Article Archive
Static Charge
NTSB Blames 2003 Glenpool Fire On Non-Lightning Spark
Vol 21 No 4

Glenpool, OK, has been the scene of major storage tank fires twice in the last 39 months. Fire Captain Clay Ward can in-stantly recall the date of the first from memory -- April 7, 2003.

"Apparently they were filling a tank with diesel and static electricity sparked it off," Ward said. It happened in a tank farm next door to the Explorer Pipeline terminal that was the scene of the June 12 storage tank fire in Glenpool.

Tank 11, destroyed in the fire, was a 48-foot-high, 109-foot-diameter welded steel storage tank with a cone-type fixed roof with an internal aluminum floating roof. A report issued by the National Transportation Safety Board concluded that the owner had allowed Tank 11 to be switch loaded at flow velocities significantly higher than those set by company procedures and industry-recommended practices.

"The high velocity of the diesel in the tank fill piping and the turbulence created in the sump area resulted in the generation of increased static charge and, combined with the very low electrical conductivity (static accumulating) liquid, an elevated risk for a static discharge inside the tank," the report states.

On the afternoon of the accident, Tank 11, with a capacity of 80,000 barrels, had been emptied of gasoline in preparation for a diesel delivery. At about 8:33 p.m., the delivery commenced at an initial flow rate of 24,000 to 27,500 barrels per hour. Approximately 22 minutes later a high product level alarm sounded. Almost simultaneous with this an explosion engulfed Tank 11 in flames 75 feet high.

"The fixed roof separated from the tank shell and was blown northward and folded over on itself, coming to rest on top of the collapsed north wall of the tank," the report states. Shortly thereafter a second explosion collapsed the north side of the tank. Power lines nearby fell into the dike, igniting unburned diesel that had collected there. Later, above ground piping containing crude oil under pressure failed due to the fire and had to be shut down.

Experts estimate that Tank 11 contained between 7,397 and 7,600 barrels of diesel and a small amount of gasoline at the time of the blast.

The emergency response eventually involved 13 fire departments including Glenpool and fire fighting personnel from other refineries and Williams F&HC, the report states.

"Initially, the fire departments applied foam from the west side of the dike ... and placed it around the burning tank so that the wind would disperse the foam to contain the ground fire," the report states. "However, the manner in which the tank collapsed hindered the application of foam to the tank."

Firefighters applied water to a nearby tank to protect the exposure. Meanwhile, the fire in the dike caused a seal fire in a neighboring internal floating roof tank containing naphtha. That fire went out unaided.

A refinery in Ponca City provided the foam to fight the fire. As it continued, about 300 families living near the tank farm were evacuated. Nearby schools would be closed for the next two days.

At about 3:43 a.m. on April 8, the wind shifted to the east. Within an hour the fire in Tank 11 had worsened. Shortly before 6 a.m. additional power lines fell into the diked area east of the tanks, igniting diesel collected there. The Tank 11 fire would continue for 21 hours.

The NTSB report noted that Tank 11 operations before the explosion had included a partial draining that landed the floating roof and partial fillings before draining dry, allowing a large amount of gasoline vapor to be generated and distributed within the tank. This created a flammable fuel-air mixture both above and below the floating roof.

"All the conditions necessary for fuel vapor ignition were present in the storage tank at the time of the accident, and the explosion most likely occurred when a static discharge ignited a flammable fuel-air mixture in the space between the surface of the diesel and the floating roof," the report states. "The extensive damage to the tank is consistent with the flammable fuel-air mixture above the floating roof contributing to the force of the explosion."

The report also cited the local electrical provider for failure to recognize the risk the tank fire posed to the nearby power lines and to take effective action.

Williams F&HC returned to the area near Tulsa in September 2004 to extinguish an 80,000-barrel storage tank fire caused by lightning.

 
 

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