Issue 40: Does The Construction Of The Pallet Pose A Warehousing Issue?
By Peter J. Gore Willse P.E., FSFPE
Pallets were first used in the US in the late 1800's as a simple skid
constructed of wood. Today, they are constructed of various materials,
wood, plastic, steel, aluminum, and cardboard; as either 2-way pallets
known as stringer pallets or 4-way pallets know as block pallets. A
higher percentage of pallets are constructed of wood, but pallets
constructed of plastic and cardboard are increasing in use. Plastic
pallets are now being used in the food industries due to health
requirements where the pallet is required to be cleaned after every use.
Cardboard pallets are being used when products are being shipped from
overseas to minimize the chance of insect infestation. Depending on the
type of pallet (configuration and material), the commodity
classification of the product stored can be increased.
Wood pallets can be constructed as a 2-way or 4-way pallet. A 2-way
pallet is open on opposite sides and has solid pieces of wood or
stringers on either side and one in the middle (see Figure 1). A 4-way
or block pallet is open on all sides with nine posts to separate the top
deck from the base. These posts are located at the corners of the
pallet and in the center of all sides and one in the center of the
pallet (see Figure 2). Some pallets are a hybrid of the 2-way to allow
the pallet to be lifted from all sides (see Figure 3). Plastic pallets
are usually 4-way pallets whereas cardboard pallets are usually 2-way.
Plastic pallets are mostly made of HDPE or recycled PET (beverage
bottles). Some plastic pallets are reinforced with metal bars to give
them more load capacity.
Wood pallets can be constructed of either softwood (pine) or hardwood
(oak). Softwood pallets are usually a 4-way pallet and are considered
expendable; they can be thrown out after each use because they are so
cheap in price. The blocks can be either solid wood or pressed wood.
Hardwood pallets are usually a 2-way pallet. Wood pallets can be reused
but usually do not last more than one year.
Originally, cardboard pallets were used for light loads, but they are
now capable of caring the loads that would be on a wood pallet. While
their use is not prevalent in the US, they are being used in Europe and
Asia because of insect problems.
The protection schemes shown in Chapters 14 through 17 of NFPA 131
(previously in NFPA 231 and NFPA 231C) are based on fire tests. These
fire tests were conducted on products on a 2-way hard wood pallet.
XL GAPS conducted tests2,3 comparing commodities stored on
2-way hardwood and 4-way softwood pallets to determine if the 4-way
softwood pallets affected the commodity classification. These tests were
conducted under a heat release calorimeter. Six Class II commodity
tests were conducted, three of which used 2-way hardwood pallets and
three used 4-way softwood pallets. A delivered water density of 0.11
gpm/ft2 (4.5 L/min/m2), 0.21 gpm/ft2 (8.6 L/min/m2), and 0.31 gpm/ft2 (12.6 L/min/m2) in accordance with "SP Report 1993: 70 Commodity Classification - A more Objective and Applicable Methodology" was used.4 This test method ranks a product in accordance to its commodity classification index. Table 1 shows the commodity ranking.
Standard Commodity Used As Reference
Glass jars in compartmented cartons
≥ 1.0 and < 2.0
Double tri-wall cartons with steel liner
≥ 2.0 and < 3.0
Paper jars in compartmented cartons
≥ 3.0 and < 4.0
Polystyrene and paper jars in compartmented cartons
Cartoned Group B Unexpanded Plastic
≥ 4.0 and < 5.0
Cartoned Group A Unexpanded Plastic
≥ 5.0 and < 6.0
Polystyrene jars in compartmented cartons
Cartoned Group A Expanded Plastic
≥ 6.0 and < 7.0
Based on SP Report 1993: 70, Commodity Classification – A More Objective and Applicable Methodology4
The results of these tests ranked the Class II commodity on a 2-way
hardwood pallet as 1.7 and the Class II commodity on a 4-way softwood
pallet as 2.1. These tests show there is a difference in commodity
classification between the pallet types.
In that same test series, tests on the storage of idle pallets was
conducted. Six tests were conducted on 2-way hardwood and 4-way softwood
pallets stacked to 8 ft (2.4 m) and 12 ft (3.6 m) high with a sprinkler
density of 0.3 gpm/ft2 (12.2 L/min/m2) and 0.6 gpm/ft2 (24.4 L/min/m2)
using a K-8 (115) sprinkler. The tests on the 4-way softwood and 2-way
hardwood pallets at the same height and sprinkler density highlighted
some issues between these pallets. The 4-way softwood pallet stack
burned to all ends of the array while the 2-way just burned to the open
end of the array. The 4-way softwood test was terminated early after 40
sprinklers operated and the fire was still growing. The maximum heat
flux was 21 kW/m2 (24 in. (609 mm) above the floor, 8 ft (2.4
m) from the array) at the time of termination. The 2-way hardwood test
went the full thirty-minute test time; only 8 sprinklers operated and
the maximum heat flux observed was 12 kW/m2.
Peter Wilse is with XL GAP Services
NFPA 13, Standard for the Installation of Sprinkler Systems, National Fire Protection Association, Quincy, MA, 2010.
Report #03CA45183, Underwriters Laboratories, Northbrook, IL, 2006.
Report #04CA56618, Underwriters Laboratories, Northbrook, IL, 2006.
Persson, H. "Commodity Classification - A more Objective and
Applicable Methodology," SP Report 1993: 70 Swedish National Testing and
Research Institute, Boras, Sweden, 1993.
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