burning of superheated gases in a fire caused when oxygen rapidly enters a hot, oxygen-depleted environment; for example, when a window or door to an enclosed space is opened or broken. Backdrafts are typically seen as a blast of smoke and/or flame out of an opening of a building. Backdrafts present a serious threat to firefighters. There is some debate concerning whether backdrafts should be considered a type of flashover.

A diving air compressor is a breathing air compressor that can provide breathing air directly to a surface-supplied diver, or fill diving cylinders with high-pressure air pure enough to be used as a hyperbaric breathing gas. A low pressure diving air compressor usually has a delivery pressure of up to 30 bar, which is regulated to suit the depth of the dive. A high pressure diving compressor has a delivery pressure which is usually over 150 bar, and is commonly between 200 and 300 bar. The pressure is limited by an overpressure valve which may be adjustable.

The fire triangle or combustion triangle is a simple model for understanding the necessary ingredients for most fires.

Gas blending for scuba diving is the filling of diving cylinders with non-air breathing gases such as nitrox, trimix and heliox. Use of these gases is generally intended to improve overall safety of the planned dive, by reducing the risk of decompression sickness and/or nitrogen narcosis, and may improve ease of breathing.

An oxygen tank is an oxygen storage vessel, which is either held under pressure in gas cylinders, referred to in the industry as high pressure oxygen cylinders, or as liquid oxygen in a cryogenic storage tank.

In electrical and safety engineering, hazardous locations are places where fire or explosion hazards may exist. Sources of such hazards include gases, vapors, dust, fibers, and flyings, which are combustible or flammable. Electrical equipment installed in such locations can provide an ignition source, due to electrical arcing, or high temperatures. Standards and regulations exist to identify such locations, classify the hazards, and design equipment for safe use in such locations.

An oil burner is a heating device which burns #1, #2 and #6 heating oils, diesel fuel or other similar fuels. In the United States, ultra low sulfur #2 diesel is the common fuel used. It is dyed red to show that it is road-tax exempt. In most markets of the United States, heating oil is the same specification of fuel as on-road un-dyed diesel.

Spontaneous combustion or spontaneous ignition is a type of combustion which occurs by self-heating, followed by thermal runaway and finally, autoignition. It is distinct from pyrophoricity, in which a compound needs no self-heat to ignite. The correct storage of spontaneously combustible materials is extremely important considering improper storage is the main cause of spontaneous combustion. Materials such as coal, cotton, hay, and oils should be stored at proper temperatures and moisture levels to prevent spontaneous combustion.

A parts washer is a piece of equipment used to remove contaminants or debris, such as dirt, grime, carbon, oil, grease, metal chips, cutting fluids, mold release agents, ink, paint, and corrosion from workpieces. Parts washers are used in new manufacturing and remanufacturing processes; they are designed to clean, degrease and dry bulk loads of small or large parts in preparation for assembly, inspection, surface treatment, packaging and distribution. Parts washers may be as simple as the manual "sink-on-a-drum" common to many auto repair shops, or they may be very complex, multi-stage units with pass-through parts handling systems. Parts washers are essential in maintenance, repair and remanufacturing operations as well, from cleaning fasteners, nuts, bolts and screws to diesel engine blocks and related parts, rail bearings, wind turbine gears boxes and automotive assemblies.

Ultrapure water (UPW), high-purity water or highly purified water (HPW) is water that has been purified to uncommonly stringent specifications. Ultrapure water is a term commonly used in manufacturing to emphasize the fact that the water is treated to the highest levels of purity for all contaminant types, including: organic and inorganic compounds; dissolved and particulate matter; volatile and non-volatile; reactive, and inert; hydrophilic and hydrophobic; and dissolved gases.

Plasma-activated bonding is a derivative, directed to lower processing temperatures for direct bonding with hydrophilic surfaces. The main requirements for lowering temperatures of direct bonding are the use of materials melting at low temperatures and with different coefficients of thermal expansion (CTE).

Gas blending is the process of mixing gases for a specific purpose where the composition of the resulting mixture is specified and controlled. A wide range of applications include scientific and industrial processes, food production and storage and breathing gases.

Hazmat diving is underwater diving in a known hazardous materials environment. The environment may be contaminated by hazardous materials, the diving medium may be inherently a hazardous material, or the environment in which the diving medium is situated may include hazardous materials with a significant risk of exposure to these materials to members of the diving team. Special precautions, equipment and procedures are associated with hazmat diving so that the risk can be reduced to an acceptable level. These are based on preventing contact of the hazardous materials with the divers and other personnel, generally by encapsulating the affected personnel in personal protective equipment (PPE) appropriate to the hazard, and by effective decontamination after contact between the PPE and the hazardous materials.

Diving hazards are the agents or situations that pose a threat to the underwater diver or their equipment. Divers operate in an environment for which the human body is not well suited. They face special physical and health risks when they go underwater or use high pressure breathing gas. The consequences of diving incidents range from merely annoying to rapidly fatal, and the result often depends on the equipment, skill, response and fitness of the diver and diving team. The classes of hazards include the aquatic environment, the use of breathing equipment in an underwater environment, exposure to a pressurised environment and pressure changes, particularly pressure changes during descent and ascent, and breathing gases at high ambient pressure. Diving equipment other than breathing apparatus is usually reliable, but has been known to fail, and loss of buoyancy control or thermal protection can be a major burden which may lead to more serious problems. There are also hazards of the specific diving environment, and hazards related to access to and egress from the water, which vary from place to place, and may also vary with time. Hazards inherent in the diver include pre-existing physiological and psychological conditions and the personal behaviour and competence of the individual. For those pursuing other activities while diving, there are additional hazards of task loading, of the dive task and of special equipment associated with the task.

The mechanism of diving regulators is the arrangement of components and function of gas pressure regulators used in the systems which supply breathing gases for underwater diving. Both free-flow and demand regulators use mechanical feedback of the downstream pressure to control the opening of a valve which controls gas flow from the upstream, high-pressure side, to the downstream, low-pressure side of each stage. Flow capacity must be sufficient to allow the downstream pressure to be maintained at maximum demand, and sensitivity must be appropriate to deliver maximum required flow rate with a small variation in downstream pressure, and for a large variation in supply pressure, without instability of flow. Open circuit scuba regulators must also deliver against a variable ambient pressure. They must be robust and reliable, as they are life-support equipment which must function in the relatively hostile seawater environment, and the human interface must be comfortable over periods of several hours.

Diving equipment may be exposed to contamination in use and when this happens it must be decontaminated. This is a particular issue for hazmat diving, but incidental contamination can occur in other environments. Personal diving equipment shared by more than one user requires disinfection before use. Shared use is common for expensive commercial diving equipment, and for rental recreational equipment, and some items such as demand valves, masks, helmets and snorkels which are worn over the face or held in the mouth are possible vectors for infection by a variety of pathogens. Diving suits are also likely to be contaminated, but less likely to transmit infection directly.

References

1 2 3 Rosales, K. R.; Shoffstall, M. S.; Stoltzfus, J. M. (2007). Guide for Oxygen Compatibility Assessments on Oxygen Components and Systems. NASA/TM-2007-213740 (Report). Johnson Space Center; White Sands Test Facility: NASA. Archived from the original on June 4, 2012. Retrieved 4 June 2013.{{cite report}}: CS1 maint: unfit URL (link)
1 2 3 4 5 6 Harlow, Vance (2001). Oxygen Hacker's Companion (4th ed.). Warner, New Hampshire: Airspeed Press.
↑ Diving Advisory Board. Code Of Practice Inshore Diving (PDF). Pretoria: The South African Department of Labour. Archived from the original (PDF) on 9 November 2016. Retrieved 16 September 2016.
↑ "Oxygen Cleaning Specifications". Harrison Electropolishing. Retrieved 28 July 2020.
1 2 3 4 5 6 Safety Advisory Group (2008). "Safety principles of high pressure oxygen systems". Brussels: European Industrial Gases Association. Retrieved 18 June 2018.
1 2 3 "Product bulletin 59:045 - Material Guidelines for Gaseous Oxygen Service" (PDF). www.Fisher.com. October 2006. Retrieved 18 June 2018.

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[Rosales_et_al_2007-1] 1 2 3 Rosales, K. R.; Shoffstall, M. S.; Stoltzfus, J. M. (2007). Guide for Oxygen Compatibility Assessments on Oxygen Components and Systems. NASA/TM-2007-213740 (Report). Johnson Space Center; White Sands Test Facility: NASA. Archived from the original on June 4, 2012. Retrieved 4 June 2013.{{cite report}}: CS1 maint: unfit URL (link)

[Harlow_2001-2] 1 2 3 4 5 6 Harlow, Vance (2001). Oxygen Hacker's Companion (4th ed.). Warner, New Hampshire: Airspeed Press.

[CoP_Inshore-3] Diving Advisory Board. Code Of Practice Inshore Diving (PDF). Pretoria: The South African Department of Labour. Archived from the original (PDF) on 9 November 2016. Retrieved 16 September 2016.

[4] "Oxygen Cleaning Specifications". Harrison Electropolishing. Retrieved 28 July 2020.

[EIGA-5] 1 2 3 4 5 6 Safety Advisory Group (2008). "Safety principles of high pressure oxygen systems". Brussels: European Industrial Gases Association. Retrieved 18 June 2018.

[Oxygen_service_materials-6] 1 2 3 "Product bulletin 59:045 - Material Guidelines for Gaseous Oxygen Service" (PDF). www.Fisher.com. October 2006. Retrieved 18 June 2018.

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