A power plant chemist/engineer has many choices when selecting tubing materials for his condenser, feedwater heater or balance-of-plant application. The wide variety of alloy choices available (ASTM lists over 75 stainless steel alloys) gives him or her greater flexibility to choose the best candidate to meet budgetary constraints and still provide the performance needed for the lifetime of the plant. At this conference in 2012, we discussed the needs for the feedwater heater tubing. This paper is focused toward the needs for tubing primarily exposed to the cooling water circuits. This water can be quite aggressive. Upset conditions common in power generation combined with this can result in premature unexpected failure of tubing and piping materials. The upsets may include differences in operation modes from design, changes in water chemistry due to leaks in other parts of the system, corrosion from unexpected sources, impact of improper lay-up practices, and the effect of corrosion product transport to other parts of the system. The motivation to build modern combined-cycle, coal and nuclear power plants for the lowest cost per kilowatt has stretched the envelope for materials performance resulting in many tube failures.
This paper provides an overview on a number of factors known to cause failure of a tube material. Knowing the limitations of material is crucial when making a selection for a specific application. This paper helps to identify the factors that need to be considered when selecting a material. Properties compared in this paper include corrosion resistance, stress corrosion cracking potential, thermal and mechanical properties, erosion resistance, vibration potential, and temperature limitations. The property comparison guides are intended to be quick tools to assist the user in selecting a cost-effective material for a specific application. Additionally, the paper includes failure mechanisms which were relatively unknown 10 years ago but have become common today.
Author: Dan Janikowski