Severity
Non-Event
Leak
No
Ignition
No

A control room received a tank lower flammability limit (LFL) analyzer low sample flow alarm. The control room operator initiated the appropriate alarm response procedure and the facility entered limiting conditions of operation. At the time of the alarm, the facility was experiencing severe weather and the field operator was unable to investigate the alarm in the field. After the severe weather cleared, the field operator investigated the alarm and found the sample flow to be low and out-of-limits.

At the given facility, composite lower flammability limit (CLFL) monitors are used to detect the presence of hydrogen and other flammable gases in waste tank vapor spaces. Maintaining the concentration of flammable vapors in tank vapor spaces below CLFL levels maintains tank integrity by preventing the possibility of fires, deflagrations and explosions. The CLFL point of a combustible gas is defined as the minimum concentration of that particular vapor that is combustible in a dry air atmosphere. In gas detection systems, the amount of gas present is specified in units of % CLFL and is usually displayed on a meter in these units. The flammable gas of primary concern for the tank is hydrogen.

The flammable range of hydrogen is from 4.0% to 74.2% by volume in air. A hydrogen concentration of 4.0% in air is equal to 100% LFL. Hydrogen concentrations less than 4.0% will not burn, assuming it is the only source of fuel. To alert personnel to the buildup of potentially dangerous levels of explosive gases in the tank, a Combustible Gas Detection System is used to monitor and analyze sample gas drawn from the tank vapor space. The indicating range of this unit is 0-100% LFL indication which corresponds to 0-100% of span and 4-20 mADC transmitter output.

The rotameter is a variable output device for measuring and controlling flow consisting of an orifice type valve seat and needle valve assembly. The valve seat is made of a resilient material and is machined to match the surface of the needle valve. Flow is controlled by moving the needle valve in and out of the valve seat. After flow moves through the valve seat, it enters a cone shaped chamber that is wider at the top. Graduations on the side of the chamber indicate flow (typically in scfm, standard cubic feet per minute, or cfh, cubic feet per hour). A weighted float rides on the cushion of gas or fluid flowing through the chamber. During this event, it was determined that the valve seat had worn down so that the shape of the orifice did not match the shape of the needle valve which caused irregular and uncontrolled flow.

Incident Date
Jul 23, 1998
Equipment
  • Safety Systems
  • Measurement / Sensing Device
Damage and Injuries
Contributing Factors
When Incident Discovered
Lessons Learned

Cause and effect can sometimes be predicted by observing abnormal behavior even when the behavior is within specifications. Operators log equipment data and inform shift management when specifications are exceeded or when unusual equipment behavior is noted. Engineering establishes trending when patterned behavior is noted. Rotameter sample flow failures are too erratic to establish a cost effective preventive maintenance program. Predictive maintenance establishes a method to predict imminent failure based on symptoms that may be displayed during normal (within specification) operation. Operations and Engineering will continue to assess abnormal equipment behavior that may be within specification, and implement prediction methods, like trending, when applicable. Maintenance will implement annual rotameter inspection to aide in rotameter operational assessment.

Key:

  • = No Ignition
  • = Explosion
  • = Fire
Hydrogen Incident Summaries by Equipment and Primary Cause/Issue
Equipment / Cause Equipment Design or Selection Component Failure Operational Error Installation or Maintenance Inadequate Gas or Flame Detection Emergency Shutdown Response Other or Unknown
Hydrogen Gas Metal Cylinder or Regulator   3/31/2012
4/30/1995
2/6/2013
4/26/2010 12/31/1969     3/17/1999
11/1/2001
12/23/2003
Piping/Valves 4/4/2002
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
2/7/2009 1/24/1999
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
1/1/1982 9/30/2004
10/7/2005
  10/7/2005  
Compressor   10/5/2009
6/10/2007
8/21/2008
1/15/2019
    10/5/2009 8/21/2008  
Liquid Hydrogen Tank or Delivery Truck 4/27/1989 12/19/2004
1/19/2009
8/6/2004 12/31/1969   1/1/1974 12/17/2004
Pressure Relief Device 7/25/2013
5/4/2012
1/15/2002
1/08/2007
12/31/1969        
Instrument 1/15/2019 3/17/1999
12/31/1969
2/6/2013
    11/13/73    
Hydrogen Generation Equipment 7/27/1999     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