REFLECTIONS ON HANDLING PURE OXYGEN

In the past 2 years, a widespread interest in applications for specialty gasses has swept the diving community. This includes the use of pure oxygen for in-water decompression, for teaching dive-med courses, and for preparing various mixtures such as Nitrox and Trimixes. Each week, our Technical Support Phone Line (414-774-1616) receives many calls related to the use and handling of oxygen and other breathable gasses. Unfortunately, it has become painfully apparent that many individuals are rushing headlong into the use of gas systems about which they are not knowledgeable or properly prepared to install, manage, and maintain. It is the purpose of this article to review a few common problem areas.

Pure oxygen can be a very unruly substance if incorrectly handled! The best and brightest technicians can make serious mistakes with it. For example , NASA may be the worlds most experienced organization in utilizing exotic materials. Yet two major accidents, the deaths of 3 Apollo Program Astronauts during launch pad tests and close call of Apollo 13 in space were oxygen-related incidences. Even now , the latest space shuttle flight is devoted to studying how to safely managing oxygen atmospheres in the Space Station. If such an august agency can err with oxygen, little has to be said about the potential for mistakes in dive shops!

Most oxygen accidents result from a lack of knowledge, improper handling, or unsuitable equipment. More specifically these mishaps are usually attributable to: 1) excessive adiabatic compression, 2) high-speed particle impingement, 3) concentrated frictional heating, or 4) material incompatibility.

Divers often do not understand the enormous heat generated by sudden gas compression; for example, if a short section of tube is rapidly pressurized to 3,000 psi, momentary temperatures of 1500' F or more are reached. Obviously, many materials could be auto ignited at such temperatures. Under such conditions in the presence of pure oxygen, even inert substances such as Teflon or Viton can be set afire!

What causes adiabatic compression? The answer is simple: just opening a valve too fast, especially the "wrong kind" of valve. Unfortunately, the common scuba-type line valve, often used on tank filling apparatus, is a terrible valve for precise oxygen control. Although such valves can readily be prepared for oxygen-service, they are designed for high flow and swift pressurization. Rapid opening (or seat failures) in these devices can lead to soaring pressures and temperatures.

The same is true of valve on commercial storage cylinders and on scuba tanks themselves. Thus great caution should be exercised while opening valves whenever pure oxygen is involved. Slow-opening, globe-type line valves should be installed to meter pure O2 into mixing vessels! Regulators or flow constrictors are also helpful in that regard. Remember: the recommended transfer rate of pure O2 is about 70 psi per minute with 200 psi per minute with 200 psi being the maximum... that's darn slow!

Swift compression also creates supersonic gas velocities with a tube or hose. Within a supersonic oxygen stream, any hydrocarbon or other noncompatible substance is likely to be partially vaporized or ignited. High speed oxygen molecules, augmented by adiabatic compression, can even case metals like aluminum or carbon steel to be set ablaze! This is why Teflon lined oxygen hoses should be employed in such systems, and even they are not totally immune from ignition accidents. The common synthetic pressure hoses used for compressed air will burn with great gusto under such oxygenated situations. Compression accidents can be abetted by long hoses wherein high gas velocities can be achieved. More of the story: in high oxygen environments use Teflon-lined hoses, and keep them as short as possible.

Particle impingement occur when debris gets accelerated to high speed within the gas stream and then strikes some internal metallic part. Depending upon the composition of the particle itself and the tube or housing it strikes, a spark may be created. In pure oxygen systems, this can be disastrous, setting even metals like aluminum or carbon steel burning like a sparkler!

Perhaps the greatest source of particles is common rust. Therefore, carbon steel fittings should never be used in any pure or elevated oxygen systems. Over time, such fittings will rust... slowly becoming more dangerous as the metal particles form, flake off and are ejected by the gas stream. Nonrusting metals such as stainless steel, brass, or Monel must be employed in high oxygen environments.

Another source of particle impingement is the thermoplastic air pressure hose like that commonly used in scuba air stations. While the hose itself may be oxygen compatible, the end fittings usually are not. In fact over 90% of these air hoses have plated carbon steel end fittings. These fittings rust internally, slowly but surely becoming a source of high velocity, sparking particles whenever the oxygen tank is opened!

Whereas a brand-new thermoplastic hose may be safe, it progressively becomes dangerous as the steel end fittings form internal rust. Also oil is usually employed in the swaging process when the end fittings are initially installed; it's quite possible that at some future time, traces of residual oil might seep from the crimps,... and oil and oxygen go boom!

Concentrated localized heating can occur in oxygen lines when right angle bends or other restrictive apertures are present. Also, controls, like ball valves allow extreme differential gas speeds across the interior mechanism, thereby creating elevated temperatures. This can be sufficient to ignite the internal seats, seals or lubricants producing a fire or explosion. Ball valves should never be used in oxygen systems over 150 psi. Much has been made of materials compatibility relative to oxygen gas. This is a rather complicated matter since both temperature and pressure are also involved. For example, the common O-rings which are used in virtually all scuba apparatus are made of Buna N, a nitrate compound. At oxygen pressures below 250 psi, Buna seals are acceptable; above that pressure, they are no longer suitable, and Viton O-rings must be substituted. Above 2500 psi or 300o even Viton is no longer desirable!

Much has been made of materials compatibility relative to oxygen gas. This is a rather complicated matter since both temperature and pressure are also involved. For example, the common O-rings which are used in virtually all scuba apparatus are made of Buna N, a nitrate compound.

At oxygen pressures below 250 psi, Buna seals are acceptable; above that pressure, they are no longer suitable, and Viton O-rings must be substituted. Above 2500 psi or 300o even Viton is no longer desirable!

Oxygen reactivity greatly increases with high pressure; therefore, material compatibility is of significant concern at elevated pressures. This is why when dive regulators are converted to use with pure O2, great care must be taken in the high pressure first stage. The composition of all seats, seals, or diaphragms must be evaluated for O2 service, or ignition accidents are a very distinct possibility. In the low pressure second stage, a much greater latitude exists in this matter

Lubricants and sealants are another critical element in the oxygen fire equation. Since lubes are usually used in liquid or lubricant form, their propensity to be ignited by speeding oxygen gas, friction, or particle impingement is much enhanced. Therefore particular attention should be paid to using proper materials known to be oxygen-compatible. Such synthetics as Teflon or perfluoropolyether compounds are often employed. Care must even be taken in selecting Teflon tape, since cheap brands may have been contaminated during manufacture or contain incompatible additives.

The preceding brief discussion covers only some of the salient points of O2 handling. If, in reading it, you discovered facts of which you were unaware, perhaps expansion or your "oxygen background" is warranted. An O2 accident in your shop can indeed spoil your whole day and put a severe drain on your checkbook.