field tests of release, ignition and explosion from silane .../media/documents/subject...

IChemE SYMPOSIUM SERIES NO. 153 # 2007 IChemE

FIELD TESTS OF RELEASE, IGNITION AND EXPLOSION FROM SILANE CYLINDERVALVE AND GAS CABINET

Eugene Y. Ngai1, Kelvin Pai-Ping Huang2, Jenq-Renn Chen3, Chun-Cheng Shen3, Hsiao-Yun Tsai3, Shang-Kay Chen3, Shui-Chin

Hu3, Pao-Yua Yeh3, Chun-Der Liu3, Yo-Yu Chang4, Deng-Jr Peng4 and Hong-Chun Wu4

1Air Products and Chemicals, Inc., Allentown, PA 18195-1501, USA2Air Products San Fu Co. Ltd., Chu Pei, Hsinchu, Taiwan3Department of Safety, Health and Environmental Engineering, and EPA/NKFUST Southern Center for Emergency Response

of Toxic Chemicals (ENSERTS), National Kaohsiung First University of Science & Technology, Kaohsiung, 824, Taiwan; e-mail:

[email protected] of Occupational Safety, Institute of Occupational Safety and Health, Council of Labor Affairs, Executive Yuan, No. 99,

Lane 407, Hengke Rd., Shijr City, Taipei, 221, Taiwan

This work presents field test results of silane releases from the cylinder valve and into a gas cabinet.

For safety reasons the release was from a cylinder valve that was attached to a cylinder cut in half.

This was attached to the test system which was setup remotely from the test valve. The following

release tests were performed: (1) Leak from DISS valve outlet connector with and without a restrict

flow orifice (RFO), (2) Leak directly from cylinder valve with and without a RFO, (3) Leak from

cylinder valve stem retainer thread. In addition, high pressure releases from a 1/8” and 1/4” tube

into a ventilated single cylinder gas cabinet were also performed. Release pressure varied from 120

psig to the full cylinder pressure of 1250 psig. Both digital video and high speed cameras were used

to record the ignition, pop, or explosion behavior. The results clearly demonstrated the value of an

RFO and will be useful for safety improvement in the semiconductor and TFT-LCD industries.

KEYWORDS: silane, release, explosion, field test

INTRODUCTIONSilane is a well known pyrophoric gas which normallyignites upon contact with air. It is also the most commonlyused silicon source gas in the semiconductor and TFT-LCDindustries. However, a silane release from a pressure sourcemay not always lead to prompt ignition and frequently theignition occurs when the release is shut-off. In a confinedspace, significant quantities of silane can accumulate priorto ignition leading to an explosion, causing significantdamage. Although many silane release tests have been per-formed [1,2], none of them was released directly from thecylinder valves which is one of the most frequent sourcesof accidental silane release.

The testing was conducted outdoors at a fire traininggrounds in Kaohsiung County, Taiwan, over a 2 dayperiod in May 2006. For safety reasons the source cylinderof silane and the controls were located remotely from thecylinder valve or the gas cabinet.

The following release tests were performed:

(1) Leak from DISS valve outlet connector with andwithout a restrict flow orifice (RFO)

(2) Leak directly from cylinder valve with and without aRFO,

(3) Leak from cylinder valve stem retainer thread.(4) High pressure releases from a 1/8” or 1/4” tube into a

ventilated single cylinder gas cabinet.

To simulate the worst case, release pressures were allat the cylinder pressure. Tests 1–3 with the cylinder valvevaried from 1250 to 950 psig. The gas cabinet test release

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pressure was regulated between 120 to 480 psig. Bothdigital video and high speed cameras were used to recordthe ignition, pop, or explosion behavior.

TEST SYSTEM FOR CYLINDER

VALVE RELEASESThe test system for the cylinder valve releases (1–3) con-sisted of a 49 liter carbon steel cylinder (test cylinder) cutin half with a manual 316 stainless steel tied diaphragmvalve (Ceodeux, D300) with a DISS 632 outlet threadedinto the top of the cut cylinder. The valve was tapped atthe outlet for a restrictive flow orifice (RFO). For all ofthe tests requiring a RFO a 0.010” dia (0.25 mm) wasused. This is the most common size used for cylinders ofSilane. The Cv of the cylinder valve when it is fullyopened is 0.31. The test cylinder was then clamped into achain vise that was mounted to the ground to hold it upright.

A 1/4” high pressure copper tube was attached to thebottom of the cylinder valve and the other end was attachedto the supply cylinder manifold located 20 m away. A 49liter cylinder containing 1250 psig of 100% Silane wasattached to the manifold as the supply. The manifold wasalso attached to a cylinder of high pressure Nitrogenwhich was used as the purge gas. A high pressure gaugewas located in the manifold between the Nitrogen andSilane cylinders and was used to record the Silane pressureprior to each test. Quick opening valves between the mani-fold and each cylinder were used to control the flow of the

Figure 1. Setup for the cylinder valve leak tests


gases. Prior to each test, the system is purged with Nitrogento remove any air or silane.

In all tests, the test cylinder valve handwheel wasfully open to simulate the maximum silane flow rate.Figure 1 shows the setup for the cylinder valve leak tests.

TEST RESULTS

LEAK FROM OPEN CYLINDER VALVE WITH AND

WITHOUT RFOThis test aims to simulate the potential release directly fromthe cylinder valve without any restriction on the valveoutlet. Such a release, although very unlikely, representsthe catastrophic release from a total failure of the cylindervalve connection or a valve is accidentally opened afterthe removal of the vapor tight outlet cap. Silane cylindersare normally shipped with a vapor tight outlet cap to sealthe valve outlet during transportation and storage. For thetest, this was completely removed from the test cylindervalve outlet.

Figure 2. (a) Flame and smoke from fully opened cylinder va

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The first test was with a RFO installed and the silanecylinder source at a pressure of 1250 psig. As soon as thesilane was introduced it ignited immediately at the valveoutlet. A visible flame and white smoke with a flamelength of 0.5 m was formed from the valve outlet.Figure 2(a) shows the flame and the smoke from the test.Figure 2(b) is the Silicon Oxide mass formed during therelease. The solids formed a mushroom shape from thevalve outlet upward. The flames were moderate in intensityand the burning noise was subdued which is indicative of aslow release. This allowed the mass to be formed even as thesilane continued to be released. This started to slow therelease even more causing less air to be mixed with thesilane jet. It inhibited the full oxidation of the silaneleading to the brown solids which were formed at the end.

In the second test, the RFO was removed from the testcylinder valve. When silane was admitted from the silanecylinder source at a pressure of 1250 psig, no pop orflame was observed. The expansion of the released silanefrom such a high pressure cooled it to the boiling point at1 atm of 21128C forming a 2 phase jet of liquid and

lve with RFO. (b) Silicon Oxide mass formed during release

Figure 3. The silane gas jet with condensation occurring in the

jet center

Figure 5. Sequence of frames captured by digital video when

flow resumed and silane ignited. Each frame differs by 0.033

second


gaseous silane. As shown in Figure 3 the liquid silane is anarrow white jet for 10–12 cm after the valve outlet. Itvaporizes and mixes with the air quickly. When the flowis shutoff the jet immediately disappears and a fewseconds later a sudden pop occurred. Unfortunately, thepressure decrease was too quick to visual note what thepressure was when this happened. Figure 4 shows asequence of frames during flow shutoff captured by thedigital video camera. Each frame differs by 1/30 second.There was flame only in the second frame. All the sub-sequent frames showed only white smoke. The first framealso shows that the condensation disappeared before pop.

To insure that the release would ignite, the sourcecylinder valve was reopened gradually. The slowly releasedsilane ignited immediately giving an intense and noisy fire.Figure 5 shows the sequence of frames captured by digitalvideo when flow resumed and silane ignited. The sequenceof frames clearly indicates that ignition occurs at the valveoutlet when the released silane first contacts air. At fullflow from a source pressure of 1250 psig, the flame lengthreached about 3 meters as shown in Figure 6(a). Afterflow shutoff, there was not a large accumulation of Silicon

Figure 4. Sequence of frames captured by digital video when

flow stopped and pop occurred. Each frame differs by 0.033

second

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Dioxide solids near the valve outlet due to the high flowof Silane, only a light dusting. See Figure 6(b).

In summary, the above tests confirmed previous lit-erature results that ignition of the silane release is stronglyinfluenced by the release conditions. A high pressurerelease with the valve being immediately opened did notignite while a slow opening of the same valve immediatelyignited. The addition of an RFO increases the likelihood ofignition. As expected the RFO was also found to be veryeffective in limiting the potential silane release rate withflames of 3 meters (No RFO) versus 1

2meter (RFO). As

reported by others the silane jet that was not burningignited at flow shutoff with a pop.

The exact condition of ignition or no ignition remainsto be found by more extensive tests. More delicate instru-mentation is also required to monitoring the release con-ditions. These studies are outside the scope of the presentwork and have become part of our ongoing studies.

LEAK FROM CAPPED CYLINDER VALVE OUTLET

WITH AND WITHOUT RFOThis test aims to simulate the potential release from theDISS seal cap installed on the cylinder valve outlet. Pyro-phoric gas cylinders are shipped with the cylinder valve

Figure 6. (a) Flame and smoke from open cylinder valve

without RFO. (b) Small Amount of Silicon Oxide formed

after the silane source shutoff

Figure 7. (a) Flame and smoke from capped cylinder valve with RFO. (b) Silicon Oxide mass formed during release


outlet capped. This release is possible when the cylindervalve is leaking or in the extreme case cylinder valve is acci-dentally opened and the outlet cap is not installed correctly.The release can also occur in use if the cylinder is not con-nected properly to the pigtail in the gas cabinet. Leakshave occurred as a result of improper torque, operator mista-kenly removing the wrong cylinder, damaged DISS bead,damaged washer, etc. Most of the leakage from the DISSnut will occur at the 2 leak check holes positioned 1808 apart.

Setup of the test is the same as previous test exceptthat the valve outlet is sealed with a DISS 632 nut but nowasher is used. In the first test, the test cylinder valve isequipped with a RFO. When silane flow was admittedfrom the silane cylinder source at a pressure of 1250 psig,prompt ignition and a visible flame was observed. Whitesmoke with flame lengths of 0.3 m was formed from the 2leak check holes, one was directed upwards and the otherdownward impinging onto the cylinder collar. The noisewas subdued. Figure 7(a) shows the flame and the smokefrom the test. Figure 7(b) is the Silicon Oxide formed.

The release continued for over 1 minute and theburning was similar to the RFO release in Test 1. It was

Figure 8. (a) Flame and smoke from capped cylinder valve

without RFO. (b) Silicon Oxide mass formed during release

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slow flame which led to partial oxidation forming thebrown solids. The mass stayed intact above the DISS leakhole, the one on the bottom broke off during the release.

In the second test, the RFO was removed. Whensilane flow was admitted from the remote silane cylindersource at a pressure of 1250 psig, prompt ignition and avisible flame was observed. Significant white smoke witha flame length of 1 m was directed upwards. The flamedirected downward toward the cylinder collar was sointense that it wrapped onto the cylinder shoulder. Therewere also small flames behind the DISS nut. The burningwas intense and loud. Figure 8(a) shows the flame and thesmoke from the test. Figure 8(b) shows the light dustingof Silicon Oxide. Only slight brown powder formedaround the cap. This is similar to Test 1 and attributed tothe larger release aperture and thus higher release ratewhich prevent a gradual powder built up. The cylinderpaint and label were burned

In summary, both tests showed prompt ignition andflame even though the release pressure was 1250 psig. Theprompt ignition is probably attributed to the flow restrictionfrom the seal cap. The RFO was again found to be veryeffective in limiting the potential silane release rate. Equip-ping a RFO should limit the extent and potential damage tothe operator should such a release occurred when the cylin-der valve is opened in a gas cabinet.

LEAK FROM CYLINDER VALVE RETAINERThis test aims to simulate the potential release from thecylinder valve retainer thread. Typical silane cylindervalve used in semiconductor industries is a tied diaphragmvalve, where the valve lower stem and diaphragm aremechanically linked to the upper stem and operating hand-wheel. The retainer is a nut threaded into the valve bodyto retain and seal the diaphragm. A very high torque(.100 Nm) is applied to the retainer for tightness. Theapplied torque is much larger than the required torque for

Figure 9. The sequence before and after pop at the end of flow shutoff from the valve retainer nut thread


turning the hand wheel or the valve stem. A human handcannot loosen the retainer nut; a large wrench must beused. A potential problem with tied diaphragm valves isthat once the retainer is loosened, the retainer can bemoved in the same direction by continuing to open the hand-wheel after the valve has been opened. The linked actionsbetween the retainer and the handwheel will pose a hazardshould the retainer be loosened in the first place. A tied dia-phragm valve has a leak check port added for leak testing ofthe valve diaphragm and retainer. This test is to determine ifa silane leak from a loose retainer nut will ignite or not. Anignited leak will be immediately uncovered by the operatorand is considered safe compared with an unignited leakwhich may lead to delayed ignition and explosion.

In the first test, the retainer nut was loosened 1/4 turnand the valve outlet pressurized with Nitrogen to 1200 psig.There was no leak with the valve opened or closed. Theretainer nut was then loosened to 1/2 turn and no leakwas found with the valve opened or closed. The retainernut was then loosened 3/4 turn, no leak was found for aclosed valve. When the handwheel was fully opened, thelinked action between the retainer and the handwheel was

Figure 10. (a) Flame and smoke from retainer thre

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visually confirmed. Even with the diaphragm under signifi-cant Nitrogen pressure, the valve handwheel was easy toturn counterclockwise by hand which also turned the retai-ner nut. At this point a leak developed at the retainer nutthread. Silane with a source pressure of 950 psig was thenadmitted to the valve. No ignition was observed in theleak. To make sure that a small flame was not missed, aninfrared camera was used to check the temperature in thearea of the leak and confirmed that there was no flame.The Silane source was then shutoff slowly while watchingthe pressure gauge. The leak continued without ignitionuntil the source pressure reached about 50 psig, a popoccurred and small silane flame jets were coming out allaround the retainer nut. Figure 9 showed the sequencebefore and after the pop.

After the pop, the silane flow was resumed by gradu-ally opening the source cylinder valve. Since the silaneresidue was still burning at the retainer nut, no ignitionwas required to develop flames which shot upward fromaround the retainer nut. At a cylinder pressure of 950 psig,the flame length reached about 0.4 meters as shown inFigure 10(a). After flow shutoff, large powder built up

ad. (b) Silicon Oxide mass formed during release

Figure 11. The gas cabinet test setup


near the retainer thread outlet as shown in Figure 10(b). Aswith Tests 1 and 2 with an RFO, the slow burning Silaneflame was smooth and not noisy. It developed long thinSilicon oxide tubes which extended from the circumferenceof the retainer nut like spaghetti.

In summary, the present tests showed that the retainernut can be loosened without immediate detection at the leakcheck port. This could then continue to be loosened whenthe valve handwheel is opened. The retainer nut threadscreated a flow restriction similar to that of an RFO exceptthat there was no ignition with the high pressure release.The results confirmed that release geometry affects thesilane ignition behavior. More studies are needed in orderto clarify the peculiar ignition characteristics of silanereleases.

LEAK INTO CABINETThis test aims to reproduce the explosion of unignited silanein a confined space. It has been shown by others [1,2] andabove tests that a high pressure silane release may notignite immediately in the air. This then ignites uponshutoff of the silane flow. Ideally the unignited silane israpidly diluted by the ventilation to below LEL forming asmall flammable mass or in the worst case it can form a sig-nificant mass of flammable gas due to insufficient ventilationand/or obstructions. The flammable mixture can be ignitedby an external source or during flow shutoff. In a gas cabinetthe ignition of a large flammable mass could produce anexplosion similar to a vapor cloud explosion in a confinedspace. Tests by Hazards Research Co. found that a 2 cylin-der gas cabinet detonated 5 seconds after the flow of silanewas stopped and the cabinet had already had 2 complete airexchanges [2]. The present test will try to reproduce thisresult with five video cameras, including one high speedcamera, viewing from different angles.

Figure 11 shows the arrangement of the cabinet test.The cabinet is a high flow single cylinder gas cabinet from

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Air Products and Chemicals, Inc. The cabinet has a doorwith dimension of 0.406 m wide and 1.778 m height.Upper part of the door has a reinforced glass window withsize of 0.33 m in width and 0.457 m in length. Lower partof the door has a louver which has an array 10 � 14 of0.02 m square opening. The cabinet is bolted to ground atits four corners. A 5-in venturi eductor was secured to the6” diameter exhaust opening at the top of the cabinet. Thisgenerates exhaust suction for the cabinet with the use ofNitrogen. The exhaust flow can be varied by changing theNitrogen pressure.

The Silane source and purge cylinders were located50 meters away. In the system manifold a bypass loop con-tained a high pressure regulator to allow Silane to flow at aset pressure. High pressure 1/4” copper tubing connected thesource cylinders and the gas cabinet. Immediately outside ofthe cabinet was a high flow pneumatically actuated valveinline and a tee with another pneumatically actuated valveleading to a vent stack. This setup allowed the system tobe purged with nitrogen and then fill the system withsilane up to the gas cabinet by using the vent. To insurethat the system is adequately purged with Silane it is press-urized and vented of Silane 3 times. The system is then filledwith Silane to the test pressure. The Silane is introduced intothe cabinet through a tube which enters the left side of thecabinet at a height of 1.2 meters. The tube outlet ispointed upwards at a 45o angle. Prior to each test theventuri eductor is activated and the ventilation flow rate ismeasured at the outlet and recorded.

In the first test a 1/8” dia. tube was used. Silane wasintroduced at 120 psig with the air flow in the cabinetestimated to be 8.8 m3/min (320 cfm). No flame wasobserved during the release, it was then shutoff and noflame was observed. The tube was immediately inspectedprior to any purging and the outlet had silicon oxidecoating on the outlet. With sufficient ventilation and a lowflow there was not enough flammable mass to create a sig-nificant fire or overpressure.

Figure 12. Sequences of cabinet explosion taken by video camera. Each frame differs by 0.033 second


So as to not prematurely ignite the Silane, the 1/8” diatube end was cut off to remove the Silicon Oxide coatingbefore the next test. The second test was at the same airflow rate of 8.8 m3/min (320 cfm) but at a much highersilane pressure of 480 psig. No flame was observed duringthe release, it was then shutoff and a flash was observedwhen the video was reviewed later. The tube was immedi-ately inspected prior to any purging and the outlet hadsilicon oxide coating on the outlet.

In the third test, the 1/8” dia tube was replaced by1/4” tube. The test was at the same air flow rate of8.8 m3/min (320 cfm) and same silane pressure of 480psig. No flame was observed during the release, it wasthen shutoff and a flash was observed when the video wasreviewed later. The tube was immediately inspected priorto any purging and the outlet had silicon oxide coating onthe outlet.

Figure 13. Sequences of cabinet explosion taken by hi

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The 1/4” tube end was cut off and the cabinet venti-lation was lowered significantly to 4.0 m3/min (144 cfm).The gas cabinet air baffles were also removed. The silanepressure was maintained at 480 psig. No flame was observedduring the release, it was then shutoff and the ignition pro-duced a significant explosion which not only blew off thecabinet door but also sheared the bolts attaching the gascabinet to the ground. The explosion occurred at about 3seconds after the flow shutoff. At these flow conditionsthere was sufficient flammable mass to cause a significantdeflagration which over pressurized the cabinet. Figures12 and 13 show the sequences of cabinet explosion takenby video camera and high speed camera. As seen fromFigure 13, the high speed camera captured successfullythe ignition and deflagration before the cabinet door wasblown out. This occurred in 0.008 – 0.010 seconds, the2nd frame shows the ignition and the 3rd frame shows it

gh speed camera. Each frame differs by 0.002 second

Figure 14. Distribution of cabinet fragments after the explosion


propagating out the top and an intense flame in the window.The reaction continues in the 4th frame and the gas cabinetdoor is just starting to be pushed out. Finally the 5th frameshows the reaction at the cabinet top while the lower partof the window is dark. The last frame of Figure 12 alsoshows that the flame continued from the 1/4” tube evenafter the cabinet door blew out. The blast wave blew thecabinet door 15 meters away as shown in Figure 14. Boththe window and air louver also separated from the door bythe blast. The results confirmed that significant damagecan occur from the ignition of large flammable silane/airmixture in a gas cabinet that is under constant air ventilation.

CONCLUSIONSThe above results confirmed the unique behavior of silanereleases. In particular, the ignition behavior is stronglyrelated to the release pressure, flow rate, aperture, and geo-metry. In these tests we demonstrated how silane can easilybe released without ignition. As flow rate and/or pressure isreduced it can ignite. The ignition will pose a significantexplosion hazard in cases of a high pressure, unignited

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release in a confined space such as a gas cabinet or a gasroom. As it is impossible to prevent the ignition atreduced flow, evacuation of all personnel including emer-gency response team is recommended should such arelease occur in a confined space like a gas room orcabinet. A silane release with immediate ignition is con-sidered the safer condition as the formation of a potentialexplosive mixture is prevented. The RFO is also found tobe very useful in significantly reducing the silane flow andincreasing the chances of ignition regardless of the releasepressure. Finally, more research is required in order to com-pletely elucidate the peculiar ignition behavior of silanerelease.

REFERENCES1. Tamanini, F., J.L. Chaffee, and R.L. Jambor, “Reactivity

and Ignition Characteristics of Silane/Air Mixtures,”

Process Safety Prog., 17 (1998) 243–258.

2. Cruice, W. J., “Leakage of Silane in Gas Cabinets and

Ducts”, Hazards Research Corporation, Report # 5038 to

IBM, May 11, 1982.

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