The CSB’s investigation into the failure of a 40-year-old heat exchanger at the refinery determined that the explosion and fire was caused by the HTHA damage mechanism, which severely cracked and weakened the carbon steel (CS) heat exchanger over time, leading to a rupture.
The CSB’s report released in May 2014 concluded that the standard used by industry to determine the vulnerability of equipment to HTHA is inadequate. To address its findings, the CSB made recommendations intended to prevent HTHA to the industry group that issues guidance on HTHA, the American Petroleum Institute (API) (Washington, DC).
“The CSB is concerned that the risk of equipment failure due to HTHA may not be fully known or appreciated within the industry,” says Vanessa Allen Sutherland, chairperson of the CSB. “Our goal is to help inform and educate industry of potential risk in order to prevent future accidents and to protect workers and the public.”
Updated guidance covering this issue—recently published by API—provides incremental improvements, but it fails to address important elements of the CSB’s recommendations, the board says.
The standard uses Nelson curves to predict the operating conditions where HTHA can occur in different types of steels. The curves are based on process data voluntarily reported to API, and are drawn beneath reported occurrences of HTHA to indicate the safe and unsafe operating regions.
The CSB says that API’s updated CS Nelson curves do not take into account all of the estimated process conditions where the failure occurred due to HTHA at the Anacortes refinery.
As a result, the new curves allow refinery equipment to operate at conditions where HTHA severely damaged the Tesoro heat exchanger, the CSB says, adding that the use of a curve not incorporating significant failure data could result in future catastrophic equipment ruptures.
“In the absence of industry guidance that incorporates findings from the Tesoro Anacortes failure, the CSB is issuing a safety alert to provide additional direction for industry,” Sutherland says. The alert provides the following guidance for industry:
1. Identify all CS equipment in hydrogen (H) service that has the potential to harm workers or communities due to catastrophic failure;
2. Verify actual operating conditions (H partial pressure and temperature) for the identified CS equipment;
3. Replace carbon steel process equipment that operates above 400 °F (204 °C) and greater than 50 psia of H partial pressure; and
4. Use inherently safer materials, such as steels with higher chromium and molybdenum content.
The CSB is an independent U.S. federal agency charged with investigating serious chemical accidents. The agency's board members are appointed by the U.S. president and confirmed by the U.S. Senate. CSB investigations look into all aspects of chemical accidents, including physical causes such as equipment failure as well as inadequacies in regulations, industry standards, and safety management systems.
The board does not issue citations or fines, but it does make safety recommendations to plants, industry organizations, labor groups, and regulatory agencies.