Low-pressure auxiliary and exhaust gas boilers on ships need water treatment to prevent scale and corrosion. Many treatment programmes control boiler water alkalinity, and caustic soda is one of the chemicals used to do that. It is also used for boil-out of new or repaired boilers. This guide explains its role and the precautions involved.
Why boiler water needs alkalinity
- Corrosion control: steel corrodes much faster in acidic or neutral water at boiler temperatures. Keeping the water alkaline protects the steel surfaces.
- Scale control: alkalinity helps keep hardness salts in a form that can be conditioned and removed by blowdown, rather than forming hard scale.
Where caustic soda fits
Many ships use proprietary combined boiler treatments that already contain alkali. Where a separate alkalinity booster is used, caustic soda solution may be dosed to keep alkalinity within the range set by the treatment supplier. The engine team tests boiler water regularly, typically for alkalinity, phosphate or other reserve chemicals, chloride and condensate quality, and adjusts dosing.
Too much alkalinity can cause foaming, carry-over into steam, and in extreme cases caustic attack of boiler steel. Always stay within the limits given in your treatment programme.
Boil-out of new or repaired boilers
After construction or major repairs, boilers contain oil, grease and mill scale. A boil-out with an alkaline solution, often containing caustic soda and other alkalis, removes these deposits before the boiler goes into service. Follow the boiler maker's or treatment supplier's procedure, then drain, flush and refill with treated water.
Handling on board
- Prepare solutions in a dosing tank by adding caustic slowly to water, never the reverse.
- Wear goggles with a face shield, gloves and protective clothing.
- Store caustic in the chemical locker, secured and away from acids.
- Keep the SDS on board and eyewash near the dosing point.
Supply for vessels in Oman
Ship managers with vessels calling in Oman can source caustic soda flakes 99% from Muscat Chemical, with ready stock in Barka and next-day delivery within Oman.