All About Engines

Raising Steam 95 But provided the Fig. 51.—Ferranti’s valve it. The space between the opening and the disc is closed by a sleeve, which is raised or lowered by a wheel and screw in the ordinary way. This type of valve, by the way, is used for other purposes, and we shall meet with it again in the next chapter. The outline of the valve in elevation is not unlike a crown —hence the name, double-beat crown valve. The objection to too small a valve in the steam pipe is that the flow of steam is hampered at the con- striction, and the pressure beyond it falls. In other words, the steam is throttled contraction of the pipe is gra- dual, narrowing slowly towards the smallest bore and widening at the same rate beyond it, the steam adapts its flow, as it were, to the shape of the pipe, and emerges beyond with an imperceptible reduction of pres- sure. On this principle Mr. Ferranti has designed a valve which, though having a very small area, is capable of passing a very large quantity of steam. A section of the valve is shown in Fig- 5i- The shape is based on the form of the stream-lines in fluids when they pass through a small opening, and is determined strictly on scientific principles. There are occasions when a reduction of the boiler pressure is desirable. For example, a small