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Does The Construction Of The Pallet Pose A Warehousing Issue?
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Issue 40: Does The Construction Of The Pallet Pose A Warehousing Issue?

By Peter J. Gore Willse P.E., FSFPE

Background

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.


Testing

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.

 

TABLE 1
Commodity Classification

 

Commodity Class Product Rank Standard Commodity Used As Reference
I < 1.0 Glass jars in compartmented cartons
II ≥ 1.0 and < 2.0 Double tri-wall cartons with steel liner
III ≥ 2.0 and < 3.0 Paper jars in compartmented cartons
IV ≥ 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  
Extra Hazard ≥ 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

  1. NFPA 13, Standard for the Installation of Sprinkler Systems, National Fire Protection Association, Quincy, MA, 2010.
  2. Report #03CA45183, Underwriters Laboratories, Northbrook, IL, 2006.
  3. Report #04CA56618, Underwriters Laboratories, Northbrook, IL, 2006.
  4. 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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