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Chuck wrote a very well put together post on some of the possible failure modes that may occur when introducing pure O2 into a scuba cylinder. I will add that I'm in full agreement with same. I'm truely amazed by the amount of diverse knowledge of many members of this list. I'm equally amazed by many of the best divers in world who are so Methodical....Can at the same time be so careless with such a simple thing. With all that is known by man.... How much of it is learned the "hard way". I believe it is better to be safe than sorry. Why say why ? How long does it take to O2 clean scuba equipment???? Minutes at best. Much, much less time than it would take to.... say explain things to an Insurance co. or recover in a burn center if the injury were not fatal. Ever try to catch your breath with a gaping chest wound? "SILT HAPPENS"JD JEFF DISLER SAFE DIVING NSS 26000 > >Jim Cobb writes of his reasons for believing that O2 cleaning of scuba >tanks for oxygen service is an unreasonable requirement (BS) . . . . . > > >>The main dangers with O2 are in high temp/pressure situations, so >>compressors do require special treatment if you are running high pps of >>O2 through them. You should never use a ball valve with 02. > > High temp / pressure situations can exist whenever there is turbulence i= n >the flow of a gas. Turbulence is caused whenever there is a change in >direction, objects projecting into the path of flow, or a sudden change in >the shape of the passageway and it increases with the velocity of the >gas. Turbulence is characterized by areas of higher and lower velocity th= an >that of the main flow as well as areas of compression and expansion. > As a gas passes by these obstructions and nonconformity=EDs tiny >areas of much higher pressure are created for reasons similar to those >that cause a wing to provide lift, turbulence behind a car, and others. >Because the gas is moving through these areas rather than eventually >becoming static, as when filling a tank, new gas is constantly being >compressed into the area and bringing more heat into the area to be >absorbed by the walls of the container. > We are talking about an area the size of a pinhead or a pinpoint inside >the >tubing of a compressor system, a tank, or tank valve and the phenomenon is >referred to as "adiabatic" in nature because temperatures rise or fall >without >adding or removing heat from outside the system. Temperatures can rise >hundreds of degrees in an instant if flow and/or turbulence is high enough >regardless of pressure. Air conditioning and refrigeration are controlled >adiabatic processes. > > A tank valve has sharp bends, protrusions, and noncomformitys and the >tank threads have many more. Gas must squeeze into the space between >these threads if they are not filled with a noncompressible lubricant and n= o >one "fills" these threads with lubricant. Currents inside the tank during >filling >result in gases moving along the walls at very high velocities. > Any pits, scale, or particles of rust or aluminum oxide in or on the sid= e >of >the tank contribute to the opportunity for these adiabatic processes as wel= l >as to moving particles that create points of heat or sparks from friction. >As the tank is filled the walls stretch and these tiny pits, imperfections, >or >points of corrosion (even invisible ones) snap, crackle, & pop as microscop= ic >pieces of corrosion fly off under the influence of vibration and friction >from >stretching. Much of this mechanical energy is converted to heat and if t= he >walls of the tank are impregnated with traces of hydrocarbons in a high O2 >environment the stage is well set for fireworks. > >Every pyrotechnic explosion starts with a detonation at the microscopic >level. > > Filling slowly greatly reduces the risks of ignition and virtually >eliminates >any problem from the threads because they represent a dead end space >that does not receive a constant supply of new gas (they fill up) but the >valve >and the sides of the tank remain vulnerable. > > Note that matter contains heat and any time matter is removed from or >added to a system heat is also removed or added. When a tank is filled it >gets warm for two primary reasons. >One; because you are squeezing the heat of many air molecules into a >compact space (more atoms banging on the side of the tank and each >other). The opposite happens when you empty it. >Two; you are stretching the metal of the tank walls as it expands like a >balloon. > > When you compress a gas into a tank or any closed container the rate of >compression and flow slows to an eventual stop so that a finite amount of >heat (carried by matter) is added to the elements of the system. When a >gas is compressed by turbulence as a result of flow, however, the rate of >compression is sustained and it is always new gas being compressed into >an open area. The difference in the amount of heat transferred to and >concentrated in adjacent material is astronomical. Spacecraft heat up upo= n >reentry not because of friction with air but because of compression of the >air >under them. There is some heat from friction but nothing to compare with >that of compression. > > The static electrical charges that result in lightning is caused by >friction >between air, water vapor, dust, . . . . The same thing happens between th= e >material ejected by volcanoes and between propeller blades and air. >Though it is much less likely to occur in a "normally" clean scuba tank whe= re >everything is usually well connected electrically it is not impossible for >high >velocity gas movement to build charges on particles that are electrically >isolated from the tank walls. Even the tiniest invisible spark may be al= l >it >takes to start the show. > > Pumping pure O2 on top of air does not mean that the O2 will mix with th= e >present contense of the tank without setting up currents of isolated gas th= at >allow parts of the tank to be exposed to pure O2 moving at high velocities. > > It is considered safe to pump mixes of up to 40% O2 into tanks that have >not been oxygen cleaned and some shops have large banks of this "pre-mix" >(usually 32% or 36%) that can be used in any tank. The problem arises >when pure oxygen and air are mixed in a tank because at some point you >must run pure O2 through the valve and because whether the O2 comes >before or after the air the tank walls are still exposed to 100% O2. > > So the point here is that you admit that "The main dangers with O2 are >in >high temp/pressure situations" and the virtual guarantee that there will b= e >some degree of adiabatic processes at work in the valve and around the >threads of the tank, that heat and possibly high velocity particles will be >generated as the metal is stretched during filling, and that friction betwe= en >particles and gas will create static charges, makes pumping high O2 >percentage gases into a scuba tank a potentially very high temp/pressure >situation on the microscopic scale where everything gets started anyway. > >>Pressure vessels carrying your primary air supply should be clean period. >>Any tank which is clean enough for your breathing air is clean enough for >>nitrox. If it is not clean enough for air then you need to get it cleaned. > > Yes they should be clean but clean enough for breathing is not necessari= ly >clean for over 40% oxygen. Hydrocarbon is not a dirty word biologically - >we >breath them all the time in quantities that the body handles with no sweat. >We lust after them at the dinner table and we smear them all over ourselves >in the interest of attracting other similarly slimy humans. There are man= y >compounds that we use regularly or are harmless that simply ignite easily >and violently in high O2 atmospheres. Silicone grease is normally used on >O'rings in normal scuba but is a no no with O2. > >With few exceptions, only vapors and gases will burn in air but solids and >liquids will burn in high O2 environments. > > >>But for the most part the 02 is added downstream, so any quality >>compressed air source can be used for pp fills. > > What you are talking about here is atmospheric entrainment mixing or >continuous blending. > These are mixing systems, developed by Dr. J Morgan for NOAA, that mix >gas as the tanks are being filled; one gas coming from an "oil free" >compressor and the other from a bank or tank of compressed gas such as >Oxygen and in these cases the O2 is indeed injected down stream of the >compressor and everything is mixed by the time the tank valve is reached. >This method, however, is not partial pressure filling and is still limited = to >about 40% oxygen or less unless the scuba tank is cleaned for O2 service. >Very few, if any shops use this method; it is primarily seen in commercial >diving systems. It is considered the best and safest method for >transferring >high pressure oxygen. > In partial pressure filling pure O2 is added to a tank to a calculated >pressure and then air is added on top of that to the final pressure and gas >percentages. Adding O2 after the air still exposes the tank interior and >the >tank valve to pure O2. > > No matter how high the quality of the compressor and filter system ther= e >is no such thing as 100% filtration. Some minute amount of vapors get >through and build up on the walls of the piping with time. Compressors >designed for O2 use either no lubrication, water, or special lubricants in >the >gas compression elements because of this accumulation problem and >because undetected blow-by or filter problems could easily contaminate the >system without the operators knowledge. > When you pump air into an O2 clean tank from a hydrocarbon lubricated >compressor you are introducing at least minute quantities of oil vapor into >the tank interior and valve. > These materials; water, oil, carbon monoxide, etc, are not always mixtur= es >or particles but often vapors and gases in solution with the air and they c= an >not be reliably filtered out as you filter dirt or dust from air and water. > These >must be removed by "adsorption" onto substrates such as activated carbon, >or by combining with other compounds, or altered by catalytic actions. >These types of filtration do not normally restrict the flow of gas as >impurities >are pulled out so that when loaded to capacity they often allow everything = to >pass unchanged and unimpeded with no visible indication that the filter is >overloaded. Also as some filters load up they become a source of >contaminants themselves. With many types of filtration, the only practica= l >way to insure a working filter is changing it on a schedule based on hours = of >use or quantity of gas / contaminants passed through it. > >Sintered filters are great for keeping particles out of your tank but if no= t >periodically replaced as they wear and corrode they can actually become >a source of particles. Check out the filling stations as you dive around >the country and see how many have these filters in their filling adapters >and the condition of those that do. > > I have found water in my tanks many times. Some came through >compressor systems as vapor and then condensed in the tank. This is not >serious in itself because as a vapor it can not carry anything in solution >with >it so it is clean water. But if that vapor condenses in a filter or in fi= ll >station >lines it will pick up accumulated hydrocarbons and can arrive at the tank a= s >atomized droplets with the oil in tow. Any water, of course, causes rust = in >a >steel tank - often resulting in those tiny pits mentioned earlier. > > >Jim, in another post you say : >"Well now, here's a newsflash. Now sand reacts with oxygen." > >It's not the sand, it's what might be adhering to the surface of the sand a= nd >the turbulence, friction, or static electricity the grain will cause. Als= o, >if the >grain of sand made it into the tank there is no telling what else might hav= e >gotten in. > >Normally, in an air environment, only vapors and gases will burn but in a >100% oxygen environment there is enough sustained contact with oxidant to >support the combustion of solids and liquids. > In the case of hydrocarbons in a scuba tank full of oxygen this fuel can >have worked into a pseudo-solution with the metal to some depth or at least >packed into porous spaces. Any heat build up in the tank walls can easily >create tiny local areas of explosive atmospheres against the walls of the >tank >as vapors rise out of these microscopic reservoirs. > When these hydrocarbons ignite there can be enough heat and explosive >force generated in the local shallow depths of the metal to pulverize and >ignite some of the metal itself. If enough heat is generated by this >initial >ignition to sustain combustion everything in the tank that can will >eventually >vaporize and combine with oxygen so that the final explosion releases far >more than the potential energy of just the instigating fuel itself. > This may be why some agencies will not fill aluminum tanks with oxygen = - >perhaps aluminum is more easily pulverized and ignited than steel alloys. > > >>If you could see how oxygen is handled in a shipyard, you would >>understand why I think all this nitrox hoohaa is a joke. While we treated >>02 with respect, we sure as hell didn't tippy-toe around it like it was a >>bomb ready to go off, as scuba rules seem to decree. We would regularly >>drag oxy-acetylene hoses into bunker C tanks which were not exactly what >>you would call "oxygen clean". And these hoses carried 100% 02. We were >>more scared of methane from leaking sewage systems. > >>Even with the shipyard experience, it seems that the rules for handling >>oxygen were derived primarily from industrial usage, for lack of a better >>source. 02 is used in different fashion in scuba. > > The danger is not in moving or handling the tanks but in filling them. >These nitrox and oxygen tanks can be safely dragged around just like any >other once they're full. The "scuba rules" as you call them only pertain = to >procedures for "filling" tanks during which they are, indeed, bombs waiting >to >go off. > Did you think nitrox tanks were supposed to be kept on padded cradles >and the outside wipped down and scrubbed ? Someone else wrote of his >experience dragging oxy-acetylene tanks through mud and grime, wiping the >valves off and using them. None of his welding or your shipyard examples >have anything to do with the reasons for O2 cleaning nitrox tanks. > When they speak of "handling oxygen" in OSHA and other regulations >they are talking about filling tanks or moving gas from one container to >another under pressure - not about rolling or dragging the tank to the job >site through mud puddles. > >>02 is used in different fashion in scuba.< > > Yep! It is more often handled by untrained, undisciplined, unbelieving= , >people - the general public ! All the more reason for the shops to >carefully >enforce the standards for "handling oxygen" ! What better source for thes= e >standards than industry where it has been safely handled for decades and >why spend money and time to come to the same inevitable conclusions all >over again ? > > It seems to me that the chances of hydrocarbons or other "fuel" enterin= g >most industrial application tanks is not as great as that for scuba tanks >because they are rarely opened and are only filled by facilities that do so >under more uniform and reliably safe conditions than can be expected in a >typical dive shop operation involving the general public. > I doubt that welding or medical tanks are ever filled directly from a >compressor, rather from a very large holding tanks. If this is true then >the >regulations specific to scuba probably take into account the fact that a >source of ignitable fuel is much closer to the final vessel handled by the >consumer and, therefore, considerably more likely to make it's way into the >tank either by accumulation, by undetected compressor and filter problems, >by standard "Joe Blow on the street" carelessness and ignorance, or by >people who just don't believe it or misunderstand the intent of the regs. > > >>There is a long track record of oxygen being used with standard scuba >>equipment with no failures due to oxygen induced oring degradation. >>0rings used in Scuba gear are changed regularly as a matter of course, >>never giving them the chance to degrade. It seems that the degradation is >>something which takes place over a period of time which makes it >>irrelevant to scuba. > > This is probably true where equipment receives regular maintenance but >where it does not, if a seal failure occurs you have the same situation as = a >ball valve opening in a compressor system - sudden turbulent flow. The >only place this is likely to happen is inside a regulator ; can a >detonation >occur inside your regulator during a dive and if so would it be life >threatening? Yes and possibly ! > The environment inside the regulator is no different from the filling >station >system as far as bends and obstructions go. If flow suddenly becomes >constant and/or high due to a seal failure you want it to be O2 clean if th= e >tank contains a high percentage of O2 (over 40%). > > >>In my own experience I have seen partial pressure fills done in shops, on >>boats, and in cars, on tanks which have had no special preparation and >>never once has the world come to an end. > > Well! I=EDll have to take your word for this. If these were real par= tial >pressure fills I am impressed with your luck and I hope it holds out. > Keep in mind that the space shuttle held out well for many flights befor= e >the design flaw in the solid booster O=ED ring blasted several people and o= ne >damn expensive ship into oblivion. The same situation exists on airliner= s >that eventually drop engines off or are incinerated when oxygen canisters >that have surely been smuggled on board for years finally light off under >=ECjust >the right conditions=EE. How much do you think the unlikelyhood of this >happening means to the surviving wives, husbands, and kids? > > You and every other diver out there using oxygen owes it to every other >diver and filling station attendant to follow these procedures whether you >understand the underlying principles or not. It is only because of the >cooperation of thousands of people, none with a complete understanding of >all the details, that you are able to dive at all. > How can you be willing to take the word of the people who have studied >the physiological consequences of breathing gases under pressure and >follow their decompression recommendations but publicly debunk the word >of those telling you of the touchy nature of high oxygen environments with >nothing to back it up but the fact that you=EDve never seen it happen ? > > Let me say that I am not a maintenance nut with unreasonable oxyphobia >and that I am not taking the =ECside=EE of dive shops or anyone else here. >I hate having to O2 clean anything, I hate government regulations, I hate >industry manipulation of markets, but more than this, I hate having body >parts blown off. > >Please note that I have said nothing about =EClabeling=EE here ! > > >Chuck > > > >-- >Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'. >Send list subscription requests to `techdiver-request@aquanaut.com'. -- Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'. Send list subscription requests to `techdiver-request@aquanaut.com'.
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