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Small Hydrogen Explosion Inside a MOCVD System Burn Box

Severity
Incident
Was Hydrogen Released?
Yes
Was There Ignition?
Yes
Incident Date
Incident Attributes
Setting
Damage and Injuries
Probable Cause
Contributing Factors
When Incident Was Discovered
Describe the incident, including corrective steps taken and their result.

While attempting to light the hydrogen flare inside a Metalorganic Chemical Vapor Deposition (MOCVD) system burn box, a small explosion occurred, blowing the back section of the burn box off. Hydrogen flow was shut down immediately, and this MOCVD operation was suspended. Researchers made the determination that this was a minor incident and there were no injuries.

The follow-up investigation determined that the MOCVD HEPA filter had become sufficiently loaded to the point where performance of the burn box exhaust ventilation system was significantly degraded. The static pressure created across the "loaded" HEPA filter equaled the operating static pressure of the exhaust ventilation system servicing the burn box. This resulted in a region of "dead air" in the burn box. Thus, without sufficient dilution, the volume of hydrogen that accumulated in the burn box exceeded the 4% lower explosion limit (LEL) for hydrogen. When the operator attempted to light the hydrogen flare, the accumulated hydrogen/air mixture ignited. This is identified as the "direct" cause of the incident. The researchers monitor the pressure drop across the filter as an indicator of when to replace the filter. At the time of initial installation, static pressure across the filter was measured -1.0" w.g. The pressure drop indicated at the time of the occurrence (-2.5" w.g.) was within the normal/nominal operating parameters established by researchers for this apparatus.

However, in retrospect, pressure drop alone is not a sufficient or reliable indicator of air flow (exhaust ventilation system performance). This is cited as a contributing cause.

The root cause of the occurrence is identified as a design error. Exhaust fan performance available to the apparatus was not sufficient to overcome the maximum level of filter loading created by this experiment. This requirement was not sufficiently evaluated during the conception, design and installation of this experiment.

 

Lessons Learned
  1. There should have been greater awareness during the design and installation of the burn box regarding the static pressure limitations of the exhaust fan in relation to the anticipated static pressure buildup across the HEPA filter.
  2. Monitoring the pressure drop across the HEPA filter is not a sufficient or reliable indicator of ventilation system performance (air flow). An air flow monitor incorporating a low-level alarm would have provided a reliable indication of ventilation system performance.

Key:

  • = No Ignition
  • = Explosion
  • = Fire
Hydrogen Incident Summaries by Equipment and Primary Cause/Issue
Equipment / CauseEquipment Design or SelectionComponent FailureOperational ErrorInstallation or MaintenanceInadequate Gas or Flame DetectionEmergency Shutdown ResponseOther or Unknown
Hydrogen Gas Metal Cylinder or Regulator 3/31/2012
4/30/1995
2/6/2013
4/26/201012/31/1969  3/17/1999
11/1/2001
12/23/2003
Piping/Valves4/4/2002
2/2/2008
5/11/1999
4/20/1987
11/4/1997
12/31/1969
8/19/1986
7/27/1991
12/19/2004
2/6/2008
10/3/2008
4/5/2006
5/1/2007
9/19/2007
10/31/1980
2/7/20091/24/1999
2/24/2006
6/8/1998
12/31/1969
2/7/2009
9/1/1992
10/31/1980
10/3/2008 
Tubing/Fittings/Hose 9/23/1999
8/2/2004
8/6/2008
9/19/2007
1/1/19829/30/2004
10/7/2005
 10/7/2005 
Compressor 10/5/2009
6/10/2007
8/21/2008
1/15/2019
  10/5/20098/21/2008 
Liquid Hydrogen Tank or Delivery Truck4/27/198912/19/2004
1/19/2009
8/6/200412/31/1969 1/1/197412/17/2004
Pressure Relief Device7/25/2013
5/4/2012
1/15/2002
1/08/2007
12/31/1969    
Instrument1/15/20193/17/1999
12/31/1969
2/6/2013
  11/13/73  
Hydrogen Generation Equipment7/27/1999  10/23/2001   
Vehicle or Lift Truck 7/21/2011    2/8/2011
12/9/2010
Fuel Dispenser 8/2/2004
5/1/2007
6/11/2007
9/19/2007
 2/24/2006
1/22/2009
   
Fuel Cell Stack      5/3/2004
12/9/2010
2/8/2011
Hydrogen Cooled Generator   12/31/1969
2/7/2009
   
Other (floor drain, lab
anaerobic chamber,
heated glassware,
test chamber,
gaseous hydrogen
composite cylinder,
delivery truck)
 11/14/1994
7/21/2011
7/27/1999
6/28/2010
8/21/2008
12/31/1969
3/22/2018
  6/10/2019
  • = No Ignition
  • = Explosion
  • = Fire
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