The standards used to design low pressure, welded storage containers are specifically intended to address the stresses and strains placed upon them by liquids. This means that depending upon which bulk solid is to be stored, the container’s usefulness will vary; bulk solids come in a variety of states, from grains, powders and pellets to wood and metal shavings, and even waxes and warm (malleable) plastics. Not every container intended for liquids will necessarily be capable of doubling as a bulk solids container.
Let’s assume that a welded, low pressure container was made specifically to carry water. The density of water is roughly 1g/cm³. Now let’s assume that you wanted to convert that container to hold titanium dioxide powder, which has density of 4.23g/cm³. Materials of differing density will exert different degrees of pressure upon the structure of the containers they’re housed in. In the case of titanium dioxide powder versus water, the difference in pressure is 4.23 times.
On paper, these differences in density may seem negligible. What is of greater concern is the manner in which stored bulk solids behave versus liquids. Fluids will only exert pressure perpendicular to a container’s walls, but solids will also exert pressure parallel to them. This behavior in solids creates a form of compressive stress that is never designed for in the creation of fluid containment vessels. When the compressive stress exerted by a solid on the walls of a container designed to store a fluid exceeds that container’s ability to resist such forces, compressive buckling will occur, and the container will fail.
Another important consideration in choosing to modify a liquid tank for bulk solid storage has to do with how the stored material is discharged for use. The discharge point on a liquid container can be placed perpendicular to the vessel’s walls, and often is, because as the liquid flows out there is no change in the distribution of pressure against the container’s walls. The same is not true of the discharge dynamics of stored solids. There is a corresponding change in pressure; a lowering of pressure at the flow outlet and a simultaneous increase in pressure about the interior circumference. These dynamics will weaken a container designed to store fluid. In fact, the cracking caused by such forces can lead to catastrophic failure.
Stroud, England’s STB Engineering offers materials handling solutions that may negate any need to take unnecessary and potentially costly risks. STB manufactures innovative products, including silos, vessels and conveyor systems.