| 120 | pounds, steam pressure, | |
| 8 | inches, area of valve, | |
| 960 | ||
| 60 | pounds, weight of valve and lever, | |
| 900 | ||
| 5 | ||
| 100) | 4500 | (45 length of lever. |
Every boiler should be provided with two safety valves, one of which should be put beyond the control of the attendant.
Safety valves that stick will do so even though tried every day, if they are simply lifted and dropped to the old place on the seat again. If a boiler should be found with an excessively high pressure, it would be one of the worst things to do to start the safety valve from its seat unless extra weight was added, for should the valve once start, it would so suddenly relieve the boiler of such a volume of steam as would cause a rush of water to the opening, and by a blow, just the same as in water hammer, rupture the boiler.
Such a condition is very possible to occur of itself when a safety valve sticks. The valve holds the pressure, that gets higher and higher, until so high that the safety valve does give way and allows so much steam to escape that the sudden changing of conditions sets the water in motion, and an explosion may result.
The noise made by a safety valve when it is blowing off may be regarded in two ways. First, by it is known that the valve is capable of performing its proper function, and that there is, therefore, a reasonable assurance that no explosion will result from excessive pressure of steam or other gas, and on the other hand too much noise of this kind indicates wasted fuel.
The hole of the safety valve may be 2, 3 or 4 inches; that does not say that the area is 3.1416, 7.06 or 12.56 square inches, but the area is that which is inside of the joint. The valve opening may be, say 2 inches, but the circle of contact of valve to seat may be of an average diameter of 21⁄8 inches, if so, all the close calculations otherwise will not avail. In the first place, the area of 2 inches equals 3.1416; that of 21⁄8 diameter equals 3.5466, showing a difference of .4 square inches.
Note.
Very extended rules issued by the U. S. Government for calculating the safe working pressure, dimensions and proportions of the safety valves for marine boilers are reprinted in “Hawkins’ Calculations” for engineers.