Why is boiler water treated on a ship and how?

Boiler water treatment gets less daily attention than it deserves, because its failures are slow and invisible until they are not: scale and corrosion both build up over weeks before showing up as reduced performance or, in the worst case, a tube failure that can take a boiler out of action or injure someone nearby.

Scale forms when dissolved hardness salts, mainly calcium and magnesium compounds, come out of solution as the water is heated and deposit as a hard layer on the heat transfer surfaces. Because scale conducts heat poorly compared to steel, it insulates the metal from the water that would otherwise carry heat away, so the metal beneath runs hotter than it should for the same firing rate, and if thick enough in a high-heat-flux area, the tube or shell can overheat past its safe design temperature and fail, which is why scale is treated as a safety issue and not just an efficiency one. Corrosion works differently but is just as destructive: dissolved oxygen carried in with feed water attacks steel directly, and either overly acidic or overly caustic water chemistry attacks the metal, with caustic conditions concentrated at welds and stressed points able to cause embrittlement or gouging.

Prevention starts with the feed water itself, ideally condensate returned from the steam system plus make-up water that has been demineralised to minimise impurities entering the boiler in the first place. Chemical dosing does the rest: oxygen scavengers, commonly sodium sulphite or a hydrazine-based compound, bind dissolved oxygen before it can attack the metal; alkalinity is controlled to keep the water within a defined pH band, high enough to prevent acidic attack but not so high as to risk caustic problems; and any residual hardness is converted into a soft, non-adherent sludge rather than baked onto the tubes as scale.

Routine testing of chlorides, alkalinity, pH and hardness is carried out on a regular schedule, with dosing adjusted to the results rather than left fixed, since feed water quality and steam demand both change. Blowdown, both surface blowdown to remove concentrated dissolved solids and bottom blowdown to clear settled sludge, is carried out regularly to keep the water within its specified limits and prevent carryover of solids and moisture into the steam, which would otherwise foul turbine blades or superheater tubes downstream. A boiler receiving consistent, correctly dosed treatment and regular blowdown runs for years without scale or corrosion problems; one that does not can develop serious damage within a surprisingly short time.