THE last quarterly report of the proceedings of the Engineers’ Club of Philadelphia, contains a valuable paper on water pipes, by A. H. Howland, which presents much interesting data relative to this important topic.
Mr. Howland points out that there are about one hundred times as many miles of cast iron pipes in the ground as there are of all other kinds, the other kinds that have received any considerable attention and use being, in the order of their proportionate use, a riveted wrought iron shell, lined and coated with cement mortar, lap welded or riveted wrought-iron pipes, coated with asphalt urn or protected with various forms of metallic coating or oxidation of the surfaces; wooden pipes, earthen-ware pipes, and composition pipes.
Referring to cast-iron pipes, which have come to be accepted as a so-called standard, our author correctly maintains that in determining what weight of cast-iron pipe should be used, the same factors must be considered that are considered in determining the dimensions of a bridge or other structure, viz., what is the probable maximum load, what is the form, and what is the quality of the material available. Cases are cited in illustration to show that this correct method of judgment is the exceptional one. As to the probable maximum load, this can only be determined by the circumstances connected with each particular place. In one sense, fortunately, it has been found impracticable to make cast iron pipes of given diameters of less than certain thickness, or if it were possible to make them of less thickness, the great danger of breakage in handling or transportation comes in to prevent extreme thinness, and Mr. Howland thinks he is within the bounds of reason when he states that the minimum thickness of any pipe under twenty-four inches in diameter, gives sufficient material so that the pipe when made will withstand 300 pounds to the square inch.
But though there are few if any works having 300 pounds pressure to the square inch, pipes are none the less subjected to extraordinary pressures, far in excess of this amount One cause is an external pressure caused by the unequal bearing of the pipe in the trench, and the heavy load placed upon its upper surface, especially in the streets of cities, where there is heavy traffic. Safety in this particular can only be secured by looking to the men who lay the pipes, and making sure that they obtain full and sufficient beds for the pipes in all cases. Nothing need therefore be added for this to the minimum thickness. The next and most important of these extraordinary pressures is that caused by water-hammer, and from extended observation, Mr. Howland concludes that an allowance of 100 per cent in excess of the static pressure is an allowance which should prove ample, and cover the cases of careless use.
In relation to the strength of pipes, our author shows that it is a simple matter to calculate the resistance of a perfect cylinder, made of a certain quality of material, against an internal pressure; but until one is willing to pay a fair price, and insist upon good material and good workmanship, one must pay for an excess of material sufficient to insure against the poor quality of the material and the carelessness of the workman. From the careful examhmtion and study ot all the data obtainable from 500 different works in the United States and Canada, our author concludes that for all we have to guard against, either in quality of material, carelessness in manufacture, carelessness in handling or laying or against water-hammer, a factor of safety of five is ample, and this only on the larger pipes.
On the basis that it is perfectly feasible to obtain cast-iron pipes made of material having a tensile strength of 18,000 pounds to the square inch, Mr. Howland calculates the following table, showing the minimum thickness of various sizes of pipes, together with their weights and ultimate strengths:—
Although Mr. Howland considers the above table as ample for the pressures named in the last column, still giving way to custom and prejudice, he has adopted as standard weights for all cast-iron pipes used in works contracted for by himself as follows:
Mr. Howland points out that it is to the selfish interest of the pipe manufactuier to sell as heavy pipe as possible for any given service. As a protection against corrosion, he advocates the method of Dr. Smith, of immersion in a hot bath of deodorized coal tar and linseed oil, but adding a small percentage of asphaltum and heating the pipes as well as the mixture. He says that when p’pes, thoroughly cleaned, are dipped in such a mixture, the result is a smooth, hard, glossy coating, which it is difficult to remove, and which is not acted upon by the usual waters confined in the pipes or found in the soils in which the pipes are buried.
In regard to cement-lined pipes, of which the cheapness is the principal attraction, Mr. Howland says that, having written personal letters to every department whose address he could obtain throughout the United States and Canada that had ever used wrought-iron cement-lined pipe, he found from the replies received that the average life of the cement pipe as usually made, was eight years, and that no place, with two exceptions, which had these pipes in use for a longer period than eight years, recommended them. The objection to this class of pipe is stated to be that the cement mortar, composed of one-half sand and one-half cement, is not an impervious material, and that water, under ordinary pressure, is forced to a greater or less extent through it, and so comes in contact with the shell, which it eats until it has no strength left, when the pipe is destroyed. From observation, it is the author’s belief that a wrought-iron shell, lined and coated with neat cement, would have greater durability and life than one lined and coated with cement mortar.
In regard to wrought-iron coated pipes, Mr. Howland refers to the satisfactory use of wrought-iron riveted pipe simply covered with asphaltum, in the Western part of the country, the only objection being the cost and the increased loss of head by friction on the lapped and riveted seams. The employment of lap-welded wrought-iron pipes, which have begun to be used in the last few years, is declared to overcome all these objections, and to prove even preferable to cast-iron pipes of same diameters.
The best method of joining these pipes, which come in lengths of from eighteen to twenty feet, is a simple cast-iron sleeve, locked on the rivets in the ends of the pipe, a joint between the two being made with hot lead poured and calked in the same method as on cast-iron pipes.
In regard to the chemical processes to protect the iron, Mr. Howland does not ascribe to the Bower-Barff the high credit usually accorded it, but expects much of the Wallen process, a process of coating with copper similar to electro-plating, but having the advantage over plating, in that by it the copper seems to become a part of the surface of the iron, and is not liable to be peeled ur tlaked off by rough usage or blows. The Kalemein process, which has been extensively used in this country, is declared to have given good results when thoroughly done.
As disadvantages of wooden pipes, are cited their bulkiness, their short lengths, the difficulty encountered in joining them together and maintaining a joint under all conditions. The diameter increases with use. Their durability is questionable.
Earthenware pipes are said to make a clean, durable conductor, reasonable in cost, but have the objectionable features of reducing capacity, on account of friction, caused by the frequent number of joints, and the irregularity of their shape.
Composition pipes made of various substances, but principally of asphaltum and canvas, are declared to have been found entirely inadequate as to capacity, durability and convenience in handling.
Glass pipes of large diameters have not yet been successfully produced, but Mr. Howland places faith in the prediction of several large glass manufacturers that it will not be long before some method of casting these pipes successfully and cheaply will be devised. Then, made in form similar to our present cast-iron pipes, with some suitable device for a joint, and of a malleable glass, he declares they would form a water pipe to which there would scarcely be an objection ; strong, tough, smooth and indestructible.