"ON LOADING SAFETY-VALVES BY DIRECT SPRINGS. "It has been shown that valves having half an inch of area per square x foot of grate surface require to lift 2 × diameter of valve Р -in order per fectly to relieve the boiler; and if proportioned as is recommended in 36. "Having determined the requisite lift, it remains to fix any reasonable or desired percentage of the load, which is not to be exceeded by the additional load due to the compression or extension of the spring, caused by the lift of the valve. Let this, for example, be restricted to 21 per cent. of the original load. "Then the spring loading the valve should be so proportioned that the compression or extension, to produce the initial load, shall be 40 times the lift of the valve. "So that with valves having half an inch area per foot of grate surface, the initial compression or extension of spring would be = 80 x diameter of valve Р With valves as recommended, the initial compression or extension would be 1.11 x diameter of valve. The following formula refers to spiral springs, made of steel in the usual way : E d = Compression or extension of one coil in inches. Diameter from centre to centre of steel composing spring in inches. w = Weight applied in pounds. D = Diameter or side of square of steel of which the spring is made in 16ths of an inch. C = A Constant which, from experiments made, may be taken as 22 for round steel and 30 for square steel. The total compression or extension of such a spring is equal to that of one coil into the number of effective coils, which may be taken as two less than the apparent number, the end coils being usually flattened to serve as bases for the spring to rest upon. "The relation between the safe load, size of steel, and the diameter of the coil has been deduced from the works of the late Professor Rankine, and may be taken for practical purposes as follows: "The application of the above formula may be illustrated by the following calculations of three different proportions of springs, all designed to give the same result. Diameter of valve, 4" = 12.5 area in square inches. Boiler pressure 60 lbs. per square inch. Omitting weight of valve, spindle, and spring; load required 125 × 60 = 750 lbs. Then, assuming that this valve is in the proportion of half a square inch area per foot of grate surface, the lift of valve would be = = 2 × 4 75 = •106, say 1". Initial compression of spring, 75 80 × 4 = 4" 26, say 4 inches. "1st. Supposed diameter of spring, or d, equal 4 in. D = 3/750 × 4 3 = 10, diameter of spring steel = 10-16ths. E = 4 = 18.3, say 18. •218 Pitch of spiral, Effective number of coils allowing between each coil a distance equal to twice the intended compression= 1"-061, say 1 inch; effective length of spring 18 x 1 = 18", and allowing for two end coils as bases, say 191", = the length of spring before compression. "2nd. Supposed diameter of spring, 6 in. D = say 12-16ths. 4 required, .355 11-2, say 11. Pitch of spiral, 1.46"; effective length of spring 1.46 × 11 16.06", and allowing for two end abutment coils, say 17" the length of spring before compression. 1.53 = 2.61. Pitch of spiral, 3.9"; effective length of spring, 8-9×2.61 10.17", say 10", and allowing for two end abutment coils, say 114" the length of spring before compression. = "In cases where it is desirable or perhaps necessary to employ springs acting at the ends of levers, the same formulæ can be employed for determining the proportion of springs, bearing in mind that the lift of the end of the lever where the spring is attached, is to be taken instead of the simple lift of valve. "ON LOADING SAFETY-VALVES BY DIRECT SPRINGS. "It has been shown that valves having half an inch of area per square 2 x diameter of valve -in order perР foot of grate surface require to lift fectly to relieve the boiler; and if proportioned as is recommended in diameter of valve this report, then the lift would be in all cases 66 36. 'Having determined the requisite lift, it remains to fix any reasonable or desired percentage of the load, which is not to be exceeded by the additional load due to the compression or extension of the spring, caused by the lift of the valve. Let this, for example, be restricted to 21 per cent. of the original load. "Then the spring loading the valve should be so proportioned that the compression or extension, to produce the initial load, shall be 40 times the lift of the valve. "So that with valves having half an inch area per foot of grate surface, the initial compression or extension of spring would be = 80 × diameter of valve With valves as recommended, the initial Р compression or extension would be 1.11 x diameter of valve. The following formula refers to spiral springs, made of steel in the usual way : E Compression or extension of one coil in inches. d Diameter from centre to centre of steel composing spring in D = Diameter or side of square of steel of which the spring is made in 16ths of an inch. CA Constant which, from experiments made, may be taken as 22 for round steel and 30 for square steel. The total compression or extension of such a spring is equal to that of one coil into the number of effective coils, which may be taken as two less than the apparent number, the end coils being usually flattened to serve as bases for the spring to rest upon. "The relation between the safe load, size of steel, and the diameter of the coil has been deduced from the works of the late Professor Rankine, and may be taken for practical purposes as follows: "The application of the above formulæ may be illustrated by the following calculations of three different proportions of springs, all designed to give the same result. Diameter of valve, 4" = 12.5 area in square inches. Boiler pressure 60 lbs. per square inch. Omitting weight of valve, spindle, and spring; load required = 12·5 × 60 = 750 lbs. Then, assuming that this valve is in the proportion of half a square inch area per foot of grate surface, the lift of valve would be = 2 × 4 75 = •106, say '1". Initial compression of spring, 80 × 4 = 4" 26, say 4 inches. "1st. Supposed diameter of spring, or d, equal 4 in. D = 3/750 × 4 3 = 10, diameter of spring steel = 10-16ths. E = allowing between each coil a distance equal to twice the intended com == the length of pression = 1"-061, say 1 inch; effective length of spring 18 × 1 = 18", and allowing for two end coils as bases, say 191", spring before compression. "2nd. Supposed diameter of spring, 6 in. D 216 × 750 20736 × 22 = 3/750 x 6 11-447, √ 3 = ='355". Effective number of coils 11.2, say 11. Pitch of spiral, 1.46"; effective length = of spring 1.46 x 11 16.06", and allowing for two end abutment coils, say 17" the length of spring before compression. 2.61. Pitch of spiral, 3.9"; effective length of spring, 10-17", say 10", and allowing for two end abutment coils, the length of spring before compression. "In cases where it is desirable or perhaps necessary to employ springs acting at the ends of levers, the same formulæ can be employed for determining the proportion of springs, bearing in mind that the lift of the end of the lever where the spring is attached, is to be taken instead of the simple lift of valve. "The above illustrative calculations have all reference to springs made of round steel, and used in compression. In many cases, two or more springs, one within the other, may be used with advantage." After consideration of the whole of the experimental information obtained, and the necessities required in practice, the Committee have come to the following conclusions:— 1st. The present practice in this country of constructing safetyvalves of uniform size for all pressures is incorrect.* "2nd. The valves should be flat-faced, and the breadth of face need not exceed one-twelfth of an inch.t "3rd. The present system of loading valves on marine boilers by direct weight is faulty, and ill-adapted for sea-going vessels, a considerable quantity of steam being lost during heavy weather, in consequence of the reduced effect of direct load-the result of the angle or list of the vessel, and also of the inertia of the weight itself, the latter not being self-accommodating at once to the downward movements of the vessel, and, moreover, the impossibility of keeping the valves when so loaded in good working order.‡ "4th. That two safety-valves be fitted to each marine boiler, one of which should be an easing-valve. "5th. The dimensions of each of these valves, if of the ordinary construction, should be calculated by the following rule : A A G HS square inches. = Area of valve in Grate surface in square feet. Heating surface in square feet. P = Absolute pressure in lbs. per square inch. "6th. The Committee suggest that only one of the valves may be of the ordinary kind, and proportioned as above, and that it should be the Good. But as the Board of Trade rule is proved to be quite right for pressures between 70 lb. and 75 lb., and in favour of the steam user at lower pressures, is it worth while to alter it ? If so, it must be on the ground that it should be altered for pressures above 75 lb.; but the committee have not shown in their Table what the sizes for those pressures should be. Surely, 75 pounds is a high enough pressure for any steamer to carry when lying at a quay. Whatever be the working pressure originally allowed on the boiler, the valves ought to be large enough to be safe when that pressure has been reduced to 75 pounds.-ED.] [† This recommendation is a wise one; and we already made it; and as there is not, and never has been, any rule against it, it will, probably, be adopted.—ED.] [We are glad to read this paragraph. The Nautical has always been in favour of trying spring-valves, and was the first to take up this subject in earnest. We offered a prize for the best spring safety-valve, and awarded it to one with a direct spring.-ED.] |