SECT. VII.]
STEAM ENGINES.
251
If a boiler flue rise from the fire, and then descend again before it enters the
chimney, it will in particular states of the fire be liable to fill with inflammable
gas, which takes fire and explodes. The effect of such an explosion in the flues of
a boiler must cause an impulsive strain on the boiler, under which it may fail.
The danger may be avoided by making the flues lead off to the chimney without
depression, and constructing the damper so that it cannot be perfectly closed; and
it should either rise so as to close the upper part of the aperture last, or move
horizontally.
Hydrogen gas may be, and frequently is, formed in steam boilers, through the
water being in contact with a part of the boiler which is red hot; and it seems to
be regularly produced during the formation of steam at very high temperatures:
and though it appears to me that it would not add to the risk of an explosion
happening, it undoubtedly would render it more destructive if it should take
place. 1
Boilers formed of Plates.
524. Having determined the resistance of plates of any curvature, it is easy
to apply these rules to rectangular boilers; remarking, that it is indifferent whe
ther the curve be convex or concave to the pressure, provided it have either abut
ments as an arch, or forms a complete circle. I doubt the efficacy of the usual
abutments, and I think the fact that boilers fail round the seats is greatly owing to
the strain and motion of the parts at every change of force or temperature.
A rectangular boiler may be considered as a cylinder, taking the greatest dia
gonal line of its section for the diameter; and for strength the thickness will be,
(art. 521.)
3‘81 a p
t= —J— -
For wrought iron, f = 17800 lbs.
a p
1 = 4060 '
1 In a letter I received from Mr. W. Williams, of Cyrfartha Iron Works, he attributes the
destructive effects of an accident in that neighbourhood to an accumulation of hydrogen inflaming
when the boiler burst. The boiler was constructed of the old spherical form, twenty feet in dia
meter ; the thickness of the plates when new was, top plates a full quarter of an inch, bottom
plates half an inch ; load on the safety valve 7 lbs. per circular inch. Many lives were lost
by this explosion ; and the boiler was thrown to a distance of 150 feet, to a place 30 feet above the
level of its former seat. The upper plates were undoubtedly too weak.