Because it’s a recent refinement of the tried and tested MSF process, the materials selection process for LT-MED remains a largely unwritten chapter.
|
Until today, the most commonly used material for bigger plants has
been solid stainless steel 1.4404, although the highly alloyed 254 SMO
has been a popular choice in Saudi Arabia.
|
 |
Feed water piping
Experience has shown that 254 SMO and
SAF 2507 are suitable for piping, including
sealing surfaces in connection with flanges.
Feed water heater
254 SMO and SAF 2507 are good alternative
for tubes and tube sheets. |
 |
Evaporators
The cylindrical shape of the evaporator shells lends itself well to cost-effective design if you employ high-strength duplex. The use of strong, corrosion resistant grades like e.g 2205 will reduce the thickness required and also eliminate the need for external painting and maintenance of the shells. An even more cost-effective idea would be to apply the DualDuplexTM concept and limit the use of 2205 to the lower part only (where conditions are hostile) and employ LDX 2101 or 2304 for the upper part of the evaporators.
|
MED evaporator shell made of 254 SMO.
(Courtesy of Aqua-Chem ICD).
|
Cell condensers
For tubes and tube sheets, the best choices are 2205, SAF 2507, 254 SMO and 4565.
Vent systems
Even though stainless steel 1.4404 is commonly used for vent piping, this grade has suffered severe pitting and stress corrosion cracking in several plants. Higher alloyed grades like 2205, SAF 2507
or 254 SMO are far more reliable. |
 |
Fresh water handling
Use either 1.4307 or 1.4404 depending on the chloride content of the water and the desired safety margins, see material selection chart. Duplex LDX 2101 and 2304 are cost-effective alternatives to both these grades if the design calls for strength, as is the case for large water storage tanks. The external environment must also be considered and may occasionally play an important role in materials selection. |
 |