A Case Study of a TFEExplosion in a PTFEManufacturing FacilityAli Reza, P.E. and Erik Christiansen, Ph.D., P.E.Exponent Failure Analysis Associates, Los Angeles, CA 90066; echristiansen@exponent.com (for correspondence)

Published online 10 November 2006 in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/prs.10174

A 1999 explosion in the purification area of a fluo-rinated polymer manufacturing plant in the UnitedStates caused fatal injuries to three employees. Anotheremployee was severely injured. The affected plant per-sonnel were attempting to locate a suspected polymerplug in lines containing tetrafluoroethylene (TFE)monomer upstream of the polymerization plant. In anattempt to dislodge a possible plug in an elbow leadingoff a pressure vessel, one of the employees rapidly actu-ated a manually operated ball valve. High-pressureTFE upstream of the valve rushed into the downstreampiping, which likely contained air at subatmosphericpressure. Adiabatic compression against a blind flangecreated temperatures sufficient to ignite a TFE/air mix-ture. The initial deflagration provided ample energyfor a self-sustaining decomposition reaction to propa-gate within the TFE line into the upstream pressurevessel. Liquid TFE had been collecting in the pressurevessel as the result of a deliberate increase in the flowof cooling brine associated with a separate process.This allowed the decomposition to continue, causing arapid increase in pressure inside the vessel, sufficientfor the bolts on the upper flange to fail. The fouremployees were exposed to burning TFE sprayed fromthe pressure vessel and were also injured by shrapnelcreated by the explosion.

Exponent Failure Analysis Associates was retainedto conduct an engineering investigation of the inci-dent, including a technical analysis of the conditionsleading to the explosion. We also reviewed the HAZOPperformed by the employer that failed to note criticalparameters that contributed to the incident. This arti-cle discusses findings from the investigation as well aslessons on handling TFE that can be learned from theincident. � 2006 American Institute of ChemicalEngineers Process Saf Prog 26: 77–82, 2007

1. INTRODUCTION

1.1. BackgroundTetrafluoroethylene (TFE) is manufactured for the

purpose of creating polytetrafluoroethylene (PTFE)polymer, which has applications as a coating in non-stick cookware and other items. TFE vapor can formflammable mixtures in air, with a lower explosionlimit of 10% at standard temperature and pressure [1].The upper flammability limit at one atmosphere hasbeen measured to be anywhere between 40 and100%, depending on the size of the apparatus andthe strength of the ignition source used. The TFE/aircombustion reaction is [1]:

C2F4 þO2 ! 2COF2 �H ¼ �150 kcal=mol

Pure TFE can undergo a self-sustaining exothermicdecomposition reaction under certain conditions ifthe ignition source is sufficiently strong:

C2F4 ! CF4 þ C �H ¼ �66 kcal=mol

The reaction front can propagate through TFE vaporin a manner similar to a deflagration. TFE decomposi-tion fronts have also been shown to transition to det-onation in long pipelines when the initial pressure is>65 psig [1].

1.2. TFE Purification ProcessAfter production, TFE monomer is typically stored

with terpene added as an inhibitor to prevent prema-ture polymerization [2]. However, any additives mustbe removed before the polymerization reactors. Inthe subject process, most of the terpene was removedin a distillation tower followed by an absorptiontower packed with silica gel to remove any remainingadditive.� 2006 American Institute of Chemical Engineers

Process Safety Progress (Vol.26, No.1) March 2007 77

The plant has two parallel trains to handle the TFEflow as shown in Figure 1. Each train includes a dis-tillation tower and two silica gel towers in parallel toallow continuous throughput during silica gelreplacement. Flow through each train is controlled bya valve downstream of the silica gel towers. High-pressure nitrogen, vacuum, and low-pressure TFE re-covery connections attached to the top of the silicagel towers are used to dilute and purge the linesbefore they are opened.

The silica gel towers are ASME Section VIII, Divi-sion 1 pressure vessels, 10 ft tall and 16 in. in diame-ter. They are constructed from 3/8-in. thick stainlesssteel pipe and rated for a maximum allowable work-ing pressure of 300 psig at a temperature of �408 F.The top and bottom of each tower consist of flatflanges held on with 20 3/4-in. bolts. Each tower issurrounded by a 20-in. diameter brine coolant jacketthat allows operating temperatures of 15 to 258 F toprohibit polymerization and runaway reactions.

Each tower holds about 225 pounds of silica gel,sufficient to process roughly 500,000 pounds of TFEbefore it has to be replaced. Because TFE reacts exo-thermically with silica gel, the tower is isolated andthe replacement media is slowly saturated with TFEto prevent overheating or a runaway reaction. Thesaturation process typically takes 24 h to complete.

1.3. Incident DescriptionWorkers at the plant noticed that the flow of TFE

through one of the trains was very low. This was aknown occurrence in the uninhibited TFE lines andtypically occurred as a result of excess polymerbuildup. Accordingly, they shut the train down tocheck for restrictions in the lines. They locked outand tagged out the ball valve at the exit of the online

silica gel tower (#4), the ball valve upstream of theoffline silica gel tower (#3), and the ball valve justahead of the ‘‘Y’’ connection where the two TFEtrains converge. They used the offline silica gel tower(#3) to vent the high-pressure TFE from the lines tothe low-pressure recovery system and then pulled avacuum of 25–27 inches of mercury before pressuriz-ing with nitrogen up to 10 to 30 psig. The workersrepeated the nitrogen replacement and vacuum onthe line several times before disassembling the linebetween the locked valve at the end of the trainupstream of the ‘‘Y’’ connection and the control valve.No restrictions were found and the control valve wasfound to operate properly when manually stroked.The line was reassembled and nitrogen replacementwas performed. The line was left under vacuum andthe employees unlocked the ball valve between thecontrol valve and the ‘‘Y’’ connection. An employeecracked open the ball valve to allow TFE to flowbackwards into Train B from Train A. The employeesheard TFE flowing and thus confirmed that there wasno blockage in the ball valve.

The ball valve was closed and locked out again.The employees then focused their attention on theline at the exit of silica gel tower #4. They performedthe vacuum and nitrogen purge procedure onceagain and then removed a 4-ft spool section of 2-in.diameter pipe immediately downstream of the valveat the exit of silica gel tower #4. No polymer plugwas found and they reassembled the line and onceagain performed a purge by introducing nitrogen at30 psi and pulling a vacuum. The purge process wasrepeated several times.

Suspecting that the plug might be in the shortelbow connecting the silica gel tower to the exit ballvalve, the workers decided to open the ball valve

Figure 1. Schematic of terpene removal system (illustrative only).

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rapidly with the line under vacuum in an attempt todislodge the plug. They removed the lock-out/tag-outlocks and rapidly opened the valve about halfwaywhen the explosion occurred. Three workers in thevicinity of the silica gel tower were killed and onewas severely inured when the top flange of silica geltower #4 failed, releasing burning TFE vapor andshrapnel. The one surviving worker reported hearinga ‘‘pop’’ and seeing a flash within 1 or 2 s of the valvebeing open. He saw flames come out of the valveand was knocked down by the overpressure.

2. ACCIDENT INVESTIGATION

After the incident, the upper flange on silica geltower #4 was found to have blown off as a result ofthe force of the explosion, with all 20 3/4-in. diame-ter retaining bolts failed in tension. Evidence of sootwas found on the underside of the flange. The tower

itself was pushed downward through the expandedmetal floor by the force of the blast.

Disassembly of the ball valve that was being actu-ated at the time of the accident also established evi-dence of soot deposition in the interior of the valve,as shown in Figure 2. The PTFE seal removed fromthe valve had soot deposits and evidence of blow-byof hot gases, as shown in Figure 3. Disassembly of allof the lines found no evidence of a polymer plug.The control valve upstream of the ‘‘Y’’ connectionwas disassembled and found to have relatively lesssoot inside of it. The control valve is shown in Figure4 and a cross section of this valve is shown in Figure5. The ball valve located between the control valveand the ‘‘Y’’ connection was also disassembled, asshown in Figure 6. The downstream side of the ballis clean, indicating the valve was closed at the timeof the incident and did not leak downstream. ThePTFE gasket removed from the valve shows smallsoot stains consistent with the edges of the ball, asshown in Figure 7.

Figure 2. Ball valve immediately downstream of silicagel tower #4. [Color figure can be viewed in the onlineissue, which is available at www.interscience.wiley.com.]

Figure 3. PTFE seal removed from ball valve down-stream of silica gel tower #4. [Color figure can beviewed in the online issue, which is available atwww.interscience.wiley.com.]

Figure 4. Control valve upstream of ‘‘Y’’ connection.[Color figure can be viewed in the online issue,which is available at www.interscience.wiley.com.]

Figure 5. Cross section of control valve upstream of‘‘Y’’ connection (control plunger removed). [Colorfigure can be viewed in the online issue, which isavailable at www.interscience.wiley.com.]

Process Safety Progress (Vol.26, No.1) Published on behalf of the AIChE DOI 10.1002/prs March 2007 79

A vent line with a rupture disc was attached to thetop of each of the silica gel towers. The rupture discswere 1 in. in diameter and rated for 280 psig. Therupture disc attached to silica gel tower #4 was foundto have failed, as had the rupture disc on an unusedsilica gel tower that was connected to the same ventline. The connected tower did not fail, but did showevidence of soot in the interior.

3. DISCUSSION

3.1. Liquefaction of TFE in the Silica Gel TowerReview of internal documents indicated that an

operator had increased the rate of cooling brine flowthrough the silica gel towers to speed the saturationof silica gel that had recently been replaced in silicagel tower #1. The cooling brine loops through all ofthe silica gel towers without individual controls. Theincrease in cooling brine flow through the towers suf-ficiently decreased the temperature in silica gel tower#4 to liquefy TFE and cause it to accumulate in thetower. The silica gel towers were originally designedto operate at a pressure of 157 psig and a tempera-ture range of 15 to 258 F. In 1998, a process changewas implemented to increase the operating pressureto 167 psig to increase throughput from the trains. Areview of the process change management documen-tation indicates that no consideration of liquefactionwas made. Liquefaction of TFE in the silica gel towerswas also not considered during the original processhazard analysis. Consequently, workers were notaware of the possibility of liquid TFE accumulation.

During the period preceding the incident, the tem-perature within silica gel tower #4 was found to be108 F, which corresponds to the intersection of thevapor dome with the operating pressure of 167 psig(see Figure 8). This verifies that the observed reducedthroughput through Train B was a result of liquidaccumulation as opposed to a plug or blockage. Thebrine flow rate had been increased nearly 24 h beforethe accident occurred, allowing enough liquid accu-

mulation to nearly fill the silica gel tower to the exitopening by the time the incident occurred. The ab-sence of liquid level indicators in the tower, or othercontrols such as temperature and pressure alarms thatwould indicate the presence of liquid TFE, allowedthis condition to go unnoticed.

3.2. Ignition MechanismEyewitnesses indicate that the initiating event for

the subject explosion was the rapid opening of a ballvalve at the exit of silica gel tower #4. The partiallyopen ball valve allowed high pressure (167 psig) TFEvapor to rapidly expand into piping that had beenevacuated to a vacuum of 25–27 inches of mercury.Although the piping had previously been purged withnitrogen, witnesses report that a leak check was notperformed and gaskets may have been reused. Inaddition, a final leak check had not been performedon silica gel tower #3, which was used to perform thevacuum and nitrogen purge procedure on the subjectpiping. Therefore, it is likely that ambient air had beendrawn into the piping while it was under vacuum.

The high-pressure TFE compressed the air insidethe piping, causing an increase in temperature. Calcu-lations by Exponent confirmed that, at this compres-sion ratio, the air temperature can reach 13608 F,which exceeds both the autoignition temperature ofTFE/air mixtures and the temperature required to ini-tiate the self-decomposition reaction in pure TFE.The process is sufficiently rapid that heat transfereffects are negligible and therefore the compressionphenomenon can be considered adiabatic until igni-tion is achieved. The calculated temperature profile atthe interface between the TFE and air is shown inFigure 9 for time increments after compression. Theresults indicate that a TFE/air deflagration can be ini-tiated at the interface. The deflagration can initiate aself-decomposition reaction front in the pure TFE thatwill propagate through the piping and into the silicagel tower. Previous work by Couture [1] demon-

Figure 6. Ball valve located between control valve and‘‘Y’’ connection. [Color figure can be viewed in theonline issue, which is available at www.interscience.wiley.com.]

Figure 7. PTFE gasket removed from ball valvebetween control valve and ‘‘Y’’ connection. [Colorfigure can be viewed in the online issue, which isavailable at www.interscience.wiley.com.]

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strated that rapidly compressing air in 3-in. pipescould create temperatures that are sufficient to igniteTFE. This mechanism is also consistent with the wit-nesses observing a flash of flame at the valve 1 to 2 safter the operator opened it.

The soot deposition on the interior of the piping,valves, and the silica gel tower is consistent with thisignition mechanism. Sooting at the ball valve justdownstream of the control valve, the likely locationof hot, compressed air necessary for ignition, is lightcompared to piping closer to the tower and the tower

itself. TFE decomposes into fluorine and carbon, so itis expected that there will be large amounts of sootproduced by pure decomposition and less by TFE/airdeflagrations.

Other ignition mechanisms were considered butruled out as unlikely. For example, TFE can form ex-plosive levels of peroxide [3]. However, peroxide for-mation is associated with exposure to heat, light,and—most important—oxygen. There is no evidencethat there was any oxygen in silica gel tower #4because a successful leak check had been performed.No evidence of peroxide or internal damage to thetower associated with a peroxide explosion was foundafter the incident. Additionally, the increase in coolingof the tower would tend to inhibit peroxide formation.Finally, long induction times can be necessary for per-oxide formation [4], which is not consistent with thetimeline of events before the explosion. Spontaneouspolymerization was also considered and discountedfor similar reasons. The timing of the event with theopening of the ball valve makes it highly unlikely thatthe polymerization reaction coincidentally experiencedrunaway at the moment the valve was opened. Thelow temperature and lack of any evidence of polymer-ization further discount this possibility.

3.3. Silica Gel Tower FailureCalculations were performed to determine the in-

ternal pressure necessary for failure of the 20 3/4-in.bolts that secured the upper flange to the tower. Thebolts were constructed of SA-320-L7 steel, with ayield strength to 105 ksi [5]. The pressure inside thesilica gel tower required for the bolts to fail was cal-culated to be 3840 psig. The pressure required forthe vessel itself to fail, using a thin-walled shellassumption, was calculated to be about 1500 psig.The vessel is surrounded by a brine jacket, whichitself has a failure pressure of about 1300 psig. The

Figure 8. Vapor pressure of TFE with silica gel tower operating conditions. [Color figure can be viewed in theonline issue, which is available at www.interscience.wiley.com.]

Figure 9. Transient temperature profiles of TFE/airinterface. [Color figure can be viewed in the onlineissue, which is available at www.interscience.wiley.com.]

Process Safety Progress (Vol.26, No.1) Published on behalf of the AIChE DOI 10.1002/prs March 2007 81

combined strength of the coupled inner shell andouter jacket allowed the vessel to sustain pressures inexcess of the failure pressure for the top flange.

The pressure required for the bolts to fail is muchgreater than the maximum pressure produced by a TFEvapor decomposition, which in an unvented vesselwill reach a final pressure of 1770 psig. In actuality,TFE vapor decomposition inside the subject silica geltower will produce a lower pressure as the result ofventing through the vent line after the rupture discbursts. Calculations were performed using the methodof Epstein et al. [6] to determine the maximum pressurein the vessel while modeling venting through the 3/4-in. line connected to the rupture disc. The reducedpressure was calculated to be 1560 psig.

Failure of the bolts attaching the top flange to thesilica gel tower can be reconciled only in the contextof liquid TFE accumulation in the tower. When thedecomposition reaction front propagated back intothe silica gel tower, it created an overpressure in theheadspace above the liquid TFE of nearly 1500 psig.However, the decomposition also produced a largeamount of heat that boiled off liquid TFE, with theresulting vapor decomposing in the head space. Theliquid TFE essentially acted as a source of fuel for thereaction, leading to super-adiabatic pressures. Thispressure-piling effect is a documented hazard of liq-uid TFE storage, and explosion vents have beenshown to be ineffective in mitigating overpressurearising from this phenomenon [7]. Because of theextreme difference in densities between liquid andvapor TFE, very large overpressures can be achievedfrom vessels containing liquid TFE. The presence ofsoot found in the unused tower that shared a ventline with silica gel tower #4 is an indication that theTFE decomposition spread to this tower. However,this unused tower did not fail because it had no liq-uid TFE present to produce super-adiabatic pressures.

4. CONCLUSIONS

1. The original process hazard analysis did notaddress the hazards of liquid TFE accumulation inthe TFE purification area. The accumulation of liq-uid TFE in the silica gel tower was the primarycause of this incident.

2. The management of process change review whenthe operating pressure was increased from 157 to167 psig was defective. Although this changemoved the thermodynamic state of TFE in the silicagel towers closer to the vapor dome and createdadditional constraints on the temperature control,the hazards associated with liquid TFE accumula-tion were not explicitly identified or addressed.

3. There were no liquid level indicators or tempera-ture and pressure alarms or controls installed onany of the silica gel towers. Accordingly, workershad no way of determining that liquid had accu-mulated within the towers.

4. Using a vacuum purge procedure allowed for thepossibility of air infiltrating the lines if a proper seal

was not achieved. The workers failed to performan adequate leak check or replace all gaskets afterbreaking the lines. This likely allowed air to leakinto the lines while they were under vacuum. Oxy-gen sensors were not used to check the lines for airbefore introducing TFE. Using a purge process thatdid not involve drawing a vacuum inside the linescould have potentially avoided this problem.

5. Improper operation of a manual ball valve (rapidopening) allowed adiabatic compression of the airthat had leaked into the piping to occur, resultingin temperatures high enough to ignite a TFE/airmixture at the interface. The TFE/air deflagrationinitiated a self-decomposition reaction front thatpropagated through the piping and back into silicagel tower #4.

6. Accumulation of liquid TFE in the silica gel towercaused a pressure-piling effect to occur, producingsuper-adiabatic pressures inside the vessel. Theresulting pressure exceeded 3840 psig, resulting inthe failure of the 20 3/4-in. bolts securing theupper flange of the silica gel tower. Without thepresence of liquid in the vessel, the silica geltower would not have failed.

7. The rupture disc and vent connected to the silicagel tower were insufficient to vent the explosion.The literature indicates that TFE decompositionreactions fed by liquid TFE create pressure-pilingeffects that cannot be vented reliably by a rupturedisk arrangement.

LITERATURE CITED

1. M.J. Couture, TFE hazards, Plastics Department,E. I. Du Pont de Nemours & Co., Wilmington, DE,January 27, 1969.

2. P.G. Urban (Editor), Bretherick’s handbook of re-active chemical hazards (5th edition, Volume 1),Butterworth–Heinemann, Oxford, UK, 1995.

3. R.R. Kelly, Review of safety guidelines for peroxi-dizable organic chemicals, Chem Health SafetySeptember/October (1996), pp. 28–36.

4. F. Gozzo and G. Camaggi, Oxidation reactions oftetrafluoroethylene and their products—I: Auto-oxidation, Tetrahedron 22 (1966), pp. 1765–1770.

5. American Society for Testing and Materials(ASTM), Specification for alloy steel bolting mate-rials for low-temperature service, SA-320/SA-320M,ASTM Specification A 320/A 320M-99, ASTM, WestConshohocken, PA, 1999.

6. M. Epstein, G.M. Hauser, J.B. Tilley, C.H. Barron, J.L.Wise, P. Thistleton, R.L. Harper, M.J. Couture, andB.R. Blair, A computer model for the estimation ofpeak pressure for sonic-vented tetrafluoroethylenedecompositions, J Loss Prev Process Ind 3 (1990),pp. 370–380.

7. Ya.A. Lisochkin, S.I. Ozol, V.I. Poznyak, and V.A.Rykunov, Explosion-proof method of storage of liq-uid tetrafluoroethylene, Russ J Appl Chem 68 (1995),pp. 920–921.

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