Steel tanks are essential structures used for storing various liquids such as water, petroleum, diesel, and kerosene oil. These tanks are typically made from steel plates, which offer durability and strength, ensuring that they can safely store a range of fluids under different conditions. Steel tanks are primarily categorized into two types based on their support structure: ground-mounted and elevated tanks. Both types serve crucial roles in various industries, from municipal water storage to fuel storage and industrial applications.
Types of Steel Tanks
1. Surface Tanks
Surface steel tanks are designed with a vertical cylindrical surface, a flat bottom, and ground support. These tanks are often placed on the ground and supported by either cement concrete foundations or steel grillage foundations. The flat bottom of the tank provides a stable base, and the cylindrical shape is ideal for distributing internal pressure evenly. Surface tanks are commonly used for water storage in urban and rural areas, where the liquid is required to be stored at ground level without the need for elevated pressure.
2. Elevated Steel Tanks
Elevated steel tanks are positioned on towers, allowing them to maintain a required pressure head for distributing the stored liquid. These tanks are particularly useful in pumping stations and water supply systems, where pressure is necessary to ensure the flow of liquid over long distances or to higher elevations. By being elevated, these tanks use gravity to create the necessary pressure, reducing the need for mechanical pumps in some systems.
Design Standards and Codes
The design of steel tanks is governed by specific standards to ensure their safety, reliability, and efficiency. The primary code of practice for designing steel tanks is IS: 805-1981, which provides detailed guidelines for the construction and material specifications for gravity water tanks. According to these guidelines:
- The minimum thickness of steel plates used in tank construction is 6 mm, except for the roof, which may have different requirements based on design considerations.
- When a tank stores liquids containing salts (such as saltwater or chemicals), the thickness of the steel plates should be increased by 1.5 mm to account for corrosion, ensuring the tank remains durable over time.
By adhering to these standards, engineers can design steel tanks that can withstand the physical and environmental stresses placed on them over their operational life.
Rectangular Steel Tanks
Rectangular steel tanks are commonly used for a range of storage purposes, particularly in industrial settings. These tanks are constructed from steel plates with flat bottoms. The width of the steel plates typically ranges from 1.20 m to 1.30 m, depending on the availability of materials. These plates must have a minimum thickness of 6 mm to ensure the tank’s strength and durability.
In rectangular tanks, the bottom plates are arranged transversely, which helps to distribute the weight of the stored liquid more evenly. The ends of these plates are turned up, and tee sections are placed on the inner side of the tank to form butt-joints with the side plates. This arrangement ensures that the joints are secure and capable of withstanding the internal pressure exerted by the stored liquid.
Elevated Circular Steel Tanks
Elevated circular steel tanks are a specialized type of storage tank, typically used for higher-pressure applications or where space limitations demand a vertical structure. The design of these tanks ensures their structural integrity under the pressures of both the liquid they store and the forces of gravity.
The plates used for the cylindrical shell of these tanks are specially shaped to suit the curvature of the tank. The plates are arranged in overlapping courses, both inside and outside the tank, ensuring that the joints are tightly sealed and able to withstand pressure. The minimum thickness of the plates in the suspended bottom is typically designed to match the thickness of the lowest course of the cylindrical part of the tank to maintain uniform strength and durability.
Additionally, the edges of the plates are sheared or planed to create a bevel that facilitates proper caulking, ensuring that there are no leaks or weak spots where the plates meet. This attention to detail in construction is essential for ensuring the long-term performance of the tank under varying operational conditions.
Pressed Steel Tanks
Pressed steel tanks have become increasingly popular due to their ease of construction and maintenance. These tanks are prefabricated, meaning the components are manufactured off-site and then assembled on-site. Pressed steel tanks are valued for their ability to be easily transported, erected, dismantled, and re-erected as needed. This makes them particularly suitable for temporary storage solutions or applications where the tank may need to be relocated.
The modular nature of pressed steel tanks allows for standardized construction, which reduces costs and time spent on installation. Their design flexibility and relatively low maintenance requirements have made them a preferred option in industries requiring large-scale liquid storage.
Permissible Stresses in Tank Design
To ensure the structural reliability of steel tanks, designers must adhere to permissible stress limits outlined in IS 800-1984. These guidelines specify the maximum allowable stresses that can be exerted on the steel plates and joints during the tank’s operation. Adhering to these stress limits is crucial for preventing the failure of the tank due to excessive internal pressure, external loads, or environmental factors.
The guidelines in IS 800-1984 are used to determine the appropriate material selection, plate thickness, and joint design to ensure the tank can safely store its contents throughout its service life. These standards help engineers design tanks that can withstand the forces of internal pressure, wind, seismic activity, and temperature changes without compromising their structural integrity.
Conclusion
Steel tanks are vital components in many industries, providing reliable and safe storage for liquids ranging from water to petroleum products. The design and construction of these tanks are governed by strict standards to ensure their strength, safety, and durability. Whether they are ground-mounted surface tanks, elevated tanks, or pressed steel tanks, the design considerations such as material thickness, joint construction, and stress limits are all carefully planned to ensure long-term functionality.