Safety Assessment of Unignited Hydrogen Discharge in Low Ventilation Garages
Explore the safety assessment of unignited hydrogen discharge in garages with low ventilation levels focusing on pressure peaking and potential risks. Motivated by the need to ensure safety in indoor hydrogen applications, this study analyzes the effects of onboard hydrogen storage releases and pressure relief devices on garage structures. The methodology, results, and conclusions of the assessment conducted in September 2011 are presented, highlighting the critical importance of understanding overpressure risks in indoor hydrogen environments.
Download Presentation
Please find below an Image/Link to download the presentation.
The content on the website is provided AS IS for your information and personal use only. It may not be sold, licensed, or shared on other websites without obtaining consent from the author. If you encounter any issues during the download, it is possible that the publisher has removed the file from their server.
You are allowed to download the files provided on this website for personal or commercial use, subject to the condition that they are used lawfully. All files are the property of their respective owners.
The content on the website is provided AS IS for your information and personal use only. It may not be sold, licensed, or shared on other websites without obtaining consent from the author.
E N D
Presentation Transcript
ICHS 4 San Francisco 12-14 2011 Safety Assessment of Unignited Hydrogen Discharge from Onboard Storage in Garages with Low Levels of Natural Ventilation S le Brennan, Vladimir Molkov Hydrogen Safety Engineering and Research Centre (HySAFER)
Outline Motivation for the work Pressure peaking Description of the problem Methodology Results Conclusions 12th September 2011
Motivation Safety levels in H2 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 12th September 2011
On-board H2 storage 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 12th September 2011
Pressure peaking (1/3) 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 12th September 2011
Pressure peaking (2/3) 70 CFD 60 Phenomenological model , C=0.6 50 Over pressure in garage (kPa) Garage destroyed in seconds 40 30 20 10-20 kPa safety limit for civil structures 10 0 0 10 20 30 40 50 60 Time (s) 12th September 2011 30.4 m3garage, brick vent, mass flow rate 390 g/s (350 bar, 5 mm orifice)
Pressure peaking (3/3) 60000 Hydrogen 50000 Methane H2 only! Propane Over pressure (Pa) 40000 30000 20000 10-20 kPa safety limit for civil structures 10000 0 0 5 10 15 20 25 30 Time (s) 12th September 2011 30.4 m3garage, brick vent, mass flow rate 390 g/s (350 bar, 5 mm orifice)
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 12th September 2011
Problem description (2/2) 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 12th September 2011
Methodology 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 tank Input 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 12th 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 = 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 = 2 P / Qs CA 12th September 2011
ACH & Vent Size (2/2) = 2 P Bernoulli: Fix volume and ACH > find Q (per hr & per s) / Qs CA 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 12th September 2011
Effect of P on Vent Area 12th September 2011
Current & safe PRD Pressure dynamics in 30m3 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 12th September 2011
Nomogram: 5 Kg H2 @ 700 bar 1. Volume > ACH f(P) 2. ACH > diameter 3. diameter > time 12th September 2011
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. 12th September 2011
Thank you for your attention Any questions? sl.brennan@ulster.ac.uk 12th September 2011
