1. Why desalination plant piping deserves a dedicated spec — not a re-used "marine" or "seawater" package

A seawater desalination plant is not one system. It is a stack of systems with very different pressure, temperature, chloride, fouling and biological regimes, all sharing the same seawater stream. The pipework that survives 30 years in a 70 bar RO high-pressure train is not the same pipework you want in a 4 m/s open intake. Treat the whole scope as "seawater piping" and you will over-spec two thirds of the plant and under-spec the third that actually drives the lifecycle cost.

A typical hybrid SWRO + MSF plant of 250,000 m³/day has, very roughly, the following material exposure profile (for one reference site; every project will differ):

• Intake screen to pump well — 1.5–4 m/s open seawater, screens, low-pressure FRP / 90/10 Cu-Ni.
• Low-pressure feed (1.5–4 bar) to break-tank — large-bore 90/10 Cu-Ni or GRP transition piece to duplex.
• Pre-treatment (MMF / UF) backwash and air-scour lines — uPVC-C / FRP / 316L depending on chemistry.
• High-pressure pump suction (4–8 bar) — typically 904L or 6Mo to handle residual chlorine, plus full vacuum for pump NPSH.
• RO pressure vessels (60–80 bar) — super-duplex 2507 or 6Mo high-pressure piping, energy-recovery device (ERD) tie-in, recycle loop.
• Permeate (low pressure, low TDS) — 316L or even carbon steel internally lined.
• Brine reject / energy recovery outlet (4–8 bar, high-TDS, high-Cl⁻) — 6Mo / super-duplex 2507 / titanium Grade 2 / FRP for tail sections.
• MSF or MED recycle, distillate and cooling — 90/10 Cu-Ni tubes, 316L / Cu-Ni shell-side, titanium for severe TBT-free sections.

Get the material map right at the engineering phase and the plant will run on planned maintenance. Get it wrong and you will be replacing elbows, flanges and ERD tie-in spools during the second monsoon. This guide walks through how a 2026 desalination EPC or plant owner should think about the major material bands, what standards to cite, and how to close the supply-chain loop through a mill-direct pipe fittings + flange + bolting package.

2. The four seawater exposure bands and what they ask of pipe, flange and fitting

Before we discuss alloys, it is worth separating seawater service into four engineering bands, because the corrosion driver, not the material name, is what picks the grade:

• Band 1 — Ambient, open, low-Cl⁻ biological risk. Intake piping, screening wash, low-pressure transfer to break-tank. < 1 m/s is bad (biofouling + silt); 1.5–2.5 m/s is good; > 3 m/s is a wear problem. 90/10 Cu-Ni (UNS C70600) is the workhorse, with FRP (vinyl-ester, AWWA M45) for the largest diameters above DN 1200.
• Band 2 — Pressurized, moderate temperature, biofouling + chlorination risk. Low-pressure feed, pre-treatment backwash, chlorinated seawater. Stainless 904L (N08904), 6Mo (S31254) or super-duplex 2507 (S32750) for piping under continuous 0.5–1 ppm free chlorine residual; 90/10 Cu-Ni tolerates 0.5 ppm residual but accelerates under continuous chlorination.
• Band 3 — High pressure, 60–80 bar, hot or cold, continuous service. RO feed, inter-stage, energy-recovery tie-in. Super-duplex 2507 is the default for SWRO; 6Mo (S31254 / 254 SMO) is the alternative for chlorinated or warm feed (> 35 °C). 904L is the lower bound; below this you will see pitting in the first 18 months.
• Band 4 — High TDS brine reject, energy-recovery outlet, distillate / cooling. 6Mo, super-duplex 2507, titanium Grade 2, or FRP for tail sections, depending on temperature. Avoid 316L in any continuous brine above 50 °C.

3. Why copper nickel alloy still owns the cold seawater side

It is easy to over-reach to super-duplex for the entire plant, but 90/10 Cu-Ni (UNS C70600, EEMUA 145 / 234) is still the most cost-effective alloy for low-pressure cold seawater. It releases copper ions that suppress macrofouling, it tolerates a moderate level of sulfide and ammonia, and it can be field-fabricated with conventional Cu-Ni welding procedures. Our mill produces 90/10 and 70/30 Cu-Ni seamless tubes and pipes per EEMUA 144 / 145 / 234, ASTM B466, ASTM B111, BS 2871 and JIS H 3300 — the four standards that desalination EPCs actually ask for.

Two practical cautions from our own project experience with EPCs in the Middle East and the Red Sea:

• Stay above 1 m/s, stay below 3.5 m/s. Below 1 m/s, silt settles, biofilm grows and under-deposit corrosion attacks the tube wall. Above 3.5 m/s, single-phase erosion / impingement starts. For 90/10 Cu-Ni, the design sweet spot is 1.5–2.5 m/s.
• Limit continuous free chlorine to 0.5 ppm. Short shock chlorination (1–2 ppm for 30 min) is fine; 0.5–1 ppm continuous accelerates wall loss. Where chlorination is continuous, run 70/30 Cu-Ni or move to super-duplex 2507.

For the structural side of the same loop — vessel shells, channel covers, intake bell mouths — we supply matching copper nickel flanges to EEMUA 234 and ASME B16.5 Class 150 / 300, so the flange facing and the pipe chemistry are guaranteed to come from the same heat. This is the single biggest practical risk we see: mismatched Cu-Ni flanges shipped from a stockist on a different standard, with a different trace of iron and manganese, ending up as a galvanic cell in the field.

4. Why super-duplex 2507 is the default for SWRO high-pressure trains

For SWRO plants of 50,000 m³/day and above, the high-pressure section between the high-pressure pump and the energy-recovery device is the most corrosive line in the plant. Pressures run 60–80 bar, temperatures can hit 35–45 °C in hot regions, and the water is full of dissolved oxygen and 60,000–70,000 mg/L chloride. Under these conditions 316L fails by pitting in months, 904L lasts 3–5 years, 6Mo lasts 10–15 years, and super-duplex 2507 is the 25-year answer.

ASTM A789 / A790 UNS S32750 (and the cold-worked variant S32760) is the de facto specification. The wall thickness is typically Sch 20S / 40S for the main run, Sch 80S for high-velocity tie-ins to ERDs, and Sch 160 / XXS for the section between the throttle valve and the reject port. For our 2026 deliveries, we ship these out of Changsha as solution-annealed and quenched tubing, with a ferrite number verified at 30–55% in the delivered condition (per ASTM E562 or ASTM A1082) and a 100% PMI + hydrostatic test at 1.5 × design pressure.

A second, often forgotten alloy in this section is 6Mo super-austenitic (UNS S31254, 254 SMO). It is more tolerant of continuous chlorination than super-duplex, and it is easier to weld in the field with no risk of sigma-phase embrittlement in the wrong heat-input window. Where the high-pressure pump is also handling chlorinated feed (which is becoming common for biofouling control), 6Mo is the safer pick. Where chlorination is intermittent and the budget is tight, super-duplex 2507 is still the best material-per-dollar answer.

5. The bolted joint — why steel flanges + super-duplex tube is the failure point that nobody tracks

In SWRO high-pressure service, the flange and the bolting fail long before the pipe. The reasons are mechanical, not chemical:

• Flange facing mismatch. The tube is super-duplex, the flange is often supplied by a different vendor in F51 / F53, and the gasket (typically a graphite or PTFE spiral-wound with super-duplex inner ring) sits on top of three different hardness levels. Under bolt-up, the softest face creeps first.
• Bolt creep and chloride stress corrosion cracking. Standard B7 / 2H studs and nuts are fine for non-aggressive service but will fail by SCC in continuous wet-chloride conditions above 50 °C. For RO trains, B8M Class 1 (A193 B8M / A194 8M) studs and nuts in 6Mo or super-duplex are the safer specification.
• Galvanic mismatch. Carbon steel stud with super-duplex tube + super-duplex flange is acceptable when dry, but in continuous wet service the stud thread crevice becomes the anode. The "studs, nuts and washers in the same alloy family as the flange" rule is worth applying in any SWRO Class 600 / 900 joint.

We routinely supply pipe flanges in F51 / F53 / F55 (duplex / super-duplex / super-super-duplex) per ASME B16.5, B16.47 Series A and B, EN 1092-1, GOST 33259 and JIS B2220, with matching F53 stud bolts and nuts, spiral-wound gaskets with super-duplex inner and outer rings, and the MTC 3.1 / 3.2 certificates to back the whole stack. This is the "joint as a system" approach that we have found saves the EPC the most on commissioning punch-list items.

6. The standard stack to write into the datasheet

For SWRO high-pressure and brine-reject sections, the most robust datasheet stack we write for our EPC customers looks like the table below. Note that we have moved away from F51 duplex in this section in 2024; 2507 is the new floor for anything above DN 200 in continuous RO service.

All flange ratings are based on ASME B16.5 Class 150 (PN 20) unless otherwise stated. Stud and nut material follows ASME B16.5 Appendix G (preferred bolting materials) and ASTM A193 / A194. The 90/10 Cu-Ni figure is for ambient seawater below 25 °C and free Cl₂ below 0.5 ppm; above these limits, move to 70/30 Cu-Ni or 6Mo.

7. The MSF / MED case — when thermal desalination changes the alloy map

Multi-Stage Flash (MSF) and Multi-Effect Distillation (MED) plants are not pressure plants, they are thermal plants. The pipes see 70–110 °C top brine temperature, plus the same chloride load, plus a chemistry that often includes antiscalants, antifoam, and (in some regions) hydrazine. The corrosion map shifts accordingly:

• Brine recirculation (top temperature 90–110 °C). Titanium Grade 2 is the safe pick for high-TDS, high-temperature brine; super-duplex 2507 has a tight upper temperature limit (~ 70 °C in continuous service) and will fall off rapidly above that. 6Mo (S31254) extends to 80–90 °C but with rising pitting risk in oxygenated brine.
• Distillate (low TDS, 40–60 °C). 316L is fine. No need to over-spec.
• Heat-recovery section tubes. This is where 90/10 Cu-Ni tubes still win — they are easy to roll-expand into tube sheets, they tolerate antifoam, and they release copper ions that keep the shell side clean.
• Seawater cooling (inlet to reject). 90/10 Cu-Ni is the workhorse for tube-side cooling water. Cooling-water outlet (warmer side) sometimes gets upgraded to 70/30 or 6Mo for scale-sensitivity control.

A common pitfall in MSF / MED projects is the transition spool between the cold intake 90/10 Cu-Ni and the hot brine 6Mo / titanium. This is one of the most expensive items in the plant to field-fabricate, and the most common place where mismatched flanges and bolts get installed. We supply the transition spool as a single piece with the bimetallic weld already done in the shop, fully heat-treated, and 100% NDT tested, with the MTC covering both alloys in one document.

8. Stainless steel pipe for the auxiliary systems — chemical dosing, CIP, instrument air

Outside the seawater loop, the desalination plant has a sizeable stainless steel piping scope that EPCs sometimes underestimate: chemical dosing (HCl, NaOH, antiscalant, sodium metabisulfite for dechlorination), Clean-In-Place (CIP) loops, instrument air, and service water. For these, the rule of thumb is simple:

• 316L for general chemical dosing, instrument air and service water up to 60 °C.
• 904L (N08904) for hot concentrated HCl (above 10%) and for hot sulfuric acid dosing lines.
• Alloy 20 (N08020) for concentrated sulfuric acid and CIP loops that see hot phosphoric / nitric blends.
• 6Mo (S31254) for seawater-side chemical injection quills that get continuous chloride exposure.

For this scope we typically supply ASTM A312 TP316L / TP304L welded and seamless pipe, EN 10216-5 and EN 10312 for the sanitary / hygienic side, plus ASTM A403 WP316L butt-weld fittings and ASME B16.5 F316L flanges. Material Test Certificates 3.1 per EN 10204 are standard, and 3.2 with TPI (BV, DNV, SGS, TÜV) is available on request for the project QA package.

9. Standards you should be citing in 2026

Desalination is one of the few areas where ASTM, EN, EEMUA, BS, JIS and DNVGL all show up in the same datasheet. For an EPC document package, the working stack we use is:

• Pipe / tube. ASTM A312 / A358 / A789 / A790, ASTM B466 / B111, EN 10216-5, EN 10217-7, EN 12451, EEMUA 144 / 145 / 234, BS 2871, JIS H 3300, JIS G3459, DNVGL-ST-F101 (offshore-bundled projects).
• Flanges. ASME B16.5 (up to NPS 24), ASME B16.47 Series A and B (larger), ASME B16.48 (spectacle blinds), EN 1092-1, GOST 33259, JIS B2220, EEMUA 234 (Cu-Ni).
• Fittings. ASME B16.9 (butt-weld), B16.11 (socket-weld and threaded), B16.28 (small-bore returns), EN 10253-2 / 10253-4 (European butt-weld), MSS SP-43 (stainless lightweight), MSS SP-97 (forged integral outlets).
• Gaskets, studs and nuts. ASME B16.20 (spiral-wound and RTJ), ASME B16.21 (non-metallic flat), EN 1514, EN 12560, ASME B18.2.1 / 2.2, ASTM A193, ASTM A194, EN 1515-1 / 1515-2, ISO 4014 / 4017 / 4032 / 4034.
• Process / plant. EN 12255-3 (process piping in desalination), ASME B31.3 (process), DNVGL-ST-F101 (offshore), NACE MR0175 (sour service, not normally a desalination concern but appears in the gas-treating section of hybrid plants).
• Quality. EN 10204 3.1 / 3.2, ISO 9001, ASME U2 / U3 (where required for pressure-containing parts).

A clean datasheet that hits every one of these is what allows a 2026 desalination RFQ to be answered with a mill-direct package instead of a stock-and-source package. The mill-direct package is typically 8–15% cheaper on FOB China basis and 4–8 weeks faster on delivery, because the mill, the fitting shop, the flange shop, and the gasket / stud shop are all integrated into a single shipping plan.

10. Bolted-joint engineering: facing, torque and re-tightening

Even with the right materials, the bolted joint will leak if the assembly is wrong. The two most common errors we see on desalination startup punch-lists are:

1. Wrong facing for the service. RF (raised face) is the default for Class 150 / 300, but for Class 600 and above, RTJ (ring-type joint) or large male-female (LMF) is the safer pick. For Cu-Ni flanged joints, RF with a soft gasket is conventional; for super-duplex Class 600 and above, RTJ is increasingly the default for new plants because it provides a metal-to-metal seal in the centre of the joint and is tolerant of gasket relaxation.
2. Wrong torque sequence. ASME PCC-1 remains the bible for cross-pattern multi-pass bolt-up. The mistake is to torque to 100% on the first pass; the right method is four passes (30 / 60 / 90 / 100%) in a star pattern, with a final round of circumferential verification after 24 hours. In super-duplex 2507 flanged joints, this is also the only way to keep the residual gasket stress where the gasket manufacturer expects it.

Our flange datasheets always include the recommended bolt-up sequence, the target torque value (derived from the stud size, the nut factor K and the gasket stress), and the re-torque schedule (typically 24 h and 7 days after first hydrostatic test). This is the kind of practical detail that turns a 2% leak-rate at commissioning into a 0.2% leak-rate, and it is built into every flange + stud + gasket bundle we ship.

11. Welding and fabrication in the field

Welding super-duplex 2507 in the field is a different discipline from welding 316L. The two failure modes that show up in the first year are:

• Sigma-phase embrittlement from too-high heat input. Super-duplex tubes need a controlled heat input in the 0.5–1.5 kJ/mm range and a fast cooling cycle. If a field welder parks the joint to "soak" the puddle, the ferrite / austenite balance shifts, and the HAZ loses corrosion resistance and impact toughness. The fix is a written WPS, an interpass temperature limit (typically 150 °C), and a calibrated heat-input monitor on the welding rectifier.
• Pickup of carbon-steel contamination. Carbon-steel dust on a super-duplex tube, plus a humid site, equals rust staining that becomes a permanent corrosion nucleus. The fix is segregated tooling, dedicated stainless wire brushes, and a white-glove visual at handover.

Our mill is qualified to ASME IX and EN ISO 15614 for super-duplex, Cu-Ni, 6Mo and titanium welding, and we provide matching weld procedure specifications (WPS) and procedure qualification records (PQR) for the field crew. We also pre-weld the largest possible assemblies in the shop — 6 m spools, transition spools, ERD tie-in spools — so the field welding burden is reduced to a minimum. This cuts site man-hours, removes weather dependence, and gets the plant into water three to six weeks earlier.

12. Inspection, NDT and documentation

The minimum inspection stack for SWRO high-pressure super-duplex pipework is:

• 100% PMI (positive material identification) on every spool before delivery, with a digital record tied to the heat number.
• 100% hydrostatic test at 1.5 × design pressure (or per ASME B31.3, whichever is higher).
• 100% radiographic or phased-array UT on every shop circumferential weld, with 10% field-weld radiographic re-check.
• Ferrite number check on a sample basis (per ASTM A1082 or ASTM E562) on the delivered tubing and on the HAZ of shop welds.
• Intergranular corrosion test (ASTM A262 Practice B or E) on the HAZ of every shop weld, in 10% of heats.
• Surface roughness (Ra) verification on internal wetted surfaces for hygienic / drinking-water service (typically Ra ≤ 0.8 µm).

The MTC 3.1 / 3.2 package, the NDT reports, the ferrite maps and the dimensional reports are all compiled by heat number and shipped as a single PDF / DocCARE-style dossier with the material. For EPCs with a TPI requirement (BV, DNV, SGS, TÜV), our QA team coordinates the third-party inspection on the mill floor at the heat-loading, NDT and final inspection stages.

13. Sustainability, water-source energy and the coming regulatory push

Two trends are reshaping desalination plant specifications in 2026 and beyond, and both of them are starting to show up in the datasheet:

• Low-carbon concrete, low-carbon steel. A growing number of EPCs (especially in the Gulf, the EU and California) are starting to request Environmental Product Declarations (EPDs) and product carbon footprints on the steel and alloy side. Super-duplex 2507 has a higher embodied carbon than carbon steel, but its 25-year life makes the lifecycle number favourable. The way to handle this in 2026 is to ask the mill for an EPD on the steel-side products and to work with the EPC on a cradle-to-grave number for the alloy side.
• Brine discharge and titanium-grade selectivity. Some new permits are starting to limit thermal desalination brine temperatures, which forces the alloy map up the temperature curve. This is a structural shift towards titanium Grade 2 in MSF / MED top-temperature sections, and away from 6Mo / super-duplex. We can supply titanium Grade 2 pipe and tube to ASTM B861 / B862 from our Changsha mill on a project-specific lead time.

For projects in jurisdictions with EPD requirements, our mill can issue an EPD for carbon steel, low-alloy steel, stainless steel and copper-nickel products on request. The expected delivery impact is +2 weeks on the documentation side, but the impact on the EPC's permitting and ESG reporting side is significant.

14. The bundled sourcing model — one PO, one mill, one shipping plan

The single biggest practical lesson from 30+ years of supplying the desalination market is that the piping package should be sourced as one bundle, not as separate line items. A typical SWRO 100,000 m³/day plant has 60–80 line items of pipe / tube across 5–6 material bands, 200+ flange line items, 1,000+ fitting line items, and 5,000+ stud / nut / gasket line items. Sourcing these from 8–12 vendors (as is still common) creates five predictable problems:

1. Document fragmentation — 8–12 sets of MTCs, 8–12 sets of QA programs, 8–12 sets of NDT reports, all in different formats.
2. Material-mismatch risk on the bolted joint — flanges from one heat, studs from another, gasket from a third.
3. Logistics fragmentation — 8–12 separate shipping plans, 8–12 sets of export documents, 8–12 chances of a hold-up at the port.
4. Cost — every vendor has its own overhead and minimum-order-quantity, which inflates the unit price on the small-bore / oddball items.
5. Schedule risk — the longest vendor sets the schedule, and it is almost never the vendor with the largest line-item value.

Bundling the entire piping package into a single purchase order with a single mill-direct supplier solves all five. It also gives the EPC a single point of accountability for the MTC stack, the NDT stack, and the logistics stack.

15. What we ship from Changsha for a typical SWRO 100,000 m³/day package

The following list is a real, anonymized example of what an SWRO 100,000 m³/day mill-direct package from our factory looks like. It is not a hypothetical; it is a recent delivery pattern:

• ~ 3,200 m of super-duplex 2507 ASTM A789 / A790 S32750 seamless tube, Sch 20S / 40S / 80S, DN 50 to DN 300, for the high-pressure train.
• ~ 4,500 m of 90/10 Cu-Ni ASTM B466 / B111 C70600 seamless pipe, Sch 10S / 40S, DN 100 to DN 900, for the intake, low-pressure feed and brine-out sections.
• ~ 1,200 m of 6Mo ASTM A312 S31254 welded and seamless pipe, Sch 10S / 40S, DN 25 to DN 200, for chlorinated feed and chemical injection.
• ~ 2,800 m of 316L ASTM A312 TP316L welded pipe, Sch 10S / 40S, DN 25 to DN 600, for chemical dosing, CIP, service water and instrument air.
• ~ 950 F53 / F55 super-duplex weld-neck flanges, Class 150 to Class 900, with RTJ and RF facings.
• ~ 1,200 90/10 Cu-Ni weld-neck and slip-on flanges, Class 150, RF facing.
• ~ 1,800 F316L and A105 weld-neck flanges, Class 150 to Class 300, RF facing.
• ~ 3,000 butt-weld fittings (ASME B16.9) in super-duplex, Cu-Ni, 6Mo and 316L, including 90° / 45° elbows, equal and reducing tees, concentric and eccentric reducers, and caps.
• ~ 1,200 socket-weld and threaded fittings (ASME B16.11) in 316L and A105, for the chemical dosing and instrument air sub-systems.
• ~ 3,500 spiral-wound and ring-joint gaskets with the matching inner / outer ring alloy for the section.
• ~ 12,000 stud bolts and nuts in B7 / 2H, B8 / 8, B8M / 8M and F53, with washers, all packaged in heat-numbered boxes.

The whole package ships in three to five 40 ft open-top and flat-rack containers, on a single bill of lading, with a single MTC dossier, and lands on site ready to lift. This is what we mean by "mill-direct bundled sourcing" — it is not a slogan, it is a specific operating model that has been refined over three decades.

16. RFQ checklist — what to send to the mill for a fast and accurate quote

The most common reason desalination RFQs go slow is missing data. To get a fast and accurate mill-direct quote, send the following with the inquiry:

1. Plant capacity (m³/day), process type (SWRO / MSF / MED / hybrid), and location (climate zone and water source).
2. Piping material specification with the standard stack (ASTM / EN / EEMUA / JIS / DNVGL) per service band.
3. Flange schedule with class, facing, and bolting material — or let the mill propose a stack based on the service.
4. Quantity by line item — pipe length by size and wall, flange count by size and class, fitting count by type and size, gasket / stud / nut count by size and material.
5. Project delivery window — including any phased delivery requirements (e.g., intake first, RO train second, brine reject third).
6. Quality / documentation requirements — MTC 3.1 or 3.2, TPI (which agency?), ASME U2 / U3 (yes / no), EPD requirement (yes / no), specific NDT scope.
7. Logistics — destination port, incoterm (FOB / CIF / DAP), and any unusual logistics (e.g., final delivery to a remote desert site over a specific road).

With these seven items, our engineering team can turn a full mill-direct package quote around in 5–7 working days, including the bolt-up torque schedule and the MTC dossier template. Without them, the conversation takes three rounds and the same calendar time, plus the risk that the package is mismatched to the project.

17. Common mistakes that an experienced mill-direct supplier can help you avoid

The most expensive lessons in the desalination piping market tend to repeat themselves project after project. The ones we have seen the most:

• Specifying F51 (2205) duplex for the high-pressure section, where 2507 is required for 25-year life. F51 has its place (e.g., lower-temperature pre-treatment and permeate-side piping), but not at 60–80 bar in continuous RO service. Saving 8% on the alloy up front and paying 100% on the retrofit two years in is not a good trade.
• Mixing Cu-Ni flanges from EEMUA 234 and JIS H 3300 stockists with Cu-Ni pipe from an EEMUA 145 producer. The flange and the pipe may both be "90/10 Cu-Ni" in the MTC, but the iron and manganese trace levels can be 0.5% and 0.7% respectively, which is enough to set up a galvanic couple in the field.
• Running 316L on the chlorinated feed because the rest of the plant is 316L. The chlorinated feed section is one place where 316L fails by pitting, and it fails fast. Use 904L or 6Mo.
• Specifying spiral-wound gaskets with carbon-steel inner / outer rings in a super-duplex flanged joint. The gasket is the cheapest item in the joint, but a carbon-steel ring in a super-duplex flange face will become a permanent corrosion site. Always match the gasket ring material to the flange face material.
• Forgetting the transition spool. The most expensive item in the entire seawater loop is the spool that connects cold 90/10 Cu-Ni intake to hot 6Mo / super-duplex RO. Specifying it as a "field weld" turns a 2-day shop job into a 2-week site job and adds a permanent quality risk. Specifying it as a shop-fabricated, pre-welded, post-weld heat-treated, 100% NDT-tested spool collapses the risk to near zero.

These five mistakes account for the majority of the warranty calls and field rework hours across the projects we have seen. None of them are exotic; they are the kind of small judgement calls that a mill-direct supplier with 30 years in this market can help you avoid before they become a punch-list item.

18. Closing: the right way to think about desalination piping in 2026

Desalination is a capital-intensive, long-life, chloride-heavy, energy-conscious infrastructure business. The right piping strategy is the one that picks the cheapest material that survives 25 years in each section, specifies the joint as a system, and ships the whole package on a single, mill-direct, MTC-backed purchase order.

For the cold seawater side, copper nickel alloy still wins on cost and biofouling. For the high-pressure RO side, super-duplex 2507 (or 6Mo where chlorination is continuous) is the 2026 default. For the auxiliary systems, 316L remains the floor. For the bolted joint, the gasket, the stud and the flange must come from the same alloy family as the tube they sit on. And for the whole package, a mill-direct bundle with a single MTC dossier, a single shipping plan and a single point of accountability is the simplest way to deliver a plant that starts up on time and runs for 25 years on planned maintenance.

That is the model we have built EZ STEEL INDUSTRIAL on since 1994 — 500+ employees, 480,000+ tonnes of annual output from Changsha, China, qualified to API, EN and ASME requirements, with an ISO 9001-accredited lab and ASME / AWS-certified welding procedures. Whether your project is a 5,000 m³/day SWRO for a small island community or a 500,000 m³/day hybrid plant for a Gulf coast, the same engineering discipline applies.