Nitrous Oxide BLEVE Tests (BLEVE = B oiling L iquid E xpanding V apor E xplosion )

1. Nitrous Oxide BLEVE Tests(BLEVE = Boiling Liquid Expanding Vapor Explosion) Yvonne Tran and Don Sargent January 18, 2013

2. Contents of Presentation • Test Program at WSTF • Test plan to determine BLEVE conditions for nitrous oxide • Test objective, approach, and procedure • Test configuration • Instrumentation • Test results • Transient pressure data in exhaust pipe • Long range video of exhaust gases • Close range video of liquid phase transition • Free field (external) overpressure and shock wave data • Summary of observations and findings • Technical background - description and characteristics of a BLEVE • Hazards of a BLEVE and mitigation measures • The potential for a nitrous oxide BLEVE • Importance of the Superheat Limit Temperature (Tsl) in avoiding a BLEVE • Thermodynamics of the Superheat Limit Temperature (Tsl) • Alternative experimental methods for determining the Tsl • Backup information • Temperature effects upon density and vapor pressure of liquid nitrous oxide

3. TEST PROGRAM: PLAN AND RESULTS

4. Test Objective • A test program was planned for NASA’s White Sands Test Facility to empirically determine the superheat limit temperature for nitrous oxide, and to demonstrate that a BLEVE would not occur if the liquid is maintained at temperatures below this superheat limit temperature

5. Testing Approach • Conduct a series of tests, each at a predetermined liquid temperature, where the test series spans the expected temperature range for nitrous oxide storage, transfer, and feed in commercial rocket systems • Intentionally create the conditions for a BLEVE to be initiated, by very rapidly relieving the pressure in a vessel that contains pressurized nitrous oxide liquid at a predetermined temperature • A burst diaphragm achieves the sudden release of pressure in a millisecond time scale • Observe and visually record the behavior of the liquid upon relieving the pressure, through a window in the test vessel, in an attempt to distinguish if homogeneous nucleation of bubbles (the defining characteristic of a BLEVE) occurs at the predetermined test temperature, instead of the much less rapid heterogeneous bubble formation • Record the pressure excursion just after the BLEVE, with high speed pressure transducers, to characterize an expected overpressure shock wave resulting from a BLEVE • Pressures in the free field, as well as pressures within the exhaust pipe of the vessel, would be recorded • Photograph the exhaust plume from the test apparatus with high speed video

6. Testing Procedure • The initial liquid nitrous oxide temperature, at near saturated pressure, was set for each test • Nitrogen was introduced to increase the pressure to ~750 psi, where a burst disc ruptured to suddenly release the pressure to the surrounding atmosphere • High speed pressure measurements and visual observations were made to detect the formation of blast waves, to indicate whether or not a BLEVE had occurred • 11 tests were conducted in August 2012, at initial liquid N2O temperatures ranging between +40oF and -8oF

7. Pressurized Vessel for Conducting BLEVE Tests • The vertical chamber is an 8-inch SS pipe segment, 24.0 inches high. The total available volume is approximately 21.5 liters. • Two 4-inch SS pipe segments, 4 inches long, are welded to the 8-inch segment to form a cross. The 4-inch segments are each fitted with a tempered borosilicate sight glass so the behavior of the liquid can be observed and recorded with standard video (30 fps). • The top of the 8-inch pipe segment is fitted with an 8-in Class 600 socket weld flange. An 8-inch burst disc (750±50 psi) is inserted between this flange and a matching 8-in upper flange. The upper flange is welded to an 11.75-inch high exhaust section of 8-inch pipe, to direct the gases and any carried liquid upward after the burst disc is ruptured. • The bottom of the 8-inch pipe segment is closed out with an 8-inch Class 600 socket weld blind flange.

8. Dynamic PT Burst Disc Liquid N2O from Dewar Configuration for BLEVE Tests

9. Test Stand Configuration for BLEVE Tests • A very long length of stainless steel tubing conveys liquid N2O from a storage Dewar to the lower portion of the test vessel via a fitting in the lower blind flange • The Dewar is self-pressurizing – the saturated vapor pressure of N2O is 278 psia at 0oF • A solenoid valve in this supply line is used to start and stop the flow of N2O to the test vessel • A check valve in this supply line prevents back flow of liquid N2O from the test vessel, which reaches higher pressures than the Dewar once the test liquid warms up to the intended test temperature • The test vessel is fitted with a vent line using a fitting in the upper flange, below the burst disc • The test vessel pressure is controlled using a back pressure regulator in this vent line • During the liquid N2O filling procedure, some liquid evaporates to cool the remaining liquid in the test vessel to approximately -20oF • After the test vessel is filled to the level of the vent line, the liquid N2O is permitted to warm to the intended test temperature via solar radiation upon the test vessel. Electric heating tape installed around the vessel exterior walls was not activated during the test series. • A nitrogen supply line to the upper flange, below the burst disc, is the auxiliary pressure source that is activated to rupture the burst disc upon command, to initiate a simulated pressure vessel failure scenario • The escaping vapor is released through the upper pipe section to the atmosphere

10. Temperature Instrumentation for BLEVE Tests • The pressurized vessel is fitted with three internal Type K thermocouples for measuring the liquid temperature. One thermocouple (TT-008) is at the mid-point of the vertical vessel, at the height of the sight glass. One thermocouple (TT-007) is at the bottom of the vessel, and one (TT-009) is at the top of the vessel • For many of the tests, the top thermocouple was in the vapor space, above the liquid level • There are also three external thermocouples, measuring the temperature at the outer walls of the vessel. One is near the top, one is at the vertical mid-point, and one is near the bottom.

11. Pressure Instrumentation for BLEVE Tests • A pressure transducer (PT-014) is in the N2O supply line from the Dewar, upstream of the check valve • A pressure transducer (PT-016) is in the vent line, before the back pressure regulator, measuring the test vessel pressure during the filling and settling period prior rupturing the burst disc • Two piezoelectric high speed pressure transducers (PT-019 or PCB1, and PT-020 or PCB2) were mounted to the inner wall of the 11.75-inch exhaust pipe of 8-inch pipe above the burst disc • These transducers were diametrically opposed and were 6.0 inches below the top opening of the exhaust pipe section • PCB1 was not entirely flush with the inner surface of the pipe, and consistently read lower than PCB2. All reported high speed data (Table 1) was the PCB2 reading. • A piezoelectric pressure gauge (“pencil gauge”) is positioned above the top opening of the exhaust pipe section, pointed down into the test vessel, to record the free field blast history • The pencil gauge was ~6 inches above the pipe for Tests 1-3, and 12 inches above the pipe for Tests 4-11

12. Video for BLEVE Tests • High speed digital video (4000 fps) showing a long-range view of the exhaust plume from the test vessel • Standard video camera (30 fps) showing a close-up of the sight glass

13. Photo of Vessel for Conducting BLEVE Tests Piezoelectric pressure transducer (“pencil gauge”) for free-field blast history Lead for piezoelectric pressure transducer at inner wall of duct 8 inch dia exhaust duct 8 inch dia burst disk positioned between flanges Video camera aimed at rear sight glass Vent line and N2 supply line 4 inch dia sight glass Liquid N2O supply line

14. Typical Exhaust Pipe Pressure Trace Third Peak Second Peak Second Minimum Initial Peak First Minimum

15. Table 1 - Summary of Transient Exhaust Pipe Pressures

16. General Test Observations from Pressure Data • Nitrogen causes the burst disc to rupture, and the nitrogen escapes with a sharp pressure pulse of ~125 psi, lasting about 2 milliseconds, as recorded by the pressure transducers at the inner wall of the exhaust pipe • The overpressure shock wave recorded at the free field transducer is approximately 5 psi • The pressure in the exhaust pipe drops, reaching a minimum at about 7 milliseconds after the burst disc has ruptured • A second pressure pulse begins, peaking at 160 to 230 psi, at about 35 milliseconds after the burst disc has ruptured • For several tests starting with higher liquid temperatures, this second pressure pulse generates a second shock wave recorded at the free field transducer • This second pulse at the free field sensor is believed by FAA and WSTF* to be the BLEVE shock wave • The pressure in the exhaust pipe drops again, before rising again to a slower steady state flow from the vaporized residual nitrous oxide • This third peak pressure in the exhaust pipe occurs at about 400 to 600 milliseconds after the burst disc has ruptured, and lasts for more than 1 second * Christopher Keddy, FAA Nitrous Oxide Boiling Liquid Expanding Vapor Explosion (BLEVE) Characterization Testing: WSTF August 2012

17. Effect of Liquid N2O Temperature Upon Peak Pressure

18. Effect of Liquid N2O Temperature Upon Time to Peak Pressures

19. Usefulness of Exhaust Pipe Pressure Data • The data from the high speed pressure transducers on the inside wall of the exhaust pipe showed no discernable correlation with initial liquid nitrous oxide temperatures • The exhaust pipe peak pressure data were qualitatively the same, regardless of the initial liquid nitrous oxide temperature • The time delays for the second and third peak pressures were qualitatively the same, regardless of the initial liquid nitrous oxide temperature • These pressure data were not helpful in discriminating between BLEVEs and non-BLEVEs

20. Still Shots from Long-Range Video of Test 5 Exhaust Plume Time = 11 msec Time = 46 msec

21. Still Shots from Long-Range Video of Test 5 Exhaust Plume Time = 146 msec Time = 266 msec

22. Long-Range Video of Test 2 Exhaust Plume High Speed Video of Exhaust Plume

23. Still Shots from Video of N2O Phase Change t = 0.0 seconds t = -0.2 seconds External N2 purge for window Internal thermocouple t = +0.2 seconds t = +0.4 seconds

24. Final Still Shot from Video of N2O Phase Change t = +0.9 seconds

25. Standard Video (30 fps) of Nitrous Oxide Phase Change at the Time the Burst Disc Ruptures

26. Usefulness of Video Data • The long-range videos of the exhaust plume appeared qualitatively similar for all tests in this series • These videos were not helpful in discriminating between BLEVEs and non-BLEVEs • The close-up videos of the sight glass showed nearly identical behavior for all tests • Liquid was present just prior to the burst disc rupture, but within a few frames (<1/15th of a second) liquid is no longer present • The window becomes very dark, indicating almost instantaneous phase change from liquid throughout the vessel, blocking all light transmission • In all tests, white nitrous oxide “ice” crystals were formed in the test vessel • White crystals were first observed in a cloudy continuous phase ~0.3 seconds after the window first became dark • The time delay for crystals to first be observed was the same (0.3±0.1 seconds) for all tests, with initial N2O temperatures ranging from 29oF to -8oF • The continuous phase became clear at ~0.9 seconds after the window first became dark • The time delay for continuous phase to become clear was the same (0.9±0.1 seconds) for all tests, with initial N2O temperatures ranging from 29oF to -8oF • After the continuous phase became clear, some white crystals remained adhered to the inner surfaces of the window • The N2O ice crystals could have been sites for heterogeneous nucleation, enhancing the vaporization rates of the liquid nitrous oxide • These videos were not helpful in discriminating between BLEVEs and non-BLEVEs

27. External “Pencil” Gauge - Free Field Pressure – Test 5 Liquid N2O Temperature 29.1oF The external free field pressure for Test 5 exhibited a distinct “double shock” pattern, showing the initial nitrogen pulse followed by a second shock. This second shock indicates that an explosion (BLEVE) took place.

28. External “Pencil” Gauge - Free Field Pressure – Test 6 Liquid N2O Temperature 25.1oF The external free field pressure for Test 6 exhibited a distinct “double shock” pattern, showing the initial nitrogen pulse followed by a second shock. This second shock indicates that an explosion (BLEVE) took place, but it was weaker than the shock in Test 5.

29. External “Pencil” Gauge - Free Field Pressure – Test 7 Liquid N2O Temperature 11.6oF The external free field pressure for Test 7 exhibited a distinct “double shock” pattern, showing the initial nitrogen pulse followed by a second shock. This second shock was weaker than the shocks in Tests 5 or 6, indicating that the explosion was borderline on whether a true BLEVE took place.

30. External “Pencil” Gauge - Free Field Pressure – Test 10 Liquid N2O Temperature +1.5oF The external free field pressure for Test 10 exhibited a distinct first shock pattern corresponding to the initial nitrogen pulse, but the negative pressure characteristics at ~130 ms do not appear to be characteristic of a true shock event (a BLEVE)

31. External “Pencil” Gauge - Free Field Pressure – Test 11 Liquid N2O Temperature -8.1oF The external free field pressure for Test 11 exhibited a distinct shock pattern corresponding to the initial nitrogen pulse. However, this was not followed by a second shock, indicating that an explosion (BLEVE) did not take place.

32. Usefulness of Free-Field Pressure Data • Useful high speed free-field (above the exhaust pipe exit plane) pressure data were obtained only for Tests 5, 6, 7, 10, and 11; covering initial liquid nitrous oxide temperatures ranging from 29.1 to -8.1oF • These free-field pressure data were helpful in discriminating between BLEVEs and non-BLEVEs

33. Summary of Observations and Findings • Based upon the review of free-field high-speed pressure data: • A BLEVE with the formation of a shock wave did occur with liquid nitrous oxide temperatures of 29.1oF and 25.1oF • A BLEVE did not occur when the liquid nitrous oxide temperatures were +1.5oF and -8.1oF • Although a weak shock wave was formed, the test results were not definitive to determine whether a BLEVE occurred at a liquid nitrous oxide temperature of +11.6oF • The strength of the shock wave increased as the liquid nitrous oxide temperature increased • The conclusion reached from these results is that the superheat limit temperature for nitrous oxide lies between +1.5oF and +25.1oF • The formation of crystals throughout the liquid was observed in all tests • The crystals then can act as additional nucleation sites, promoting rapid heterogeneous nucleation that could resemble homogeneous nucleation • All the measures were consistent: even without a shock wave that may indicate a BLEVE event, a large volume of rapidly expanding gas was generated for all tests at all liquid nitrous oxide temperatures, between -8.1oF and +29.1oF • The rapid phase change from liquid to vapor within 400 milliseconds may occur via homogeneous bubble nucleation (characterizing a true BLEVE), or alternatively via heterogeneous nucleation promoted by nitrous oxide crystals

34. TECHNICAL BACKGROUND: DESCRIPTION AND CHARACTERISTICS OF A BLEVE

35. Hazardous Characteristics of a BLEVE • A BLEVE (Boiling Liquid Expanding Vapor Explosion) occurs when a vessel, containing a liquefied gas stored above its normal boiling point, fails and so is depressurized to atmospheric pressure • Vessel failure can result from a metallurgical failure,mechanical impact, corrosion, fatigue, stress corrosion, an external fire, or excessive internal pressure • A vessel failure results in a sudden release of a large mass of pressurized superheated liquid to the atmosphere • The driving force for a BLEVE is sudden vaporization due to non-equilibrium between the liquid and gaseous phases • Sudden flashing of the liquid into vapor may lead to catastrophic vessel rupture as the internal pressure rapidly rises, with subsequent damage due to ejection of the rapidly expanding vapor, an overpressure shock wave, and impact of vessel fragments • The end result may be a blast that is very similar to a gaseous pneumatic vessel burst event or a chemical explosive event

36. Potential for a Nitrous Oxide BLEVE • Nitrous oxide is one substance that is stored under pressure as a liquefied gas, typically (in aerospace applications) at 0oF, where the equilibrium vapor pressure is 278.3 psia • Nitrous oxide tanks in flight configurations may be operated at 750 to 800 psia, pressurized with an inert gas • This typical storage temperature is well above the normal boiling point of nitrous oxide, -127.3oF • The boiling point of nitrous oxide at 12.4 psia, the atmospheric pressure at White Sands Test Facility (WSTF), is -132.3oF • These conditions imply that a BLEVE should be considered as a potential hazard in nitrous oxide storage operations • Mitigating conditions should be observed and mitigating procedures should be practiced

37. Mitigating the Hazard of a Nitrous Oxide BLEVE • The most obvious mitigating measure is to prevent vessel failure, by imposing an appropriate design safety factor for possible overpressure, by verifying the structural integrity of the vessel, and by imposing operational procedures that avoid failures from corrosion, mechanical impact, or external heating • Another mitigating measure in industrial practice is to maintain the nitrous oxide storage temperature at or below 0oF • A BLEVE can occur only if the fluid temperature is above a “superheat limit temperature”, Tsl

38. The Superheat Limit Temperature • The superheat limit temperature, Tsl, or the homogeneous nucleation limit, represents the highest temperature below the critical point a liquid can sustain without undergoing a phase transition from liquid to vapor • Below this threshold temperature, heterogeneous bubble nucleation at the liquid phase interface with another substance or vessel wall becomes normal boiling, and vapor explosions may not occur • The superheated state of a liquid owes its existence to an energy barrier, which is the work required for a gaseous nucleus to expand in the higher-density liquid to sustain its growth • The vapor pressure inside the gaseous nucleus, and the evaporation rate of the liquid, both increase as the liquid temperature increases, promoting growth of the nucleus • At higher external pressures, the energy barrier for a gaseous nucleus to expand is greater, inhibiting growth of the nucleus • The growth of a bubble in a superheated liquid is also limited by the restraining effect of surface tension • If the energy below the threshold temperature is not sufficient to reach this barrier, a vapor embryo collapses and nucleate boiling is inhibited

39. Initiation of a BLEVE by Homogeneous Nucleation • At or above the superheat limit temperature, explosive boiling suddenly occurs, when active nuclei are evenly formed throughout the liquid • Flash vaporization of a large portion of the liquid to gas can occur within milliseconds, at rates than can be orders of magnitude faster than typical heterogeneous rapid boiling • This is called “homogeneous nucleation”, and is the mechanism for a BLEVE to occur • No significant bulk nucleation occurs until a definitive temperature is reached • Then, in a range of only a few degrees, the rate changes from a negligible value to a very large value • When a BLEVE occurs, gases may be evolved by homogeneous nucleation throughout the entire liquid mass in a millisecond timeframe, such that existing venting capabilities (e.g., burst diaphragms or pressure relief valves) may be insufficient either in response time or in volumetric capacity (or both)

40. Initiation of a BLEVE by Heterogeneous Nucleation • Explosive behavior may be exhibited even when the temperature is below Tsl • Homogeneous nucleation may not always be required – under some circumstances, heterogeneous nucleation may be hard to distinguish from homogeneous nucleation • Heterogeneous nucleation may be attributed to the formation of crystals (analogous to carbon dioxide dry ice) throughout the liquid, and the crystals then act as additional nucleation sites • The formation of crystals throughout the liquid was observed in all 11 tests • The freezing point of nitrous oxide is -90.9oC (-131.5oF), which is virtually the same as its boiling point at 12.4 psia

41. Superheat Limit Temperature for Nitrous Oxide • The ratio Tsl / Tc has been determined* to fall within the narrow interval 0.89 to 0.90 at low liquid pressures for a wide range of industrial compounds, where Tc is the critical temperature of the fluid • The critical temperature of nitrous oxide is 309.6oK (or 97.5oF), so the expected value for Tsl at low pressures is 275oK or 36oF • The superheat limit temperature can be calculated from thermodynamics when the equation of state is known, to determine both the vapor density inside a gaseous nucleus, and the density of the liquid phase, at a given pressure and the temperature • The fluid properties of the vapor phase in the vicinity of the critical temperature of the substance, however, behave in a distinctively non-ideal fashion • There is no totally satisfactory correlation among , P, and T in the superheated liquid region • Consequently, the superheat limit temperature may be different from the value calculated simply as 0.89 times the critical temperature • The superheat limit temperature for atmospheric pressure was estimated to be 0.84 times the critical temperature, using the Van Waals equation of state • The superheat limit temperature for atmospheric pressure was estimated to be 0.895 times the critical temperature, using the Redlich-Kwong equation of state

42. Superheat Limit Temperature for Other Substances

43. Superheat Limit Temperature for N2O at Higher Pressures • For a wide range of compounds, Tsl / Tc increases with pressure** • 750 psia is the burst disc rating, and is also the N2O tank pressure in some commercial rockets • The critical pressure of N2O is 71.43 atmospheres, so at ~ 750 psia (51 atmospheres), the ratio of pressure / critical pressure is 0.71 • For many non-polar liquids, the value of Tsl / Tc is estimated** to be 0.95 at a 0.71 ratio of pressure / critical pressure • For many organic liquids, the value of Tsl / Tc is represented empirically*** by Tsl / Tc = 0.11(P/Pc) + 0.89, so at P/Pc = 0.71, Tsl / Tc = 0.97 • If the value of Tsl / Tc is approximately 0.95, the expected value for Tsl at high pressure is 294oK or 70oF * Guidelines for Evaluating the Characteristics of Vapor Cloud Explosions, Flash Fires, and BLEVEs, Center for Chemical Process Safety of the American Institute of Chemical Engineers (1994), Page 158 ** Refer to previous chart of Tsl / Tc vs. P / Pcrit *** R.C. Reid, Rapid Phase Transitions from Liquid to Vapor, Advances in Chemical Engineering 12, 105-208 (1983)

44. The locus of points of neutral stability on the p-T diagram is known as the liquid spinodal line, which is the locus of points for which • The limit of superheat is observed to occur very near to, if not at, the liquid spinodal line • The state at which the spectrum of bubbles first contains a large number that are growing is defined as the superheat limit • If the density fluctuations  are interpreted as fluctuations of phase, the superheat limit is determined by • where V is the volume of the sample,  is the density, v is the specific volume, p is the pressure, T is the temperature, and k is Boltzmann’s constant Thermodynamics of the Superheat Limit Temperature Source: J.E. Shepherd and B. Sturtevant, Rapid Evaporation at the Superheat Limit, J. Fluid Mech. (1982), vol. 121, pp. 379-402

45. Alternative Experimental Methods for Determining Tsl • One method is to observe the homogeneous nucleation rate in the liquid at a given pressure and temperature • The rate of bubble formation, J, is the number of vapor nuclei formed per second, per unit liquid volume • The nucleation rate increases by 3 or 4 orders of magnitude as a result of increasing the liquid temperature by little more than 1 degree K, as the superheat limit temperature is approached • This drastic increase in nucleation rate is readily observed and photographed, • A high nucleation rate also may be detected acoustically by a sensitive piezoelectric transducer • For a BLEVE, a measured nucleation rate is typically greater than 105 nuclei/ sec /cm3 • A second method is to measure the waiting time for nucleation to begin in the metastable superheated liquid, after the pressure is suddenly released • The waiting time,  = 1/(JV), where V is the volume of liquid • The waiting time is observed to abruptly decrease with a comparatively small increase in temperature

46. BACKUP INFORMATION

47. Density of Liquid N2O is Highly Dependent upon Temperature

48. Vapor Pressure of Liquid Nitrous Oxide