Molten salt storage tanks
- Posted by: César Boró Martín
- Category: Static Equipment
The storage of energy is something convenient and even necessary in various industrial sectors. Energy can be stored in the form of electrical, mechanical or thermal energy. For this purpose, among others, devices such as batteries, flywheels or substances in a different phase than the one at ambient temperature are used. Regarding the last case, we will focus in molten salts, which are used in some thermosolar plants in order to store thermal energy that will allow to generate electricity when radiant energy from the sun is not available.
For the storage of mentioned molten salts, metallic storage tanks are utilized. These tanks have similarities with other tanks installed in other sectors but, at the same time, due to the nature of operation that they carry out during their useful life, there are many details that make these tanks very special and are discussed as follows.
First of all, it should be noted that these equipment normally operate with a variable loads’ regime, therefore, this fact has an important influence on the design stage. For this, the engineer involved in the design must have a deep knowledge of the process that will take place inside the tank throughout its whole life. On the other hand, it should be borne in mind that molten salts storage tanks work at very high temperatures that can reach several hundred Celsius degrees. This second fact implies that thermal expansions that will occur in the different parts of the tank must be considered, both in the design calculations of the equipment and during its service period. Likewise, the high temperatures must be withstood by existing materials, so they have to be properly selected.
Naturally, molten salts stored must not be reach a cooler temperature than their freezing point at any time, because of this would shut down the process and the tank would be severely damaged. Therefore, for this kind of tanks, the thermal insulation or any other method that allows to keep the salts in liquid phase in critical. In addition, ensuring a proper mix in the molten salts inside the tank will avoid the existence of thermal gradients in the tank materials, which could generate excessive thermal stresses. If it is not possible to find an applicable method to remove thermal stresses, these will have to be reduced as much as possible and, obviously, included in the calculation in a convenient way. Once the tank has been started up, it must be ensured that the acting temperature gradients do not exceed the allowable values considered during the design and calculation stage.
In relation to all mentioned above, and since the molten salts must always be at considerably high temperature, when molten salts are poured into the tank for the first time this needs to be at a similar temperature than the target for the salts. . To prevent that thermal stresses, generated when the salts contact tank materials, involve an excessively harmful effect, the tank must have at all times a temperature as similar to the salts as possible. Moreover, the temperature difference between the components of the tank should be as small as possible, which implies an operation that is not so easy to carry out considering the dimensions of the molten salts storage tanks that are in service worldwide nowadays.
It is necessary to point out that the substance stored inside these tanks has a density (at normal operation temperatures) considerably higher than water density and much higher than the density of petroleum products. This is a factor that should not be ignored when thicknesses and the configuration of some components of tank are being checked.