Tank design

Author: Design of tank structures

Tank design

Oil and oil products are most commonly stored in cylindrical steel tanks at atmospheric pressure or at low pressure. The tanks are flat bottomed and are provided with a roof which is of conical or domed shape. Water is also sometimes stored in cylindrical steel tanks. When used to store potable water they are of a size suitable to act as a service reservoir for a local community; they have a roof to prevent contamination of the water. Cylindrical tanks are also used in sewage treatment works for settlement and holding tanks; they are usually without a roof. The sizes of cylindrical tanks range from a modest 3m diameter up to about 100m diameter, and up to 25m in height. They consist of three principal structural elements - bottom, shell and roof.

Tanks designed for storage at nominally atmospheric pressure must be suitable for modest internal vacuum (negative pressure). Tanks may also be designed to work at relatively small positive internal pressures. Non-refrigerated tanks are designed for a minimum metal temperature which is based on the lowest ambient air temperature (typically, ambient plus 10°C) or the lowest temperature of the contents, whichever is the lower. No maximum service temperature is normally specified.

Tank loads

A minimum superimposed load of 1,2 kN/m2 (over the horizontal projected area) is applied to the roof of the tank. This load is commonly known as the 'snow load', but in fact represents, as well as a nominal snow load, any other imposed loads, such as maintenance equipment, which might be applied to the roof, and it includes the internal vacuum load. It is therefore applicable even in locations where snow is not experienced. Non-pressure tanks are often fitted with valves which do not open until the vacuum reaches a value of 2,5 mbar, to contain vapour losses. By the time a valve is fully open, a vacuum of 5 mbar (0,5 kN/m2) may have developed. Even without valves a tank should be designed for a vacuum of 5 mbar, to cater for differential pressure under wind loads. In pressure tanks the valves may be set to 6 mbar vacuum, in which case a pressure difference of 8,5 mbar (0,85 kN/ m2) may develop.

Wind loads are determined on the basis of a design wind speed. Maximum wind speed depends on the area in which the tank is to be built; typically a value of 45 m/s is taken as the design wind speed, representing the maximum 3-second gust speed which is exceeded, on average, only once every 50 years. Wind loads can be calculated according to Eurocode 1. In some areas, a tank must be designed to withstand seismic loads. Earthquake resistance can be calculated according to Eurocode 8.

Tank design

Vertical cylinder tanks carry the hydrostatic pressures by simple hoop tension. No circumferential stiffening is needed for this action. The circumferential tension in the shell will vary directly, in a vertical direction, according to the head of fluid at any given level. For practical reasons, it is necessary to build up the shell from a number of fairly small rectangular pieces of plate, butt welded together. Each piece will be cylindrically curved and it is convenient to build up the shell in a number of rings, or courses, one on top of the other. This technique provides, at least for deeper tanks, a convenient opportunity to use thicker plates in the lower rings and thinner plates in the upper rings.
The lowest course of plates is fully welded to the bottom plate of the tank providing radial restraint to the bottom edge of the plate. Similarly, the bottom edge of any course which sits on top of a thicker course is somewhat restrained because the thicker plate is stiffer.