Two desalination plants on the same stretch of coast, drawing the same seawater, can have almost unrelated material lists. The reason is that thermal and membrane plants fail differently: a thermal plant combines hot seawater with vapour and condensate, so the problem is temperature plus chloride, while a membrane plant runs cold at very high pressure, so the problem is chloride plus pressure plus the crevices that high pressure fittings create. This desalination piping material guide works through the process first and the water second. Ranges are underpipes and tubes.
Thermal Against Membrane
|
Thermal, MSF and MED |
Membrane, SWRO |
|
|---|---|---|
|
Governing combination |
Hot seawater plus chloride plus vapour and condensate |
Cold seawater plus very high pressure plus chloride |
|
Temperature |
Raised through the plant, which accelerates chloride attack |
Close to ambient |
|
Pressure |
Modest |
Very high on the feed side |
|
Characteristic failure |
Pitting and crevice attack accelerated by temperature, plus erosion in tube bundles |
Crevice corrosion at high pressure joints, and pitting at welds |
|
Dominant components |
Exchanger and evaporator tube bundles, large bore seawater headers |
High pressure feed piping, small bore connections, pressure vessel internals |
|
Typical tube materials |
Copper nickel 90/10 and 70/30,titanium in the hottest stages |
Not tube dominated |
|
Typical pipe materials |
Copper nickel,duplex, coated or lined carbon steel on large bore |
Temperature is the whole difference. Chloride attack accelerates sharply with temperature, which is why a thermal plant reaches for titanium and copper nickel in its hot stages while a membrane plant, running cold, can use high alloy stainless and duplex at pressures a thermal plant never sees.
Desalination Materials FAQ
Why do thermal and membrane plants need different materials?
Thermal plants combine hot seawater with vapour and condensate, so temperature accelerates chloride attack. Membrane plants run cold at very high pressure, so pressure and crevices govern instead.
Which stream is most often under-specified?
Brine reject. It is the most concentrated stream in the plant and it is frequently specified from the intake material, which was chosen for a much milder duty.
Does chlorination affect all materials the same way?
No. It raises the oxidising power of the water, which helps some alloys and hurts others, and copper alloys respond differently from stainless, so a single regime will not suit every line.
Why does commissioning matter for copper nickel?
The protective film forms over weeks in clean flowing seawater, so commissioning on stagnant or polluted water can establish a poor film that never fully recovers.
What does heat tint do to a duplex weld?
It marks a chromium depleted layer beneath the discoloration, so brushing off the colour leaves the depletion and the local pitting resistance is lost. Pickling and passivation address it.
Is a stagnant seawater line the same duty as a flowing one?
No. Copper alloys depend on flow to maintain their film, and deposits settle in still lines, so stagnant service should be identified separately in the line list.
Applying This to an Enquiry
Say which technology the plant uses and which zone each line sits in, then mark the normally stagnant lines. Those three pieces of information change the alloy on more lines than the seawater analysis does.
Quotations are available in USD, OMR, AED, SAR, QAR, KWD and BHD, on EXW, FOB, CFR or CIF terms, with EN 10204 3.1 certificates as standard and 3.2 on request. Lead time is confirmed at quotation.