Twisted duplex 22% Cr tubes installed in a Crude Unit overhead condenser have failed prematurely
due to corrosion; the service lifetime was only 18 months. The failure mechanism has been chloride
induced under deposit corrosion, exacerbated by the presence of elemental sulfur. Since the 22% Cr
duplex grade was not corrosion resistant enough hyper duplex UNS S32707 has been chosen as the
replacement grade. Qualification of UNS S32707 shows that it is possible to twist the tubes in the same
way as the lower alloyed 22% Cr grade. Thus the design of the heat exchanger and the fabrication
procedure will not have to be altered. The hyper duplex grades mechanical properties and corrosion
resistance have been shown to not be altered by the cold working procedure.
Keywords: Hyper duplex, duplex, twisted tube, overhead condenser, under deposit corrosion