Many of this country's fluid or liquid products are transported from one location to another in bulk. The preferred method of transporting such fluids is in a large tank hauled by a bulk carrier. The bulk carrier typically employs a motor-driven tractor or power unit that is coupled to a trailer on which is mounted the tank. Tanks customarily used for this purpose have either a circular cross-sectional configuration or an ovoid cross-sectional configuration, with the height being less than the width so that the exterior tank surface is convexly curved from the top to the bottom. The fill inlet, fill line attachment, valves, gauges, tank controls and other structures to which the tank operator or driver must have access in order to fill the tank with liquid are usually located on the topmost surface of the tank. This topmost surface of a typical tank trailer may be well over twenty feet from the ground. Consequently, some type of structure must be provided to allow the tank operator to have access to the top of the tank to fill the tank and to adjust or repair the controls when required. A tank container of this same general configuration designed either to be carried on a railroad car rather than by tractor-trailer or to be stationary poses the same kinds of problems regarding access to the fill inlet, fill line attachments, valves and similar structures.
Once the tank operator can reach the fill openings and equipment, filling the tank with the tens or hundreds of thousands gallons of the liquid or flowable commodity to be carried presents additional problems. Not only does the typical tank configuration fail to provide a flat surface on which the operator can stand, but the smooth metal exterior surface can be treacherous. These surfaces, which may be highly polished and almost mirror-like on tanks designed to carry milk and other potable and edible fluids, are slippery when dry. Even a small liquid spill can increase significantly the slipperiness of such a surface. Consequently, unless the tank operator is provided a safe, flat structure on which to stand while filling the tank or working on the controls, his safety could be seriously compromised. Since such structures are typically not provided, filling a trailer or railcar tank can be rather hazardous, and accidents are not uncommon.
In addition, filling operations are very often accompanied by some spillage of the tank contents, either from overfilling of the tank or as a result of leakage from the fill hose. In such instances, the tank operator may find these contents on his shoes or boots as well as on the tank exterior. His safety is further jeopardized by the lack of a nonskid work surface that will drain away spilled liquids. Ladders or other structures that provide access to the top of the tank which accumulate, rather than drain away, spilled liquid also pose safety hazards. When a spill has been particularly bad, moveover, any contact between the operator's wet footgear and a structure such as a ladder rung could be dangerous. In this situation, some alternate means for the operator to descend safely from the top of the tank to the ground would be highly desirable.
Structures currently available for providing access between the ground and the tops of large tank carriers and stationary tanks fall significantly short of these objectives. U.S. Pat. No. 4,371,056 to Anglade discloses one of these structures, an upright ladder mounted on the rear of a tank truck to provide access to a "catwalk" located along the top of the tank. While the ladder shown in this patent provides a means of ascent and descent between the tank top and the ground, its utility is limited to those tanks having either a flat catwalk to enable the operator to reach the fill controls and equipment as described by Anglade or fill controls and equipment that are located at the extreme end of the tank. However, even assuming that either or both of these conditions was met, the Anglade ladder structure does not provide an alternate safe means of descent apart from using the ladder rungs in the event spillage of the tank's contents onto the operator's boots occurs.
It is also known to secure a ladder structure at one or more locations along the side of the tank to provide access to the top of the tank. These ladder structures, however, follow the generally convex configuration of the tank exterior surface and require the tank operator to ascend and descend along this convexly curved surface. Descent, in particular, can be quite hazardous if attempted quickly or when the ladder rungs are slippery from spillage of the tank contents.
Access to the top of the tank may also be provided by a portable ladder that is not permanently securd to the tank, such as one of the types of ladders shown in U.S. Pat. No. 3,112,010 to Mihalik, 4,044,857 to Guerette and 4,063,616 to Gutierrez. These ladders, however, are somewhat bulky to store and may be inconvenient to use, since their inclined angle requires the base of the ladder to be located some distance from the tank. A major disadvantage of such ladders, however, is that they provide access to the top of the tank only when the tank carrier is in a location where the ladder can be stored, such as in a garage or railyard. If access to the top of the tank is required for any reason when the tank carrier is on the road or away from the ladder storage location, the operator cannot get to the top of the tank unless a substitute for the ladder can be obtained.
The prior art, therfore, fails to disclose a substantially vertical ladder or similar structure that can be permanently attached to the convex exterior surface of a tank to provide quick, safe and easy ascent and descent between the ground and the top of the tank under adverse weather and working conditions. The prior art further fails to disclose a safety ladder for attachment to the convex exterior surface of a tank wherein the uppermost step of the ladder forms a safe, flat work surface at the topmost surface of the tank and the remaining steps are specifically designed to stay substantially free from liquid and nonslippery in wet conditions.