Safety Valves. Rules are given in Chapter II. for guidance in making calculations relating to spring pop valves, which are now almost universally used on boilers, and which, without doubt, are the most reliable appliance for relieving a boiler of surplus steam. A short space will be devoted to the consideration of the lever safety valve also, as it may be of interest to some students. The U. S. marine rule for lever valves is here repeated: "Lever safety valves to be attached to marine boilers shall have an area of not less than one square inch to every two square feet of grate surface in the boiler, and the seats of all such safety valves shall have an angle of inclination of 45° to the center line of their axis." In order to arrive at accurate results in lever safety valve calculations it is necessary to know first the number of pounds pressure exerted upon the stem of the valve by the lever itself, irrespective of the weight, also the weight of the valve and stem, as all these weights together with the weight of the ball suspended upon the lever tend to hold the valve down against the pressure of the steam. The effective weight of the lever can be ascertained by leaving it in its position attached to the fulcrum and connecting a spring balance scale to it at the point where it rests on the valve stem. The weight of the valve and stem can also be found by means of the scale. When the above weights are known, together with the weight at the end of the lever and its distance from the fulcrum, also the area of the valve and its distance from the fulcrum, the pressure at which the valve will blow can be found by the following rules: Rule 1. Multiply the weight by its distance from the fulcrum. Multiply the weight of the valve and lever by the distance of the stem from the fulcrum and add this to the former product. Divide the sum of the two products by the product of the area of the valve multiplied by the distance of its stem from the fulcrum. The result will be pressure in pounds per square inch required to lift the valve. Example. Diameter of value, 3 in.. Effective weight of lever, valve and stem, 20 lbs. Distance of ball from fulcrum, 30 in. Required pressure at which the valve will blow off, 50 × 30+ 20 × 3 = 1560. Area of valve, 7.0686 × 3 = 21.2058. 1560 + 21.2058 73.57 lbs. pressure. = When the pressure at which it is desired the valve should blow off is known, together with the weights of all the parts, the proper distance from the fulcrum at which to place the weight is ascertained by Rule 2. Rule 2. Multiply the area of the valve by the pressure and from the product subtract the effective weight of the valve and lever. Multiply the remainder by the distance of the stem from the fulcrum and divide by the weight of the ball. The quotient will be the required distance. Example. Area of valve, 7.07 sq. in. Effective weight of lever and valve, 20 lbs. Distance of valve stem from fulcrum, 3 in. 510.25 × 350 = 30.6 in., distance from fulcrum at which to place the ball. When the pressure is known, together with the distance of the weight from the fulcrum, the weight of the ball is obtained by Rule 3. Rule 3. Multiply the area of the valve by the pressure and from the product subtract the effective weight of the lever and valve. Multiply the remainder by the distance of the stem from the fulcrum and divide by the distance of the ball from the fulcrum. The quotient will be the required weight. Example. Area of valve, = 545.63 30 54.56 lbs., weight of ball. Safety valves, especially those of the lever type, are liable to become corroded and stick to their seats if allowed to go any great length of time without blowing. Therefore it is good practice to raise the steam pressure to the blowing off point at least two or three times a week, or oftener, for the purpose of testing the valve. If it opens and releases the steam at the proper point all is well, but if it does not, it should be looked after forthwith. Generally the mere raising of the lever by hand, or a few taps with a hammer if it be a pop valve, will free it and cause it to work all right again; but if this treatment has to be resorted to very often the valve should be taken down and overhauled. In too many steam plants not enough importance is attached to the safety valve. The fact is, it is one of the most useful and important adjuncts of a boiler, and if neglected serious results are sure to follow. Feed Pumps. A good engineer will always take a pride in keeping his feed pump in good condition, and if he has two or more of them, which every steam plant of any consequence should have, he will have an opportunity to keep his pumps in good shape. The water pistons of most boiler feed pumps are fitted to receive rings of fibrous packing. The best packing for this purpose and one that will stand both hot and cold water service is made of pure canvas cut in strips of the required width, 2, 5%, 3/4 in., etc., and laid together with a water proof cement having the edges. for the wearing surface. This packing is called square 'canvas packing, and can be purchased in any size required for the pump. The size is easily ascertained by placing the water piston, minus the follower plate, centrally in the water cylinder and measuring the space between the piston and cylinder walls. This packing should not be allowed to run for too long a time before renewing, for the reason that pieces of it are liable to become loose and be forced along with the feed water on its way to the boiler and lodge under the check valve, holding it open and causing no end of trouble. If the feed pump has to handle hot water, or has to lift the water several feet by suction, the packing rings should be looked after at least once a month. Hard rubber valves are, all things considered, the best for a boiler feed pump, as they are not affected by hot water and do not hammer the seats like metallic composition valves do. Every boiler feed pump should be fitted with a good sight-feed lubricator for cylinder oil. The steam valve mechanism of a steam pump is very sensitive and delicate and requires good lubrication in order to do good work. In too many cases feed pumps are fitted with an old style cylinder oil cup and there is generally more oil on the outside of the valve chest than there is inside, while the valve is bulldozed into working by frequent blows from a convenient club. The steam valves of all steam pumps are adjusted before they are sent out from the factory, and most of them are arranged SO that the stroke may be shortened or lengthened as the engineer desires. It is best as a rule to allow a pump to make as long a stroke as it will without striking the heads, because then the parts are worn evenly. Sometimes an engineer is called upon to set the valves of a duplex pump which have become disarranged. In such a case he should proceed as follows: Place both pistons exactly at mid-stroke. This may be done in two ways. First, by dropping a plummet line alongside the levers connecting the rock shafts with the spools on the piston rods. Then bring the rods to the position where the centers of the spools will be in a vertical line with the centers of the rock shafts. the piston to The second method is to the extreme end of the stroke until it comes in contact with the cylinder head. Then mark the rod at the face of the stuffing box gland. Next move the piston to the other end of the stroke and mark the rod at the opposite gland. Now make a mark on the rod exactly half way between the two outside marks and move the piston back until the middle mark is at the face of the gland and the piston will be at mid-stroke. Having placed both pistons at mid-strike, remove the valve chest covers and adjust the valves in their central position, viz., so that they cover the steam ports. The |