7650 Banks St. Justice, Illinois 60458
One of the questions many ask about is safety valves; what size and how to adjust. The study of the safety valve has been the first step of many a man in scientific engineering. Induced to its study by the necessity of solving its problems before the examiner, his consideration of this simple device has led him into the computation of areas, into a study of the principle of the lever, of moments of forces, of the velocity of flow of steam and other fundamental principles of mechanics. This applies to those who have studied the subject intelligently, not to those who have attempted to get over it by learning a rule by rote, simply to be confounded when confronted by another rule, or a case to which their rule would not apply. The whole subject is so simple that an hour's study will put a man in possession of the fundamentals so that he can make his own rules or solve any problem without a rule, from a sheer understanding of the principles involved.
Safety valves should always be large enough to permit the escape of all the steam a boiler is capable of making and each boiler should have its own safety valve rather than connecting two or more boilers together and depending on one valve for the whole. The valve and seat should be made of hard gun metal, or any other composition that will not rust and stick fast. At one time it was quite a common thing to see a brass valve fitted to a cast-iron seat; this is wrong, for the rusting of the iron would fix the valve so tightly that the boiler would be in constant danger of rupture from over pressure.
For satisfactory results with stationary boilers the common ball and lever safety valves are used. For stationary boilers it is immaterial whether the safety valve be fitted with a lever and weight, or whether it be fitted with a spring. The former is the usual manner of loading a safety valve and has but few objections. For portable engines and locomotives, safety valves are loaded with springs, which by suitable adjustment may be made to blow off at any desired pressure.
Let us next consider the capacity of valves; how large a valve is required for a given boiler. Most of the rules deal with grate surface and the area of the valve; the rule adopted by the U. S. Board of Supervising Inspectors being one square inch of valve area for each two feet of grate area. That the valve should be proportioned to the grate surface seems proper because it is the grate surface, and not the heating surface, which determines and limits the capacity of a boiler. To a given grate surface, however, we should apportion a sufficient amount of area of opening, and this area of opening is not proportional to the area of the valve but to the diameter and lift. A valve 1 inch in diameter has an area of 0.7854 of a square inch, but that does not mean that there will be an opening of 0.7854 of a square inch for the steam to escape. If the valve is flat, the area opened for the discharge of steam will be the circumference of the valve multiplied by the lift. When a flat valve has lifted a quarter of its diameter it has reached the limit of its capacity to discharge steam. It doesn't do any good to lift higher, for the area around the edge of the valve is already as large as the area of the valve itself and the capacity of the valve is proportional to the area or the square of the diameter.
In practice, however, the lift of valves is much less than one-quarter of their diameter, and for a given lift the area for the escape of steam is proportional to the circumference or the diameter rather than to the area. Most of the rules, however, as above stated, allow a given amount of valve area to a square foot of grate surface, and make the allowance liberal enough to include all conditions. For instance, the rule of the U. S. Board of Supervising Inspectors calls for one-half a square inch of valve area for each square foot of grate surface. A 4-inch valve has about 12 square inches of area and would thus take care of 24 square feet of grate. It would not be possible to burn over 25 pounds of coal per square foot of grate per hour with natural draft, nor to evaporate over 12 pounds of water with a pound of coal, so that the boiler could not possibly make more than:
25 x 12 x 24 = 7200 pounds of steam per hour.
Here is a rule which will give you the diameter of the valve in inches at once:
Multiply the square root of the grate surface by 0.8.
This would be particularly handy when the grate is square, or nearly so, for then the length would be the square root of the area.
Next time we get together, we will talk about water in the use of boilers.