1. V. Shentsov, M. Kuznetsov, V. Molkov
6th International Conference on Hydrogen Safety,
19-21 October, 2015, Yokohama, Japan
The pressure peaking phenomenon: validation for unignited releases in laboratory-scale enclosure
V. Shentsov, M. Kuznetsov, V. Molkov
Hydrogen Safety Engineering and Research Centre (HySAFER)

2. Presentation outline What is pressure peaking phenomenon (PPP)
Calculation of PPP: 2 steps methodology
Experimental facility and validation tests at KIT
Ulster theory versus KIT experiments
Conclusions

3. The problem Hydrogen stored as compressed gas @ 350-700 bar
Even if unignited, the release of hydrogen has been shown to result in unacceptable overpressures indoors capable of destroying the structure (pressure peaking phenomenon, see Brennan & Molkov, 2013)
The experimental results were absent for PPP validation before the project
This study presents the experimental validation of PPP existence and the Ulster theory validation

4. The pressure peaking phenomenon
Example of garage
Typical garage of size LxWxH=4.5x2.6x2.6 m with a “brick” size vent. Mass flow rate 390 g/s (350 bar, 5.08 mm TPRD orifice).
Pressure limit for structures to withstand
Engineering solution: decrease PRD orifice size (by increasing fire resistance of storage tank)

5. 2-steps methodology Cyber-laboratory (free access) Step 1:
Make an estimation of PPP existence inside the compartment of interest.
PPP exists only when hydrogen leak and vent size are such that there will be only outflow of hydrogen through the vent outside the enclosure, i.e. 100% hydrogen accumulation with time will be reached.
Calculate hydrogen concentration in an enclosure using nomogram/model (Molkov et. al 2014) or engineering tool
If the calculated concentration is 100% then the second step has to be applied to calculate the PPP overpressure.
Step 2:
Use the model/nomogram (Brennan and Molkov 2013) implemented as an engineering tool at to calculate the over-pressure resulting from PPP.

6. Experimental facility (KIT)
Validation tests
19 experiments on PPP were carried out by KIT (Germany) in the enclosure with sizes HxWxL=1x0.98x0.96 m.
Round vent of diameter either 11 mm or 16.5 mm was located centrally at the top of the front panel or at the bottom.
Three gases were tested: air, helium and hydrogen
Internal diameter of the release nozzle is specified to 5 mm, located at the centre of enclosure 10 cm above the floor directed vertically.
Release rates ranges g/s.
Rear wall
Left wall
Front wall

7. Validation Air and helium releases Air, 1.444 g/s, CD=0.72
Helium, 0.22 g/s, Top vent 0.95 cm2, CD=0.72
Helium, 0.22 g/s
Bottom vent, CD=0.72
WP1 - Task Presentation title

8. Validation Helium releases PPP is independent on vent location
Helium, g/s Bottom vent Vent 0.95 cm2, CD=0.85
Helium, g/s,
Top vent 0.95 cm2, CD=0.82
Helium, 1 g/s,
Vent 2.14 cm2,CD=0.72
Helium, 0.5 g/s,
Vent 0.95, CD=0.72
WP1 - Task Presentation title
PPP is independent on vent location

9. Validation Hydrogen releases Hydrogen, 0.108 g/s Vent 0.95 cm2 CD=0.65
Due to higher pressure the experimental enclosure starts to “breathe” resulting in the additional vent area increase of 0.8 cm2
Hydrogen, 1.08 g/s
Vent 2.14 cm2, CD=0.9
Hydrogen, 1.08 g/s
Vent 0.95 cm2,CD=0.72
WP1 - Task Presentation title

10. Validation Effect of “breathing” on CD
Hydrogen, g/s
Vent 2.14 cm2, CD=0.54
Hydrogen, g/s
Vent 0.95 cm2, CD=0.56
The CD scatter is thought due to the “breathing” of the enclosure, i.e. the existence of “additional openings” (non-controllable process) will be “compensated” in the calculations by larger value of discharge coefficient CD.
WP1 - Task Presentation title

11. Leak in fully “closed” box
“Breathing” effect
Closed camera
Leak in fully “closed” box
Hydrogen, g/s
Maximum opening of 0.67 cm2 with CD=0.72
WP1 - Task Presentation title

12. Concluding remarks
Experiments with release of air, helium and hydrogen performed by KIT proved the existence of the pressure peaking phenomena predicted theoretically at Ulster in 2013.
The discharge coefficient was found to be in the range CD= for performed experiments (“breathing effect”). The average value of CD through the series of test is CD=0.72, the conservative value is CD=0.54.
PPP should be considered as an essential part of hydrogen safety engineering for all indoor use of HFC systems.

13. Acknowledgements Thank you for attention!
The authors would like to acknowledge the financial support of Fuel Cells and Hydrogen Joint Undertaking under the HyIndoor project (grant agreement No ), and the European Commission for funding through H2FC project.