[Illustration: Fig. 94.
(SECTIONAL VIEW.)]
_The safety valve_ is a circular valve seated on the top of the boiler,
and weighted to such an extent, that when the pressure of the steam
exceeds a certain point, the valve is lifted from its seating and
allows the steam to escape. Safety valves can be loaded directly with
weights, or the load can be transmitted to the valve by a lever. Again,
the end of the lever is sometimes held down by a spring, or the spring
may be applied directly to the valve seat.
Fig. 94 (2 views) exhibits _a spring loaded safety valve_. These are
generally provided with a reaction lip, surrounding the seat, which
causes them to open much further, and thus enables them to discharge a
larger volume of steam than a lever valve of equal diameter.
The operation can be easily understood by examining the figures. As
soon as the steam pressure is high enough to lift the valve disc clear
from its seat, the steam will escape around the valve seat as in an
ordinary lever safety valve, but instead of escaping directly into
the atmosphere, the current of steam is turned downward against the
reaction lip, by the curved projection on the valve disc, which can be
seen in the figure. The steam pressure is thus assisted in holding the
valve open, as well as raising it much higher, giving a larger opening
than would be the case if the valve were lifted by the pressure alone.
Spring loaded valves are mostly used on marine boilers, locomotives and
portable boilers, and wherever outside disturbances interfere with the
action of a weight.
_A “pop” safety valve_ is a common form of safety valve and takes its
name from the fact that it takes a little more pressure to raise it off
its seat than what it is set at, consequently it releases itself with a
“pop.”
[Illustration: Fig. 95.]
Fig. 95 shows a form of dead weight safety valves when _a_ is the valve
which rests on the seating _b_.
The valve is attached to the circular casting A, A, A, so that both
rise and fall together. The weights W, W, etc., are disposed on the
casting in rings, which can be adjusted to the desired blow off
pressure. Owing to the center of gravity of the casting and weight
being below the valve, the latter requires no requires no guides to
keep it in position. This is a great advantage as guides frequently
stick, and prevent the valve from acting. Another advantage of this
form of valve is, that it is difficult to tamper with. For instance, a
four-inch valve, intended to blow off at 100 lbs. per square inch would
require weight of over 1,200 lbs., which require a considerable bulk.
An unauthorized addition of a few pounds to such a mass would make no
appreciable addition to the blowing off pressure, while any effectual
amount added to the weight would be immediately noticed. It is quite
different with the lever safety valve about to be described, a small
addition to the weight at the end of the lever is multiplied several
times at the valve.