This is the second blog in a 5-part series about tanks. If you want to read the previous entry check here
Now that we have learned about existing tanks and why it’s important to take care of them, let’s learn a little bit more about what is involved in designing a tank. As discussed in the last blog, there are many forces that are acting on a storage tank, but the largest one by far is caused by hydrostatic forces, which are the weight of the contents of the tank acting on its walls and floor.
Although an involved process there is some guidance out there in the form of standards, most commonly API 650 and API 620 are used in the design and construction of storage tanks, and offer detailed guidance on some of the problems and difficulties one might face. This article will not be nearly as comprehensive as either of those two documents, however, might offer a good starting place as to some of the considerations that need to be taken into account when designing a tank.
Storage Tanks – Material Selection
Selecting the material to be used in tank design is going to depend on a few different factors, the first of which is the expected load, smaller tanks can be made out of fibreglass or different types of plastic these tanks don’t need to contain the massive forces that are held in by larger tanks and thus a weaker material is suitable for large tanks that carry hundreds of thousands of litres tank material is usually chosen between various types of steel and some form of concrete due to the large loads imposed.
Once we have considered our load we must consider our operating environment and conditions, our conditions can have a strong impact on what the best material is, for example in a high salinity environment stainless steel may be considered due to its ability to prevent corrosion on the other hand tanks designed for earthquake prone regions will likely tend towards steel as it’s more likely to flex than crack during earthquake activities.
Another key consideration for our tank material is what is being stored in it, corrosive materials might not be suitable for steel, or might require some form of rubber lining to cover the tank to ensure that excessive corrosion isn’t caused.
Storage Tanks – Designing the shape
Once the material is selected we can determine the shape of the tank, often this is dictated by practical requirements rather than anything that can be worked out with a calculator, meaning tanks are often sized based on where they’re going to be located; in large areas, it’s possible to how wide and flat tanks, whilst if space is a concern then a taller and thinner tank might be more suitable. Additionally, accessibility is also a concern, is the tank going to be built in an area where people are working, do these people need access, and how often will they need access. The choice of material will also have some impact on what can be achieved within a reasonable cost.
Part of this process will also be determining whether the tank will be covered or have an open top, in general, a covered tank will have greater strength as it will increase the stiffness of the walls, however in many cases due to access or some other operational requirement tanks will be required to have an open top
Storage Tanks – Designing for Strength
Designing storage tanks to be strong enough to hold appropriate loads is perhaps one of the easier parts of the tank design. An engineer will likely look at all the forces acting on the tank and all the forces that may be possible, such as those caused by extreme weather events, and determine what is the maximum load case that the tank may find itself experiencing.
Once the maximum load is determined they will look at the strength of the material the tank will be built from and then determine exactly what thickness the material needs to be to ensure the tank can maintain the load. It important that appropriate environmental conditions of the tank are taken into account, in hot areas or in processes where hot liquid is stored within the tank the material properties of both the tank structure and also the fluid within can change resulting in different results.
Any good design will include a factor of safety which is put in place to account for unknown factors such as variability in the strength of the material, variability in environmental conditions or other load factors.
Storage Tanks – Designing for Longevity
As discussed in our previous article, tanks can and do eventually fail if not cared for properly, but at the design stage we can do a lot to prevent this from happening early, the first thing is to ensure we have considered any cyclical factors that may cause excessive fatigue. For example, a tank has liquid being agitated inside experiences a cyclical load as the weight of the fluid moves throughout the tank, this can eventually cause fatigue to build up in the material that comprises the walls of the tank and eventual failure. One more thing to note is that cyclical loads are not always caused by movement, in any process that causes a tank to undergo numerous hot and cold cycles these cycles will also cause a build-up of fatigue in the material.
Storage Tanks – Inspection and maintenance
Many people think the tank design stops with the tank; however, this is not true. Under the current Western Australian legislation, and under good design practices it is critical for the designer to give some consideration to the inspection and maintenance requirements of the tank. The designer needs to outline exactly how the tank must be cared for, and how it can be used to ensure that it will not fail, we will discuss some best practices when it comes to inspection and maintenance in a future article, however know
