Oxygen handling


The following article covers the impact of Oxygen in pressurized breathing equipment environments.This is for your information only. It should give you the knowledge to know what to ask for in a scuba service center, and gives you an appreciation of what a service center should do for you. 
This article is NO substitute for proper oxygen service training administered by a certifying agency (e.g. ANDI, TDI, PADI, ...). This article can be used as additional reading, to supplement your training. 

Oxygen Handling

Before you can start blending gases, you need to be aware of the impact of Oxygen-rich hyperbaric conditions on diving equipment and how to handle Oxygen-rich gases. More specifically it covers the following topics:
  • Oxygen-compatibility of materials in diving equipment
  • Oxygen-cleaning of diving equipment
  • flow speed of Oxygen

Oxygen Compatibility

Oxygen is highly reactive with a lot of materials, especially under higher partial pressures. Exactly the environment scuba diving operates under. In order for all the equipment in the breathing loop (tank, tank valve, first and second stage regulator, hoses) to be able to pass high concentrations of Enriched Air (i.e. higher Oxygen content) under high pressure, all combustion-promoting factors must be eliminated. 

Combustibility is a measure on how easy a material can ignite and sustain burning.

Oxygen Compatibility of materials refers to the level of combustibility of that material when exposed to higher levels of oxygen. These refer to materials that burn more violently as the oxygen level increases. Some materials even will not burn in air (21% oxygen) but will burn in higher oxygen environments. On top of that, incompletely burned materials will result in increased levels of CO.  

Surprisingly, most standard diving equipment was never designed with Enriched Air environments in mind. In fact, a lot of Oxygen-INcompatible materials were used in standard diving equipment. 

Metals

Extra care needs to be taken in the selection of metals in the design of diving equipment that get exposure to high concentrations of Oxygen. All metals can burn in pure Oxygen environments. Stainless Steel is not a bad choice, but it is not the best. Stainless Steel can still burn in high Oxygen mixtures, but it is very hard to ignite. Once ignited, it burns very well. Steel can be used in high pressure gas storage, but is considered less suitable for low pressure oxygen-rich gas flow systems.

More appropriate metals are Copper, Nickel and their alloys (Brass, Bronze and Monel). They have a higher ignition temperature and produce less heat once burning than Iron alloys. They are considered to be self-extinguishing and are used in the low-pressure gas flow systems, like valves, orifices, branch connections. Copper and Brass are the metals of choice. In high-pressure flow systems, the metals Monel and Stainless Steel 316 (which has a good corrosion resistance).
Beware: Titanium is not a suitable material for oxygen-rich hyperbaric environments.

Following is a list of metals, ranging from "more compatible" to "less compatible":

More Compatible
Less Compatible

Non-metals

As for Non-metals, also assume these can burn heavily in Oxygen-rich environments. Remember the Apollo 1 fire
Here is a list of common materials found in diving equipment, which promote combustion:


Teflon (e.g. Teflon O-rings) and Kel-F are the only plastics that can be used in oxygen systems.

Also be aware that high-pressure flex hoses are commonly fitted with steel end-fittings. Ensure you use hoses with stainless steel or brass fittings.

Viton O-rings are a better choice for O-rings than the common Buna-N O-rings. The latter is only rated for incidental exposure. Ensure the O-rings are the specified size for the application (don't just guess the size, but use an O-ring measuring cone). O-rings need to be smooth.

Standard compressors are oil-lubricated. Synthetic oils are better than Petroleum-based oils, but preference should be given to compressors that minimize the amount of lubricants in the gas stream. 

Silicone grease (Dow Corning 111) is the traditional lubricant for scuba o-rings. It is acceptable in Air (21% O2) pressures up to 2500 psi (170 bar), but not appropriate for 3500 psi (240 bar) systems. Furthermore, it is unacceptable in Enriched Air applications. ANDI recommends Crysto-lube MCG for Enriched Air applications.   

Do I need special equipment?

When you started diving and bought all your gear, checking for Oxygen-compatible materials was probably the last thing on your mind, and you were most likely ignorant about these issues. Does this mean that your gear is not suitable for use with Enriched Air? Not completely. NOAA, who defined the standards of using NITROX in diving applications, state that Enriched Oxygen levels up to 40% can be used with standard diving equipment. This allows for the use of standard NITROX I (32%, up to a depth of 130ft) and NITROX II (36%, up to a depth of 94ft) mixtures.  

According to the Compressed Gas Association (CGA), the National Fire Prevention Association (NFPA), diving equipment manufacturers, any gas mixture with Oxygen percentage of more than 23.5%, should be treated as 100% pure Oxygen and recommend Oxygen cleaning.

Oxygen Cleaning

Breathing equipment refers to :
  • scuba tanks
  • tank valves
  • flexi hoses
  • first stage regulators
  • second stage regulators
Rendering equipment "Oxygen Clean" means ensuring "absence of contaminants". Examples of contaminants can be:
  • silicone grease
  • machining oils
  • industrial thread lubricants 
  • cleaning solvents and detergents
  • paints
  • burrs and metal filings
  • chrome plating chips
  • metal oxides
  • carbon dust from filtration systems
  • airborne dust and soot
  • oily fingerprints
  • pipe thread sealants
  • soapy water used in leak checks
  • cloth lint
All these compounds can be the fuel that starts a reaction. Pressurized enriched oxygen environments make it worse. There is a similarity to dust explosions. Oxidation can manifest in several forms: fire, smoldering, charring, sizzling. Incomplete combustion of hydrocarbons produce CO, which can be lethal under certain diving conditions.

Never assume a product is Oxygen-clean. It must be labeled as such. Assume that any new equipment is NOT Oxygen-cleaned. Also, a product can be oxygen-cleaned, but that does not make it oxygen-compatible. Check the literature for oxygen-compatibility of the materials used in the product.

Design and Engineering of Hyperbaric Oxygen equipment

It is not sufficient for equipment that is exposed to hyperbaric Oxygen environments to consist of Oxygen-compatible materials and to be Oxygen cleaned. It also needs to be specifically designed to work in hyperbaric oxygen environments. E.g. certain medical oxygen equipment is designed to provide a constant flow of oxygen at room temperature on the surface (1 ATA). This equipment, although Oxygen clean and containing only Oxygen-compatible materials, is not designed to be used under the conditions common to diving, i.e. hyperbaric and cold temperatures. 

Incorrectly designed equipment can damage the equipment and cause life-threatening situations (e.g. explosion, structural failure, ...). The type of damage inflicted to the diving equipment can be:
  • temperature related
  • mechanical related

Temperature related damage

Temperature related damage is caused by rapid temperature changes which can cause:
  • explosion (very fast oxidation)
  • fire (fast oxidation)
  • smoldering (slow oxidation)
In reference to the elements that contribute to fire, we use the Fire Triangle. The hyperbaric Oxygen provides the oxidizer, the impurities (non-Oxygen-compatible materials) provide the fuel and a high temperature provides the ignition. Traditionally, an oxidation can be extinguished when any of the tree elements is eliminated. Unfortunately in these applications, the Oxygen is both the oxidizer and the origin of the ignition ( see below).

The Ignition can be caused by: 
  • frictional heating
  • adiabatic compression
  • flow impediment

1) frictional heating: rapidly flowing Oxygen molecules are restricted by non-smooth surfaces, restrictions and angles. In valves this occurs mainly near valve seats and stems

Tanks used for pure Oxygen, have specially designed valves that comply with all the criteria listed in this article. They are used for Medical, Aviation and filling station applications. They are not meant to be used in hyperbaric conditions (diving).

Unfortunately, traditional scuba valve design (which dates back to the 40-ties and 50-ties) never considered the possibility of elevated Oxygen levels. They have constrictions and sharp turns. They can be used for hyperbatric Enriched Air application, but equipment is kept Oxygen-clean and are annually inspected and serviced (see below). Also, be careful in rapidly changing the flow speed. Maintain a maximum flow rate of 200 psi/min (see below).

Ball valves are completely out of the question. They allow rapid flow gradients (much higher than the allowed 200 psi/min), and pose a severe risk in ignition. 


 
Oxygen valve
(good)

Scuba tank valve
(less suitable)

Ball valve
(very bad)

 
This is a valve for high concentrations of Oxygen (more than 50%). There are few restrictions and sharp turns.

These are not available for diving applications.
 
A regular scuba tank valve has sharp turns, restrictions and angles.

Beware, ball valves, have few restrictions, turns or angles.
Yet they are NOT suitable for Oxygen applications because they open rapidly (within a 1/4 turn), and accelerate the flow of Oxygen to the point of ignition. Only needle valve designs should be used, which open up slowly with multiple turns.

 
 

 



2) Adiabatic compression: when Oxygen under pressure is injected in an environment at a significantly lower pressure, then Oxygen molecules accelerate rapidly to fill out the available space. These molecules can approach the speed of sound. This causes the temperature to rise at obstructions. The temperature can rise above the ignition temperature of metals, like Stainless Steel. That is why metals like Copper and its alloys are recommended, because their ignition temperature is much higher.
The following table shows the theoretical maximum temperature that can be reached from 20 oC and 1 bar:
 psi bar oF oC
 100 453 234
 1000 70 1303 706
 2000 140 1688 920
 4000 280 2158 1181
 5000 350 23330 1287

E.g. If we increase the pressure from 1 bar to 140 bar (2000 psi), then the temperature can increase from 20 oC to 920 oC (1688 oF), which is a significant increase, that can ignite certain metals.


3) flow impediment: a sudden change in flow passage diameter (like orifices) can cause rapid temperature rise.

Mechanical-related damage

A bad design and engineering of diving equipment can cause mechanical damage. The following types of damage can occur:
  • sonic resonance
  • particle impingement
 A gas flow can reach the speed of sound. This causes the gas to vibrate. If the oscillation of these vibrations approaches the natural frequency of the equipment, the oscillations get magnified (similar how you can make a swing go higher and higher by pushing at the right moment). This causes sonic damage which manifests itself by fittings and components resonating until damaged. Most vulnerable are: springs, seats and washers.

This can be avoided by design and careful operation of the valve. When you hear a high pitched noise, reduce the flow immediately until the sound is gone. Keep the flow rate below 200 psi/min.

2) particle impingement
Sonic resonance can lead to particles being released from components (e.g. corrosion particles, dust, chips, burrs). These particles get in the gas stream and accelerate to the speed of that gas. These particles impact with the gas pathways: tubing, fittings, angles, obstructions. The higher the mass of the particle, the higher the damage. This damage can impact the integrity of the pathway. Even worse, these impacts can cause sparks, which in turn ignite and cause an accelerated oxidation. 


Oxygen Reaction

The following factors attribute to an Oxygen Reaction:

  • Total Pressure of the gas
  • Partial Pressure of Oxygen
  • Percentage of Oxygen
  • Temperature change
  • Pressure change
  • Compatibility of materials
  • Particle impingement
  • contaminants (quality and quantity)
  • flow impediment
  • gas velocity

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