SAFETY VALVES.
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.”
