A pressure vessel is ideally designed to hold liquids and gases at a much higher pressure than the normal atmospheric pressure. Industrial pressure vessels can be extremely large and pose the threat of fatal accidents. Therefore, the design of pressure vessels is strongly regulated around the world, especially in high-risk industries. Corrosion can be a real problem in industrial pressure vessels; affecting its performance and compromising the strength of its outer shell, which can pose a health and safety risk.
A pressure vessel can be subject to corrosion and erosion from without, arising from environmental conditions, or from within, due to the fluids used, as well as operating temperatures and pressures. Both inner and outer surfaces of a pressure vessel may need treatment to guard against corrosion and erosion.
In this article, we look at some of the available design methods to prevent corrosion and erosion in commercial applications.
INERT CONSTRUCTION MATERIALS
High quality pressure vessels use improved materials that decrease the chance and rate of corrosion by a noticeable margin. The use of stainless steel is common to improve the anti-corrosion performance of a pressure vessel. However, it may not always be feasible or affordable to use a corrosion resistant material, especially where small margins may mean the difference between a successful or unprofitable business venture.
Some cheaper vessels therefore use materials that are more susceptible to corrosion than others. In a lot of cases this is a false economy, increasing the risk associated with the vessel and increasing your maintenance costs. However, when choosing a material, consider the level of corrosion the vessel will be exposed to. This will depend on the environment and whether the vessel is sited indoors or outdoors, as well as the process fluids used. Your vessel is at a greater risk from corrosion when acidic gases or fluids are present, especially at high temperatures and pressures.
Cathodic protection is a method of erosion prevention that adds a more reactive material to the surface of the pressure vessel. This is a cheaper option than using a completely inert material to construct the outer shell. The protection barrier corrodes in place of the vessel material itself, avoiding damage to the vessel, but necessitating replacement every few months to offer continuous protection. Iron vessels are frequently treated with hot zinc (galvanisation) for this purpose.
A coating is any layer that is placed over the metal surface of the pressure vessel to protect it from the surrounding environment. There are several different options available for this protection system. Below, we discussed some of the most common coatings that you can apply at the manufacturing stage to protect your industrial pressure vessel:
- CERAMIC COATINGS
The most commonly used coatings to protect steel tanks and containers are fashioned from ceramic. These coatings are excellent at protecting metallic surfaces from corrosive fluids, and so are used on the inside of a pressure vessels that utilise acidic fluids or gases. A ceramic coating insulates the steel structure with an alkaline surface layer. This slows down the rate of corrosion, with the level of protection depending on the presence of free acidic radicals. Ceramic coatings are inorganic and extremely non-reactive, making them perfect for use in a variety of conditions. A ceramic coating will need to be checked and replaced occasionally as part of routine maintenance.
- INHIBITOR COATINGS
Inhibitors are compounds that chemically reduce the reactiveness of the surface of the pressure vessel. Zinc oxide excellent for this purpose. It is ideal for use with large iron vessels as it provides a strong cathodic protection. This inhibitor creates a hermetic seal that ensures the substances present in the vessel cannot reach the iron surface.
- ORGANIC COATINGS
Recently, a range of organic coatings have become available, especially for aluminium-based pressure vessels. Aluminium is chemically different from other commonly used metals and therefore, needs protection from water, free ions and oxygen. If water persistently remains present on the external layer of the pressure vessel, it may create a microscopic blister, which increases the rate of corrosion in aluminium. This poses a problem for pressure vessels used in outdoor environments, due to the effects of rain, sea spray and snow.
An organic coating is perfect in this regard, because the natural reactivity of aluminium renders other coatings – such as zinc oxide – useless.
In extreme environments where unusually high or low pressures are used, or when a highly corrosive process fluid is used, it may not be enough to just use a coating. You may need to invest in a high-quality inert construction material, such as stainless steel, as well as one or more cathodic protection layers.