How to Select Corrosion-Resistant Materials for Seawater Desalination Systems?
When selecting materials for seawater transport piping, which operating conditions should be checked first?
Mark the raw seawater, reverse osmosis feed water, concentrate, and permeate along the process flow. Conventional filtration removes suspended solids, while dissolved salts remain in the feed water. Inside the membrane modules, part of the water passes through the membrane as permeate, while most of the salts remain on the concentrate side. When evaluating concentrate piping, use concentrate data; do not simply copy the chloride ion concentration at the intake.
It is also necessary to consider where water may remain. During operation, seawater in the main line flows continuously, but the branch piping for standby pumps may hold water for long periods; crevices can also readily form under flange gaskets and in areas covered by deposits. In addition to the maximum water temperature, clarify whether the branch piping can be drained after shutdown. If chlorination is used for disinfection, specify the injection point, frequency, and residual chlorine in that section; chloride ion concentration cannot be used in place of residual chlorine data.
The L in 316L stands for low carbon, which helps reduce the risk of intergranular corrosion after welding. It does not mean that the material can withstand raw seawater over the long term. The advantage of 2507 lies in its combination of strength and resistance to localized corrosion, making it suitable for comparison when piping is both exposed to seawater and required to withstand high pressure.
254 SMO and UNS S31254 refer to the same grade, a super austenitic stainless steel containing approximately 6% molybdenum. It is sometimes abbreviated as “31254” in lists, but formal inquiries should use the full designation UNS S31254. Writing only “6Mo” is also insufficient, because other alloys containing approximately 6% molybdenum may also fit this description.
If the drawings allow a choice between 2507 and 254 SMO, first determine which condition is more demanding. At the high-pressure pump outlet, compare the allowable stress at design temperature and the required wall thickness for the two materials; for flanges that still contain concentrate after shutdown, focus on evaluating crevice corrosion under the gasket. Once both options meet the requirements, compare the lead time and cost of the complete piping package.
How Should Materials Be Selected for Seawater Desalination Pumps and Evaporators?
If the pump outlet is marked 2507, should the pump casing also use the same steel?
If the high-pressure pump outlet spool is marked 2507, this requirement applies only to that section of piping. The suction side should be verified against its own design pressure; for the materials of the pump casing, impeller, and shaft, check the pump manufacturer's data sheets. The spool inquiry should also specify diameter, wall thickness, and the mating flange sealing face; otherwise, even if the material is correctly selected, the flanges may still not fit together.
The seawater reverse osmosis high-pressure piping at Singapore’s SingSpring Desalination Plant uses super duplex steel, while the next section, which handles water with lower salinity, uses conventional 300 series stainless steel. In the case, the part explicitly marked as 2507 is the energy recovery system. This distinction also applies when referring to material lists from other projects: first confirm which component the grade corresponds to, then determine whether it can be used as a reference.
Can the evaporator shell and heat exchanger tubes use the same grade?
There are engineering examples of multi-stage flash (MSF) evaporators using 2205 duplex stainless steel shells. Heat exchanger tubes separate two fluids; when selecting materials, separately verify what medium is in contact with the inside and outside of the tubes and the respective operating temperatures. The “2205” shown on the shell drawing alone cannot determine the grade of the heat exchanger tubes.
How Should Material Selection Results Be Written into the Procurement List?
Use line numbers to link water quality and design conditions to the drawings, then list item by item the grade, product standard, and delivery batch for straight pipe, elbows, and flanges. Through the integrated manufacturing base of MT ALLOY PIPING, MTSCO produces pipes, fittings, and flanges, and can also supply plates to match project requirements. If pump piping and evaporator plates arrive in separate batches, the manufacturing records and inspection documents can be checked against the heat number and product batch.
Frequently Asked Questions
If the seawater transport pressure is low, can 316L be used?
This cannot be determined by pressure alone. Even for ambient-temperature raw seawater, the chloride ion concentration, stagnation time, and joint crevices must be checked. Reducing the pressure in the pipe does not eliminate the risk of pitting corrosion and crevice corrosion of 316L in seawater.
Can the supplier use 2507 to replace 254 SMO specified on the drawing?
It cannot be directly substituted. The allowable stress at design temperature, corrosion resistance requirements, and welding procedure should be checked, and approval from the designer must be obtained. Even if both materials meet the corrosion resistance requirements, the wall thickness and connectors specified on the original drawing still need to be reviewed.
What inspection documents can MTSCO provide for seawater desalination projects?
Material traceability, dimensional inspection, and corresponding test records can be provided according to the order and product standards. If third-party witnessing specified by the customer is required, please list the inspection items in the inquiry.
To purchase seawater transport pipe or desalination pump piping, send the material list, pump interface drawing, and design conditions to MTSCO to verify materials, connections, and lead time.