FRP Filter Housings: What Non-Corrosive Construction Actually Buys You
Stainless steel housings need a duplex or super duplex grade, or a rubber lining, to survive seawater without corroding. FRP housings sidestep the question, because the material itself does not corrode in the first place.
Why the conventional answer to seawater corrosion has a gap
Stainless steel duplex and super duplex grades resist seawater corrosion well, but at a higher cost and a higher weight. A rubber lining on a lower grade is a cheaper way to attempt the same protection, and it is not foolproof: seawater's abrasive effect can wear a pinhole through the lining over time, and once breached, corrosion begins underneath it, in a location that is difficult to inspect and where it can propagate before anyone notices.
What FRP changes
FRP housing compared with stainless steel for seawater duty
| Criteria | FRP | Stainless steel |
|---|---|---|
| Corrosion resistance in seawater | Inherent to the material | Requires duplex or super duplex grade, or a rubber lining |
| Weight | Lighter | Heavier |
| Impact and abrasion resistance | Good | Good |
| Failure mode under abrasion | No propagating corrosion path | A lining pinhole can propagate corrosion beneath it |
| Typical relative cost | Lower | Higher |
Fibreglass-reinforced plastic housing material is inherently non-corrosive, which removes the pinhole-and-propagation failure mode entirely rather than mitigating it with a coating. It is also lighter than stainless steel, which matters for transport and installation on a project moving multiple housings into place, and it retains good impact and abrasion resistance despite the lower weight.
Where FRP fits, and where it does not
FRP suits applications where the fluid is corrosive but the duty does not require the mechanical strength that only metal provides, seawater desalination, brine handling, and electroplating solutions among the common examples. Where mechanical strength genuinely is the deciding factor, a stainless steel housing in the correct grade remains the right specification. FRP is not a universal replacement for metal housings; it is the better-fitting material for a specific combination of corrosive chemistry and moderate mechanical demand.
What a well-specified FRP housing looks like
A standardised FRP housing built for this duty is designed to a recognised pressure vessel code, ASME Section X for FRP construction being the relevant one, rated for a defined flow rate, commonly around 100 cubic metres per hour per housing, with additional housings banked in parallel where a project needs higher total flow. Closure bolts are still specified in a corrosion-resistant metal, stainless steel super duplex, since the bolted closure is a mechanical connection point rather than a wetted surface the FRP body itself protects. Held to a recognised design code and available ex-stock rather than built to order, a standardised FRP housing removes both the corrosion question and the lead-time question from a seawater project at the same time, which matters on projects where a custom stainless steel housing would otherwise sit on a multi-week build schedule before it could even begin resisting the corrosion it was ordered to resist.
Direct answers
Why not just use a rubber-lined stainless steel housing for seawater?
A rubber lining is not a foolproof solution because the abrasive effect of seawater can create a pinhole in the lining, which then corrodes and lets that corrosion propagate under the lining where it is hard to inspect. FRP avoids the failure mode entirely, because the material itself is non-corrosive rather than protected by a coating that can be breached.
Is FRP suitable for anything other than seawater desalination?
Yes, it suits any application where the fluid is corrosive to metal but does not require the mechanical strength only metal provides. Brine and electroplating solutions are two other common examples.