The Tribological Behavior of Tube Supports in Helium Heat Exchangers

A closed-cycle gas turbine plant for high-temperature gas-cooled reactor application contains a number of heat exchangers for low to medium temperatures (up to 500°C). The tubes of these heat exchangers are held in multiple spacers. Since thermal expansions lead to relative motions between tubes and spacers, a predictable tribological behavior of this sliding pair is important. The search for a low-cost treatment to achieve good tribological properties has led to manganese phosphate conversion layers on ferritic steel. Laboratory tests in helium with representative tube/spacer configurations have shown very small wear rates, complete absence of scuffing, and moderate and predictable coefficients of friction. Mechanical testing of phosphated tube material did not reveal any detrimental influence from the phosphate treatment. This work was performed under the joint German/Swiss development agreement for the helium high-temperature turbine project, HHT.