Prd Failing to Open
A cryogenic hydrogen laboratory had a power outage. The cryogenic hydrogen storage was no longer being cooled, therefore temperatures and pressures within the storage system started rising. There were several relief devices on the system, one with a set point of 150 psi (approximately 1 MPa) and a second at a set point of 165 psi (1.1 MPa). The system was monitored by site personnel so that when the hydrogen pressure increased to 120 psi (approximately 0.5 MPa), a manual vent was opened. However, the manual vent rate was not able to control the rising pressure. Pressure continued to increase until a 165 psi relief valve opened to control the pressure. The 150 psi relief valve failed to open. This incident was classified as a near miss.[Source: Burgess et al., Technical Report NREL/TP-5400-67381]
Event Date
August 1, 2016
Record Quality Indicator
Region / Country
Event Initiating System
Classification of the Physical Effects
Nature of the Consequences
Causes
Cause Comments
The immediate cause was the power outage, which stopped the cooling of the cryogenic storage. The root cause was the installation of a relief valve not suitable for cryogenic temperature, because its location was not expected to be affected by these temperatures. Inspection of the stuck relief valve with a 150 psi set point showed an opening pressure of 230 psi (approximately 1.6 MPa, 50% above set point). It is suspected that vacuum grease solidified (froze) in the valve seat. In this case the combined manual venting and secondary 165 psi relief venting were sufficient to control the overpressure condition. The 150 psi set point valve was in a section of the system that was not expected to see a cryogenic temperature so was not rated for cryogenic service. The relief valve was replaced with a valve rated for cryogenic hydrogen. The design improvement allows for migration of cryogenic boil off to the relief system.
Facility Information
Application Type
Application
Specific Application Supply Chain Stage
Components Involved
cryogening tests, PRD, vent stack
Storage/Process Medium
Location Type
Location description
Inhabited Area
Operational Condition
Pre-event Summary
A cryogenic hydrogen laboratory had a power outage.
Lessons Learned
Lessons Learned
The system was equipped with redundant safety system. Therefore the investigation concluded that the combined manual venting and secondary 165 psi relief venting were sufficient to control the overpressure condition. Nevertheless, the 150 psig relief valve, was replaced with a valve rated for cryogenic hydrogen. The design improvement allows for migration of cryogenic boil off to the relief system.The source of this event concluded that pressure relief devices should be put on a predictive maintenance schedule that depends on the severity of service. Further information on predictive maintenance and failure mode determination can be found in the American Petroleum Institutes Recommended Practice RP-576 (American Petroleum Institute 2009).
Event Nature
Emergency Action
The system was monitored by site personnel. When they saw the hydrogen pressure increased to 120 psi (approximately 0.5 MPa), they opened a manual vent. However, the manual vent rate was not able to control the rising pressure. The pressure continued to increase until a 165 psi relief valve opened to control the pressure, because the 150 psi relief valve failed to open.
Release Type
Release Substance
Hydrogen Release Concentration (%)
100.00
Release Pressure (bar)
165.00
Detonation
No
Deflagration
No
High Pressure Explosion
No
High Voltage Explosion
No
Flame Length (m)
0.00
Source Category
References
References