A ship cannot operate if the distilled water for feedwater contains more than the maximum allowable amount of what?

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Keep feedwater purity high. Distilled water used for boiler feed must have very low dissolved solids. If the water contains more than the allowable amount of salts, the water’s electrical conductivity rises, which accelerates corrosion in pipes and boiler components and promotes scale formation on heat‑transfer surfaces. That combination reduces heat transfer efficiency, can lead to overheating, and risks boiler failure, so operation cannot continue safely. Salt is used as the general measure of dissolved solids in this context, since it encompasses the various ions that can cause those problems. Specific impurities like chlorides or calcium are components of that overall salt content, but the limit is set on the total dissolved salts.