When it comes to storage tanks, size poses a unique challenge. Typically, smaller storage tanks are prefabricated and shipped to the location. However, larger tanks with a capacity of hundreds of thousands of gallons or more require a different approach. With limitation on road transport due to width and height restrictions, these tanks are assembled on-site using prefabricated sections, referred to as Field-Erected Tanks.
Field Erected Tanks and Their Usage Across Different Industries
The oil and gas industry are perhaps the most visible users of field-erected tanks. Any image of a refinery will include a tank farm consisting of dozens of large, multi-million-gallon tanks storing various hydrocarbons. However, they are not the only industry to utilize these structures. Chemical plants also have field-erected tanks on their campuses, to contain the amounts of components and finished products needed to meet demand on an industrial scale. While not as numerous, even the food and beverage industry have a need for these multi-story, high volume storage tanks. Large fermentation tanks mark the exterior of the largest breweries, and major food processors use them to store large quantities of various ingredients including vegetable oil and water.
Advantages of Field-Erected Tanks
The size of a field-erected tank depends on its location and its use. This flexibility is an advantage in that it can be customized to a very specific need. Because of the previously mentioned transportation restrictions, the overall dimensions of the tank aren’t limited by infrastructure concerns in that regard. Rather, it comes down to the amount of land set aside for the tank along with the number of pieces that will need to be fabricated and transported to build it. With a properly sited location, the tank owner can source the most cost-effective options for prefabrication from nearly anywhere.
Field-erected tanks offer a long-term advantage over their in-ground counterparts. Because they are above the ground, they are easier to maintain and inspect. Corrosion or other storage problems can be identified and remediated in a timely manner. This can prevent serious environmental spills and contaminations. For this reason, many states are prohibiting construction of new in-ground tanks and even requiring their replacement with above-ground storage.
Preparing Field-Erected Tanks: Customized Approach
Preventing a tank from failing begins at the construction phase. While these large tanks can be constructed from plastic or a composite material, the majority are constructed of carbon steel or stainless steel. The choice of material is primarily driven by what will be stored. But the fact remains that the surface – or at least the welds – must be properly treated before commissioning to maximize the service life of the tank as well as preventing the weld sites from becoming the source of corrosion or failure in the first place.
Surface Treatment Options: Pickling, Electrolytic Cleaning, & Passivation
Because each tank is unique in structure, size and purpose, there is no one-size-fits-all approach. Tanks that will hold corrosive material will need to be treated differently than those that will store neutral contents. The physical size of a tank will also determine how it is prepared. The sheer amount of surface area may make treating the entirety cost prohibitive. In such cases, only the welds are pickled and passivated to remove the heavy oxide film that results from welding. In some cases, this involves applying a chemical and passing an electric current through the electrolyte and the metal to remove the free iron and weld color, also known as Electrolytic Cleaning. But, sometimes it is more cost effective to perform a pickling gel onto each weld and the surrounding heat-affected area. On more than one job the welds on one section of a tank are being treated even as other sections are being welded to the structure.
In other cases, there is no other choice but to clean the entire surface. A tank destined to hold liquid oxygen must be thoroughly cleaned to remove any chance of contamination. Even a technician entering the tank runs the risk of adding foreign debris to the interior.
Because of the variety of situations and conditions, it pays to bring in the experts from Astro Pak early in the planning stages. The institutional knowledge developed over the past six decades makes it highly likely that many unforeseen issues can be prevented before they occur. Furthermore, they can assist in planning the treatment program that will ensure that the tank is ready for commissioning on-schedule.
Maintaining and Servicing Field-Erected Tanks
Like other, smaller steel and stainless steel systems, tanks operating in certain conditions benefit from regularly scheduled passivation and cleaning as part of their ongoing maintenance. While a tank for storing crude oil may not need cleaning, a tank used for fermentation or storage of ingredients will absolutely require an ongoing schedule of servicing. The Food Safety Modernization Act of 2011 sets standards that all food and beverage companies must comply with.
Rouge Formation and Preventative Measures:
Modern tanks constructed to hold ingredients or final products are typically built with an integral Clean-in-Place (CIP) system to remove any leftover contents between batches. But even with such systems, more thorough treatments are recommended. The appearance of rouge or rust – typically orange-red in color – indicates that somewhere within the system, a steel part has failed or lost its resistance to corrosion. The source must be located, and the problem addressed. If the rouge has spread through the system, it requires a higher level of cleaning than can be self-performed. Failing to address the problem can lead to failure of the system or contamination of the product. An expensive option either way.
Proactive Approach in Maintaining Field-Erected Tanks
As we have noted elsewhere on this blog, stainless steel is not rust-proof, but instead highly rust-resistant thanks to its passive layer. While naturally occurring, it can be strengthened through the passivation process. This reduces the chance of rouge formation, and aids in increasing the life of the physical structure of the system as well.
Additionally, should any changes to the tank be made over its service life, the affected areas should be treated – through pickling, electrolytic cleaning, and passivation – to ensure that they do not serve as a weak spot and a potential source of failure.
Being proactive in maintaining the tank is a case where an ounce of protection is worth several thousand gallons of cure.
Protecting the Investment and Maximizing the Return
Because of their ability to be custom designed for a specific operation, location, and purpose, along with the relative ease of construction, field-erected tanks are likely to remain a common solution to the bulk storage of liquids required for a variety of industrial-scale uses. Since they constitute a considerable investment and are integral for the operational plans of a facility, steps should be taken during the construction phase as well as during the operational lifespan of the structure, to ensure the maximum availability and longevity.
By properly preparing the welds and interior surfaces of the tank, the chance of a failure due to a construction defect or a material weakness can be reduced. This not only helps protect against the possibility of costly environmental issues, or product loss, but it protects the investment in the tank itself as well as maximizing the return on its investment.