HTHA heat exchanger rupture Nelson Curve Chemical Safety Board
HTHA heat exchanger rupture Nelson Curve Chemical Safety Board

Contributing factors to an accident that occurred during startup of a refinery’s naphtha hydrotreater unit included reliance on a Nelson Curve that does not reliably predict the occurrence of high temperature hydrogen attack and the failure to use actual process conditions when calculating the risk of HTHA.

In a 14-minute safety video, the U.S. Chemical Safety Board simulates the conditions that led to the fatal April 2, 2010, explosion and fire at the Tesoro refinery in Anacortes, Wash., where a nearly 40-year-old heat exchanger violently ruptured, causing an explosion and fire that fatally injured seven workers

The CSB’s investigation report, approved in May 2014, found an immediate cause of the incident to be long-term, undetected high temperature hydrogen attack (HTHA) of the steel equipment, which led to the vessel rupture on the day of the accident. The CSB found the industry’s standard for determining vulnerability of equipment to HTHA, to be inadequate.

“Nelson Curves predict the susceptibility of the carbon steel heat exchangers to HTHA damage. They take into account process temperature, the amount of pressure contributed by hydrogen and the kinds of materials used in constructing the equipment. Above each Nelson Curve, HTHA was thought to be possible. Below each curve, HTHA was not predicted to occur. But after conducting detailed process simulations, the CSB concluded that the portion of the heat exchanger that ruptured actually had operated below the curve for carbon steel — in the zone that industry guidance considered safe. And, the CSB has learned of at least eight other refinery accidents where HTHA has reportedly occurred below the carbon steel Nelson Curve. As a result, the CSB determined that the carbon steel Nelson Curve is inaccurate and cannot be trusted to predict the occurrence of high temperature hydrogen attack,” says the CSB in “Behind the Curve.”

The CSB’s investigation report points to their findings of a substandard safety culture at Tesoro, which led to a complacent attitude toward flammable leaks and occasional fires over the years. The CSB also called on the Environmental Protection Agency to use its authority under the general duty clause to promote wider use inherently safer designs.