Only 25 minutes after the normal work shift ended, an explosion occurred at a hydrogen storage and use facility that had been in a non-operational mode for several months while undergoing modifications for future tests. No one was in the facility at the time of the explosion. The event was viewed about 30 seconds after the explosion by two engineers in a blockhouse 1000 feet away. Authorities were notified and calls were placed to other personnel needed to secure the area. About 8 minutes later, the engineers moved to a vantage point about 450 feet from the facility. There they viewed heat waves rising from a central location on the test pad, heard popping sounds similar to gaseous hydrogen (GH2) venting on a burn pond, and suspected that a hydrogen fire was in process. They returned to the blockhouse and then traveled via a safe route to where the hydrogen storage vessels were located. Two large manual supply valves were closed, isolating the GH2 source about 30 minutes after the explosion. The facility's other supply systems and utilities had been severed or ruptured. Shrapnel and debris were ejected up to 540 feet away. Teams of firefighters and emergency medical personnel were sent to the area to verify that no one was injured and to extinguish small residual fires.
Damage was significant, including destruction of two support buildings, one 27 x 33 ft, the other 28 x 48 ft. One building, a single-story structure with a basement, served as an electrical control and instrumentation terminal. It was constructed of 14-inch-thick reinforced-concrete walls designed to withstand an external explosion of 4500 lbs of TNT 600 feet away. The second building was a shop and support building constructed of 8-inch-thick concrete blocks, filled with concrete and reinforced with steel rods. The support building was located about 90 feet from the terminal building and the two were interconnected by underground air conditioning and electrical ducts. Other structures and support systems in the vicinity were also extensively damaged. Costs incurred from the explosion were estimated to be approximately $5.9 million. Detectable levels of GH2 were recorded at several locations adjacent to the concrete pad for a five-day period following the event.
Findings of the investigation board were as follows:
- The explosion was the result of a massive GH2 leak.
- A GH2 leak occurred in an underground ASTM A106 Grade B, Schedule XX carbon steel pipe with a 3.5-inch diameter and a 0.6-inch wall thickness. The pipe was coated with coal tar primer and coal tar enamel, wrapped with asbestos felt impregnated with coal tar, coated with a second coat of coal tar enamel, and wrapped in Kraft paper, in accordance with American Water Works Association Standard G203. The source of the leak was an oval hole about 0.15 in x 0.20 in in at the inner surface of the pipe and about 2 in in diameter at the outer surface of the pipe. Upon excavation of the pipe, it was noted that the coating was not present at the leak point. This resulted in galvanic corrosion over a 15-year period and the eventual rupture when high-pressure gas was applied to the thin pipe membrane. The pipe was 8 ft 9 in below the concrete pad.
- Prior to the pipe rupture, a pneumatically operated GH2 isolation gate valve, designed for 6000 psi service and located about 280 feet from the facility, failed in the open position. Pneumatic pressure had been removed earlier in the day and failure analysis indicated that the valve had been damaged during recent field servicing. Leakage across the main seals of the valve over time, due to operations at another facility, permitted a pressure buildup within and downstream of the valve, sufficient to drive the valve to the open position. The failure was duplicated during failure analysis testing. This failure allowed GH2 in excess of 4000 psi to enter the 3-inch GH2 line buried beneath the concrete apron surface next to the facility.
- GH2 was trapped in large quantities in sand and gravel under the apron surface (a 1-foot-thick concrete pad about 160 x 140 ft). GH2 then entered the basement of the electrical control and instrumentation terminal building, located immediately adjacent to the facility, through penetrations in the basement wall, including cable ducts, cable pulls, and two 24-inch-diameter air conditioning ducts. GH2 was transported through the air conditioning ducts to a support building about 90 feet from the terminal building.
- An explosion originated in the basement of the terminal building. A shock wave traveled through the air conditioning ducts and caused a second explosion of lesser magnitude in the support building. The actual ignition source in the terminal building is unknown, however an electrical arc from a sump pump was considered to be the most likely source.
- The TNT equivalent of the blast was calculated to be between 100 and 475 pounds, depending on the location.
Active GH2 sensors should be installed and continuously monitored in all enclosed buildings near GH2 sources. All buildings near areas where hydrogen is used should be designed to preclude GH2 entrapment (e.g., sloping roof with ventilation at the highest point).
Underground carbon steel lines beneath concrete pad areas should not be used for GH2 transmission. All GH2 lines are now stainless steel and above ground. - Any GH2 transmission lines buried underground should be proof-tested and leak-checked on a periodic basis.
Any below-grade piping installation should be in open trenches covered by grating.
Facilities should be protected from GH2, at a safe distance, by manual isolation valves. If remote-operated valves (ROVs) are required for operational isolation purposes, the ROVs should be in series with and downstream of the manual isolation valve.
The pressure between isolation valves and stand shut-off valves should be routinely monitored on a daily basis.
Field repair of mechanically severable valves in high-pressure systems should be eliminated.
Valves repaired in the field should be subjected to functional and leak checks, including actuator and valve seals at simulated operating conditions. A written procedure should be prepared and used.
Valves utilizing pneumatic actuators should have actuator piston and piston nut staked (or locked by other positive means) in the installed condition.
All high-pressure gas lines scheduled to be inactive for periods greater than 6 months should be physically isolated by blind flanges from active systems.
Supply system status of pressure vessels and lines (pressure and/or quantity) should be recorded at the start and completion of operations each day. All reservoirs should be isolated at close of business each day, and before weekends and holidays.
Corrosion protection systems for underground lines should be reviewed and tested to confirm the adequacy of the systems.
Operational and support buildings at hazardous sites should be isolated (i.e., interconnecting air conditioning systems should be avoided). Buildings connected to hazardous sites by tunnels and/or conduits should be physically isolated by seals. If physical isolation is not practical, then positive air flow should be maintained in tunnels and conduits.
Explosive gas detection meters should be included in the equipment carried by firefighters and emergency medical personnel.
Fire alarm transmitters should be located at all hazardous locations.
Emergency instructions for isolating GH2 and utilities for hazardous locations should be permanently posted with names and telephone numbers of key individuals to be contacted.