Process Safety: Dust Flash Fires

Tuesday, March 22, 2016 @ 03:03 PM gHale


Just around five years ago five people died and three suffered injuries in a series of iron dust flash fires and a hydrogen explosion at the Hoeganaes facility located in Gallatin, TN.

The plant specialized in melting and converting scrap metal to various metal powders. The Hoeganaes facility’s main product is a powder that is 99 percent iron. The process involves melting the iron, then cooling and milling it into a coarse powder. The powder then goes through an annealing furnace on a 100-ft-long conveyor belt. The furnace has a hydrogen atmosphere to reduce oxides and prevent oxidation. Hydrogen is supplied through pipes located in a trench in the floor, which is covered by metal plates. The product from the furnace, called a cake, is sent to a cake breaker and then crushed into a powder with a particle size of 45-150 pm.

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Process Safety: Lessons Learned

When it comes to safety incidents, there are the huge incidents that live on forever like Bhopal, India or BP Texas City to name a few.

But then there are incidents that make headlines, but quickly fade away, but they still can end up used as a great learning experience. In an occasional series, ISSSource will edit reports on lessons learned about incidents that showed how a small mistake led to a big safety incident.

First Incident
On Jan. 31,2011, operators thought that a bucket elevator used to transfer the powder was off track (the belt had become misaligned, which can cause the motor to overheat due to the increased torque). After shutting down the motor, a maintenance mechanic and an electrician inspected the equipment. They did not believe the belt was off track and requested the operator to restart the motor. When the motor started, the vibrations dispersed powder that was on the equipment and floor. A flash fire occurred almost immediately and engulfed the two workers, killing both.

Second Incident
On March 29,2011, a Hoeganaes engineer and a contractor were replacing igniters on an annealing furnace. They had difficulty reconnecting a gas line, and the engineer used a hammer to force the connection. Large amounts of dust on surrounding surfaces ended up dispersed by the hammering and ignited almost immediately. The engineer suffered first- and second-degree bums, while the contractor was able to escape. The engineer was wearing flame-resistant clothing (FRC), which may have helped prevent more serious bums. A photo taken at the Hoeganaes plant on Feb. 3, 2011, about two months before the showed much dust had piled up on the plant’s surfaces.

Third Incident
On May 27, 2011, operators near an annealing furnace identified a gas leak coming from a trench that contained hydrogen, nitrogen, and cooling water runoff pipes, in addition to a vent pipe for the furnaces. Mechanics ended up dispatched to find and repair the leak. One area operator stood by as the mechanics searched for the source of the leak. Although maintenance personnel knew hydrogen piping was in the same trench, they presumed the leak was nonflammable nitrogen because of a recent leak in a nitrogen pipe elsewhere in the plant. However, in this case the source of the leak was a line containing hydrogen.

The trench covers were too difficult to lift without machinery, so workers used a forklift to lift a cover near the leak. As workers pulled up the cover via the forklift, friction created sparks and an explosion ensued. The hydrogen explosion dispersed large quantities of iron dust from rafters and other surfaces in the upper reaches of the building. Portions of this dust ignited, creating multiple dust flash fires in the area. Three employees died from the burns they suffered in the fire.

Key Lessons
Understanding hazards and risks is one of the pillars of risk-based PSM. After the incidents, combustibility tests indicated the iron dust was a weak explosion hazard and relatively hard to ignite.

These findings were similar to results Hoeganaes obtained after an insurance audit in 2008. A lesson here is even a weakly explosive and hard-to-ignite dust is still combustible, and therefore, still hazardous and capable of causing fatalities when ignited. In this case, even though the company had the necessary information, personnel did not fully understand the hazards and risks of combustible dusts.

Learning from experience is another pillar of risk-based PSM. The plant experienced an incident in 1992 that was very similar to the third incident in 2011. A hydrogen explosion in a furnace dispersed accumulated dust and created a flash fire that severely burned an employee (burns covered more than 90 percent of his body, and he spent a year in a burn unit). Hoeganaes did not learn from its own incident.

The importance of housekeeping in a facility that handles solids goes without saying.

All three of these incidents were the result of large quantities of combustible dust present. Poor housekeeping has been involved in most, if not all, high-consequence dust explosions. At the Hoeganaes plant, control of dust emissions and housekeeping were ineffective. Baghouse filtration systems installed to control dust were frequently out of service, and the CSB investigators observed the baghouses leaked when the bags ended up pulsed. The 2008 insurance audit also noted that housekeeping needed improvement in several areas. The ineffective dust control and housekeeping enabled dust layers with more than enough dust to fuel the flash fires to accumulate. These deficiencies were contributing factors to all three incidents.

Suffice it to say engineers have learned PSM lessons the hard way, but their mistakes can help you to avoid similar situations in the future.

Most processes have more than one layer of protection. However, no protection or safeguard is 100 percent perfect, and, like slices of Swiss cheese, there are holes in every layer. Incidents occur when multiple failures — or holes — line up. The goal of PSM is to make the holes as small and as few as possible.

Technical competence is not enough to prevent an accident, but rather management systems and company culture also play a key role in process safety.