If one can decrease the intensity , initiation, or rate of fire spread, one “buys time” so that what otherwise might be a disaster could be prevented by having enough time to take corr·ective action. It is suggested that in submarines the oxygen concentrati OI• be decreased, and that the upper limit or 19%, instead of the present 21%, be made the norm for routine operations. The value of 19% is chosen because cjgarettes will still smolder at about this level.
Fire in a submarine is a constant menace to the welfare of submarine crews, particularly during peacetime and wartime patrols. In 19801984, 1 04 ~ubtr1arine fires — ~t 8ea and in port -were reported, and they coat the Navy in excess of six million dollars. Perhaps those damages could have been greatly reduced by altering the composjtion of the inhabited atmosphere to retard flames. Case histories and fire research have both shown that the destructive power of flanJes is proportional to the concentration of oxygen in the atmosphere.
Recently, simulations of submarine hullinsulation fires in air 21% oxygen — have shown that temperatures in a sealed compartment can reach fatal levels in less than two minutes, suggesting that a conflagration can easily incapacitate a submarine within a very short period of time.
Fire damage aboard submarines may be diminished by reducing the concentration of oxygen with rapid or delayed additions of nitrogen to the atmosphere. Human activity, moreover, should not deteriorate from a deficiency of oxygen, since the total amount of oxygen in the compartment is unchanged by supplemental nitrogen. In other words, the rate of flame-spread depends on oxygen concentration and human activity depends on the absolute Bllrount of oxygen in the atmosphere.
It is apparent that fire is dependent on the percent of oxygen whereas life is dependent on the partial pressure of oxygen. Crews presently maintain their oxygen, equivalent to living at any altitude between Washington, DC, and El Paso, Texas. But even lower concentrations of oxygen may be safe, since it is common knowledge that residents of Denver, Colorado, perform many complex tasks without ill-effects from living at an altitude where there is only .175 atm. of oxygen partial pressure in the air breathed. Similarly, students at the U.S. Air Force Academy appear to study effectively at their high altitude. Thus, submarine crews could operate with lowered concentrations of oxygen as long as their partial pressures of oxygen are maintained at acceptable physiological levels.
As opposed to fire, the sustaining of life is oxygen partial pressur•e dependent not oxygen concentratjon dependent. Humans show a ver·y wice:
tolerance to large changes in oxygen partial pressure. It can be shown that humans can live in from 1.09 to .12 atm. partial pressure of oxygen. In the Apollo disaster, the atmosphere was pure oxygen at 16 psi. (and we all remember the almost explosive Apollo fire at Cape Canaveral which killed three astron~ut~) while people in Bolivia live at .12 atm. partial pressure of oxygen. The intplication is tllat humans could easily tolerate 19J oxygen concentration in a submarine. The 1.0 atm. total pressure, this gives a partial pressure of oxygen of .19 atm .• roughly equivalent to that in the Blue Ridge mountains of Virginia.
The Naval Research Laboratory bas proposed that submarines operate continuously at 19S — at about 1 atmosphere of pressure or sljghtly below — rather than at the uaximum of 21S permitted now. The reason for ohooain£ 19J is based on cigarettes still being able to smolder somewhat. Thus, the crew would not have to forego smoking. After all, a smoldering cigarette is also a fire, and at lower oxygen levels it tends to go out -causing interesting psychological effects on the crew. For non-smoking crews the 19% restriction might not apply. That 19% is quite acceptable to submarine c ·rews. has been r·epeatedly shown by submarines operating under these conditions for stretches of 24 hours or longer. often without the crews being aware of it. This has been documented by the atmosphere habitability logs of operating submarines.
One concern is that if the upper limit of oxygen is set at 19S for submarines, excursions below .19 atm. partial pressure might occur, and the physiological significance of this is not kr.own. Indeed, the physiological impact of an upper limit of 19% for 90 days in a submarine is also not known. particularly when one considers t hat ~ublllarines can ope1•ate with carbon dioxide l t:vcl ~· tJP to • 8J and other gasf;S and vapors are much higher than that in “air” and must be taken into account.
In a sublllarine:, the atmosphere is not “air.” ‘rl!e composition varies. According to the Submarine Atmosphere Control Manual there are limits ror many gases and vapors. It is noted that only for 1-hour emergency conditions is the oxygen concentration allowed to be above 21J. Excursions in the other direction, however, are com~on. For a 24-hour period in one submarine, the concentration or oxygen averaged somewhat below 19J and the crew was not perturbed about it. Concentrations as low as t7.5J oxygen have been permitted in submarines for many years. However, at this particular point, the partial pressure is still .184 atm., even though the total pressure was above 1 ~tnosphere, accounting ror the low concentration of oxygen.
Additionally, rayon, cotton, and rilter paper won’t ignite at about 18.5% oxygen, and at 19~ oxygen the burning rate of these materials would be about 10% less than at 21% oxygen — but also, at 19J there would be a greater delay in flame induction time, there would be both slowed and lessened amounts of heat release, and fire spread would be slowed considerably. All of these factors “buy time” for damage control actions.
Two very significant problems with fires in confined spaces are that they get out or hand very much faster than norDJal and that temperatures quickly reach lethal levels.
There is an induction period b~fore the fire becomes violent. In the Apollo disaster, the “bend in the knee” was less than 15 seconds. If orte drops a cigarette on a sofa in the evening, the bend in the knee might occur at 0300 the next morning. Ir there is a break in a hydraulic line, the ignition of the enlitting mist might also “bend t he ltnee” in seconds. In many Cires it is oftP.n only minutes. The important thing is to “buy time” by postponing or eliminating the “bend” by being able to have a long enough induction time to put the fire out before it gets out of hand. Reduced oxygen will “buy time.”
In connection with the concept of sudden extinguishment, a system has been proposed that, in the event of a runaway fire in a submarine, .5 atm. of nitrogen be dumped suddenly into the compartment. Adding .5 atm. of nitrogen raises the total pressure to 1.5 atm. — and the concentration of oxygen drops to 14% but the partial pressure of the oxygen stays the same.
The bottom line is that we can indeed slow fires down markedly by diluting the atmosphere with an inert gas, such as nitrogen, as long as we stay within physiologically acceptable levels. This buys time, if nothing else, and could spell the difference between an incident and disaster.
Dr. Homer Carhart and
CAPT Douglas Knight, MC, USN