double-ported flat slide valve, p, fitted with a relief ring is used. Piston valves are employed with high pressure because in them the pressure is on all sides and there is no force holding the valve against its seat. They are fitted with spring rings to make them tight. Valves are made of cast iron, cast steel, or forged steel. The valve seats are generally liners of cast iron. The valve stem is secured to the valve by its taper or shoulder and the nut q and the upper end r of the rod is fitted as a guide. The lower end of the valve rod is connected by means of a bearing to the link block s, which works in the link t. To the ends of the link are attached the eccentric rod u, and the lower ends of these rods are bolted to the eccentric straps v, which move around eccentrics w. The eccentrics are secured to the shaft and fitted so that the position can be slightly changed. There are two eccentrics, one to give go-ahead motion and the other backing. The link is moved by an arm attached to the reversing shaft x, which is operated by the reversing engine y, and this engine is controlled by the reversing lever. The link arm is attached to the adjustable cut-off block, by means of which the cut-off can be varied from .5 to .75 of stroke. This is the Stephenson gear, which is most generally used; others are Marshall's, Joy's, Morton's, etc.
In the cut shown, the air pump A is operated by a cross beam B attached to the L. P. crosshead. In many merchant vessels the circulating, bilge, and feed pumps are operated from such a beam. For large installments these pumps are as a rule independent and of the Blake, Worthington, Snow, or other patent type. The office of the air pump is to pump the condensed water and vapor from the condenser to the feed tank and produce a vacuum. Surface condensers are now always fitted, and the steam and condensed fresh water are kept separate from the circulating sea water. This keeps salt out of the system. The condenser consists of an approximately cylindrical vessel, having a water chamber and a tube sheet at each end. Brass tubes connect the two tube sheets and cold sea water is pumped through the tubes by means of the circulating pump, thus cooling and condensing the exhaust steam surrounding the tubes. Condensers are made of bronze or cast steel and the tubes of brass. Circulating pumps are centrifugal and operated by a vertical simple or compound engine.
The feed tank is generally fitted with a filter chamber for purifying the water. Feed heaters, using auxiliary exhaust steam to heat the water before reaching the boilers, are fitted for purposes of economy and make the service less hard on boilers. The feed pumps are vertical, single, or duplex plunger pumps. Cylinders are lagged (i.e. covered with non-heat-conducting material) to prevent loss of heat. All cylinders are fitted with relief valves set at appropriate pressures, drains for conducting off any water that may accumulate, and indicator pipes, cocks, and reducing motion for taking indicator cards. Pressure gauges are supplied to indicate the pressure in the steam pipe, the various receivers, and the vacuum in the condenser. Revolution counters are attached, which automatically record the number of revolutions of the engines. A water service is supplied, consisting of a system of piping by means of which sea water can be circulated through such parts as the thrust bearing and crosshead guides, or sprayed on various other bearings where heating is likely.
The oil services on a modern engine are very elaborate, as all working bearings must be supplied with a lubricant. The best practice is the manifold system, where each bearing has a small pipe leading up to one of the several manifolds where it is fed by means of a wick. The manifold can be filled from a reservoir placed above the level of the engine. Besides oil, graphite and various preparations of tallow and grease are used for lubrication.
The steam pressures now used are 150-300 pounds. It is not likely that much higher pressures than 250 will be soon used, on account of the great strength of parts necessary to withstand the pressure, the difficulty of keeping tight joints, and the high temperature of steam, which heats the working surfaces and prevents proper lubrication.
Steam is expanded in triple-expansion engines 6 to 9 times; in quadruple, 8 to 12 times. The ratio of the area of the H. P. to that of L. P. cylinder varies from 1 to 5 to 1 to 10, there being a greater ratio with increased pressures. The revolutions vary from 80 or 100 per minute in very long stroke engines to 400 to 500 in high-speed torpedo boats. The piston speed is limited to about 1000 feet per minute. The length of stroke for large merchant vessels is four to six feet; for naval engines not over four feet; with smaller engines the stroke is less. A relatively long stroke results in greater economy.
Of late the steam turbine is beginning to be introduced in place of the reciprocating engine for fast vessels. See Steam Turbine.
Boilers. Modern marine practice is either to use the cylindrical fire-tube boiler carrying pressures of 150 to 200 pounds per square inch, or some form of water-tube boiler using pressures of 160 to 300 pounds. W.T. (water-tube) boilers are more largely used for naval purposes and fast passenger vessels and cargo vessels in fresh water, cylindrical boilers for general merchant service.
The substantial advantages of the cylindrical boiler are: reliability, simplicity; it is well made and generally understood; it can use salty or dirty water, and it will stand hard usage without serious loss or injury. The disadvantages are: great weight; steam cannot be gotten up or taken off quickly; it does not readily adjust itself to change of output; and heavy forced draught cannot be used.
The advantages of the water-tube boiler are: lightness; adaptability to high pressure; rapidity of raising steam or taking it off; it is readily adjusted to sudden change of output; forced draught can be used (in nearly all types); and repairs or removals are more easily made. Its disadvantages: it requires great care and attention; it cannot use salty or dirty water or experience hard usage; corrosion takes place very readily; it is complicated and many types require a large number of mechanical attachments; and being new, it is not well understood by men who handle it and best results are not obtained. The economy of fuel is about the same in the best of each type. For average running the cylindrical is probably the more economical.
The general form of a cylindrical boiler is the
