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CSB Urges Overhaul of Steel Standards

Wednesday, November 5, 2014

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Federal investigators are calling for drastically improved standards for steel in petroleum facilities, where equipment is vulnerable to potentially catastrophic hydrogen attack.

Current methods for predicting and inspecting for high-temperature hydrogen attack (HTHA) are flawed and inaccurate—too often, with deadly consequences, the U.S. Chemical Safety Board contends.

The warnings are part of "Behind the Curve," a new 14-minute CSB video that details the inadequacies of the current standards.

'It Should Not Have Happened'

The video follows CSB's investigation into the 2010 explosion and fire that killed seven workers at Tesoro Corp.'s refinery in Anacortes, WA. The blast was triggered by the rupture of a nearly 40-year-old heat exchanger and caused the largest loss of life in five years at a U.S. refinery.

The new CSB safety video uses 3D animation and interviews with investigators to detail the steps leading up to the fatal explosion and fire at Tesoro's Anacortes, WA, facility.

"The CSB is seriously concerned by the number of deadly refinery accidents in recent years," said Chairperson Dr. Rafael Moure-Eraso. "We have concluded that extensive improvements must be made in how refineries are regulated at the state and federal level.

"Seven lives were lost at Tesoro. It should not have happened. Companies, workers, and communities would all benefit from a more rigorous regulatory system that is focused on continuously lowering risks."

'Common Hazard'

At the facility, raw naphtha is treated to remove impurities and pre-heated inside heat exchangers, or pressure vessels. These have to be taken offline every six months to be cleaned because of fouling.

The CSB's investigation determined that long-term undetected HTHA in the steel equipment had led to the rupture of a pressure vessel rupture and the massive release of highly flammable hydrogen and naphtha.

HTHA occurs when hydrogen atoms diffuse into steel under high temperatures and react with carbon in the steel to form methane gas.

Methane molecules then accumulate, stressing the steel and causing large internal cracks. At Tesoro, one such crack was 48 inches long and extended one-third of the way through the vessel's inch-thick shell, CSB said.

HTHA "is a common hazard that has long been known within the petrochemical industry," CSB investigator Lauren Grim explains in the video. "However, Tesoro engineers and corrosion experts did not believe it could occur within the heat exchanger that ultimately failed."

The reason: Tesoro, like many companies, relied on data by the American Petroleum Institute to predict susceptibility of the heat exchangers to HTHA damage.

Nelson Curve

The prediction tool, called the Nelson Curve, factors in process temperature, the amount of pressure from hydrogen, and the material used to construct the equipment. Above the curve, HTHA is considered possible; below it, HTHA is not predicted, the video explained.

However, after detailed process simulations, the CSB concluded that the part of the Tesoro heat exchanger that ruptured had operated below the curve for carbon steel, "in a zone that industry guidance considered safe."

Additionally, the CSB identified eight other refinery accidents where HTHA reportedly occurred below the carbon steel curve.

"As a result, the CSB determined that the carbon steel Nelson Curve is inaccurate and cannot be trusted to predict the occurrence of [HTHA]," the video stated.

Design vs. Actual Conditions

At Tesoro, the CSB found that the company had not measured actual operating temperatures and pressures in the exchanger that failed. Instead, corrosion experts hired by the company relied on design operating conditions to calculate HTHA probability based on the Nelson Curve.

Nelson Curve
Images, video: CSB

The American Petroluem Institute's Nelson Curve is "inaccurate and cannot be trusted to preduct the occurrence of [HTHA]," CSB investigators said.

However, the actual temperatures of the fouled exchangers were likely much higher than design conditions, the CSB discovered.

"Had Tesoro used actual process conditions when determining HTHA susceptibility of the heat exchangers, their internal policies would have required the heat exchangers to be inspected for HTHA," Grim said.

But inspections for HTHA can also overlook deadly flaws, as equipment damage can be microscopic or limited to small areas.

"The best way to prevent HTHA is by using inherently safer materials of construction" that are more resistant to attack, said Grim. The industry has already identified such steels, she added.

CSB recommends that the American Petroleum Institute revise its standards to prohibit carbon steel equipment in HTHA-susceptible service and to require verification of actual operating temperatures.

Regulatory Shortfalls

"Our Tesoro findings are all too similar to those of other CSB refinery investigations," Moure-Eraso said.

Tesoro Anacortes explosion

"Seven lives were lost at Tesoro. It should not have happened," said CSB chairperson Dr. Rafael Moure-Eraso.

These similarities include:

  • Weaknesses in industry standards for safeguarding equipment;
  • A deficient refinery safety culture; and
  • A regulatory system in need of reform.

Citing shortfalls in current state and federal regulations, the CSB has asked the U.S. Envrironmental Protection Agency to revise rules under the Clean Air Act to:

  • Require facilties to analyze and use inherently safer technologies to the greatest extent feasible; and
  • Use the "hierarchy of controls," with safe design at the top of the hierarchy, to reduce process hazards.

The CSB's also recommended that the governor and legislature in Washington state "significantly strengthen" its oversight of refineries.

'Landmark Report'

CSB's investigation report on the Anacortes incident "is one of the most extensive and complex refinery explosion investigations," the agency has said. The final report was approved May 1, 2014.

Moure-Eraso called the investigation "a landmark report, which recommends extensive improvements to the way in which refineries are regulated at the federal and state levels."


Tagged categories: Explosions; Fatalities; Steel; U.S. Chemical Safety Board

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