Selection of materials for heat exchangers
Heat exchangers are commonly used in various processes such as chemical, fertilizer, petrochemical, fossil and nuclear power production, refrigeration, desalination and other. They are always of special importance and their significance has increased with increasing energy demands. It is essential to make the maximum use of energy resources by using economical and efficient heat exchangers. Needs for heat exchangers for example better efficiency, stringent environmental needs and cost effectiveness demand reliable materials particularly for heat exchanger tubing and hence there has been a regular search for enhanced materials.
Heat exchangers are a key role of problem in several applications causing poor plant performance. For steam generators used in nuclear power plants common tube failure rate is 0.24% per year on alone plugging rate and 0.4% to 0.5% per year following plugging and sleeving. Utilities that have replaced steam generators were usually plugging tubes at a rate much higher than average rate.
Standard factors for materials selection
An engineer responsible for materials selection should know the aspects of construction, operation and maintenance of heat exchanger. An engineer should consider the regular service parameters, startup and shutdown, upset conditions and special conditions such as product purity, risk of combining shell and tube side fluids, radioactivity and related maintenance and others.
While following the strategy for deciding candidate materials, the main and considerable factors are cost and reliability. It means using economical materials, resolving problems when they occur and choosing the most reliable material irrespective of the price. Main factors that influence material selection are:
- High heat transfer coefficient
- Low coefficient of thermal expansion and fit with the materials used in tubesheet, tube support and shell to resist thermal cycling.
- Good tensile and creep characteristics
- Good fatigue and corrosion fatigue and creep-fatigue behavior
- High fatigue toughness and impact strength to prevent fast cracking
- Nominal corrosion rate to decrease the attack
- Resistance to corrosion from off normal composition caused by leak in upstream heat exchanger or failure in the chemistry control
- Withstand chemistry developed by mixing shell and tube fluids
A nuclear plant consists of more than 100 types of heat exchangers among which the steam generators are of crucial importance. These are connected with long history of tube cracking or failure and resulted in forced outages.
Steam generator tubing material for Nuclear plants
Considering the types of damage, steam generator tubing causes problems that are rarely observed in an industrial context. The recommended materials of construction for steam generator tubing are:
- Heat treated Inconel 600 wire
- Incoloy 800
- Monel 400
- Stainless steel 304
- Stainless steel 316/316L
Major tube defects are caused by:
Intergranular stress corrosion cracking, pitting, thinning, denting, tube fretting and wear, fatigue cracking and erosion-corrosion.
It is evident from application experience on fast reactors that steam generators are the key to successful commercial operation of nuclear reactors. So the selection of right materials for the heat exchanger tube is of utmost importance.