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.
U. S. Rules Relating to Safety Valves.
Extract from rules and regulations passed and approved Feb. 25, 1885, by the United States Board of Supervising Inspectors of Steam Vessels:
Section 24. “Lever safety valves to be attached to marine boilers shall have an area of not less than one square inch to two square feet of the grate surface in the boiler, and the seats of all such safety valves shall have an angle of inclination of forty-five degrees to the centre line of their axis.
“The valves shall be so arranged that each boiler shall have one separate safety valve, unless the arrangement is such as to preclude the possibility of shutting off the communication of any boiler with the safety valve or valves employed. This arrangement shall also apply to lock-up safety valves when they are employed.
“Any spring-loaded safety valves constructed so as to give an increased lift by the operation of steam, after being raised from their seats, or any spring-loaded safety valve constructed in any other manner, or so as to give an effective area equal to that of the aforementioned spring-loaded safety valve, may be used in lieu of the common lever-weighted valve on all boilers on steam vessels, and all such spring-loaded safety valves shall be required to have an area of not less than one square inch to three square feet of grate surface of the boiler, and each spring-loaded valve shall be supplied with a lever that will raise the valve from its seat a distance of not less than that equal to one-eighth the diameter of the valve opening, and the seats of all such safety valves shall have an angle of inclination to the centre-line of their axis of forty-five degrees. But in no case shall any spring-loaded safety valve be used in lieu of the lever-weighted safety valve, without first having been approved by the Board of Supervising Inspectors.”
The following size “Pop” Safety Valves are required for boilers having grate surfaces as below:
| 2 inch “Pop” Valve for | 9.42 | square feet of grate surface. |
| 21⁄2 inch “Pop” Valve for | 14.72 | square feet of grate surface. |
| 3 inch “Pop” Valve for | 21.20 | square feet of grate surface. |
| 4 inch “Pop” Valve for | 37.69 | square feet of grate surface. |
| 5 inch “Pop” Valve for | 58.90 | square feet of grate surface. |
| 6 inch “Pop” Valve for | 84.82 | square feet of grate surface. |