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Selecting Feedwater Heater Tube Materials for Greatest Efficiency and Reliability

In today’s competitive marketplace, it is imperative to minimize the heat rate, and maximize reliability and MW production. Copper alloy tubing in power plant systems create significant restrictions while attempting to do this. Copper will slowly dissolve into the condensate and eventually plate in other parts of the system. When it coats the boiler tubes, it can create a lower melting alloy that can cause premature boiler failure; this is called liquid metal embrittlement. In many plant designs, the copper preferentially coats the HP steam turbine blades which results in significant derates. Additionally, copper alloys are more sensitive to multiple pitting mechanisms and tube leaks are responsible for many unplanned forced outages to plug those tubes.

Modern condensate chemistry control to prevent FAC of the carbon and alloy steel components relies on tight oxygen control and higher pH than traditional values. The chemical additives used to control this cause accelerated corrosion and dissolution of the copper tubing. Although much of the copper is supplied by the condenser, feedwater heater’s higher temperatures accelerate this corrosion.

Today, austenitic and ferritic stainless steels dominate use new and replacement feedwater heater tubing for coal fired power plants. ASTM/ASME specification requirements are not sufficient to ensure the reliability needed for an efficient stainless steel heater. In addition to the standard eddy current (ECT) and pressure testing, the user should consider specifying more sensitive ECT testing, ultrasonic testing, maximum residual stress levels, cold working requirements, and limiting delta ferrite in the austenitic alloys for in-service testing. This paper discusses the failure mechanisms, lists the advantages and limitations of each alloy alternatives, and details the requirements needed for new FWH tubing.

Author: Dan Janikowski

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