For more than 40 years, Tnemec coating consultants have been addressing the questions and concerns of municipal architects, engineers, owners and contractors as they consider the various options open to them as they plan for the construction of new water storage tanks for their communities. Among the topics recently raised is whether an elevated welded steel tank or an elevated glass-coated bolted tank is better suited for a community’s potable water supply. In order to address this issue, Tnemec water tank specialists offer the following insights to some frequently asked questions.

What is the expected life cycle of welded steel elevated tanks compared to elevated glass-coated bolted water tanks?

High-performance coating manufacturers indicate that with proper maintenance, welded steel elevated tanks offer longer life cycles when compared to elevated glass-coated bolted tanks. Typically, welded steel tanks that are coated to industry standards offer 25 or more years of corrosion protection before any coating maintenance is required. The life cycle extends to 50 years or longer with just one coating maintenance cycle. The Steel Tank Institute/Steel Plate Fabricator’s Association has identified more than 20 historic welded steel water tanks that have been in continuous use for more than a century. The average lifespan of a glass-coated bolted water tank is approximately 40 years, according to the American Water Works Association (AWWA) D103-09 Factory-Coated Bolted Steel Tank for Water Storage, which is the standard for ground-level bolted tanks.

What can cause the life cycle of elevated glass-coated bolted water tanks to be shortened?

Every bolt used to assemble a bolted water tank represents a potential point of weakness. Bolted tanks can be constructed with 0.094-inch gauge steel compared to thicker 0.250-inch gauge steel for a welded steel tank with the same capacity, so the areas around the thousands of bolts are susceptible to corrosion and can be damaged if the bolts are overtightened during assembly. A urethane elastomeric coating gasket that is used to prevent corrosion of vertical and horizontal seams on bolted tanks requires maintenance and can be difficult and expensive to repair. The glass coating on bolted tanks is also susceptible to cracking when impacted by an object.

How do the construction standards for welded steel elevated tanks compare with elevated glass-coated bolted water tanks?

The American Water Works Association (AWWA) D100-05 Welded Steel Tanks for Water Storage is the established standard that provides guidance to facilitate the design, manufacture, and procurement of welded steel tanks. Sections of the standard cover material specifications, welding, erection, wind loads, stress and overload conditions that can be caused by hurricanes, high winds and seismic activity for both ground and elevated welded steel tanks. The AWWA D103-09 Factory-Coated Bolted Steel Tank for Water Storage was written for ground-level bolted tanks, but elevated glass-coated bolted tank construction is not included within the scope of that standard. This leaves the structural integrity of elevated bolted tanks to the discretion of the manufacturer.

Are welded steel elevated tanks and elevated bolted water tanks available in the same sizes?

Welded elevated steel tanks can be constructed up to 4.1 million gallons capacity and ground storage tanks up to 34 million gallons. Bolted tanks, which originally were designed for ground storage of dry agricultural products, are typically available in standard, prefabricated shapes and sizes that are determined by the manufacturer instead of the customer. The average maximum capacity of ground storage glass-coated bolted tanks is approximately 2 million gallons with much smaller tank sizes for elevated bolted tanks.

How do welded steel elevated tanks and elevated bolted water tanks compare in terms of their aesthetics?

Welded steel water tanks offer an ideal surface for today’s high-performance exterior coating systems that allow for unlimited color choices and boast outstanding color and gloss retention. Applying designs or logos on elevated bolted tanks can be challenging due to the presence of seams and thousands of exposed bolts. In addition, graphic alterations to prefabricated glass coated bolted tanks can result in added cost to the owner.

Since elevated glass coated bolted tanks are prefabricated, are they easier to assemble?

Elevated glass coated bolted tanks are made of smaller sections that are usually easier to erect than welded steel tanks. Although easier to assemble, bolted tanks can be difficult and expensive to repair due to the preparation required to replace the glass-coated panels.

What about the cost of construction for welded steel tanks vs. elevated bolted water tanks?

Welded steel tanks are initially more expensive to build than glass-coated bolted tanks, but the cost advantage disappears when life cycle comparisons, design flexibility, aesthetics and conformance to industry standards for quality, reliability and safety are considered. Today’s high performance coating systems have extended the life cycle of elevated welded steel tanks considerably over the average life span of elevated glass-coated bolted water storage tanks.