Hydrocarbon or mineral oils are
now much used for lubricating the cylinders of engines, and especially
of condensing engines, and that for two reasons--first, because they are
neutral bodies, which have no action on metals; and, second, that they
are not liable to deposit on the boilers, if they should happen to be
introduced with the condensed water so as to produce burning of the
ironwork over the flues.
Animal or vegetable oils or fats are composed of fatty acids in
combination with glycerine, and these, under the influence of
high-pressure steam, are decomposed or dissociated, the fatty acids
being liberated from the glycerine, leaving the former to act upon or
corrode the iron of the cylinder. But here their objectionable influence
does not end. They form with the iron hard, insoluble compounds called
iron soaps, which increase the friction between the cylinder and piston,
and in some cases gradually collect into the form of hard balls inside
the cylinder.
When the water is used over and over again a considerable proportion of
the fatty acids of the oils used for lubricating the piston is carried
over with the steam and is found in the condensed water which is
introduced into the boiler along with the water. Here it commences
action, which proves quite as injurious to the boiler as it does to the
cylinder, but in a different way.
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