Explosion and fire at a metal powder metallurgy plant.

The event occurred at one of the unit producing iron powder, consisting of a furnace with a running belt for the powder, kept under hydrogen atmosphere to reduce and purify the metal. The hydrogen was supplied to the furnace through a pipeline running in a trench below the furnace. A corroded section of this pipeline released hydrogen which could cumulated in the covered trench for at least 30’ before igniting. The following hydrogen explosion projected into the atmosphere a large quantity of iron dust, which also ignited.
The consequences were three fatalities and additional two injured workers.

DETAILED DESCRIPTION from the CSB REPORT (se references)
Operators near one of the belt furnaces heard a hissing noise that they identified as a gas leak. The operators determined that the leak was in a trench, an area below the band furnaces that contains hydrogen, nitrogen, and cooling water runoff pipes, in addition to a vent pipe for the furnaces.
The operators informed the maintenance department, and six mechanics were dispatched to find and repair the leak. Although maintenance personnel knew that hydrogen piping was in the same trench, they presumed that the leak was nitrogen because of a recent leak in a nitrogen pipe elsewhere in the plant and started removing trench covers by using an overhead crane. They determined that the leak was near a trench covers which the crane could not reach. Therefore they acquired a forklift and started removing that covers. During this operation a powerful explosion.
A corroded section of piping carrying hydrogen was the cause of the hydrogen leak (the CSB investigation revealed a large hole of approximately 3 x 7 inches. The hydrogen was probably ignited by friction sparks produced when lifting the trench cover. The resulting explosion overpressure dispersed large quantities of iron dust from rafters and other surfaces in the upper reaches of the building. Portions of this dust subsequently ignited. Eyewitnesses reported embers raining down and igniting multiple dust flash fires in the area. According to the CSB report, it is difficult to attribute the final damages to one of the two fire sources.
The consequences were three fatalities and additional two injured workers.

The INITATING CAUSE the hydrogen leak, cumulation and explosion from a pipe. This event escalated when the iron powder was involved in the following fire.

The domino effect occurred becasue two independent hazards were present at the same time at the facility.

IRON POWDER: iron powder is flammable and explosive. This was not correctly recognised by the operator of the facility. On top of that, two previous accidents in the same year caused by iron powder explosions and/or fires had made clear that iron dust was present everywhere in the facility, and not only limited to the production areas. This constituted an additional not recognised hazard.

HYDROGEN
The trench involved in the accident contained many pipes including nitrogen and hydrogen supply and vent pipes for the furnaces. In addition to housing pipes, the trench was also draining the cooling water used in the band furnaces. At the time of the accident, this water came out of the furnaces hot and drained directly onto the pipes and into the trench.
Despite the availability of national codes dealing with this type of hazards, the facility operator had not in place any inspection/monitoring of the state of health of the pipelines. Slow corrosion could develop undetected for a long period. In addition, there were no procedure for mitigating leaks. The team looking for the leak could access the trench without checking presence of explosive mixture beforehand. It assumed a nitrogen leak based on a previous case, and did not raised the possibility of a hydrogen release.

The ROOT CAUSE was a management failure in recongising the two hazards and the related high risks and in taking th erequired preventing measures. The fact that the leaking hydrogn could cumulate in the trench was also a safety design failure. The fact that iron powder was everywhere in the premises highlights shortcoming in cleaning procedures.

pipeline

The local Fire Department had responded to 30 incidents of various types over the past 12 years at the facility, including the January 31, March 29, and May 27 incidents. The last two were caused by ignition of iron power accidentally dispersed in the atmosphere, but without any role for hydrogen in the fire. On May 13, 1992, a similar hydrogen explosion had already occurred, injuring severely one worker.

PROCESS DESCRIPTION
The facility produces high purity iron powders. The main steps are the melting of scrap iron, cooling it and milling into a coarse powder processed in long annealing furnaces to make the iron more ductile. The furnaces are band furnaces consisting of a 100 foot conveyor belt running through them. A hydrogen atmosphere is required to reduce the iron by removing oxides and preventing oxidation. The hydrogen is supplied to the facility by a contract provider, onsite.
Hydrogen is conveyed to the furnaces via pipes located in a trench under the floor and covered by metal plates. In the process of going through the furnace, the coarse powder becomes a thick sheet called “cake.” The cake is sent to a cake breaker and ultimately crushed into the fine powder metallurgy product. The majority of the finished PM product has a particle diameter between 45-150 microns.

Ayi at al. assumes a pressure in the pipeline of 1 MPa.

The investigation found an impressive number of shortcomings and safety-related failure in the design of the facility and its management:

(1) Hydrogen was supply through pipes in the confined space of a covered trench. This allowed for the formation of an explosive mixture before igniting. No fixed detection system was in place which could have warned workers of the presence of hydrogen. Not to mention an interlock system able to automatically shut down the facility in case of a hydrogen leak. Even portable monitors were on available, so that the workers approached the trench without any possibility to early detect the leak.

(2) The facility management weas aware of the iron powder combustibility hazard, but did not take necessary action to mitigate the hazard through engineering controls and housekeeping.

(3) There was an impressive lack of procedures: no prescribed regular inspection of the integrity of critical components, such as the pipes, so that the leak of the water pipe could cause the corrosion of the hydrogen pipe; and no prescribed action to handle the iron powders and to limit its presence.

(4) Inadequate personal protection equipment: the flame-resistant clothing available to the employees were not protecting against the combustible iron dust flash fires and the hydrogen explosion that caused the fatalities.

(5) The personnel were not informed and trained on the hazards of their activities. There was probably also a lack of awareness at every level of the leadership structure.

(6) Despite some inspection from the local authorities, they did not have a clear role in enforcing comprehensive and rigorous NFPA standards for the prevention of dust fires and explosions.

CORRECTIVE MEASURES REQUESTED
Based on the investigation finding, the CSB report issued several recommendations. The most directly relevant recommendations for the avoidance of further occurrence were directed to the facility operator:
(1) To conduct periodic independent audits for compliance with the applicable NFPA standards, using knowledgeable experts, and implement all recommended corrective actions;
(2) To develop training materials that address combustible dust and plant-specific metal dust hazards and train all employees and contractors. Require periodic refresher training for all employees and contractors;
(3) To Implement a preventive maintenance program and leak detection and leak mitigation procedures for all flammable gas piping and gas processing equipment;
(4) Develop and implement a near-miss reporting and investigation policy.

Also the city fire department responsible for the area was asked:
(a) To ensure that all industrial facilities of the city are inspected periodically against the International Fire Code, and to document all inspections;
(b) To implement a program to ensure that fire inspectors and response personnel are trained to recognize and address combustible dust hazards.