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. |
Professor Rankin’s Rule.—Multiply the number of pounds of water evaporated per hour by .006, and the product will be the area in square inches of the valve.
The U. S. Steamboat Inspection Law requires for the common lever valve one square inch of area of valve for every two square feet of area of grate surface.
United States Navy Department deduced from a series of experiments the following rule: Multiply the number of pounds of water evaporated per hour by .005, and the product will be the area of the valve in square inches.
Rule adopted by the Philadelphia Department of Steam Engine and Boiler Inspection:
1. Multiply the area of grate in square feet by the number 22.5. 2. Add the number 8.62 to the pressure allowed per square inch. Divide (1) by (2) and the quotient will be the area of the valve in square inches. This is the same as the French rule.
The maximum desirable diameter for safety valves is four inches, for beyond this the area and cost increase much more rapidly than the effective discharging around the circumference.
There should not be any stop valve between the boiler and safety valve.
The common form of safety valve is shown in [Fig. 96].
Here the load is attached to the end B of the lever A, B, the fulcrum of which is at c. The effective pressure on the valve, and consequently the blowing off pressure in the boiler can be regulated within certain limits, by sliding the weight W along the arm of the lever. In locomotive engines, as well as on marine boilers, the weight would on account of the oscillations, be inadmissible and a spring is used to hold down the lever.
In the calculations regarding the lever safety valve, there are five points to be determined, and it is necessary to know four of these in order to find the fifth. These are: (1) The Steam Pressure, (2) The Weight of Ball, (3) The Area of Valve, (4) The Length of Lever, (5) The Distance from the Valve Centre to the Fulcrum.
Fig. 96.
In making these calculations it is necessary to take into account the load on the valve due to the weight of the valve-stem and lever. The leverage with which this weight acts is measured by the distance of its centre of gravity from the fulcrum. The centre of gravity is found by balancing the lever on a knife edge, and the weight of the valve-stem and lever can be found by actual weighing. This load can also be found by attaching a spring balance to the lever exactly over the centre of the valve stem when they are in position. The following examples will be computed under these conditions: (1) Steam Pressure, 120 pounds; (2) Weight of Ball, 100 pounds; (3) Weight of Valve and Lever, 60 pounds, weighed in position; (4) Length of Lever, 45 inches; (5) Length of Distance from Valve Centre to Fulcrum, 5 inches; (6) Area of Valve, 8 square inches.
To find the area of the valve:
Rule.—Multiply the length of the lever by the weight of the ball, and divide the product by the distance from the valve centre to the fulcrum, and to the quotient add the effective weight of the valve and lever, and divide the sum by the steam pressure.
Example.
| 45 | inches, length of the lever, | |
| 100 | pounds, weight of the ball, | |
| Fulcrum, 5 in. ) | 4500 | |
| 900 | ||
| 60 | pounds, weight of valve and lever, | |
| Steam pressure 120 lbs. ) | 960 | (8 square inches, area of valve. |
| 960 |
To find the pressure at which the valve will blow off:
Rule.—Multiply the length of the lever by the weight of the ball; divide this product by the distance from the valve centre to the fulcrum, and to the quotient add the effective weight of the lever and valve, and divide the sum by the area of the valve.
Example.
| 45 | inches, length of lever, | |
| 100 | pounds, weight of ball, | |
| Fulcrum, 5 in. ) | 4500 | |
| 900 | ||
| 60 | pounds, weight of valve and lever, | |
| Area of Valve 8 ) | 960 | |
| 120 | pounds, pressure at which valve will blow. |
To find the weight of ball:
Rule.—Multiply the steam pressure by the area of the valve, and from the product subtract the effective weight of the valve and lever, then multiply the remainder by the distance from the valve centre to the fulcrum, and divide the product by the length of the lever.
Example.
| 120 | pounds, steam pressure, | |
| 8 | inches, area of valve, | |
| 960 | ||
| 60 | pounds, weight of valve and lever, | |
| 900 | ||
| 5 | inches, fulcrum, | |
| Length of lever, 45 in. ) | 4500 | |
| 100 | pounds, weight of ball. |
To find the length of lever:
Rule.—Multiply the steam pressure by the area of the valve, and from the product subtract the effective weight of the valve and lever, then multiply the remainder by the distance from the valve centre to the fulcrum, and divide the product by the weight of the ball.
Example.
| 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.