Steam power by unknow

Author:unknow
Language: eng
Format: epub
Tags: Steam engineering
Publisher: New York, J. Wiley & sons, inc.; London, Chapman & Hall, limited
Published: 1916-03-25T05:00:00+00:00

Fig. 135.—Non-detaching Corliss Valves Located in Cylinder Head.

the cylinder heads as shown in Fig. 135. In some the wrist plate and the connecting links are also retained, but in others they are eliminated. In all engines of this type the admission valves are closed positively, the closure being effected by the same linkage that opens the valves to admit steam. Quick action is obtained by the arrangement of the operating mechanisms, the centers of rotation and the

lengths of links being so chosen that the valve travel is small when the valves are closed, that it is rapid when the valves are opening and closing, and that the valves remain practically wide open during most of the time that steam is being admitted.

The advantages of small clearance and short and separate ports are attained in these arrangements and the operation of the valves is almost as perfect as that of the trip-cut-off gear. Engines fitted with these modified Corliss gears are operated at speeds considerably higher than those permissible with the older arrangement, and they may be classed with medium-speed engines.

Engines of this type are generally known commercially as four-valve engines, but as this name applies equally well to the ordinary trip-cut-off gear and to others which will be described later, it is best to use some other designation. The term non-detaching Corliss engines seems to best describe them and is apparently gaining in favor.

Non-detaching Corliss engines generally give diagrams intermediate between those obtained with the low-speed, trip-cut-off mechanism and those obtained from slide-valve engines with the simpler forms of valves, though the later designs very closely approximate the performances of the trip-cut-off Corliss engine.

92. Poppet Valves. Attention has already been called to the fact that the use of highly superheated steam is very effective in lessening or even eliminating initial condensation. Experience has shown that large valves and valves with sliding surfaces such as slide valves and Corliss valves do not work well with highly superheated steam. The large castings warp so that contact surfaces do not remain true and the lack of moisture which acts as a seal with saturated steam leads to excessive leakage. Difficulty has also been experienced with the lubrication of these sliding types of valves when using highly superheated steam.

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