1. Pro-Science 4 th International Conference of Hydrogen Safety, September 12-14, 2011, SAN FRANCISCO, USA EXPERIMENTAL STUDY OF IGNITED UNSTEADY HYDROGEN RELEASES FROM A HIGH PRESSURE RESERVOIR Grune, J. 1, Sempert, K. 1, Kuznetsov, M. 2, Jordan, T. 2 1 Pro-Science GmbH, Germany 2 Karlsruhe Institute of Technology, Germany

2. Pro-Science 4 th International Conference of Hydrogen Safety, September 12-14, 2011, SAN FRANCISCO, USA Structure of the presentation  Introduction  Objectives  Experimental  Test results  Summary

3. Pro-Science 4 th International Conference of Hydrogen Safety, September 12-14, 2011, SAN FRANCISCO, USA Hydrogen is successfully used as energy carrier in many different applications. Introduction Accidental hydrogen releases from pipe systems are one of the main hazards that occur in the handling of pressurized hydrogen. The accidental H 2 release from the system should be detected fast and as safety consequence the main supply tank should close. So the released amount of hydrogen is limited and the release conditions are unsteady. The generated hydrogen cloud can be ignited subsequently by an external ignition source or in case of sudden hydrogen release from high pressure state the possibility of a self ignited jet fire is present. Currently no systematic studies are available on the hazard potential of transient releases of small amounts of hydrogen from a high pressure reservoir.

4. Pro-Science 4 th International Conference of Hydrogen Safety, September 12-14, 2011, SAN FRANCISCO, USA Objectives Goal of this work is to quantify the possible hazards arising from spark ignited or auto ignited free transient hydrogen jets. To study the nature of transient hydrogen jets and their combustion behaviour a simulation of an accidental hydrogen release from a high pressure pipe system was performed with two different start-up release conditions. I.A fast valve opening to produce the free gas jet. This jet is ignited with a forced spark. II.Additionally a rupture disc is installed in the exhaust pipe, which leads to a sudden hydrogen release with the possibility of an auto ignited jet fire. - Circular release opening: 3 mm, 4 mm and 10 mm (forced ignition test). 4 mm (auto ignition test) - Initial reservoir pressures of up to 200 bar. - Reservoir volume of 0.37 dm 3. The main parameters in these experiments are:

5. Pro-Science 4 th International Conference of Hydrogen Safety, September 12-14, 2011, SAN FRANCISCO, USA Experimental set-up (pneumatic needle valve DN 3 mm, driven with pressurized helium, valve opening time < 2 ms) The small volume between the rupture disc and the leak valve was filled with H 2 at 1 bar. The nozzle downstream of the rupture disc is open to the atmosphere. The burst pressure of the used aluminium rupture disc was clearly lower than the overpressure in the reservoir. Schematic of the experimental set-up. Forced ignition test: Auto ignition test: Fast leak valve opening produced the unsteady free jet. Additionally a rupture disc holder is installed behind the fast leak valve.

6. Pro-Science 4 th International Conference of Hydrogen Safety, September 12-14, 2011, SAN FRANCISCO, USA Ignition source Positioned along the jet axis on a lance High frequency electric spark (20 kHz, ~ 60 kV) with an electrode distance of ~ 6 mm. Five dynamic pressure sensors: PCB (Type 113A31) Pressure sensor line was used in parallel (50 cm) to the jet axis. (reflected pressure measurement) Integral heat flux sensors Positioned on lance along the jet axis. Unobstructed length in space of 4 m. Distance to the floor and the nearest wall was 1.6 m. Jet axis Experimental set-up Optical observation By high speed camera in different configurations.

7. Pro-Science 4 th International Conference of Hydrogen Safety, September 12-14, 2011, SAN FRANCISCO, USA Dynamics of H 2 -releases from pressurized vessels Comparison of measured and calculated pressure decay inside the vessel: - Burst pressure of the rupture disc ~ 160 bar. - 4 mm nozzle. No significant difference for the pressure decay inside the vessel.

8. Pro-Science 4 th International Conference of Hydrogen Safety, September 12-14, 2011, SAN FRANCISCO, USA Dynamics of H 2 -releases from pressurized vessels Hydrogen release via valve. Start up of the flow is controlled by the valve opening behavior. Sudden hydrogen release via valve and rupture disc ( burst pressure ~ 160 bar). 30 cm (jet is ignited!) 4 mm nozzle, 200 bar

9. Pro-Science 4 th International Conference of Hydrogen Safety, September 12-14, 2011, SAN FRANCISCO, USA Hydrogen release via rupture disc. t 60 mm Auto ignition in 2 mm glass tube: Auto ignition in 5 mm square glass tube: A view inside the release nozzle downstream the rupture disc: The auto ignition in the tube in a complex process! An extension tube downstream the rupture disc is necessary to reach auto ignition. 200 bar, 180000 f/s Burst pressure of the rupture disc ~ 120 bar 220 bar, 450000 f/s

10. Pro-Science 4 th International Conference of Hydrogen Safety, September 12-14, 2011, SAN FRANCISCO, USA Critical conditions for spontaneous ignitions inside the extension tube downstream the rupture disc. Hydrogen release via rupture disc. In this configuration all spontaneous ignition events lead to a jet fire. Length of extension pipe / mm

11. Pro-Science 4 th International Conference of Hydrogen Safety, September 12-14, 2011, SAN FRANCISCO, USA Hydrogen release via rupture disc. Example for the combustion pressure amplitude of a spontaneous ignition test. 4 mm nozzle, 200 bar The small amount of burned mixture released from the nozzle ignites the following hydrogen flow  jet fire. -Fist: shock wave from the ruptured disc leaves the nozzle exit. -Second: combustion of the turbulent free jet. Two pressure peaks were observed:

12. Pro-Science 4 th International Conference of Hydrogen Safety, September 12-14, 2011, SAN FRANCISCO, USA Spark ignition: maximum pressure Due to the start-up process of the head of the flow in combination with the transient release conditions a highly reactive hydrogen / air cloud inside the turbulent jet is formed. The volumetric size and the position of this highly reactive hydrogen / air cloud are changing during the transient release. For every configuration a distinct ignition time and ignition position exists for the generation of a maximum pressure wave due to a "local explosion" in the free jet. -Variation of the ignition time.-Variation of the ignition distance.

13. Pro-Science 4 th International Conference of Hydrogen Safety, September 12-14, 2011, SAN FRANCISCO, USA Nozzle: 4 mm, P 0 = 200 bar, ignition time and distance optimized. Example "local explosion"Spark ignition: 0.6 m Integral heat flux sensors Nozzle Ignition source H 2 jet Real and shadow image Digitally processed (pressure wave visualization)

14. Pro-Science 4 th International Conference of Hydrogen Safety, September 12-14, 2011, SAN FRANCISCO, USA Hazard potential of pressure loads. Maximum measured overpressure amplitudes from the 4 mm nozzle experiments, measured in reflected orientation in 50 cm distance to the jet axis. For 200 bar of initial release overpressure reflected amplitudes of 220 mbar in a distance of 50 cm to the jet axis were measured, this value reaches the upper limit for possible irreparable ear injuries.

15. Pro-Science 4 th International Conference of Hydrogen Safety, September 12-14, 2011, SAN FRANCISCO, USA Thermal energies from ignited transient free jets An early ignition of the released hydrogen close to the nozzle produces the highest thermal loads. Measured maximum integral heat fluxes along the jet axis. The maximum measured thermal loads decrease almost linearly with increasing distance to the nozzle. Due to the fast process: Integral heat flux sensors were used. Measured integral heat fluxes along the jet axis (auto ignition). The case of the spontaneous ignition of the first released hydrogen inside the nozzle pipe is the earliest possible ignition time. A conservative linearization of the maximum thermal loads along the jet axis is possible.

16. Pro-Science 4 th International Conference of Hydrogen Safety, September 12-14, 2011, SAN FRANCISCO, USA Assumption: The characteristic release time of the transient hydrogen jets is the conservative exposure time for the integral thermal loads. Hazard potential of thermal loads Energy threshold: A.M. Stoll et al exposure time = characteristic release time

17. Pro-Science 4 th International Conference of Hydrogen Safety, September 12-14, 2011, SAN FRANCISCO, USA In this study an accidental release of pressurized H 2 from a small reservoir of 0.37 dm 3 was investigated experimentally with two different procedures. In the first procedure a fast valve opens and tubular release nozzles with diameters of 3, 4, and 10 mm produce a hydrogen free jet in air with subsequent spark ignition. In the second release application an additional rupture disc was installed in the 4 mm exhaust pipe and the sudden release conditions initiated a spontaneous ignition of the jet. A minimum reservoir overpressure of 25 bar for a spontaneous ignition event was observed. The transition from the auto ignition inside the exhaust nozzle pipe to the full jet fire in the ambience generates a detectable overpressure blast wave due to a "local explosion" in the free jet. For every configuration a distinct ignition time and ignition position exists for the generation of a maximum pressure wave due to a "local explosion" in the free jet. The generated thermal loads were investigated systematically for both ignition scenarios. A free jet that is ignited early and close to the nozzle generates the maximum thermal load for its ambience. The spontaneous ignition of the first released H 2 inside the tube behind the rupture disc is the earliest ignition situation of the released jet. Summary