Pressure vessels serve various purposes in the industrial world. However, a pressure vessel is only as reliable as the type of material it’s made from. That’s why industrialists must choose the right materials from the outset, considering the factors that will determine what those materials will be.
Pressure vessels constructed with poorly selected materials are prone to failure. These failures, also known as failure modes, can assume different forms, including overload, buckling, and fracture. External conditions and forces, such as structural fatigue, corrosion, and creep may overwhelm the vessel’s material and weaken it. This may lead to a total failure of the pressure vessel.
A failure of the tank compromises its performance, leading to costly repairs or replacement and production delays. But the most concerning consequence of pressure vessel failure is an elevated safety risk, which can turn dangerous, even deadly.
An example of a deadly pressure vessel failure is the tragic industrial accident at the Loy-Lange Box Company in St.Louis, Missouri, on April 3, 2017. Staff members ignored a leak in the bottom of a pressure vessel, which led to a massive explosion that launched the tank 150 meters. Four people died, one working at Loy-Lange, and the other at a neighboring facility.
Negligence is no doubt a major contributor to this accident. However, a detailed report published on July 29th, 2022, revealed extensive oxygen pitting and general corrosion weakened the vessel’s semi-closed receiver (SCR), which was made of steel. The vessel failed, triggering a boiling liquid expanding vapor explosion (BLEVE) that turned it into a projectile.
Had the Loy-Lange staff chosen a more corrosion-resistant metal, could they have averted this tragedy? Possibly. This accident highlights the importance of choosing the most suitable materials for a pressure vessel (as well as the importance of thorough maintenance).
There are three key factors to consider when deciding what metal is most appropriate for a pressure vessel. They include both internal factors, such as the purpose of the vessel, as well as external ones that may increase or decrease the tank’s performance.
The location of the pressure vessel and its surrounding environment have a direct impact on its structural integrity. Ambient temperatures, vapors and liquids, flammable particles, and other hazards can compromise seals and valves made with the wrong materials.
One of the contributing factors to the Loy-Lange explosion mentioned in the report was the introduction of oxygenated water into the SCR daily. The issue wasn’t the water itself but rather, the failure to remove oxygen from the water, the oxygen being corrosive.
This is an example of why industrialists need to assess their working environment and identify threats before installing pressure vessels. Here at Wattco, we consider your environment and forces or elements that may pose a threat to your pressure vessel. These considerations allow us to recommend the most suitable metal for the construction of your vessel, among other recommendations.
The load strength of a pressure vessel simply refers to a material’s potential capacity to withstand loads placed upon it without failure or damage. There are various types of loads (or loadings).
They include transverse loads, which refer to perpendicular forces applied to a longitudinal axis, or torsional loads, which refer to twisting action caused by equally applied and oppositely directed external forces.
Also, there are different types of stress. That includes compressional stress, which refers to a squeezing of the material, which is best resisted by a material with high compressional strength. There is also tensile stress which refers to an elongation of the material, best resisted by a material with high tensile strength.
The right material for a pressure vessel, in terms of load strength, will depend on the type of loads and stresses the tank has to withstand. Also, certain materials are more likely to conform to certain shapes, which may be required for different processes. Naturally, this correlates with the industrial application for which the pressure vessel must serve.
Here at Wattco, we recommend pressure vessel materials based on the loading and stress factors inherent to your industrial application. This ensures that your pressure vessel is structurally sound enough to fulfill your production needs without compromising performance or safety.
Pressure vessels also have temperature and pressure limits. There is maximum allowable working pressure (MAWP), the maximum pressure the vessel can handle at its weakest point at a given temperature during standard operation.
There is also maximum design temperature, the maximum temperature a metal can sustain at a given MAWP. Excesses in either pressure or temperature are detrimental to the vessel’s performance, not to mention workers’ safety.
One of the failures cited by the Loy-Lange report was water pressure and temperature exceeding the limits of the vessel’s SCR. The risk of boiling liquid explosions rises substantially when the tank stores liquids at temperatures and pressure levels beyond its capacity.
For this reason, it’s important to calculate your application’s needed pressure and temperature and build a pressure vessel to facilitate these levels. Ideally, the tank should withstand slightly more heat (or cold) and pressure than the application needs to provide a buffer zone. We factor in your application’s MAWP and design temperatures when building your pressure vessel. That ensures your chosen materials can withstand and perform in the conditions they’re subject to.
Pressure vessels come in a wide range of materials, many of which are metals or alloys. That said, it’s important to clarify that no metal is necessarily superior to another. The best choice depends on the pressure and temperature in the vessel and the application it’s supporting. Nevertheless, some materials are often used because they have a reputation for performing in a certain manner.
Selecting the material for pressure vessels boils down to finding a happy medium, where they’re able to provide maximum performance and safety for your application.
Of course, finding a balance of pressure vessel performance and safety requires thorough and meticulous calculation. You may need to adjust the grade of your materials, especially if your application presents unique performance and safety challenges.
The materials should also demonstrate some versatility to withstand more than your operation demands. With that said, the shape and location of your pressure vessel will also contribute to its performance and safety.
It’s better to frontload these design and structural considerations rather than to address them later. Accidents such as the tragic explosion at the Loy-Lange are examples of what happens when fundamental factors are ignored. Addressing the factors in the pre-construction phase can greatly reduce performance and safety hazards later.
Here at Wattco, we design custom pressure vessels for various industries and applications. We consider all aspects of performance and safety, to ensure the materials we recommend are the most suitable ones for your production needs.
Get a quote for your pressure vessel today. Our representatives will help you find the exact setup needed for your industrial needs.