From the very early days of the steam locomotive, the quality of water used has been of prime concern.
No water is perfect. Water obtained from lakes, rivers and reservoirs may contain organic matter while water taken from boreholes is usually very hard. Hardness, caused by calcium and magnesium salts is a major problem in a locomotive boiler as the previously soluble salts become insoluble as the boiler temperatures and pressures increase. Some of these salts fall on hot surfaces where they bake and form a scale while other salts sink to the bottom of the boiler as sludge. Since scale has a low heat conductivity and hampers the efficiency of the boiler, it can lead to the tubes in the boiler to overheat, resulting in buckling or in extreme cases collapse.
Another less common scale that can cause problems is silica, however, even carbon dioxide from surface water is undesirable since under the influence of heat, it gives off pure oxygen which causes pitting and corrosion.
Organic impurities could be removed by effective filtration and lineside water softening plants were used where the water was hard. In this instance, chemicals such as hydrated lime, sodium carbonate or sodium aluminates were added to the water. In British Railways Standard classes, these plants were replaced by water softening blocks that were added to the tender or tank water from perforated cylinders.
In recent years, preserved locomotives have had problems with excessive nitrates in the water supply. Acidic water is corrosive and reducing acidity by chemical means is quite simple, however some degree of acidity is essential to reduce the risk of foaming in the boiler, leading to priming when water is carried with the steam into the cylinders.
All steam locomotives needed a periodic boiler washout to remove sludge, where jets of pressurised hot or cold water are fed into plug holes in the side of the firebox. Due to the variation of water quality in Britain, boiler washouts were performed every 500 to 5000 miles and typically this work would be carried out every eight to ten days.