The Flameless Venting using Polyurethane Foam

The Flameless Venting using Polyurethane Foam Andrews, G.E.*, Fakandu, B.M., Stride, D. Witty, L. and Phylaktou, H.N. Energy Research Institute, University of Leeds, Leeds LS2 9JT, UK *Corresponding author email: [email protected]

ABSTRACT Explosion suppression foam (ESF) is used in aircraft fuel tanks and Formula 1 fuel tanks to prevent a vapour space explosion, as occurred in the TWA 800 disaster. It therefore shows potential to be used as a flame arrester using different fuels and configurations. It was investigated in a 50mm thickness to see if explosion flame propagation could be quenched and whether ESF could make a practical flameless venting system. A 10 litre L/D=2.8 vented explosion vessel was used with. The results showed the potential for use of ESF as a flameless venting device for gas explosions for methane and propane. Quenching by the foam occurred with some damage to the ESF in terms of melting and compression of the foam in line with the vent. The depth of flame penetration into the foam was found to increase for higher vent coefficients, Kv. Evidence for flame quenching and no external vented flame is presented for 10% methane and 4.5% propane-air explosions for Kv = 3.6 and 10.9.

KEYWORDS: Flameless venting, suppressant foam, flame arrester, explosion suppression. INTRODUCTION Under the EU ATEX Directive and in NFPA68 [1] explosion protection measures must not put people at risk and thus explosion protection using direct venting is limited by the length of the flame outside the vent and people have to be removed from this area. If this cannot be done then vent ducts have to be attached to take the flame to a safe discharge location [1]. These vent ducts increase the explosion overpressure, which requires large vents to be used [1, 2]. An alternative method of achieving this safety is the application of flameless venting [1], which is currently very expensive. The present work investigates the use of polyurethane foams, which are currently used in the explosion suppression for aircraft fuel tanks [3, 4]. Commercial open cell polyurethane foam, as supplied to the aircraft fuel tank protection industry, was investigated in a simple arrangement of flameless venting, so as to determine whether this material was a potential low cost flameless venting material. Commercial flameless venting usually uses wire mesh flame traps with a strong enclosure that protrudes some distance from the vent. In this work a simple arrangement was used of the 50mm thick polyurethane foam placed upstream of the vent and covering the whole end wall of the cylindrical explosion vessel. Explosion suppression foam (ESF) is a sponge-like material made from polyurethane. It was developed by the United States Air Force during the 1960s due to the large number of aircraft being lost to enemy fire during the Vietnam war. When the foam was fitted inside of aircraft fuel tanks, it prevented or significantly reduced the severity of fuel tank explosions due to small arms fire. All US military aircraft are now fitted with the foam. The investigation into the TWA 800 crash near New York in 1996 revealed that aircraft fuelled by kerosene were also vulnerable to accidental fuel tank explosions. In the TWA case the ignition source was a faulty electrical lead in the vapour space above the central wing tank. ESF has therefore also been recommended for use on civilian aircraft to prevent accidental fuel tank explosions.

Proc. of the Seventh International Seminar on Fire & Explosion Hazards (ISFEH7), pp. 1-10 Edited by D. Bradley, G. Makhviladze, V. Molkov, P. Sunderland, and F. Tamanini Copyright  2013 University of Maryland. Published by Research Publishing ISBN: 978-981-08-7724-8 :: doi: 10.3850/978-981-08-7724-8_0x-0x

Proc. of the Seventh International Seminar on Fire and Explosion Hazards (ISFEH7)

CHARACTERISTICS OF EXPLOSION SUPPRESSION FOAM (ESF) Technology in producing ESF foam has improved over the past 40 years resulting in very consistent pore sizes and foam which is very light and causes an almost negligible loss in fuel tank volume. ESF is polyurethane open cell foam and is commonly used in aircraft and other fuel tanks where the possibility of fuel tank explosions exists [5]. The product has been developed since 1965 when the USAF began using polyester / polyurethane foam to suppress fires and explosions in fuel tanks.

ESF has now been in use for over 30 years [4] and has found thousands of successful applications in aircraft, boats, military vehicles, competition and emergency vehicles. The foam is about 97% void and reduces the fuel volume in a protected fuel tank by only 2.5% whilst adding a negligible amount of net weight to the fuel tank. The foam is effective against:  Gun fire  Electrical ignition  Lightning strike  Static discharge (source www.irappa.com) Flame arresters (also known as a flame traps) are commonly used in many process industries, particularly petrochemical, to prevent a flame travelling along pipe work or through other structures, threatening property and life. There are many types of flame arrester on the market, the majority of which are constructed from some form of metal mesh. However, ESF has advantages over other materials:  Lightweight;  economic;  does not corrode;  flexible and compressible;  easily cut to shape; and  behaves as a particle filter. The ESF foam is very light with a bulk density of about 20 kg/m3 with about 700 pores/m or an average pore size of 1.4mm, which is below the quench distance of most hydrocarbon flames. The foam has the ability to behave as both a dust filter and flame arrester and may negate the requirement for two separate components. To the authors knowledge this type of foam has not previously been applied to flameless venting or flame trap use. Existing flameless venting devices are expensive, as are alternative explosion suppression mechanisms. The potential may exist for a low cost flameless venting application of suppressant foam, provided it acts to prevent a flame from passing through a vent without an undue increase in the explosion overpressure. It was the aim of this research to determine this capability of the foam. It is somewhat counter-intuitive that polyurethane foam should function well as a flame arrester, as the foam used in this work can be easily ignited by an external flame after 2-3s from flame application. The measured rate of flame propagation is 6 mm/s. However, in an explosion it is the very low residence time of the flame that is important. It will be shown later that the explosion flame approaches the flame trap at about 20 m/s which for a 50mm thick ESF gives 2.5ms residence time and for 25 cells in the 50mm width the residence time per cell is 0.1ms. The heating of a mass of flame trap is a first order time response and the ESF cell walls would not heat up much in 0.1s, but it would be a faster heating rate than for a metal flame trap due to the differences in thermal mass. Flame traps operate by cooling the flame below the critical flame propagation temperature of 1400K (based on the lean flammability limit). This requires about 700K of flame temperature reduction by the flame trap to quench a stoichiometric flame. The heat removed heats up the flame trap and the temperature rise of the foam flame trap compared with a metal flame trap for 2

7th International Seminar on Fire and Explosion Hazards (ISFEH 2013)

the same flame quenching is in proportion to the ratio of metal to foam densities multiplied by the ratio of metal to foam specific heat ratio, for the same cell wall thickness. The specific heat of the ESF was 1.5 kJ/kgK compared with about 0.4 kJ/kgK for a metal flame trap, thus the metal to foam specific heat ratio is 0.27, but the density ratio is about 400. Thus for the same heat input and cell wall thickness the foam should have 100 times the temperature rise of the metal. When this is coupled with the poor thermal conductivity of ESF foam there is a risk of the foam melting. If this did occur then there is the additional cooling mechanism of the 1.22 MJ/kg latent heat of vaporization. There was evidence of the foam melting or softening in the present work and causing an indentation in the foam that could be measured after the explosion. In related work with ethylene vented explosions there was some melting of the ESF, but still successful flame quenching. It is considered that ESF are heated more than an equivalent metal flame trap and should be considered to be sacrificial flame traps and need the foam replaced after an explosion has been suppressed. A further feature of the foam is its flexibility. Forster and Weichman [6] describe a detonation flame arrester which attempts to separate the compression wave of an explosion from the flame front, by inserting baffle plates in front of the quenching element. The baffle plates behave as a form of shock absorber. It is possible that a similar mechanism is operating with the foam. It first absorbs mechanical energy from the compression wave by virtue of its flexibility, before dealing with the decelerated flame front. Figure 1. Detailed photo of the reticulated structure of the foam, the cell size was about 2mm. FLAMELESS VENTING The danger of the jet flame exiting an explosion vent can be avoided by allowing the explosion to vent inside the building, with the vent closure designed to quench the flame and retain hot particulate matter, so as not to risk injury to nearby personnel. The first flameless venting devices were developed and tested in the late 80s and early 90s [7]. Barton [8] and Holbrow [9] describe flameless venting devices as typically comprising a vent panel, flanged housing, and a flame arrestor element. The general principle is that during the early stages of an explosion the vent cover opens, the burned dust, unburned dust and flame enter the flame arrester element. Flame propagation beyond the device is prevented by energy dissipation in the element, reducing the burning fuel below its ignition temperature. The dust is also retained within the arrestor element and gases from the explosion are vented through the device into the external atmosphere. EXPERIMENTAL EQUIPMENT The experiments were performed in the same vented vessel as used by Kasmani et al. [10] shown in Fig. 2, with a diameter of 162mm, length 460mm (L/D=2.8), and a volume of 0.01m3. This is within the limit of applicability of the US and EU explosion vent design methodologies of 2≤ L/D≤ 3 [1, 11]. The test vessel was connected to 0.5m diameter cylindrical vessel which was also 3


Figure 2. Detailed diagram of the explosion vessel and connecting vessels.

connected to a 50m3 dump vessel to safely capture the vented flames. The 0.5m diameter vessel between the vented vessel and the dump vessel was used to mount three thermocouples on the centreline of the discharge jet so that the vented jet flame velocity could be determined as a function of distance from the vent. If the flame ESF flame trap was effective then no flame would be detected downstream of the vent. This vessel was sufficiently larger than the vented vessel to give free venting conditions in the near vent area. The experiments were carried out with free venting, with the mixture confined by a vacuum gate valve before the explosion. The 50mm thick ESF was cut to the diameter of the explosion vessel, 162mm, and pushed against the vented end, upstream of the gate valve. The gate valve, when closed, allows the mixture of gas and air to be accurately made by partial pressure. The gate valve separates the test vessel from the 0.5m dia. vessel and only opens prior to ignition to allow the required mixture to be ignited before the explosion occurs. A 16 J spark was used and the spark plug was located at the centre line of end flange opposite the vent. End ignition was shown by Kasmani et al. [12] to give significantly higher overpressures in vented explosions compared with central ignition. Each test was repeated at least three times. The flame travel time was recorded by mineral insulated, exposed junction type-K thermocouples, arranged axially at the centre line of both the main test and the 0.5m dia. vessel, as shown in Fig. 1. These had a negligible dead time and the time response was irrelevant as they were not there to measure the temperature but to detect the flame arrival which was determined by the first change in temperature. Thermocouples T1, T2 and T4 were located on the centreline of the main test vessel, while thermocouples T5, T6 and T7 were on the centreline of the 0.5m dia. connecting vessel. If the vent flame trap was successful then T5 – T7 would not record a flame arrival. The time of flame arrival was detected from the thermocouples and the flame speed between two thermocouples was calculated and plotted as the flame speed for the midpoint between the two thermocouples. There was also another thermocouple, T3, located on the wall of the main test vessel to measure the time of flame arrival at the wall of the vessel. The time of flame arrival upstream of the ESF was determined from T2 and the non-arrival of the flame at the vent and outside the vent was determined from the lack of response of thermocouples T4-T7. Two piezo electric pressures transducers PT0 and PT1 were located at the end flange (PT0) opposite the vent and mid way the vessel length (PT1) respectively. In low flame speed 4


explosions these pressure transducers had identical pressure time characteristics and this was the case in the present work. However, for reactive gas explosions such as ethylene and hydrogen there were dynamic flame events that caused these two pressure transducers to record different pressure time records [10, 13]. A third transducer PT2 was located in the 0.5m dia. connecting vessel which measured the external explosion overpressure and it time of occurrence. This was of great assistance in determining when the external explosion occurred and was used in the present work together with the external thermocouples to prove that there was no external explosion. The polyurethane explosion suppressant foam was investigated with two vent coefficients (Kv = V2/3/Av) of 3.6 and 10.9. The 50mm thick Explosion Suppression Foam (ESP) was inserted across the vessel diameter flush with the upstream face of the vent, as shown in Fig. 3a. The vessel was instrumented with flame arrival thermocouples on the centreline of the vessel upstream of the vent and downstream of the vent. Successful flameless venting occurred when no flame was detected downstream of the vent. It was also shown by the absence of an overpressure due to the external flame, which is referred to as P3. The overpressure due to the unburned gas flow through the vent, driven by the expanding explosion flame, was measured using a pressure transducer mounted on the end flange of the vessel opposite the vent and is referred to as P2. All the work was carried out using free venting so that there was no overpressure peak, P1, associated with the vent static bursting pressure. The results showed successful flameless venting with little damage to the foam, as shown in Fig. 3b.

Figure 3. Polyurethane foam (a) before explosion (b) after explosion. RESULTS AND DISCUSSION The pressure time results for 10% Methane-air with vent coefficients, Kv, of 3.6 and 10.9 with and without the polyurethane foam are shown in Fig. 4. This shows that for Kv=3.6 without the ESF the peak overpressure was controlled by the external explosions, P3, at 50mb. The depression in the foam, shown in Fig. 1b, after the explosion was 11mm of the 50 mm thickness and hence was well away from penetration. However, with the suppressant foam in place the external explosion was extinguished so that the peak overpressure was that due to the flow of unburned gas through the vent with the additional flow blockage of the foam. The surprising result was that the pressure P2 was slightly lower with the foam in place than for the vented explosion, 30mb compared with 35mb. This indicates that quenching the flame 50mm upstream of the vent reduced the flow through the vent and this reduced the pressure loss more than the presence of the foam increased it. 5


Figure 4. Pressure profile for 10% Methane-air (a) Kv=3.6 and (b) Kv=10.9. The flame speeds for these explosions are shown in Fig. 5b. This confirms that the ESF quenched the flame, as none of the four thermocouples at the vent and downstream recorded a flame arriving. Without the ESF the external freely vented flame for a Kv of 3.6 reached a maximum flame speed of 65 m/s, which was responsible for the generation of the external explosion overpressure, P3. Upstream of the vent the flame speed was increased by the ESF from 25 to 40 m/s. The reason for this was the foam at the vent acted as a blockage to the flow and this forced the flame to spread radially outward, more than it did for the free vent. For this reason, the flame speed decreased on the axis, this was because the flow through the vent was decreased, and this resulted in the observed lower P2 overpressure in Fig. 4a. Further evidence, for Kv=3.6, that showed there was no external explosion with the ESF in place was provided by the pressure transducer (PT1) mounted in the wall of the 0.5m diameter vessel that connected the vented vessel to the 50 m3 dump vessel. The comparison of the pressure record inside and outside of the vessel is shown in Fig. 6 with and without the ESF in place. Fig. 6 is a different repeat explosion for Kv=3.6 than that in Fig. 4a and the P2 and P3 overpressures without the foam were lower at 20 and 50 mb in Fig. 6 and 35 and 55mb in Fig. 4a. The results with the foam in place in Fig. 6 were also a repeat and show a P2 overpressure of 28mb, which is the same as that in Fig. 4a. The increase in P2 with the foam in place (20mb > 28 mb) was expected due to the backpressure of the flow of unburned gas through the foam. It is thus the results in Fig. 4a that are rather unusual in showing a lower P2 with the foam in place. Fig. 6 shows that without the ESF the open vent showed no external pressure rise until the flame had left the vent. There was then an external pressure recorded by PT1 that aligned in time with

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Figure 5. Flame speeds with and without ESF for Kv = 10.9 (a) and 3.6 (b). The vent is located 454mm from the end flange. the external explosion pressure peak P3 and was very similar to it. There was no P2 pressure peak due to the flow of unburned gas through the vent, recorded on the external pressure transducer, PT2. With the explosion suppressant foam across the vent Fig. 6b shows successful quenching of the external explosion as PT2 records no pressure rise, whereas PT1 inside the vessel records the pressure due to the flame development and the pressure rise due to the induced flow of unburned gas through the vent. For Kv=10.9 the peak overpressure was due to the flow through the vent, P2, due to the smaller vent area and higher back pressure for a similar flow displacement from the internal flame. Thus, although the external explosion was quenched by the foam, the P2 overpressure increased slightly due to the increased back pressure of the unburned gas flowing through the foam and the vent. Without the foam present there was a clear external explosion pressure peak, P3, which was significantly lower than P2. However, with the ESF in place there was no external explosion pressure peak. Compression/burning of the foam of the type shown in Fig. 1b was more extensive at the higher Kv, due to the higher vent flow velocities at the smaller vent area. The damage extended 33mm into the 50mm depth of the foam, considerable more than the 11mm for the Kv = 3.6 case discussed above. The flame speed for the Kv=10.9 external explosion was very high at 200 m/s which was responsible for the 0.13 bar external explosion overpressure. With the foam across the vent the external explosion pressure peak disappeared and there was no flame detected at the thermocouples. This shows that the ESF had successfully quenched the flame at the high vent

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Figure 6. Comparison of overpressure measured outside of the vent (PT2) and at the wall of the vessel upstream of the vent (PT1) with and without the ESF. .across the vent.

Figure 7. Overpressure as a function of time for 4.5% propane-air for a vented explosions with Kv = 3.6, with and without explosion suppressant foam (ESF) across the vent. 8


Figure 8. Flame speeds with and without ESF for Kv = 3.6 with 4.5% propane-air. flow velocities generated by the small vent area. This was with minimal damage to the ESF, as shown in Fig.3b. Additional tests were carried out that demonstrated a very similar performance with propane-air explosions as for methane-air, with no external explosion at either of the vent areas. There was slightly more ESF damage with the propane explosions, with 12mm of compression/burning of the type shown in Fig. 1b. The explosion overpressures as a function of time are shown in Fig. 7a for 4.5% propane-air with Kv = 3.6 with and without the ESF across the vent. The results show that without the ESF in place the peak overpressure was due to the external explosion at 47 mb, which was slightly lower than the 50-55mb for 10% methane-air discussed above. However, with the ESF across the vent the external explosion was quenched as shown in Fig.7b, but the pressure loss due to the flow of unburned gas through the vent increased to 40mb, much higher than the 28mb for methane-air. This increase in P2 was due to the higher upstream flame speed with propane. The result was that the reduction in the peak overpressure with the ESF from the open vent situation was only small. Thus it is the absence of the external vented flame that is important not the elimination of the back pressure caused by the external flame. Fig. 8 shows a similar trend of flame speeds upstream, with no flame detected after the vent as that for 10% methane/air. However, the flame speed was shown to be higher when compared to Fig.5b with similar Kv of 3.6. This was responsible for high value of P2 with the foam across the vent as mentioned above.

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CONCLUSIONS Polyurethane reticulated foam (ESF) was shown to be a good flameless vent material as quenching of the flame was demonstrated for 10% methane-air and 4.5% propane-air explosions. The damage to the foam was significant and this material should be developed for practical explosion venting applications. Further work is required for the system to be demonstrated to work with more reactive gases. The relative low cost of ESF gives the potential for more widespread use of flameless venting. ACKNOWLEDGEMENTS We would like to thank Crest Foam Industries for the donation of some of the ESF used in this work. Bala Fakandu would like to thank the Government of Nigeria for a research scholarship.

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National Fire Protection Association (NFPA), “Guide for Venting of Deflagrations”, NFPA 68, 2007. F.G. Ferrara, S.K. Willacy , H.N. Phylaktou , G.E. Andrews, A. Di Benedetto, E. Salzano , G. Russo, Venting of gas explosion through relief ducts: Interaction between internal and external explosions. Journal of hazardous materials, (2008), 155(1-2), pp.358-368. National Fire Protection Association (NFPA), “Standard on Explosion Prevention Systems”, NFPA 69, 2008. R. Zalosh, Deflagration Suppression using expanded metal mesh and polymer foams, Journal of loss prevention in process industries, (2007), 20, pp.659-663. Crest Foam Industries: www.crestfoam.com Förster H. and Weichmann W. (2004) “Flame arresters” in Handbook of explosion prevention and protection. Edited by Hattwig, M. and Steen, H. pg 593 Wiley-VCH, Weinheim. Going J. and Chatrathi K., (2003), “Efficiency of flameless venting devices” in Process safety Progress Vol 22, No.1, pg 33-42 Barton, J. (2002). Dust explosions prevention and protection, a practical guide. Institution of Chemical Engineers, Rugby. Holbrow P. (2006) Explosion protection using flameless venting – a review. Health and safety laboratory at www.hse.gov.uk/research/hsl_pdf/2006/hsl06105.pdf R. M. Kasmani, B. Fakandu, P. Kumar, G. E. Andrews, H. N. Phylaktou, Vented Gas Explosions in Small Vessels with an L/D of 2, Fire and Explosions Hazards Proceedings of the Sixth International Seminar on Fire and Explosions Hazards, Bradley, D., Makhviladze, G., and Molkov, V., (Eds.), University of Leeds, Leeds, UK (April 11-16, 2010), 2011. pp. 659-670. European Standard EN 14994:2007, ‘Gas explosion venting protective systems’, 2007. Kasmani, R.M, Andrews, G.E., Phylaktou, H.N., Willacy, S.K. The Influence of Vessel Volume and Equivalence Ratio of Hydrocarbon/air Mixtures in Vented Explosions 4th International Conference on Safety & Environment in Process Industry, Florence. Chemical Engineering Transactions, Vol.19, p.463-468 (2010). Fakandu, B.M., Kasmani, R.M., Andrews, G.E. and Phylaktou, H.N., “Gas Explosion Venting and Mixture Reactivity”, 23th International Colloquium on the Dynamics of Explosions and Reactive Systems (ICDERS 2011), California, July 2011.

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