New Trends Emerge in Thermal Desalination Evaporator Materials

Abstract

Lessons Learned In the last half of the 20th century, the main evaporator body for an MSF was made of carbon steel of varying thickness, depending on the evaporator component. The general trend was to first establish the evaporator structure’s required thickness and add a corrosion allowance based on an assumption of general uniform corrosion to meet the evaporator’s expected life span. The highest corrosion allowance for an evaporator component was for areas subject to severe corrosion, such as the stage division wall and flashing brine gates deflector, which can reach 25 mm. Because carbon steel corrosion was never uniform, an evaporator was plagued by frequent shutdowns to patch leaks in the evaporator body or water boxes. The immediate impact was higher maintenance costs and production loss, eventually lowering evaporator availability. Bare carbon steel can’t withstand aerated seawater or the high salinity of flashing brine. Operating experience proved that bare carbon steel wasn’t the correct material for an evaporator shell and internal components. The selected material must exhibit high corrosion resistance to aerated seawater, hot brine recycle, and concentrated brine. Water boxes were protected by 3 mm 90/10 copper–nickel (Cu–Ni). The evaporator floor was lined with 3 mm 317L (3 percent molybdenum), which exhibits excellent resistance to pitting corrosion. The evaporator’s side walls, from stage floor to ceiling, were lined with 316L. The ceiling was protected by 304L, and the distillate trough, division walls, flashing brine gates, and deflectors were replaced by solid 316L. After using austenitic stainless steel to control corrosion of an evaporator’s internal components, it was extremely important to reduce dissolved oxygen in the evaporator, reduce dissolved oxygen in the feedwater to less than 20 ppb, and inject sodium sulfite in the brine recycle to scavenge the remaining dissolved oxygen to nil ppm. This important step controls corrosion of the evaporator’s internal components and condenser tubes. By selecting the proper materials, it was possible to reduce evaporator operating costs, increase availability to 93 percent, and achieve sustainable production. Because the price of nickel increased dramatically, reverting to lean duplex stainless steel made evaporator capital costs more competitive. A new generation of MSF and MED evaporators replaced austenitic stainless steel 304L, 316L, and 317L with lean duplex stainless steel UNS 32202, UNS 32304, and UNS 32205, respectively. Lean duplex stainless steel is characterized by low-nickel, highchromium content, which improves stress corrosion properties and provides greater pitting resistance than austenitic stainless steel. To better understand the importance of proper material selection, consider an MSF evaporator’s primary components.