Beyond The Fence Line

Mike Schmidt, Bluefield Process Safety, USA, argues that the industry is responsible for process safety even after chemical products leave the plant, and shares some ideas on how to meet that responsibility.

here is not much talk about process safety once

Thazardous chemicals are loaded and shipped. But process safety – the concern with fi res, explosions and toxic releases – does not end at the fence line. Wikipedia lists 35 incidents in its article on ammonium nitrate (AN) disasters. Of those, over a third involved the transport of AN; four were explosions of cargo ships loaded with AN; three were explosions of freight trains pulling cars loaded with AN; and six were explosions of trucks hauling loads of AN.

They follow a common pattern: an incident – sometimes a wreck, sometimes an onboard failure – ignites a fi re. The fi re burns and attempts are made to bring the fi re under control. Then, without warning, the AN explodes, with dramatic consequences. By way of example, an AN truck explosion on a highway in Arkansas, US, in 2019 was heard as far away as Little Rock, almost 100 miles (160 km) to the north (Figure 1).

Stability of AN

AN is not combustible. It is an oxidiser, however, and will accelerate the combustion of fl ammable and combustible materials. It is stable at ambient temperatures and pressures, and is reported as having very low shock sensitivity as a pure compound. Prior to the 1921 explosion at Oppau in Germany, BASF reportedly used small charges of dynamite to disaggregate solid masses of AN/ammonium sulfate mixtures as many as 20 000 times without incident.

However, AN is sensitised when contaminated with organic materials. In fact, ANFO (the bulk industrial blasting agent) typically consists of a 94% AN/6% fuel oil mixture. Spilled diesel fuel at truck accidents has been suspected as a sensitiser in some cases of AN truck explosions. Occasionally, reporters or the public will seize on terms such as ‘fertilizer grade’ versus ‘explosive grade’ to explain different stabilities or energies of explosion between various inventories of AN. The difference is in the porosity of the prill. Agricultural grade is not very porous, so it ships a little denser. Explosive grade is porous, so it can absorb fuel oil more readily to make ANFO. Without fuel oil, though, explosive grade AN is just AN, and no more or less likely to explode.

The Institute of Makers of Explosives (IME, based in Washington, D.C., US) recommends that trailers be dedicated to hauling AN, and if not, that they be washed out and dried before being used to haul AN. Contaminants from previous loads that the IME particularly warns about include: Animal fats. Baled rags, burlap or cotton bags. Charcoal. Foam rubber. Hay. Metal powders. Sawdust. Straw. Vegetable oil. Wood, wood chips or wood shavings.

Other impurities are known as sensitisers for AN, including chlorates, mineral acids and metal sulfi des.

That does not mean that uncontaminated AN will not explode. It is the sudden and catastrophic decomposition of AN that results in explosions. The melting point of AN is 169.6˚C (337.3˚F), and uncontaminated AN starts decomposing at 210˚C (410˚F).

There are two decomposition reactions. The fi rst dominates at lower temperatures, in the range of 200 ~ 300˚C (390 ~ 570˚F):

NH4NO3 N2O + 2 H2O(g)

This means that for each volume of AN prill, decomposition produces 780 volumes of gas at standard conditions. At 210˚C, that same volume of AN prill will decompose into 1280 volumes of gas when released to the open atmosphere. On the other hand, if that volume of AN prill were enclosed (say by being encased in a fused mass of AN), the volume would remain constant and the pressure after decomposition would increase to almost 1300 bar (almost 19 000 psi). An explosive release would happen long before reaching that pressure.

Then, at around 300˚C (approximately 570˚F), the reaction shifts:

2 NH4NO3 2 N2 + O2 + 4 H2O(g)

At this higher temperature and higher amount of gas generated, the decomposition of a volume of AN prill releases 1770 volumes of gas to the open atmosphere. Again, if the volume of AN prill decomposes with no way of venting, it will build to a pressure of almost 1800 bar (almost 26 000 psi).

AN decomposition does not require the presence of air; suppression efforts have to be directed to cooling the mass, not on smothering it.

Truck fi res

There are thousands of truck fi res each year involving large, over-the-road trucks. Still, the probability of any one truck catching fi re is less than 0.1% in any given year. Obviously, the probability of a specifi c truck catching fi re during a specifi c trip depends on the age of the truck, its maintenance history and the experience and skill of the driver. When trucks do catch fi re, however, trailer fi res are invariably caused by one of four reasons (Figure 2).1 The two causes that get the most attention are tyre fi res, because the other causes often set tyres on fi re as well, and brake fi res, perhaps because they are so diffi cult to put out. Tyre fi res are typically not the initiating cause of truck fi res but are a source of fuel for fi res once started by a brake fi re or wheel bearing fi re. The normal operating temperature for truck tyres is 40 ~ 65˚C (100 ~ 150˚F). At approximately 120˚C (approximately 250˚F), tyres begin to deteriorate rapidly, which can lead to blowouts. At 260 ~ 290˚C (500 ~ 550˚F), tyre decomposition produces fl ammable vapours, which can ignite if there is an ignition source. The autoignition temperature for the decomposition products of tyre deterioration is around 450 ~ 480˚C (850 ~ 900˚F). While brake fi res and wheel bearings are most often the ignition sources of tyre fi res, it is possible for truck tyres to catch fi re even when not started by overheated brakes or bearings. The most likely cause then is a fl at tyre. Flat tyres can overheat and catch fi re by spinning on the rim, rubbing against Figure 1. The smouldering crater left after an AN truck explosion was 15 ft the other tyre or frame rail or becoming stuck and deep and stretched all the way across the highway (photo by David Archer, dragging on the road. Arkansas Department of Transportation, used with permission). The temperature of a tyre fi re impinging on a load of AN prill is well above that necessary to cause the material to melt and then to decompose. Brake fi res are the most common cause of truck fi res. Most brake fi res occur within 30 to 50 miles (50 to 80 km) of the start of a trip. Brakes can catch fi re if the driver rides the brakes. This is especially a problem with inexperienced drivers on highways with steep grades. More often, though, brake fi res result when the brake drags. A driver will often not notice that brakes are dragging. Eight of the wheels on an 18-wheeler are on the trailer and each has a brake. Hopper trucks that are used to haul AN often have more wheels on the trailer. If the driver feels the drag at all, they may attribute it to a heavier than normal load or to headwinds. Brett Aquila, a US-based truck driver and Figure 2. The causes of truck trailer fires. blogger, commented that: “...brake fi res are

incredibly diffi cult to put out. The brake shoes and drums can reach very high temperatures and take a long time to cool off! Once they’ve reached high enough temperatures to ignite the shoes themselves and any grease that may be in the area, the fi re continues to re-ignite even after you’ve tried putting it out several times. The fi re won’t stay out until the brakes can be cooled down. So, it isn’t uncommon to hear about brake fi res that end up burning the trailer to the ground because there wasn’t enough water or fi re extinguishers around to put the fi re completely out. If you ever happen to see a tractor trailer with a brake fi re, make sure the fi re department is called. A fi re extinguisher will not often be able to put it out.”2

Preventing AN truck explosions

Trucks hauling AN do not catch fi re because they are hauling AN. They catch fi re because they are trucks. Tyre fi res and brake fi res are not more likely because of the load. If the load is AN, however, the load can explode, something that will not happen if the load is furniture or potatoes.

What can those at the plant do to make shipping their products safer?

Ship in well-maintained trailers

When the truck fi rst arrives to pick up a load, it should be examined carefully. Does it appear to be in good repair? Are the tyres properly infl ated? Brake fi res typically occur within the fi rst hour or two of a trip. Do the brakes smell hot? Does the lubricant in the hubs smell burnt? Is the trailer clean and dry? While quality issues are typically the only consequence of cross-contamination between loads, cross-contamination with AN can result in more than simply quality issues for the customer – it may result in catastrophic incidents on the road.

Before the truck leaves, another examination should take place. Are valves securely closed? Are caps in place and secure? Are there fi re extinguishers on board? The driver is responsible for the load, but a second set of eyes can help.

Confi rm the driver’s understanding of the hazards of AN

An experienced driver is more likely to handle unusual road conditions with greater skill, but experienced driving does not necessarily translate into experience of hauling AN. The hazards of AN should be explained.

It should be noted that a full truck-load of AN has the explosive energy of approximately 10 t of TNT. The detonation of a truck-load of AN is enough to blow a crater in the ground that stretches across the highway and to shatter windows nearly a third of a mile (half a kilometre) away. The driver should be reminded that fi re extinguishers are just for incipient fi res and that a tyre or brake fi re is hot enough to cause the catastrophic decomposition of AN, whether it is contaminated or not. It should be emphasised that the emergency response guidance is to evacuate over half a mile (a kilometre) at least in all directions when a truck loaded with AN is involved in a fi re. It is not reasonable to simply hand a driver a Safety Data Sheet and expect them to make sense of it.

The driver should be asked what they would do in the event of a brake fi re, a jackknife or a collision that results in a leak or spill. If they do not know the answers, they should be helped to fi gure out the best response.

Ensure that the load is correctly placarded

Before a truck leaves a facility, it should be ensured that it is properly placarded. The placards are neither for the facility nor the driver: the placard is for any emergency responder that must react to an incident involving the truck.

Emergency responders are trained to approach truck incidents from upwind and uphill, and then scan the truck for placarding before deciding how to approach the incident. The few seconds they take to do this can make the difference between life and death. All too often, when there are fatalities or other injuries during an incident, it is the fi rst responders that suffer them. There is not much chance of responding correctly if the placards are wrong.

What the future holds

In a plant, any safety critical process condition is typically instrumented so that a Safety Instrumented System continuously monitors it and shuts down the process automatically when conditions are unsafe, or alarms to prompt an operator to shut down the process. Increasingly, it is also possible to continuously monitor conditions in a load that is on the road. Fleet tyre pressure monitoring systems allow for both drivers and fl eets to stay aware of tyre pressures.

There are also fl eet temperature monitoring systems, which are currently used primarily to ensure that food and medicine in cold chain logistics does not spoil. Monitoring the temperature of AN loads, especially directly above the rear axles, could also be useful in preventing future incidents. Temperature monitoring should also reduce the number of truck fi res in general, regardless of the load. It may be time to apply what has been learned about Safety Instrumented Systems in fi xed locations to transportation.

Making a difference beyond the fence line now

Process safety on the road is different from process safety in the plant. That does not mean that producers and process safety professionals do not have a role to play. Not only can they make plants safer, they can also make the road safer.

When the media reports on a highway chemical spill that leads to a fi re, evacuation and shutdown of a road, the public are not thinking that the trucking industry is incapable of operating safety; the public are thinking that the chemical industry is sending dangerous products through their community.

As long as the industry must ship its products, the public are going to hold it accountable for how the products are shipped. Like it or not, the industry is expected to make a difference, both inside the fence and beyond the fence line.

References

1. FISHER, P.J., ‘Many trailer, truck fires can be prevented’, www. tirebusiness.com/opinion/fisher-many-trailer-truck-fires-can-beprevented (Accessed 17 February 2021). 2. AQUILA, B., ‘Tractor Trailer Brakes on Fire!’, www.truckingtruth.com/ trucking_blogs/Article-30/tractor-trailer-brakes-on-fire (Accessed 2 February 2021).