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. |
