250
OF THE PARTS OF
[sect. VII.
ance, and were it always provided, there would be little chance of accident if the
valves be properly constructed and attended to.
It becomes insufficient in high pressure boilers, because a common low pressure
boiler contains about ten times the volume of steam required for one stroke of the
engine, consequently the time of twenty strokes must elapse before the density of
the steam could accumulate to three times its working density, supposing the
engine to be stopped, and the valve out of order; but if the boiler contains only as
much steam as is required for one stroke, the force will be increased to three times
in the time the engine would have made two strokes. This rapidity of the increase
of force does not leave the necessary time to examine, nor even to open the valves
in this extreme case, and the hazard must be greater in consequence. In all cases
the time of accumulating power should not be shorter than it is in the common
boiler. Besides, in working an engine where the excess of force increases so fast,
the loss of steam would be considerable from any variation of the heat of the fire,
even were the valve to act properly, and therefore there is a temptation to load the
valve beyond its regular weight. To render the security on the stoppage of the
engine equal in all cases, the excess of strength should be inversely as the space
allowed for steam.
It is still more important to consider the subject, in relation to the danger arising
from unequal action of the fire; and for this the excess of strength should be
inversely as the contents of the boiler expressed in units of the power.
Thus, taking the horse power as the measure, if one boiler contains twenty cubic
feet for each horse power, and another only ten, the boiler with only ten feet of
space should be of twice the strength ; for equal powers require equal fires, and the
effect of excess of fire in raising the temperature and force of the steam is inversely
as the quantity of matter acted upon ; hence the risk of the dangerous increase of
strength is inversely as the quantity of water and steam the boiler contains.
523. The proportion for excess of strength I shall therefore consider to be two
times that which is proper for the working pressure when the boiler contains twenty
cubic feet for each horse power; and containing any other quantity as n cubic feet
per horse power, it will be
40
n : 20 : : 2 : - .
n
The effect of unequal expansion, of improper form and flexure, and of wear,
must be included in the calculation of the strength ; for these are not allowances
for risk, but actually necessary for security.
Boilers may fail from strains produced by other causes besides the force of the
steam, and these may be noticed to guard against the circumstance which produces
them.