In [Fig. 3357] we have a drawing of a safety valve shown in section, and if there was no weight upon the lever, the pressure of steam the valve would hold in the boiler would be that due to the weight of the valve and of the lever upon the valve.
To find out how much this would be, we would have to put the valve itself and the pin a on a pair of scales and weigh them.
Then put a piece of string through the hole at a in the lever, and see how much it weighed when suspended from that point.
Suppose the valve and pin to weigh 2 lbs. and the lever (suspended by the string) 10, and the total will be 12 lbs.
Next we find the area of the valve, and suppose this to be 8 square inches; then we may find how much pressure the valve would keep in the boiler, by dividing the area of the valve into the weight holding the valve down, thus:
| Lbs. | ||||||
| Weight | of | valve and pin, | 2 | |||
| „ | „ | lever, | 10 | |||
| Area of valve, | 8 ) | 12 | ||||
| Pressure the valve would hold, | 1 | .5 | lbs. | |||
The area of the valve is that part of its face receiving the steam pressure when the valve is seated, so that if the smallest part of the valve diameter is equal to the diameter of the seat bore, the diameter from which the valve area is to be calculated will be that denoted by d in the figure, and cannot in any case be less than this. But if the smallest end of the valve cone is of larger diameter than the smallest end of the seat cone (which should not, but might be the case), then it is the smallest diameter of valve cone that must be taken in calculating the area, because that is the area the steam will press against.
Now suppose we rest a 20 lb. weight on the top of the valve that is on the point denoted by i, and there will be 32 lbs. holding the valve down, thus, weight of valve 2 lbs., of lever 10, and weight added, 20 lbs., and to find how much pressure this would hold in the boiler, we divide it by the valve area, thus:
| Weight on valve. | ||||
| Valve area | = | 8 ) | 32 | |
| 4 | = pressure valve will hold. | |||
But suppose we put the weight on the lever, in the position shown in the figure, which is six times as far from the fulcrum f of the lever as the valve is, and its effect on the valve will be six times as great as it would if placed directly upon the valve, so that leaving the weight of the valve and of the lever out of the question (as is commonly done in engineers’ examinations), we may find out what pressure the valve will hold, as follows: