Rupture of a heat exchanger in a refinery, due to hydrogen attack

A petroleum refinery experienced a catastrophic rupture of a heat exchanger in the catalytic reformer / naphtha hydrotreater unit.
The heat exchanger catastrophically ruptured because of High Temperature Hydrogen Attack.

Highly flammable hydrogen and naphtha at more than 500 degrees Fahrenheit (260ºC) were released from the ruptured heat exchanger and ignited, causing an explosion and an intense fire that burned for more than three hours.
The accident fatally injured seven employees (one shift supervisor and six operators) who were working in the immediate vicinity of the heat exchanger at the time of the incident.

The INITIATING CAUSE was carbon steel degradation and seal break due to high temperature hydrogen attack (HTHA).
The reports speak of self-ignition of the naphtha/H2 mixture.

Because of the refinery’s long history of frequent leaks and occasional fires during this start-up activity, the CSB considered this work to be hazardous and non-routine.

heat exchanger PREHEATING THE NAPHTA

The workers were in the final stages of a start-up activity to put back in-service following cleaning a bank of heat exchangers.

DESCRIPTION OF THE FACILITY
The heat exchangers are used to preheat process fluid before it enters a reactor of the Naphtha Hydrotreater unit.

The Chemical Safety and Hazard Investigation Board (CSB) has issued a report which makes recommendations, summarized below, to the following recipients:

1. To the US Environmental Protection Agency (EPA) to revise the Chemical Accident Prevention Provisions under 40 CFR Part 68 to require the documented use of inherently safer systems analysis and the hierarchy of controls to the greatest extent feasible in establishing safeguards for identified process hazards.

2. To the Washington State Legislature to augment the existing process safety management regulatory framework with the more rigorous safety management attributes identified in this report for petroleum refineries in the state of Washington

3. To the Washington State Division of Occupational Safety and Health – Labor and Industries to perform verifications at all Washington petroleum refineries to ensure prevention of equipment failure due to HTHA and that effective programs are in place to manage hazardous non-routine work

4. To the American Petroleum Institute to revise API RP 941 and API RP 581 to prohibit the use of carbon steel equipment in HTHA-susceptible service and require verification of actual operating conditions. Make additional revisions to API RP 941 to establish minimum requirements to prevent HTHA failures and to require the use of inherently safer design.

5. To the Refinery to implement a process safety culture program that will assess and continually improve any identified process safety culture issues.

The Chemical Safety and Hazard Investigation Board (CSB) has issued a report which makes recommendations, summarized below, to the various recipients. To the Refinery the recommendation was to implement a process safety culture program that will assess and continually improve any identified process safety culture issues.

Moreover, since it is difficult to detect HTHA damage by inspection, an inherently safer design would be better approach to prevent HTHA.
This design approach should especially focus on heat affected zones around welds, because HTHA occurs most likely there.

The following two aspects related to management of operations:
(1) Gradual changes to operating conditions (e.g. heat exchanger fouling or catalyst deactivation) may lead to an accidental breach of operating limits.
(2) Abnormal (transient) operating conditions (e.g. startup, fouling, shutdown, etc) can create major process safety hazards.

Finally, an important technical lesson could be drawn, regarding the Nelson Curve; this is a chart used in the petroleum and chemical industries to define safe operating limits (temperature vs. hydrogen partial pressure) for steel equipment, preventing High Temperature Hydrogen Attack (HTHA), where hydrogen degrades the steel's microstructure, leading to potential failure. The heat exchangers materials had been chosen according to the curves. Wit the information delivered by the event, the Nelson curve for carbon steel was revised. For carbon steels and C-0.5 Mo steels in hydrogen service, the safe operating limit should be > 28°C (50°F) and > 3.5 bar (50 psi) below the new Nelson curve.
Refinery equipment and piping susceptible to HTHA should be replaced with inherently safer materials (e.g. low Cr-Mo alloys) to mitigate the risk.

Since the incident, the company has installed new naphtha hydrotreater (NHT) heat exchangers with upgraded materials of construction to significantly reduce the potential for High Temperature Hydrogen Attack (HTHA).
In addition, an advanced process control system is in place to minimize fouling.
The heat exchangers are also constructed using only one bank of exchangers.
The entire NHT unit now must be shut down for cleaning, eliminating the hazards of online switching and creating a much safer approach for maintenance.
The new heat exchangers also incorporate additional instrumentation to allow the monitoring of each heat exchanger for fouling and decrease the likelihood of operation in HTHA-susceptible conditions.