As the cylinder of the boiler is made to stand a certain pressure, any excess may cause it to burst. Therefore it is essential that the fireman should know when that pressure is exceeded. Various devices have been designed to give the fireman warning. Among these are the safety valve, the pressure gauge, and the water gauge.

A boiler usually has two safety valves, a water gauge, and a pressure gauge. The function of a safety valve is to relieve the boiler of all pressure in excess of that allowed. The valve is placed at the top of the boiler and piped outside. As it oftentimes becomes corroded and sticks, it should be tried every day.

The size of the safety valve is a very important matter, and is determined by the area of the grate, the weight of fuel burned, and the steam pressure. The amount of steam generated in a given time will depend upon the weight of coal burned, while the velocity of escape through the valve will depend upon the pressure. Low pressure safety will not run higher than 30 lbs. The figure stamped on the lever shows the limit,

A lever safety valve (Fig. 165) consists of a disk, a stem, and a lever with a weight hung on the end. The weight keeps the valve in its place until the steam pressure under the valve overcomes that of the weight and some of the steam escapes. If it were not for the safety valve, boiler explosions would be much more frequent.