Rzeczoznawca, ochrona przeciwpożarowa

Logistics and warehouse facilities are a specific group of buildings. Large warehouses also occur at facilities with a different primary purpose, such as commercial, manufacturing buildings, etc. As in any industry, methods for optimizing costs, operational efficiency, etc., are sought here as well. Often, people managing the operation of such facilities may not be aware that certain decisions seemingly unrelated to the building's fire safety nevertheless have a significant impact on it.

One might ask: what does the choice of pallet type used in a warehouse have to do with fire safety? Seemingly, there would appear to be no connection, yet there is—and in certain cases, a quite significant one.

I was prompted to write this article by a post on LinkedIn from someone praising the use of plastic pallets instead of wooden pallets.

From a functional perspective regarding storage, switching from wooden pallets to plastic pallets may perhaps be a better solution, although the voices of discussion participants varied in this regard. Since this is outside my area of expertise, I will not weigh in on this matter. However, I would like to draw attention to how the decision to change the pallet type can affect fire safety, alter the actual fire protection conditions, and impact the operating fire protection equipment.

The first issue is the fire load density. People with some knowledge in warehouse facility design know that one of the most fundamental factors influencing the way a warehouse facility is designed is precisely the assumed fire load density, which results from the mass of the stored goods, their characteristics regarding heat of combustion, and the area of the warehouse.

And so, fire load density affects, among other things:

the required fire resistance class of the building, and consequently the fire resistance class of its individual elements
the permissible area of a fire compartment
the permissible lengths of evacuation routes and exits* provision of internal fire hydrants
the required amount of water for external firefighting
the necessity to provide a fire access road
the necessity to enclose and smoke-vent stairwells
the required distances from neighboring facilities and property boundaries etc.

Thus, based on the above, it can be seen that in certain situations changing the fire load density can practically completely redefine the fire protection requirements for a facility.

Now let us consider how the type of pallets used affects fire load density. For better illustration, let us adopt assumptions for a reference area for further estimates:

high-bay warehouse, five levels of pallet positions on the rack
rack width: 1.1 m
aisle width between racks: 3.3 m
racks on both sides of the aisle
rack length: 100 m
reference area: 550 m²
weight of a wooden pallet: 25 kg
weight of a plastic pallet: 14–18 kg
pallet material: HDPE or PP

It seems that the above assumptions regarding the aisle width, i.e., storage-free area, are quite optimistic; sometimes warehouses with higher rack density can be encountered, which would automatically result in an increase in fire load density.

Under the above assumptions, the fire load density when using wooden pallets (heat of combustion of wood: 18 MJ/kg) can be estimated at approx. 820 MJ/m².

And here an interesting conclusion can be emphasized: a high-bay warehouse, if we store goods on pallets, in most cases cannot be a facility where a fire load density below 500 MJ/m² is assumed, yet such ideas can unfortunately still be found.

Taking into account only the load resulting from the presence of wooden pallets, under the above assumptions regarding rack placement, for the facility to feature a load density below 500 MJ/m², there could be no combustible materials in the facility other than the pallets (which is usually unlikely, because even when non-combustible materials are stored, packaging is most often combustible), and the racks would have no more than 3 pallet positions vertically.

Going further under the original assumptions, if we replace wooden pallets with plastic pallets (heat of combustion approx. 43 MJ/kg), with a plastic pallet weight of 14 kg, the fire load density will be approx. 1100 MJ/m², which is approx. 300 MJ/m² more. That is with the weight of lightweight plastic pallets; if we consider pallets weighing 18 kg—and these are not uncommon—the fire load density will be approx. 1400 MJ/m², which is approx. 600 MJ/m² more than in the case of wooden pallets.

Based on the above estimates, we see that the difference is significant, and it is not difficult in this way to jump from one fire load density range determining the fire protection requirements in a facility to a higher one.

And if we translate this, for example, into calculating the capacity of a water tank for external firefighting, an increase in fire load density by 600 MJ/m² shows the necessity to extend the assumed fire duration by about 0.7 to 1 hour. So the differences in required volume can be significant.

In the case of facilities protected by a sprinkler system, another important aspect that should be taken into account is the requirements arising from the standard based on which the system is designed. For example, in the NFPA 13 design standard, which is one of the most commonly used, we find provisions stating that when using plastic pallets that have not been proven to pose no greater hazard than wooden pallets, it is necessary to increase the hazard classification:

for Class I–IV commodities on unreinforced plastic pallets, by one class
for Class I–III commodities on reinforced plastic pallets, by two classes, and Class IV commodities in this case should be treated as Group A unexpanded plastics in cartons

The above does not apply to cases of ceiling-only protection with spray sprinklers having a K-factor of at least 240.

In the case of pallets made of materials other than HDPE or PP:

for Class I–III commodities, the classification should be established based on tests conducted by an authorized entity or increased by 2 classes
class IV commodities should be treated as Group A unexpanded plastics in cartons.

Thus, depending on the assumptions adopted for the design of the sprinkler system, it may turn out that changing the pallet type without changing the type of stored goods can cause the warehouse to be operated contrary to its design assumptions.

Finally, attention should be drawn to the potential negative consequences of failing to consider the above issues. Exceeding the fire load density in an existing facility beyond the assumptions under which the building was designed will often constitute a material change in the use of the facility due to the change in fire protection conditions. Failure to meet the requirements specified in the design assumptions of the sprinkler system means that we can formally consider the system to be "non-operational," so all fire protection aspects where the presence of a sprinkler system was utilized change, including relaxations of regulatory requirements for facilities protected by fixed water fire extinguishing systems.

Beyond the obvious change in hazard due to increased fire load or lack of effective sprinkler operation—which significantly impacts fire-induced losses—one can also imagine the consequences of the facility manager's liability for operating the facility contrary to its intended purpose, as well as—what can also be very "painful"—the lack of insurer liability for fire losses in such a facility.

The above is merely an example of how a decision regarding a change in the type of pallet used, seemingly having nothing to do with fire safety, can impact that safety. However, many more examples of such seemingly insignificant elements could be listed.

If you would like to ensure the fire safety of your facility, I invite you to cooperate! I will be happy to provide substantive support, whether in the form of consulting on specific issues or, for example, a fire safety audit in a facility aimed at identifying potential hazards and necessary solutions

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"What impact does the type of pallets have on warehouse fire safety?"

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