The safety valve is a device for relieving the boiler of steam after it has reached a certain pressure.
This it accomplishes by letting the steam escape after it has reached the required pressure.
At what pressure the safety valve will blow off depends upon the position of the weight on the safety valve lever.
The calculations referring to this part of the subject are, finding how much weight will be required to be placed at a given point on the lever, in order, with a given sized valve, to blow off at a given pressure.
Finding the position on the lever of a given amount of weight, in order to blow off at a certain pressure.
Finding, with a given sized valve and a given weight, how to mark off the lever and where the notches must be cut for given pressures.
In each of these calculations there are three elements: first, the area of the valve and the steam pressure, which constitute the effect of acting to lift the valve; second, the amount of the weight and its position upon the lever, which acts to keep the valve closed; and third, the weight of the lever and of the valve, which act to keep the valve closed.
Fig. 3357.