Sea Water Strainers: Corrosion and Biological Fouling in One Body

As a cooling medium, sea water is cheap and unlimited; the cost is paid elsewhere. The same water attacks equipment through two mechanisms: it consumes metal electrochemically and it closes flow area biologically. A sea water strainer is a body where both act at once, and selecting one means thinking about each separately.

Two different effects, one body

The corrosion side is a materials question. Salt water is a conductive electrolyte and feeds the electrochemical reaction on a metal surface continuously. In stagnant water and in oxygen-depleted crevices — under a gasket, where the basket rim touches the body — corrosion accelerates locally and goes deep.

The biological side is an accumulation question. Sea water carries shelled organisms, weed, larvae and organic matter in suspension. Some is caught by the basket; some attaches to internal surfaces and grows there. The second matters more, because a growing layer narrows the mesh from inside while the basket still looks clean from outside.

Body and basket material therefore answer both questions at once: withstanding the electrochemical action of salt water, and surviving the mechanical work of removing growth — scraping, brushing, washing — without damage.

Galvanic pairs: basket and body have to match

What gets overlooked most often is that there are two separate metal parts inside the strainer, touching each other inside the same electrolyte. Join two dissimilar metals in a conductive liquid and you have built a galvanic pair; one becomes the anode and is consumed at an accelerated rate.

In practice the consumed part is usually the basket, its surface area small compared with the body and its wall thin. The outcome is familiar: the body is sound while the basket has thinned at its edges and opened somewhere. An open basket is no longer a strainer — the line looks clean because solids no longer stop there at all. Where the fault lies becomes clear only when the heat exchanger is opened.

So when a basket is treated as a spare, it is not only the size that has to match but the material. Fitting whatever correctly-sized basket comes to hand is the quietest way to shorten a strainer’s life. The cover gasket belongs to the same decision: on a cover opened at every cleaning it is the part most likely to be re-used, and a flattened gasket draws air in on a suction line.

Differential pressure: a routine and a diagnosis

A strainer reports its blockage through the pressure difference across the basket. This is trend monitoring rather than measurement: if the difference grows at the same flow, accumulation is increasing. Its value is that cleaning can be planned before performance drops.

Neglected, the consequence does not stay in the strainer. A blocked strainer first reduces flow, and a heat exchanger’s rejection capacity falls with flow until cooling is inadequate. It then lowers pressure at the pump suction, which is more serious than lost efficiency — if suction pressure falls far enough the pump cavitates. An uncleaned strainer spoils the cooling first and the pump afterwards.

Straight or angle body

This choice is geometry rather than performance. A suction line leaving the sea chest for the pump usually turns on the way, and an angle-type strainer sits in that turn, removing the need for a separate elbow. Given that every elbow is one pressure loss and one more joint, the gain on the suction side is not trivial.

A straight body goes where the line continues straight. For both types the second criterion is the same: enough headroom to open the cover and lift the basket out vertically. A strainer placed correctly in theory but with no space above it becomes a dismantling job at the first cleaning — and after that job, cleaning intervals lengthen on their own.