November 1, 2008

Acceptable alternatives to titanium tubing

Haydée Richaud-Minier, Gérard Pascal, Hervé Marchebois, and Dennis Schume

For the past 40 years, titanium has been the material of choice for condenser tubes exposed to sea and brackish water applications. That said, rising raw material costs make it important to reconcile engineering desires with budget limits in new plant designs. As an alternative, you might consider several highly alloyed stainless steels that approach the corrosion resistance of titanium at a modest premium over the cost of more common stainless steel options.

A typical nuclear plant might have six condenser sections, each with perhaps as many as 15,000 1-inch-diameter tubes about 50 feet long. That’s over 140 miles of tubes (Figure 1). Rising commodity prices will push purchasers to review alternative materials rather than automatically selecting titanium as the condenser tube material for the new generation of nuclear plants.

1.    Tubular exchanger. A typical tubular bundle for a large thermal or nuclear power plant uses thousands of tubes. Courtesy: Alstom Power Turbomachines

Depending on the quality of the circulating water, many tube material options can be selected for the steam surface condenser. Titanium may be the material of choice for seawater, brackish water, and tertiary-treated municipal wastewater effluent. However, less-expensive super alloy alternatives can be considered when conditions permit. Table 1 compares the mechanical properties of titanium with three less-expensive super alloy alternatives now available in the market. These alloys are UNS S31254 superaustenitic alloy and UNS S44735 and UNS S44660 superferritic alloys. Their perhaps more familiar ASTM designations are included in Table 1.

Table 1.    Typical chemical composition of titanium and three super stainless steels that were tested. Constituents are in percent. Source: ASTM