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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 - PowerPoint PPT Presentation
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Safety Assessment of Unignited Hydrogen Discharge from Onboard Storage in Garages
with Low Levels of Natural Ventilation
Síle Brennan, Vladimir Molkov
ICHS 4 San Francisco 12-14 2011
Hydrogen Safety Engineering and Research Centre (HySAFER)
Motivation for the work Pressure peaking Description of the problem Methodology Results Conclusions
Outline
12th September 2011
Safety levels in H2 technologies need to be at least the same as those in existing fossil fuel applications
Necessary to consider indoor usee.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
Motivation
12th September 2011
H2 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
On-board H2 storage
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Example Release in 30.4 m3 garage from 350 bar
onboard storage 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
Pressure peaking (1/3)
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0 10 20 30 40 50 600
10
20
30
40
50
60CFD
Phenomenological model , C=0.6
Time (s)
Ove
r pre
ssur
e in
gar
age
(kPa
)
10-20 kPa – safety limit for civil structures
Pressure peaking (2/3)
Garage destroyed in seconds
30.4 m3 garage, “brick” vent, mass flow rate 390 g/s (350 bar, 5 mm orifice)
12th September 2011
Pressure peaking (3/3)
30.4 m3 garage, “brick” vent, mass flow rate 390 g/s (350 bar, 5 mm orifice)
0
10000
20000
30000
40000
50000
60000
0 5 10 15 20 25 30
Time (s)
Ove
r pre
ssur
e (P
a)
Hydrogen
Methane
Propane H2 only!
10-20 kPa – safety limit for civil structures
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
Problem description (1/2)
12th September 2011
Pressure: Onboard storage tanks @ 350 & 700 bar Mass of H2: “Typical” inventories of 1, 5 and 13 kg Garage volume: Free volumes in range 18-46 m3
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
Problem description (2/2)
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Step 1: Relate ACH to garage volume & vent size Step 2: UU blown-down model to calculate
dynamics of H2 mass flow rate from storage tankInput to phenomenological model
Step 3: For each scenario use phenomenological model to iteratively find PRD area such that:
Pgarage < 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
Methodology
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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 = Qhr/V Qhr = air flow rate (m3/hr), V = volume (m3)
Uncertainty in the literature in how to relate ACH to volume and vent area
Bernoulli: Qs = air flow rate (m3/s)A = vent area
C = coefficient of discharge = 0.6 ∆P = pressure differential between garage & atmosphere
ACH & Vent Size (1/2)
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/2 PCAQs
Bernoulli: 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
ACH & Vent Size (2/2)
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/2 PCAQs
Effect of ∆P on Vent Area
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Pressure dynamics in 30m3 garage, ACH-0.18, 5 kg hydrogen @ 350 barsPRD diameters of 5 mm and 0.5 mm
“Current” & “safe” PRD
1. Volume > ACH f(P)2. ACH > diameter3. diameter > time
Nomogram: 5 Kg H2 @ 350 bar
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1. Volume > ACH f(P)2. ACH > diameter3. diameter > time
Nomogram: 5 Kg H2 @ 700 bar
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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.
Conclusions
12th September 2011
