Fiber-reinforced plastic (FRP) tanks are becoming increasingly popular due to the many advantages over tanks constructed from traditional materials such as steel and concrete. These advantages include a high strength-to-weight ratio, high resistance to corrosion, ease of installation, and low maintenance requirements. However, the design of FRP tanks requires careful consideration of several factors to ensure the structural integrity, durability, and safe operation. Below, we will discuss some of the key aspects of FRP tanks and the approaches March engineers use to analyze and design these structures.
Materials of Construction and Manufacturing Processes
The first step in designing a FRP tank is selecting the most appropriate composite materials for construction and their constituents. This process involves selecting the resin, reinforcement, reinforcement/matrix volume ratios, and manufacturing processes. Common materials for resins used to construct FRP tanks are polyester, vinyl ester, and epoxy. Polyester resins are very economical but aren’t suitable for high temperature applications. Vinyl ester resins are more expensive but offer superior chemical resistance compared to polyester resins. Epoxy resins are the most expensive but provide excellent mechanical properties and resistance to chemicals and high temperatures.
The materials commonly used to reinforce polymer matrix composites include glass fibers, carbon fibers, and aramid fibers. Glass fibers are the most widely used reinforcement due to the low cost and high strength. Carbon fibers are more expensive but provide superior mechanical properties compared to glass fibers. Aramid fibers offer excellent impact resistance, but their use is not common in FRP tank manufacturing (they are more commonly used in military and aerospace applications).
Two typical manufacturing processes used for manufacturing FRP tanks include hand lay-up and filament winding. Hand lay-up involves manually placing the reinforcement material into a mold and then applying the resin. Filament winding involves wrapping the reinforcement material around a rotating mandrel and then applying the resin.
Other Design Considerations
In addition to the materials for construction, the design of an FRP tank generally includes consideration and determination of the tank’s geometry, loading conditions, and operating environment.
Geometry
The geometry of a tank includes its shape, size, shell wall thickness, as well as the size and location of penetrations. The shape of the tank can be cylindrical, rectangular, or any other required shape, depending on the application. Cylindrical tanks are the most common shape. Rectangular tanks are also used but need additional reinforcement due to their flat walls and panels. The size of the tank depends on the application, process requirements, available space, and layout. Larger tanks are more difficult to manufacture and are often shipped to the site in pieces requiring site assembly. The wall thickness of the tank depends on the expected loads and material properties. A thicker wall provides greater strength but also adds weight and results in higher costs.
Loading Conditions
The loadings on a tank include static loads, dynamic loads, environmental, and thermal loads. Static loads include the weight of the tank, loadings from attached platforms, the static head of the liquid contents, the soil pressure, etc. Dynamic loads include those such as fluid agitation and impact. Environmental loads include wind, seismic, and snow. Temperature changes will result in thermal loading and stresses, and deformations in the tank’s structure. Finite Element Analysis (FEA) and three-dimensional modelling can be used to simulate the tank’s structural response when subjected to various loading scenarios, assess the tank’s structural integrity, and ensure that the structure remains fit for purpose and safe to operate. Figure 1 depicts the Finite Element Model (FEM) of a FRP tank developed at March Consulting Associates Inc.
Figure 1: Finite Element Model of a Composite Fiber-reinforced Plastic Tank Developed at March Consulting Associates Inc. (It shows failure indexes on the left and deflections on the right)
Operating Environment
The operating environment of a tank includes the type of liquid stored and the process parameters such as temperature and pressure. The type of liquid stored in the tank should be considered for determining the required resin and ensuring chemical compatibility with the tank. For example, tanks that store acids or alkalis require more chemical resistance than tanks that store water. The temperature of the liquid and the ambient temperature can also affect the tank’s behaviour. Temperature fluctuations can cause thermal expansion or contraction of the tank, which can lead to excessive deformation and cracking. Low temperatures can result in a brittle fracture of the tank. The pressure of the liquid stored in the tank also affects the behaviour. High-pressure applications require thicker walls and more reinforcement to ensure the tank’s structural integrity.
Conclusion
In conclusion, the design of FRP tanks requires careful consideration of several factors to ensure the structural integrity, durability, and safe operation. The selection of appropriate materials, determination of the tank’s geometry, and development of a detailed understanding of operating environment and loading conditions are critical for the tank’s design and in-service performance. Finite Element Modelling and experiments are often the best approaches for optimizing the tank’s design to ensure its safe operation under various loading conditions. With proper design and manufacturing techniques, FRP tanks can offer superior performance and longevity compared to traditional materials.
At March Consulting Associates Inc., our engineers have the knowledge and experience to assist with designing a wide variety of FRP tanks for municipal and heavy industrial applications. We have worked closely with FRP tank manufacturers and provided engineering services and certified construction drawings for several hundred tanks in North America. Contact us today to get started; we’re ready to help.
Contributing Authors: March Structural Team