Explosion of flammable gases at a refinery

The INITIATING CAUSE of the accident was the internal structural failure and blow-out of a check (non- return) valve. A non-return valve failed due to hydrogen embrittlement. The valve was on a high-pressure light hydrocarbon gas line in the process gas compression system of an olefins unit. It released a hydrogen-rich hydrocarbon mixtures (approximately 19% vol of hydrogen) at pressure in the 20 to 30 bar range, forming a vapour cloud that ignited and started a large flammable gas leak that eventually ignited, resulting in an unconfined vapor cloud explosion (VCE).
The investigators calculated that approximately 15,000 pounds of process gas was released prior to the explosion, and that approximately 3,000 pounds of gas was involved in the explosion.

the INITIATING cause of the accident was the internal structural failure and drive shaft blow-out of a 36-inch diameter pneumatically-assisted non- return valve.
Fractography revealed typical hydrogen - embrittlement damage.
Explosion energy calculation assessed the hydrogen content in the vapour cloud around 20%.

The EPA/OHSA Shell report of 1998 identified these causes components:
1) Inadequate Valve Design
2) Failure to Learn from Prior Incidents
3) Inadequate Process Hazards Analysis
4) Inadequate Mechanical Integrity Measures
5) Inadequate Operating Procedures

Plus the following Contributing Factors
A1) No Indication of Hydrocarbon Leak / Delayed Operator Response to Leak
A2) Inadequate Communications Practices

check (non-return) valve,

Check and butterfly valves are used in refineries, petrochemical plants, chemical plants, and power generation facilities to stop and control the flow (e.g., prevent reversal of the flow's direction) of process materials through pipelines or apertures.

As a result of the explosion and fire, extensive damage occurred to the facility, and several workers received minor injuries.
Damage to the facility was estimated at US$101M.
Nearby residential property was damaged. Major transportation routes adjacent to the facility were closed for several hours, and nearby residents were advised to remain indoors.

The valve failure which caused the accidents had various design aspects which made it unsuitable for the operation of the installation.
The EPA and OSH report contains the following recommendations (for a full list see reference):

1. To review process systems to determine if installed valves may be subject to a shaft blow-out hazard, and modify or replace those valves as necessary
2. To consult valve manufacturers or other appropriate design authorities to ensure that any modifications made are safe.
3. To update and revalidate the process hazards analysis
4. To implement a more rigorous mechanical integrity inspection program for valves in extreme service
5. To develop and implement a corporate information communication system
6. To implement precautionary measures contained in OSHA’s PSM standard and EPA’s RMP rule to prevent flammable gas leaks from resulting in vapour cloud explosions.

Specific to hydrogen, the EPA stated that "valves used in hydrogen-rich or hydrogen sulphide-containing environments may be more susceptible to blow-out due to hydrogen embrittlement of critical internal components, particularly if these are made from hardened steel." (EPA enforcement Alert, 2, August 1999)