BOILER ACCESSORIES: PIPING Although piping can hardly be considered a boiler accessory, a few general remarks will not be out of place.
Pipes must not only be of sufficient size and strength, but should be so installed as to make ample provision for expansion due to the high temperature when they are .filled with steam. The supports for long pipe lines should be arranged somewhat as shown in Fig. 70, which allows the pipe a considerable amount of lateral motion.
If the pipe line is long, an expansion joint must be provided. Sometimes a curved U-bend may be inserted in the pipe line, which of itself will have flexibility enough to provide for reasonable expansion. Or, if the steam main is not all in one line, a similar bend may be provided, with elbows and nipples, as shown in Fig. 71. In this case, any expansion of the steam main will cause the nipples to turn slightly in the elbows. This motion, of course, is slight, but it is sufficient to prevent rupture. U-bends and swivel-joints are hardly practicable in large pipe; and in such cases a slip-joint, made tight by a stuffing gland, is usually provided. If this is done, great care must be taken that the steam main is straight and in perfect alignment, as the pipe may otherwise bind in the expansion joint and cause much damage from leakage.
In marine work, especial care must be taken that the pipe lines are not so rigidly connected together that they will be injured by the working of the ship. This can readily be provided for by laying the pipe in such a way as to provide a simple form of swivel-joint.
The pipe lines should be as straight as possible, to prevent unnecessary friction of the steam and unnecessary condensation; and they should, if possible, be so installed as to leave no pockets wherein condensation may collect. If such a pocket is unavoidable, a drain must be provided, leading from the pocket to the steam trap, whence the condensation may be discharged into the hot well or filter-box, because the collection of water in steam pipes is a source of inconvenience and danger.
The pipe lines should be installed with sufficient slope, so that the condensation will readily drain to a convenient point whence it may be drawn off. This slope should be in the direction of the flow
of the steam, as the water will not readily flow otherwise. Great care should be taken that the pipe lines nowhere sag, as such a depression will collect condensation. This may cause very little disturbance unless the pressure of the steam is suddenly raised, in which case the water is liable to flow bodily along the pipe; and if it does not enter the cylinder of the engine and cause damage there, it will cause a serious water-hammer which may rupture the elbows of the pipe and may endanger life.
Formerly, when low pressures were used, cast iron was a common material for a main steam pipe leading from the boiler to the engine, hut the higher pressures of to-day require the best wrought iron or steel. In marine work, copper is commonly used; but with the advent of higher and higher pressures, copper fails to give the requisite strength, and it has to be reinforced with wire or iron bands. At pressures not over 150 lbs., copper pipes may be used, by the British Board of Trade rules, 15 inches in diameter; but at 200 lbs., copper pipes are not allowed over 10 inches in diameter. For large sizes, riveted iron or steel pipe may be used. For high pressures, cast-steel fittings are required by the U. S. Steamboat Inspection rules. There was always danger that the large copper pipe would burst; and it is now the common practice to use steel for such purposes.
Large steam pipe is made in sections which can be riveted together. The small sizes are fitted with the ordinary type of flange, and the sections may be bolted together, a suitable gasket being used between the two flanges to make a steam-tight joint. The flanges are machined perfectly smooth, and the packing may consist of rubber and fiber reinforced with wire insertion, or of asbestos, or of corrugated copper.
The true inside diameter of steam, gas, or water pipe is not always the same as the size of the pipe as popularly known. For instance, what is called "3-inch" pipe has an actual inside diameter of 3.067 inches, and 3.5 inches outside diameter. The actual sizes of pipe, inside and outside, can be found in any handbook or steamfitter's catalogue.