1. Pressure Relief Devices: Calculation of Flammable Envelope and Flame Length Vladimir Molkov Hydrogen Safety Engineering and Research Centre v.molkov@ulster.ac.uk NHA Hydrogen Conference & Expo, 3-6 May 2010 Codes & Standards Workshop: International Standards Progress and Opportunities

2. Outline Pressure relief devices (where we are now?) Non-reacting hydrogen leaks (similarity law) –Flammable envelope calculation –Buoyancy- and momentum-controlled jets Reacting hydrogen leak –Jet flame length calculation (the universal correlation and the engineering nomogram) Challenges for indoor activation of PRD (research and SDO)

3. Contributions and support of: Colleagues from the HySAFER Centre at the University of Ulster Partners from the European Network of Excellence HySafe (International Association for Hydrogen Safety), other EC-funded projects HYPER, HyCourse, HySAFEST, and The European Commission are gratefully appreciated. Acknowledgements

4. Pressure relief devices (PRD) Regulation requires use of pressure relief devices for onboard storage of hydrogen. Only thermal activation of PRD is acceptable for safety provisions. Current PRDs are characterised by high hydrogen mass flow rates, e.g. 390 g/s for 350 bar storage pressure and 5.08 mm diameter orifice. Non-reacting and reacting (jet fires) releases.

5. Currently used PRD (upward) Vehicle equipped with two 34 L capacity cylinders at 350 bar (IJHE, 2007, 32:2154–2161). Back viewSide view Do we accept 10-15 m flame? No…

6. Currently used PRD (downward) Fire was initiated on the instrumentation panel ashtrays. The PRD was actuated 14 min 36 s (upward scenario ) and 16 min and 16 s (downward). Blowdown time < 5 min. Side view Back view What if car is in a garage?

7. Non-reacting release 1

8. Non-reacting expanded jets 1957, Sunavala, Hulse, Thring (buoyancy is negligible): 1961, Ricou and Spalding (averaged fuel mass fraction): 1980, Chen and Rodi (axial fuel mass fraction): Conclusion: Flammable envelope size (distance to 4% by volume) increases proportional to nozzle diameter.

9. Underexpanded jets The similarity law for expanded jets (1980, Chen and Rodi) The similarity law for underexpanded jets (1987, Birch et al.) Three differences: 1.Volume fraction (1987) instead of mass fraction (1980) 2.Density ratio is reciprocal 3.Notional nozzle (1987) instead of real nozzle (1980) Question: how to calculate decay in underexpanded jets?

10. Solution (underexpanded jets) We use the original similarity law (expanded jets) by Chen and Rodi (1980) with an “unknown” density in the nozzle. “Unknown” density is calculated by an underexpanded jet theory* developed at the University of Ulster (affects the flammable envelope size of 4% by vol. or 0.00288 mass fraction; 0.00141 - 2%, 0.0007 - 1%) NB: underexpanded theory of Birch et al. can not be applied for p>100 bar. The Abel-Noble equation should be used instead of the ideal gas equation (at 700 bar amount of released hydrogen is 50% more than real if ideal gas equation is applied). *More in MSc in Hydrogen Safety Engineering (distance learning course): http://campusone.ulster.ac.uk/potential/postgraduate.php?ppid=24

11. The similarity law - validation The similarity law is conservative to tests - effect of losses

12. Losses and flammable envelope Example: mass flow rate for two channels of D=0.75 mm and different length at storage p=105 bar (Table) Flammable envelope size is proportional to the nozzle diameter (effective). Decrease of mass flow rate on 28% is associated with about 13% decrease of the jet diameter and thus the 13% decrease of the flammable envelope size. Method of calculationL = 15 mmL = 150 mm Abel-Noble gas model without losses2.80 g/s Abel-Noble gas model with losses2.08 g/s1.50 g/s Large Eddy Simulation2.10 g/s1.52 g/s

13. Flammable envelope size Distance to 4% by volume: 4% v/v 1% v/v

14. Buoyancy VS momentum jets Momentum Buoyant

15. When a jet becomes buoyant? Fr=3.5 4.757.25 momentum buoyant downward jet

16. Reacting releases (jet fires) 2

17. New similarity group (m. D) m – mass flow rate; D – real nozzle diameter Kalghatgi (1984) data are converged L f =f(m, D)

18. Jet fires: universal correlation

19. The nomogram Derived from the universal correlation for jet fires. Special feature: No stable flames were observed for nozzle diameters 0.1-0.2 mm – flame blew off although the spouting pressure increased up to 400 bar (Mogi et al., 2005). D=3 mm P=350 bar Flame L=5 m No flame

20. Challenges for indoor activation of PRD 3

21. Garage release: consequences Conservative conditions: Hydrogen-powered car is in a small closed garage of 44 m 3 free volume. Release from an onboard storage through PRD of 5.08 mm diameter at pressure 350 bar gives mass flow rate 390 g/s (volumetric flow rate is 390/2*0.0224=4.4 m 3 /s). Estimate of consequences: Every second of non- reacting release pressure will increase 4.4/44=0.1, i.e. by 10 kPa. Civil building structures can withstand 10-20 kPa. Thus, in 1-2 s the garage is gone (reacting release – divide time by 8).

22. Pressure peaking: H2 only! Small garage LxWxH=4.5x2.6x2.6 m. Mass flow rate 390 g/s, C=0.6 for all fuels. 10-20 kPa – civil structure can withstand

23. What to do (research for SDO)? Mass flow rates should be reduced  Blowdown time increased  Fire resistance rating of onboard storage increased  … Back to stainless steel vessels with fire resistance R30 (and early PRD activation)? Innovative pressure relief devices with essentially shorter flame length at the same mass flow rate (e.g. United Kingdom Patent Application No.1005376.7, University of Ulster)