In the petroleum, chemical, and other bulk manufacturing industries, hazardous liquid and solid materials are often stored and shipped in large containers, such as Intermediate Bulk Containers (IBCs), IM 101s, and IM 102s (hereinafter referred to collectively as "bulk containers"). Because many of these materials are noxious or hazardous products, bulk containers must be constructed in accordance with certain regulations, many of which are established by international agreement. In the United States, the applicable regulations for the design and construction of bulk containers are promulgated by the U.S. Department of Transportation, and are embodied in Volume 49, Part 178 of the Code of Federal Regulations, which is incorporated herein by reference (see particularly 49 C.F.R. .sctn..sctn. 178.270-178.272-2 and 178.700).
Under the regulations, IBCs are defined as having a volume of not more than 3 cubic meters (793 gallons) and not less than 0.45 cubic meters (119 gallons) or a maximum net mass of not less than 400 kilograms (882 pounds). 37 C.F.R. .sctn. 178.700(c)(1). IMs are defined as portable tanks having a diameter no greater than 2438 mm (96 inches) that are designed to carry liquids having a vapor pressure of less than 2.97 bar-absolute (43 psia) at a temperature of 50.degree. C. 37 C.F.R. .sctn. 178.270-1(a). IMs must be cylindrical in shape, while IBCs are not required to be cylindrical. The construction and design of IMs is generally more strictly regulated than that of IBCs.
Bulk containers such as IBCs and IMs consist of a tank enclosed in a frame. The frame is designed such that the bulk containers can be stacked one-on-top-another. The frame is equipped with features which facilitate moving and accessing the tank. Such features include slots for lifting the container with a forklift, pad eyes for lifting the container with a crane, and ladders for accessing the top of the tank. The frame also provides some degree of protection to the tank.
To comply with applicable regulations, a bulk container, and particularly IBC containers, must pass certain tests, including vibration, hydrostatic, stacking, and drop tests. In order to pass these tests, a bulk container must be of substantial construction. One significant problem that arises with bulk containers of substantial construction is that is they have very heavy lids. The heavy lids of prior art bulk containers are difficult to open and can cause serious injury. The heavy lids of prior art bulk containers must either be opened by two people working together, or by one person working with the assistance of a crane. Because the people opening the lid are required to place their hands and arms near the lid and the opening, the lid can fall on the people opening the lid, resulting in crushed or even severed body parts. The prior art lids tend to close rapidly when dropped, which may not provide sufficient time to withdraw an arm, hand, or other body part from the tank opening before the lid closes. Additionally, the stress of opening a heavy lid can lead to injuries, such as muscle pulls or ruptured discs. The use of a crane to open bulk containers adds to the expense of using such containers, and can also cause injury.
Various types of counterweights have been used to assist in opening and closing heavy lids on containers. Many of the prior art counterweights mechanisms are directed to subterranean containers, such as the type which are placed below the surface of airport docking areas. One disadvantage of the prior art mechanisms for subterranean containers is that they occupy some of the internal volume of the tank, thereby decreasing the volume available for use. See U.S. Pat. No. 5,673,810 (Rothrock); U.S. Pat. No. 4,669,625 (Armstrong); U.S. Pat. No. 4,467,932 (Dabich). Other counterweight mechanisms rely upon relatively large and obtrusive external structures which would not work within the space limitations imposed by the frame of a bulk container. See U.S. Pat. No. 4,227,414 (Elkins); U.S. Pat. No. 2,424,715 (Shreve); U.S. Pat. No. 2,372,753 (Watson).
A major disadvantage of the prior art counterweights is that they are not designed to efficiently use the limited amount of space available in bulk containers. Because bulk containers are designed for storage and shipping, they must provide as much storage tank volume as possible within the space limitations imposed by the frame of the container. There is thus a need for a counterweight mechanism for opening and closing the heavy lid of a bulk container which efficiently uses the space available within the frame.