ICHS 4 San Francisco 12-14 2011 Safety Assessment of Unignited Hydrogen Discharge from Onboard Storage in Garages with Low Levels of Natural Ventilation Síle Brennan, Vladimir Molkov Hydrogen Safety Engineering and Research Centre (HySAFER)

Outline



Motivation for the work



Pressure peaking



Description of the problem



Methodology



Results



Conclusions

12 th September 2011

Motivation



Safety levels

in H 2 technologies need to be at least the same as those in existing fossil fuel applications  Necessary to consider

indoor use

e.g. forklifts, vehicles, equipment in a garage etc.

 Build on overlooked safety issue of

“pressure peaking”

to understand requirements for safe blow-down of

on-board hydrogen storage

indoors 12 th September 2011

On-board H

2

storage

 H 2    as compressed gas (350 - 700 bar) Tanks equipped with pressure relief devices Composite tanks rupture in < 6.5 min in fire Current venting area of the PRD releases hydrogen quickly from the tank before its catastrophic failure  However, even if unignited, the release of hydrogen has been shown to result in unacceptable overpressures within the garage capable of destroying the structure 12 th September 2011

Pressure peaking (1/3)

Example

 Release in 30.4 m 3 onboard storage garage from 350 bar  PRD with typical diameter of 5 mm  Steady mass flow rate release: 390 g/s of  Garage has single vent (area ~ 1 brick)  What is the overpressure in the garage?

 Simple methods predict max 18 kPa 12 th September 2011

70 60 50 40 30

Pressure peaking (2/3)

CFD Phenomenological model , C=0.6

Garage destroyed in seconds 30.4 m 3

20 10 0 0

10-20 kPa – safety limit for civil structures

10 20 30

Time (s)

40 50 60 September 2011

Pressure peaking (3/3)

30.4 m 3

60000 50000 40000

H 2 only!

Hydrogen Methane Propane 30000 20000

10-20 kPa – safety limit for civil structures

10000 0 0 5 10 15

Time (s)

20 25 30 September 2011

Problem description (1/2)

 Used phenomenological model to investigate releases indoors e.g. garage  Model based on a known volume, vent area and release rate  Characterise garage by Air Change per Hour (ACH)  Consider range of scenarios involving a release from onboard storage through a PRD in a vented garage 12 th September 2011

Problem description (2/2)



Pressure:

Onboard storage tanks @ 350 & 700 bar   

Mass of H 2 :

“Typical” inventories of 1, 5 and 13 kg

Garage volume:

Free volumes in range 18-46 m 3

Ventilation:

All natural ventilation, assume flow out: • ACH values (0.03 – 1) 

Release parameters:

the unignited hydrogen is released into the enclosure through PRDs with different areas 12 th September 2011

Methodology



Step 1:

Relate ACH to garage volume & vent size 

Step 2:

UU blown-down model to calculate dynamics of H 2 mass flow rate from storage tank Input to phenomenological model 

Step 3:

For each scenario use phenomenological model to iteratively find PRD area such that:

P garage < 20 kPa i.e. a “safe” level



Step 4:

Find blow-down time, through PRD with “safe diameter” to tank over-pressures of 100, 50, 20, 1 & 0.1 bar 12 th September 2011

ACH & Vent Size (1/2)



Air changes per hour (ACH)

is a measure of how many times the air within a defined space (e.g. a garage) is replaced.

 ACH = Q hr /V  Q hr = air flow rate (m 3 / hr ), V = volume (m 3 ) Uncertainty in the literature in how to relate ACH to volume and vent area  Q s = air flow rate (m A = vent area 3 / s ) C = coefficient of discharge = 0.6

∆P = pressure differential between garage & atmosphere 12 th September 2011

ACH & Vent Size (2/2)

  Fix volume and ACH > find Q (per hr & per s)  Fix ∆P to find vent area, A  BUT what do we take as “∆P” ??

 50 Pa commonly used in building applications “N50”  Bigger ∆P used, smaller the vent for a given volume  Thus vent size and hence “peak-pressure” sensitive to ∆P chosen 12 th September 2011

Effect of ∆P on Vent Area

12 th September 2011

“Current” & “safe” PRD Pressure dynamics in 30m 3 garage, ACH-0.18, 5 kg hydrogen @ 350 bars PRD diameters of 5 mm and 0.5 mm

Nomogram: 5 Kg H2 @ 350 bar 1. Volume > ACH f(P) 2. ACH > diameter 3. diameter > time

12 th September 2011

Nomogram: 5 Kg H2 @ 700 bar 1. Volume > ACH f(P) 2. ACH > diameter 3. diameter > time

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Conclusions

 Garages characterised by ACH and volume  Pressure-peaking model for unignited released used to calculate “safe” PRD diameters and corresponding blow-down time from on-board storage in vented enclosures  This phenomenon should be accounted for in indoor use of HFC systems and must be reflected in RCS.

 Work raises questions about current approaches to fire resistance of onboard storage and PRD parameters  Further research is needed to develop safety strategies and engineering solutions.

12 th September 2011

Thank you for your attention Any questions?

[email protected]

12 th September 2011