1991. George W. Sidebotham 1, Gerald L. Wolf 1, Jeffrey Stern 1, and Robert Aftel 1

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1 George W. Sidebothm 1, Gerld L. Wolf 1, Jeffrey Stern 1, nd Robert Aftel 1 ENDOTRACHEAL TUBE FIRES: A FLAME SPREAD PHENOMENON REFERENCE: Sidebothm, G.W., Wolf, G.L., Stern, J., nd Aftel, R. "Endotrchel Tube Fires: Flme Spred Phenomenon," Flmmbility nd Sensitivity of Mterils in Oxygen-Enriched Atmospheres; Fifth Volume. ASTM STP Joel M. Stoltzfus nd Kenneth Me Ilroy, Eds., Americn Society for Testing nd Mterils, Phildelphi, ABSTRACT: Three flme types cn result when n ignition source is pplied to tube of combustible mteril through which n oxidizer flows. If the oxygen concentrtion inside the tube is greter thn the tube mteril's limiting oxygen index (LOI), n opposed flow flme cn spred long the inner surfce of the tube (primry intrluminl flme). This flme, which requires forced flow, consumes the supplied oxygen (fully or in prt) nd produces combustible, toxic gses which emerge from the free end (nd ignition hole where pplicble). These gses cn then rect with the oxygen in the mbient environment nd thereby support the second flme type (secondry jet diffusion flme). If the LOI is exceeded outside the tube, n extrluminl flme (the third flme type) cn occur, nd be supported by nturl convection. KEY WORDS: flme spred, underventilted, endotrchel tube, oxygen index, fire sfety 1 Sidebothm; Assistnt Professor of Mechnicl Engineering, the Cooper Union for the Advncement of Science nd Art, 51 Astor Plce, New York, NY Stern nd Aftel; Cooper Union Mster of Engineering degree recipient nd cndidte, respectively. Wolf; Professor of Anesthesiology, Stte University of New York, Helth Sciences Center, Brooklyn, NY

2 The problem of operting room fires which rise from ignition (lser or electrocutery) of endotrchel tubes (ETT) continues to be reported [1, 2]. It is cler tht the high concentrtion of oxygen (02) nd/or nitrous oxide (N2O) typiclly delivered through the irwy during surgery is mjor fctor. Mesurements of the limiting oxygen index of flmmbility (LOI) for different ETT mterils [3] nd studies of the effects of using different inert gses [4] hve incresed understnding of the clinicl problem. However, there hs been little effort to chrcterize the fundmentl nture of the flme phenomen which cn occur when the LOI is exceeded nd the tube is ccidentlly ignited. Much of the current effort to solve the problem is focused on the interction of the lser source with the tube prior to ignition [2]. However, considerble development work nd testing would be required to provide n ETT which is both "lser-proof" nd stisfies the strict requirements ssocited with nesthesi. In ddition, other ignition sources could be problemtic if tube ws designed to ddress only the lser ignition problem. Until n "ultimte" solution exists, there re likely to be procedurl nd/or simple design modifictions which cn reduce the risk of operting room fires. Identifiction of such short term prtil solutions should proceed first from fundmentl understnding of the problem. Therefore, the gol of this study is to chrcterize the flme dynmics which cn occur during existing operting room procedures. In this context, the nture of the ignition source (i.e. lser or open pilot flme) plys smll role. Rther, the concentrtion of the oxidizer supplied nd its flow rte re the key vribles. The type of inert mixed with the oxidizer cn ply secondry role [4]. If n oxidizer (bove the LOI) is flowed over solid fuel nd n ignition source is brought into contct with the surfce, flme will propgte ginst the flow [5, 6]. This phenomenon, termed "flme spred," is chrcterized by gs phse diffusion flme front. Het conducted from the flme gsifies the solid fuel which diffuses bck towrd the flme. Fuel nd oxygen diffuse towrd nd re.seprted by the flme front. There re two fundmentl mesures of flme spred process: the velocity t which this flme moves reltive to the solid fuel (the flme spred velocity), nd the rte t which solid fuel is gsified by the flme (the fuel consumption rte, or burning rte). We hypothesize tht endotrchel tube fires cn be chrcterized s cylindricl geometry flme spred phenomen [7]. Three distinct flme types re identified nd described in this pper; n externl (extrluminl) flme, n internl (intrluminl) flme, nd jet diffusion flme fueled by the intrluminl flme products.

3 EXPERIMENTAL APPARATUS A brief description of the experimentl pprtus nd procedures re reported here. Further detils cn be found in Stern [8]. Dt obtined using polyvinylchloride (PVC) tubing, purchsed from New Age Industries, re reported here. Polyvinylchloride nd red-rubber endotrchel tubes were lso tested nd yielded consistent results. The PVC tubes were slipped over -horizontlly positioned 1/4 inch copper tube through which the oxidizer ws supplied. The other end of the PVC tube ws unsupported, nd is referred to s the "free end." The externl environment of the tube ws mbient ir. Oxygen, nitrogen nd helium were obtined from compressed cylinders. Flow rtes were controlled by needle vlves nd mesured with either sop-film flow meter or mss flow controller. Ignition of the tube ws ccomplished for most tests using n open pilot flme. Some tests were performed using C02 lser, which yielded identicl results. The pilot flme ws plced either ner the free end of the PVC tube, or t hole drilled into the side of the tube (used to model lser ignition). A stopwtch ws used to mesure the time for the flme to trvel mrked distnce, from which the velocity of the intrluminl flme spred ws clculted. Some experiments were recorded with video cmer, nd the velocities obtined were in good greement with the rel-time mesurements. The fuel consumption rte ws obtined by weighing the tube smple before nd fter n experiment, nd determining the time the flme existed (the distnce trveled divided by the mesured flme velocity). RESULTS AND DISCUSSION Figure 1 shows photogrphs of n experiment t three different times (pproximtely 1 second time intervl). The PVC tube mteril used does not support cndle-type combustion in ir, but does in pure oxygen. Oxygen flowed through the tube [0.25 inch (0.63 cm) inner dimeter (I.D.), 0.5 inch (1.3 cm) outer dimeter (O.D)] t constnt flow rte of 0.5 liters per minute from left to right. The tube end, to which pilot flme ignition source hd been pplied, ws freely suspended in ir.

4 Fig. 1 Photogrphs of intrluminl flme spred (left running) nd secondry jet diffusion flme (emerging from the free end). The photos re of the sme experiment t three times, pproximtely 1 second prt. The leding edge of the propgting flme is on the inner surfce of the tube, nd its til is seen emerging out of the tube in the top photogrph. The ppernce of the flme behind the leding edge is obscured due to the cylindricl geometry, internl reflections of light, luminosity due to soot prticles nd the degrdtion of the inner surfce of the tube.

5 Two distinct flmes re observed in ech photogrph. One flme propgtes inside the tube, ginst the flow, t stedy velocity, nd is flme spred phenomenon. The ppernce nd velocity of this flme depends primrily on the oxygen concentrtion nd the flow rte of oxidizer supplied. As this intrluminl opposed flow flme moves to the left, the second flme remins ttched to the free end. After this experiment, the outer surfce of the tube is virtully untouched, lthough the inside is chrred considerbly. Intrluminl Flme Spred Process Figure 2 is logrithmic plot of the propgtion velocity of the intrluminl flme spred s function of oxidizer flow velocity. Results using three different tube sizes re reported using pure oxygen [(0.25-, nd inch (0.63-, nd 0.95-cm, respectively) inner dimeters nd , 0.5- nd 0.5-inch (0.95-, 1.3- nd 1.3-cm', respectively) outer dimeter, respectively). For the 0.25 inch I.D., 0.5 inch O.D. tubes, tests using mixtures of 50 mol% oxygen re reported using two inert gses, nitrogen nd helium. 0 <u o P? O2 A 50/50 N2/O2 * 50/50 He/O in ID O2 O.SOin 00 x 0.25in ID O in w <U s fc, Oxidizer Velocity (cm/sec) Fig. 2 Intrluminl opposed flow flme spred velocity versus oxidizer velocity. Dt for pure oxygen re shown for three tube geometries, nd for 50 mol% oxygen in nitrogen nd helium, respectively. Tube size is 0.25-inch (0.63-cm) I.D., 0.5 inch (1.3-cm) O.D. except where indicted.

6 Flmes could not be estblished when the flow velocity ws below pproximtely 1 cm/sec. This result indictes tht the intrluminl flme spred requires forced flow to exist. For the lrgest inner dimeter tube, the shpe of the curves is the sme s tht observed for flme spred in n opposed flow over flt pltes [6]; s the flow velocity is decresed, the flme spred velocity pproches constnt vlue. This behvior hs been ttributed to nturl convective effects; the nturlly-induced flow component is greter thn the forced flow component [6]. In contrst, for the smller inner dimeter tubes, the flme velocity decreses shrply s the oxidizer velocity is decresed. The influence of buoyncy is reduced in the smller tubes, since the hot gses cnnot rise nd thereby induce flow into the dvncing flme. For ech cse, there is n upper limit flow rte bove which the flmes could not propgte into the tube, clled the "blow-out" limit. This phenomenon is essentilly the sme s tht which occurs when mtch is "blown out." This upper limit occurs t much lower flow rte for 50% oxygen compred with the pure oxygen cses. The blow-out limit is slightly lower when helium is used s the inert insted of nitrogen. However, the flme velocity is significntly greter with helium dilution for most flow velocities. This trend cn be ttributed to two min effects. First, the gseous diffusion flme temperture is higher with helium becuse of its lower het cpcity. Second, helium hs higher therml conductivity thn nitrogen. Both of these fctors would tend to increse the het feedbck to the solid fuel, which is controlling process in flme spred [6]. Nevertheless, it is concluded tht the type of inert used is secondry fctor compred with the oxidizer concentrtion. The observed upper- nd lower- limit behvior suggests tht s the oxygen concentrtion is reduced, the flow rnge which supports intrluminl flme spred will decrese. Ultimtely, the rnge would collpse to point which represents limiting oxygen index for intrluminl flme spred. Efforts re underwy to mesure this _pxygen index nd the corresponding flow velocity, nd compre it to the LOI obtined in the cndle-type flme test [9]. Distinct from the flme velocity, the fuel consumption rte is mesure of the net rte of solid fuel disppernce, nd is shown s function of oxidizer flow rte in Figure 3. The fuel consumption rte correltes with the oxidizer flow rte (not velocity) nd is reltively insensitive to tube inner dimeter, thickness, oxidizer concentrtion, nd inert type [8]. The fuel consumption rte is somewht higher for pure oxygen t the higher flow rtes s the 50 percent oxygen cses pproch the blow-out limit.

7 0 en If bfi "J +j cs /50 N2/O2 x 50/50 He/O2 pure O2 fi 0.01 S *H C Oxidizer Flow Rte (ml/sec) 1000 Fig. 3 Fuel consumption rte versus oxidizer flow rte. Dt for 0.25-inch inner dimeter, 0.5-inch outer dimeter PVC tubes re presented. As the oxidizer flow rte is incresed, the fuel consumption rte increses. This trend is in greement with the visul observtion of the tubes fter the experiment; the higher the flow rte, the greter the degree of chrring of the tube. In some cses, for tubes with thinner wlls, the fuel consumption rte peks, then decreses with flow'. However, the phenomenon is different with thin wlls in tht the dvncing flme completely consumes the tube mteril, nd the flme spred nd fuel consumption rtes re no longer independent. Another wy to present fuel consumption dt is to clculte n overll equivlence rtio of the intrluminl flme spred process, shown in Figure 4 s function of oxidizer flow rte. The equivlence rtio (ER) is defined s the fuel consumption rte divided by the oxidizer supply rte normlized by the fuel/oxidizer rtio required for complete combustion of PVC to CO2, H2O, nd HCl. It is ssumed tht the fuel is pure PVC. The inclusion of plsticizer would yield lower fuel/oxygen stoichiometric rtio, hence higher clculted equivlence rtio. When the mesured fuel/oxygen rtio is greter thn this vlue (ER > 1), more fuel is liberted thn cn be completely consumed by oxygen. It is seen tht this "underventilted" condition is stisfied for most flow rtes.

8 O.D- 0 rh -J r* O K_ Cu f^.d & 0) 0 o G 2 ^13 >PH > i R-, cr ^ _ X X X.X ft x x ~- X H n j j Or] ^ 50/50 N2/O2 X 50/50 He/O2 I J pure O2 O C Oxidizcr Flow Rte (ml/sec) 1000 Fig. 4 Equivlence rtio versus oxidizer flow rte. Dt for 0.25-inch inner dimeter, 0.5-inch outer dimeter PVC tubes re presented. The decrese in ER with incresing flow rte could be due in prt to end effects. The intrluminl flme cn be considered to hve length defined s the xil distnce between the flme nchor t the fuel surfce nd where the gseous flme "closes" on the centerline, the flme tip. This flme length increses with incresing flow rte. When the flme length is greter thn the distnce from the free end to the loction of the flme nchor, the flme tip emerges from the tube. Therefore, less of the solid mteril is exposed to the het of the flme thn when the dvncing flme is further from the free end. Efforts were mde to minimize these effects (by using the longest tube smples possible) but could not be eliminted. Nevertheless, the generl trend of decresing ER with incresing flow seems resonble s blow-out is pproched. Secondry Jet Diffusion Flme Interprettion of the nture of the secondry flmes (seen in Figure 1 emerging from the free end nd ignition hole) is ided by the equivlence rtio dt of Figure 4. Since the dvncing intrluminl flme spred is overll fuel rich (more solid fuel is vporized thn the supplied oxygen cn consume), the products which emerge from the tube

9 contin combustible components such s, but not limited to, crbon monoxide, hydrogen gs nd vinyl chloride. The higher the mesured equivlence rtio, the higher the concentrtion of the combustible components. Therefore, wherever flow emerges from the tube downstrem of the intrluminl flme, clssic jet diffusion flme cn exist if the mbient environment is suitble oxidizer. In ddition to the combustibles mentioned, the products of the intrluminl flme consist of inerts (N2 or He, C02, H20), mny other combustibles in smll concentrtions, toxics (HCl, CO, soot, romtics, mny others), nd 02 in certin cses (especilly when ER < 1). The secondry flme will not exist if the concentrtion of the combustibles is too low, nd/or the exit flow velocity is too high. For mny cses, prticulrly for reduced oxygen concentrtions, the secondry flme will initilly exist but then blow-out s the primry flme moves wy from the end. If there is no secondry flme, the products of the intrluminl flme re simply emitted into the environment. Visully, when the secondry flme does not exist, copious mounts of blck smoke re observed which indictes tht the intrluminl flme genertes soot. When there is secondry flme, it is observed s n intense yellow flme, the yellow being ttributed to incndescent rdition of soot prticles which re heted s they pproch the secondry flme. In ddition, the existence of the secondry jet diffusion flme depends on the distnce from the ignition hole to the free end; if the hole is too fr from the free end, the secondry flme will not be estblished [8]. In cses where the ignition hole ws further from the free end thn this minimum distnce, the secondry flme could be lit if the pilot flme ws plced ner the free end. These results confirm tht the secondry flme -requires n ignition source, which cn either be n externlly pplied source, or from the primry intrluminl flme itself. It would be possible to mesure limiting oxygen index of the mbient environment, below which the secondry flme would not be estblished. It is intriguing tht lthough the solid fuel will not support cndle-type flme in ir, the products of the primry intrluminl flme spred in mny cses hve n LOI less thn 21% oxygen (becuse they support jet diffusion flme in ir). This result cn be explined by noting tht the primry flme gsifies the fuel, which requires considerble energy; the energy relesed in the secondry flme need only het the rectnt gses up to suitble "ignition temperture."

10 Extrluminl Flme An extrluminl flme cn exist if the concentrtion of oxygen outside the tube exceeds the LOI. In this cse, even in stgnnt environment, if n ignition source is held ner the tube, flme cn be estblished which spreds long the outside surfce of the tube. This experiment is essentilly tht used s stndrd to mesure the LOI, nd the flme ppernce is similr [9], If flow were estblished inside the tube, the intrluminl nd secondry jet diffusion flmes could exist simultneously. Unlike the intrluminl flme, the extrluminl flme does not require forced flow but cn be driven by nturl convection. CONCLUSIONS The nture of the flme dynmics which cn occur during existing operting room procedures hve been investigted. We conclude tht flme spred process chrcterizes most if not ll endotrchel tube fires which involve polyvinylchloride tubes [7]. The dynmics of opposed flow flme spred in tubes re similr to those for flme spred over solid fuels in other geometries (such s flt pltes or rods). However, the intrluminl flme is unique becuse it is underventilted (except t high nd low flows), nd therefore the oxidizer cnnot be considered infinite in extent. The diffusion flme cn close on the xis, nd thereby consume ll the incoming oxygen. The products of the intrluminl flme fuel the secondry jet diffusion flme which is nchored wherever flow emerges from the tube into n oxidizing environment. Visully, the secondry jet diffusion flme is the most pprent, nd in the context of operting room fires, hs the potentil to do the most direct dmge. However, it exists only s result of the primry intrluminl flme spred. If the oxygen concentrtion of the externl environment is bove the LOI of the mteril, flme spred cn occur over the outer surfce of the tube. Unlike the intrluminl flme, bouyncy cretes the flow necessry to support extrluminl flme spred.

11 ACKNOWLEDGEMENTS The uthors grtefully cknowledge the support of Irvin Clssmn (Robert H. Goddrd Professor of Mechnicl nd Aerospce Engineering), nd Dr. Kenneth Brezinsky, both of Princeton University, for their encourgement nd for mking vilble fcilities for erly experiments. Also, the internl support of the Cooper Union is gretly pprecited. REFERENCES [1] Sosis, M. B., "Airwy Fire during C02 Lser Surgery Using Xomed Lser Endotrchel Tube," Anesthesiology, Vol.72, No. 4, April 1990, pp [2] Ossoff, R.H. "Lser Sfety in Otolryngology-Hed nd Neck Surgery: Anesthetic nd Eductionl Considertions for Lryngel Surgery," Lryngoscope, Vol. 99, No. 8, prt 2, August 1989, pp [3] Wolf, G.L. nd Simpson, J.I., "Oxidnt 02 nd Oxidnt N2O Indices of Flmmbility nd their Additive Effect," Flmmbility nd Sensitivity of Mterils in Oxygen Enriched Atmospheres; Third Volume, ASTM STP 986, D.W. Schroll, Ed., Americn Society for Testing nd Mterils, Phildelphi, 1988, pp [4] Pshyn, A.G., Grvenstein, J.S., Cssisi, N.J., nd McLughlin, G., "The Helium Protocol for Lryngotrchel Opertions with C02 Lser: A Retrospective Review of 523 Cses," Anesthesiology, Vol. 68, No. 5, My, 1988, pp [5] de Ris, J.N. "Spred of Lminr Diffusion Flme," Twelfth Symposium (Interntionl) on Combustion, The Combustion Institute, Pittsburgh, PA, 1969, pp [6] Fernndez-Pello, A.C., Ry, S.R. nd Glssmn, I., "Flme Spred in n Opposed Forced Flow: the Effect of Ambient Oxygen Concentrtion," Eighteenth Symposium (Interntionl) on Combustion. The Combustion Institute, Pittsburgh, PA, 1981, pp [7] Sidebothm, G.W. nd Wolf, G.L. "A Preliminry Investigtion of Flme Spred in Endotrchel Tubes," presented t the 1989 Fll Technicl Meeting of the Estern Section of the Combustion Institute, pper number 49, October, [8] Stern, J., "An Experimentl Study of Flme Spred within PVC Tubes with Appliction to the Endotrchel Tube Fire Problem," Mster's Thesis, Mechnicl Engineering Deprtment, The Cooper Union for the Advncement of Science nd Art, New York, My, [9] Fenimore, C.P. nd Mrtin, F.J., "Cndle-type Test for Flmmbility of Polymers," Modern Plstics. Vol. 44, no. 3, November, 1966, pp , 192.