Maxims and Instructions for the Boiler Room / Useful to Engineers, Firemen & Mechanics; Relating to Steam Generators, Pumps, Appliances, Steam Heating, Practical Plumbing, etc.

by N. Hawkins

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.”

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.