Well, we will say 5 pounds, then. You will admit
that the boiler is not cold by any means, even at only 5 pounds, and if
you know enough not to blow off at 50 pounds, you certainly know that at
5 pounds pressure the damage is not entirely avoided. As long as the
iron is hot, the dirt will dry out quickly, and by the time the boiler
is cold enough to force cold water through it safely, the mud is dry and
adheres closely to the iron. Some of the foreign matter will be blown
out, but you will find it a difficult matter to wash out what sticks to
the hot iron.
I am aware that some engineers claim that the boiler should be blown out
at about 5 pounds or I0 pounds pressure, but I believe in taking the
common sense view. They will advise you to blow out at a low pressure,
and then, as soon as the boiler is cool enough, to wash it thoroughly.
Now, if you must wait till the boiler is cool before washing, why not
let it cool with the water in it? Then, when you let the water out,
your work is easy, and the moment you begin to force water through it,
you will see the dirty water flowing out at the man or hand hole.
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