CSB probe into Philadelphia refinery disaster targets corroded pipe as cause
A ruptured pipe elbow has become the focus of the U.S. Chemical Safety Board investigation into the June 2019 explosion and fire at the Philadelphia Energy Solutions refinery in Philadelphia, PA, an information update issues Wednesday states.
“The rupture of the elbow appears to be the initiating event causing the process fluid release,” an advance copy states. That sudden loss of containment caused flammable process fluid containing hydrofluoric acid to release from the PES alkylation unit, forming a ground-hugging vapor cloud that ignited only minutes later.
Measurement of the thinnest portion of the ruptured elbow post-incident revealed a thickness of only 0.012 inch, about half the thickness of a credit card and less than seven percent of PES’ own requirement for piping to be replaced, CSB said.
CSB officials issued their initial findings during a news conference conducted in Philadelphia this morning (Oct. 16).
Within two minute of the first rupture at 4 a.m. June 21 the leaking vapor ignited causing a large fire in the alky unit. Within 13 minutes an explosion rocked the unit, followed by a second blast in the same unit four minutes later.
According to the CSB, the ruptured elbow under study was part of the piping between V-11, the depropanizer accumulator and the T-6, the depropanizer distillation column (See Figure 1).
“The elbow was on the discharge (outlet) piping from a pump (one of two pumps in this system) that was not operating at the time of the incident,” the update states. “At the time of the event, this piping was operating at a pressure of about 380 psig and a temperature of about 100 degrees Fahrenheit.”
Ultrasonic thickness measurements are taken at designated locations along the piping circuit as part of the PES inspection program to monitor metal loss due to corrosion. However, the ruptured elbow itself was not one of those locations, so its thickness was not specifically checked, the CSB reports.
Installation of the ruptured elbow has been traced to about 1973 and is thought to be part of the original refinery piping, the CSB states.
Piping failures due to corrosion played key roles in at least two major refinery fires in the last 10 years. CSB noted that the 2012 Chevron Richmond, CA refinery fire was caused by the rupture of a piping component that had become extremely thin due to sulfidation corrosion Likewise, the 2009 Silver Eagle refinery fire at Woods Cross, UT, was also traced to the rupture of a piping component worn thin from corrosion.
The havoc at PES did not end with the two explosions in the alky unit that day, the report states.
“At 4:22 a.m., a third, and the largest, explosion occurred when the V-1 Treater Feed Surge Drum, containing primariiy butylene, isobutene, and butane, violently ruptured,” the update states.
A fragment of the vessel weighing approximately 38,000 pounds landed on the opposite side of the Schuylkill River. Two other fragments, one weighing about 23,000 pounds and the other 15,000 pounds, landed in the PES refinery.
That final explosion “appears to be a secondary event caused by the fire,” the update states.
PES estimated that about 676,000 pounds of hydrocarbons were released during the event, of which an estimated 608,000 pounds combusted. Low-concentration hydrofluoric acid was also present in some the process piping and equipment during the incident.
An estimated 5,239 pounds of HF was released from piping and equipment during the incident, CSB reports. Of that, 1,968 pounds of released HF was contained by water spray within the unit and subsequently processed in the refinery wastewater treatment plant.
The remaining HF released escaped to the atmosphere, CSB states.
Interim Executive Dr. Kristen Kulinowski said, “Since 2015, the CSB has investigated three major incidents at refineries that utilize HF for alkylation. Incidents in Superior, WI, and Torrance, CA, fortunately did not result in an HF release. That was not the case here in Philadelphia. Though the main tank holding HF was not breached, HF was a component of the process fluid released from the alkylation unit. We are lucky there were no serious injuries or fatalities.”
HF is classified by the U.S. National Institute for Occupational Safety and Health (NIOSH) as “immediately dangerous to life or health (IDLH) at 30 parts per million (ppm). It penetrates the skin to cause destruction to deep tissue layers and bone.
If inhaled, HF can cause severe lung injury and pulmonary edema that may also result in death.
“The CSB is unaware of any offsite or onsite health impacts from the hydrofluoric acid release,” the update states.
Extinguishment of the PES blaze was not accomplished until about 8:30 a.m. the following day. Four days later, PES announced that the refining complex would be permanently shut down. In late July, the company filed for bankruptcy.
At the CSB, the investigation into the explosion and fire is ongoing. A final report is expected to be issued in early 2020, Dr. Kristen Kulinowski, interim executive with the CSB, said.
“At the conclusion of the investigation, the CSB will publish a final investigation report discussing findings, analysis and issuing recommendations,” CSB said.