D slide valve

The D Slide Valve was a form of rectilinear slide valve for use in rotative steam engines invented by William Murdoch and patented in 1799. It was named after the hollow central piston and was in the shape of a D.

This valve worked by "connecting the upper and lower valves so as to be worked by one rod or spindle, and in making the stem or tube which connects them hollow, so as to serve for an induction pipe to the upper end of the cylinder." This allowed two valves to do the work of four.

The above description (referring to upper and lower valves) clearly relates to an engine with a vertical cylinder, such as a beam engine. Where the cylinders are horizontal, as in a steam locomotive, the valves would be side-by-side.

In the 19th century, most steam locomotives used D slide valves to control the flow of steam into and out of the cylinders. In the 20th century, slide valves were gradually superseded by piston valves, particularly in engines using superheated steam. There were two reasons for this:

  • With piston valves, the steam passages can be made shorter. This reduces resistance to the flow of steam and improves efficiency
  • It is difficult to lubricate slide valves adequately in the presence of superheated steam

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