. A manual of marine engineering: comprising the design, construction, and working of marine machinery. engines, although they give a good opening to steam. Tocombine the advantages of the two systems the piston valve is designed(fig. 88). The port area is nearly three times that of a flat valve of thesame dimension transversely, and the pressure on the sides due to thesteam is nil. Essentially, the piston valve consists of two pistons, the faceof each being equal in length to the bars of a locomotive slide-valve, andconnected by a rod. These pistons are fitted into a cylindrical chamberhaving
. A manual of marine engineering: comprising the design, construction, and working of marine machinery. engines, although they give a good opening to steam. Tocombine the advantages of the two systems the piston valve is designed(fig. 88). The port area is nearly three times that of a flat valve of thesame dimension transversely, and the pressure on the sides due to thesteam is nil. Essentially, the piston valve consists of two pistons, the faceof each being equal in length to the bars of a locomotive slide-valve, andconnected by a rod. These pistons are fitted into a cylindrical chamberhaving ports corresponding to those in the cylinder face; the faces of thepiston cover these ports, and have the same amount of lap, (fee, as a commonvalve. Steam is sometimes admitted outside the pistons, and it exhaustsfiom the cylinder into the space between them, and from there into theSKhaust passage in the usual way. But the piston valve permits of thesteam being between the pistons and the exhaust steam passing their this case the valves are said to be with inside cut-ofi. Piston valves. Fig. 88.—The Piston Slide-valve. were used very early in the 19th century, especially in marine engines,and found to be superior to the Long D and Short D valves; theywere always of the inside cut-ofi type, and then, as now, had theadvantage of confining the steam of high pressure to the circular casingsurrounding the middle of the valve, so that no joint or stuffing-box wasexposed to its action. This quality is to-day, with steam of really highpressure and high temperature, of great value, especially when the steam isrendered very dry by superheating, &c. When the pistons are sufficiently large they are connected by a pipe orhollow casting (as shown in fig. 88), through which steam can pass from oneend to the other; if this cannot be accomplished, the two ends of the valve-case are connected by a pipe cast with or connected to it. Small engines, when fitted with such valves, have
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