Experiments have shown that pure water, into which air has been forced, on heating causes corrosion. Highly heated surfaces in contact with water containing common salt corrode and pit rapidly. The sides of the furnace, the tube plates and the hottest tubes suffer most. It is clear, then, that feed-water, free from solids, combined or in suspension, organic matter, acids of all kinds, and air, would be best for the life of boilers. In cases where water containing large amounts of total solid residue is necessarily used, a heavy petroleum oil, free from tar or wax, which is not acted upon by acids or alkalies, not having sufficient wax in it to cause saponification, and which has a vaporizing-point at nearly 600° F., will give the best results in preventing boiler-scale. Its action is to form a thin, greasy film over the boiler linings, protecting them largely from the action of acids in the water and greasing the sediment which is formed, thus preventing the formation of scale and keeping the solid residue from the evaporation of the water in such a plastic suspended condition that it can be easily ejected from the boiler by the process of "blowing off." If the water is not blown off sufficiently often, this sediment forms into a "putty" that will necessitate cleaning the boilers. Practical experience is decidedly in favor of water purification, both from the standpoint of preserving the life of the boiler and for the best efficiency in operation. Air in solution, if allowed to enter the boiler, will accelerate corrosion more than any other cause, hence water heaters should be used with open feed and careful regulation of the temperature, which should always be about 190° F. FLOW OF WATER IN PIPES The quantity of water discharged through a pipe depends on the head. If the discharge occurs freely into the air, this head is the difference in level between the surface of the water in the reservoir and the center of the discharge end of the pipe; if the lower end of the pipe is submerged, the head is the difference in elevation between the two water levels. The discharge for a given diameter depends also upon the length of the pipe, upon the character of its interior surface as to smoothness and upon the number and sharpness of its bends. The head, instead of being an actual distance between levels, may be caused by pressure, as by pumping, in which case the head is calculated as a vertical distance corresponding to the pressure, 1 pound per square inch being equal to 2.309 feet head, or I foot head being equal to a pressure of 0.433 pound per square inch. The total head operating to cause flow is divided into three parts: (1) The velocity head, which is the height through which a body must fall in a vacuum to acquire the velocity with which the water flows in the pipe. This is equal to 22 g, in which v is the velocity in feet per second, and 2 g 64.32; (2) The entry head, which is required to = entrance the entry head equals about one-half of the velocity head; with smooth, rounded entrance the entry head is inappreciable; (3) The friction head, due to the frictional resistance to flow in the pipe. In ordinary cases of pipes of considerable length the sum of the entry and velocity heads scarcely exceeds one foot; in the case of long pipes with low heads it is so small that it may be neglected. When the flow becomes steady, the pipe is entirely filled throughout its length, and hence the mean velocity at any section is the same as that at the end, when the size is uniform. This velocity is found to decrease as the length of the pipe increases, other things being equal, and becomes very small for great lengths, which shows that nearly all the head has been lost in overcoming the resistances. The length of the pipe is measured along its axis, following all the curves, if there be any. The velocity considered is the mean velocity, which is equal to the discharge divided by the area of the cross section of the pipe. The actual velocities in the cross section are greater than this mean velocity near the center and less than it near the interior surface of the pipe. The object of the discussion of flow in pipes is to enable the discharge which will occur under given conditions to be determined, or to ascertain the proper size which a pipe should have in order to deliver a given discharge. The subject cannot, however, be developed with the definiteness which characterizes the flow from orifices and weirs, partly because the condition of the interior surface of the pipe greatly modifies the discharge, partly because of the lack of experimental data, and partly on account of defective theoretical knowledge regarding the laws of flow. In orifices and weirs errors of two or three per cent may be regarded as large with careful work; in pipes such errors are common, and are generally exceeded in most practical investigations. It fortunately happens, however, that in most cases of the design of systems of pipes errors of five and ten per cent are not important, although they are of course to be avoided if possible, or, if not avoided, they should occur on the side of safety. Quantity of Water Discharged The quantity of water which flows through a pipe is the product of the area of its cross section and the mean velocity of flow. That is, Q = av, in which Q is the quantity discharged in cubic feet per second, a is the area in square feet and v is the velocity in feet per second. For U. S. gallons per second multiply by For U. S. gallons per minute multiply by 7.4805 448.83 26929.9 For U. S. gallons per hour multiply by For U. S. gallons per 24 hours multiply by 646317. The diagram, page 279, gives the discharge in gallons per minute, 50000 -100000 90000 80000 70000 60000 50000 40000 30000 Chart for Flow of Water in Wrought Pipe If any two of the three factors represented by the scales are known, the third may be found by passing a straight line through these quantities on their respective scales. This line will intersect the third scale at the number representing the desired factor. Example. For 4000 gallons per minute with 12 inch pipe, velocity = 11.4 feet per second. 0.5 0.6 0.7 0.8 0.9 1 1.5 2 2.5 3 3.5 4. 4.5 5 7 8. 9 10 15 800 250 200 ៖ ៖ ៖ ៖ 150 100 90 80 70 60 Mean Velocity of Flow The velocity of flow, depending as it does to such a great extent upon the condition of the interior surface of the pipe, is difficult to compute. Below are given the formulæ most generally accepted. In the solution of any problem a comparison of the results obtained by the use of these formulæ is advisable. There are so many conditions affecting the flow of water that all hydraulic formulæ give only approximations to accurate results. Approximate Formula (Trautwine). To find the velocity of water discharged from a pipe line, knowing the head, length and inside diameter, use the following formula: in which v = approximate mean velocity in feet per second; The above coefficients are averages deduced from a large number of experiments. In most cases of pipes carefully laid and in fair condition, they should give results within 5 to 10 per cent of the truth. = Example: Given the head, h 50 feet, the length, L = 5280 feet, and the diameter, D = 2 feet; to find the velocity and quantity of discharge. The value of the coefficient m from the table when D = 2 feet is Substituting these values in the formula, we get: To find the discharge in cubic feet per second, multiply this velocity by the area of cross section of the pipe in square feet. Thus, 3.1416 X (1)2 × 7.752 = 24.35 cubic feet per second. Since there are 7.48 gallons in a cubic foot, the discharge in gallons per second 24.35 X 7.48 = 182.1. = The above formula is only an approximation, since the flow is modified by bends, joints, incrustations, etc. Wrought pipes are smoother than cast-iron ones, thereby presenting less friction and less encouragement for deposits; and, being in longer lengths, the number of joints is reduced, thus lessening the undesirable effects of eddy currents. Kutter's Formula. This formula, although originally designed for open channels, can be used in the case of long pipes with low heads. It is the joint production of two eminent Swiss engineers, E. Ganguillet and W. R. Kutter, and is, properly speaking, a formula for finding the coefficient C in the well-known Chezy formula: in which v = CVTS, v = mean velocity in feet per second; S= mean hydraulic radius in feet; slope head ÷ length, measured in a straight line from end to end. The mean hydraulic radius is the area of wet cross-section divided by the wet perimeter, which for pipes running full, or exactly half full, is equal to one-quarter of the diameter. According to Kutter the value of this coefficient C is in which s is the slope, r is the mean hydraulic radius in feet and n is the "coefficient of roughness." The value of n varies from .010 for very smooth pipes to .015 for pipes in a very poor condition. For ordinary wrought pipe .012 can be used. For clean steel riveted pipe .015 can be used. The following table gives values of the coefficient C as obtained by Kutter's formula for different slopes, hydraulic radii and degrees of |