DSS076: Case Study - Nylon Flock Explosion in the Textile Industry

In this episode of the DustSafetyScience Podcast, we examine a case study for a nylon flock explosion in the textile industry. This is one of a series of case studies we’ve reviewed, the other three being:

A fishmeal dust explosion in Episode #69
A metal dust explosion in a 3D printing application in Episode #71
An explosion that resulted from insufficient venting during a silo fire in Episode #74.

The nylon flock explosion happened in the north of Italy in 2001. The investigation was featured in a journal paper titled Case study of a nylon fibre explosion: An example of explosion risk in a textile plant, by Dr. Luca Marmo, which was published in 2010 in Volume 23 of the Journal of Loss Prevention In The Process Industries. As we review the case, we answer the following questions:

What is nylon flock?
What does the flocking process look like?
What did the facility look like?
What did the investigation find?
What caused the explosion?
Why was there no explosion protection?

What is nylon flock?

Nylon flock is created using an assortment of small fibers that range from half a millimeter to one millimeter in length and generally have a 10 to 100-micron thickness. They are applied to a central core to create a fuzzy textile material for applications like noise reduction, insulation, and surface protection.

What does the flocking process look like?

During the flocking process, plant operators take a ream of these long threads, dye them to the desired color, cut them, and apply them to a nylon core using an electrostatic field.

After a drying process, the flocked material is ready for use.

What did the facility look like?

At the facility where the incident happened, there were three parallel drying lines. Drying line one and two processed 163 of these core threads at time while the third dryer processed 120 of these threads at the same time.

Each of the dryers is quite large: 14 meters long, two meters wide, and six meters high, with multiple sections. They have a lower part where the incoming freshly-created flock flows in with the warm air that goes through the section twice before travelling to the upper section. The flock continues to dry in the hot air, which is heated through a heat exchanger system at 270 degrees Celsius.

There is also ducting at the top of the system that leads to the dust collector system, which consists of a dust collector with 12 bag filters, each with a 30-centimetre inner diameter. This system receives air from the in-flow line where the freshly-flocked fibres are located.

The last thing is that each of the dryers has eleven inspection doors. We’ll see that this played an important role in the actual explosion as well in the injury to the workers.

What did the investigation find?

In terms of the investigation, the authors of the paper-covered multiple areas, including:

Witness statements
The properties of the flock material
Post-explosion damage to the facility
The process that led to the explosion

Witness statements indicated that at approximately 4:10 a.m. on the morning of the explosion, one worker noticed some broken threads in dryer number two. They then followed the standard procedure, which was to shut down the dryer to re-tie core threads. This process turns off the fans or shuts the fans and the valves that control the heating medium, but the heating system is still hot inside the dryer.

At around 5:45 p.m., with everything being done, employee number two started to close the inspection doors to the dryer and employee number one went to turn on the line. When he turned it on, the explosion happened immediately.

Deflagration propagated throughout the dryer system. Flames spread throughout the facility, burning employee number one, who was at the control board. Employee number two was still closing the dryer doors when he was knocked down and suffered severe burns. Another employee was also injured during the explosion and flash fires that followed.

The investigators also looked at the properties of the flocking material. They measured these properties using standard testing apparatus like the 20-litre chamber and determined that the minimum explosible concentration for the flocking material was 70 to 80 grams per meter cubed, which was the same concentration for a lot of combustible dust.

They did a thermogravimetric analysis to determine if the flock could release flammable gases at the heats that were found, and concluded that at 270 degrees Celsius, the nylon flock did release some combustible gases.

Examination of post-explosion damage included a look at the dryer assembly, dust collection system, and related areas. Burnt nylon flock was found throughout the entire dryer assembly, which indicates where the deflagration took place and propagated. Thick layers of melted nylon material appeared in the upper part of dryer number two inside the ducts, leading back to the dust collection system.

Other observations included:

One of the bags was detached in the battery of bags and the dust collector and had been for quite a long time before the incident happened, due to the dust collection system not being inspected as often as it should have been.
A lot of melted flock was on top of the heat exchanger inside that dryer, giving an idea of where the explosion originated.
Every inspection door was open in the dryers, even though the employee had closed them. This suggests that the dryers experienced overpressure, which caused the doors to open. They did not have any explosive protection on them, so the incident could have been more severe had those doors not been able to open.
A large hole was found in the ducting where the deflagration had come out of the side. Drops of plastic were found in an 18 by 18-meter radius, giving a good idea of how large the fireball had been.
They found the building itself suffered only minor damage. All glass was broken, but there is little structural damage to the roof or to the beams in the columns of the facility because it is much larger than the dryers in which the explosion happened.

What caused the explosion?

The authors proposed two ignition processes that could have occurred when the dryer was turned on after being turned off for the hour and a half.

There could have been smouldering combustion in the nylon flock that settled on the heat exchangers. When the dryer was turned back on, fresh oxygen flowing through the system could have ignited either the flock material in the vicinity of the smouldering combustion or even combustion gases that have been released from the flock material.
Electrostatic discharge inside the dryer ignited either the flock material itself or a hybrid mixture of the flock material and the combustible gases released.

By looking at factors like the damage between the ducting and the dust collector, the fact that all the dryers were pressurized and that the inspection doors were open, they came up with the following likely sequence of events.

A primary explosion occurred in the upper part of dryer number two due to one of the ignition processes above. It then ignited a secondary explosion in the flock material, with flames propagating throughout the ductwork above the dryer.

Since the flame front couldn’t propagate through the bag filter, it reverted and went in three different directions: two travelled back to the other dryers, pressurizing them and forcing the inspection doors open while the third ejected out the side of the ductwork into the facility, injuring employee number one. Employees number two and three were injured by the explosions that ejected from dryer number two.

Why was there no explosion protection?

There was no venting installed at the facility, nor was there isolation between the equipment. A risk assessment had been done at the facility, but it had never regarded the flocking material is a combustible dust hazard. No one was aware that these fibres could explode and propagate a deflagration when suspended as a cloud.

This lack of awareness is concerning, especially during these critical times. Due to the COVID-19 pandemic, a lot of facilities are either partially shut down or shutting down their operations completely. When they come back online, what steps are being followed to resume operations safely and avoid fires and explosions?

In addition to taking startup precautions, facilities need to obtain expert testing, because materials are quite difficult to assess under the standard testing conditions. A different process needs to be followed to identify things like KST and minimum explosible concentration, and creating this process requires expertise.

Conclusion

Dealing with nontraditional dusts (a term coined by Dr. Paul Amyotte) can be an uphill battle because their explosive nature is not widely recognized, even despite events like this nylon flock explosion and a 2017 incident at a flocking facility in Leominster, Massachusetts. A willingness to overcome complacency and take a different approach is the best way to move forward.

If you have questions about the contents of this or any other podcast episode, you can go to our ‘Questions from the Community’ page and submit a text message or video recording. We will then bring someone on to answer these questions in a future episode.